Suez Canal University Faculty of Science

INTRODUCTION OF

GEOPHYSICS

Prepared By Dr. El-Arabi H. Shendi

Professor of Applied & Environmental Geophysics 2007

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:Definitions

Geophysics is the application of physics to study of

the solid earth. It occupies an important position in

earth sciences. It has been taught principally in Earth

sciences departments of university. There is an

obvious need for it to introduce to engineers and

archeologists much more widely than at present.

Geophysics developed from the disciplines of physics

and geology and has no sharp boundaries that

distinguish it from either.

The use of physics to study the interior of the

Earth, from land surface to the inner core is known

as solid earth Geophysics

Solid Earth Geophysics can be subdivided into

Global Geophysics or pure Geophysics and Applied

Geophysics.

Global Geophysics is the study of the whole or

substantial parts of the planet. Geophysical methods

may be applied to a wide range of investigations from

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studies of the entire earth to exploration of a localized

region of the upper crust, such as plate tectonics, heat

flow and paleomagnetism.

Applied Geophysics is the study of the Earth's crust

and near surface to achieve an economic aim, or

making and interpreting measurements of physical

properties of the earth to determine subsurface

conditions usually with an economic objectives ( e.g.

discovery of fuel or mineral deposities).

Applied Geophysics

Comprises the following subjects:

1- Determination of the thickness of the crust (which

is important in hydrocarbon exploration.

2- Study of shallow structures for engineering site

investigations.

3- Exploration for ground water and for minerals and

other economic resources.

4- Trying to locate narrow mine shafts or other forms

of buried cavities.

5- The mapping of archaeological remains.

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6- Locating buried piper and cables

Engineering Geophysics: the application of

geophysical methods to the investigation of near-

surface physico-chemical phenomena which are likely

to have (significant) for the management of the local

environment.

The discipline of environmental geophysics needs

to bring to the attention of policy- makers and

planners.

The principal distinction between engineering and

environmental geophysics is that the former is

concerned with structures and types of materials,

whereas the latter can also include mapping

variations in pore fluid conductivities to indicate

pollution plumes within ground water, for examples:

- Geophysics can be used to investigate

contaminated land to locate polluted areas prior

to direct observations using trail pits and

boreholes. Large areas can be surveyed quickly

at relatively low cost.

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- The alternative and more usual approach is to

use a statistical sampling techniques, the

geophysical survey is used to locate anomalous

areas and there will be a higher certainly that

the constructed trail pits and boreholes will

yields useful results.

- Geophysics is also being used much more

extensively over landfills and other waste

repositories.

- Geophysics can be used to locate a corroded steel

drum containing toxic chemicals. To probe for it

poses the real risk of puncturing it and creating a

much more significant pollution incident.

- By using modern geomagnetic surveying methods,

the drum's position can be isolated and a careful

excavation investigated to remove the offending

(hurt) object without damage. Such approach is

cost effective and environmentally safer.

- Geophysics investing of the interior of the earth

involve taking measurements at or near the

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earth's surface that are influenced by the

internal distribution of physical properties.

- Analysis of these measurements can reveal how

the physical properties of the earth's interior

vary vertically and laterally.

- Exploration geophysics developed from the

methods used in global geophysics

Example:

- Exploration seismology used mainly in oil

exploration, have been used in academic studies

relating to the structure of the earth's crust and

upper mantle.

- Geophysics measurement within geographically

restricted areas are used to determine the

distributions of physical properties at depth that

reflect the local subsurface geology.

- An alternative method of geophysical investing

subsurface geology is, of course, by drilling

borehole, but these are expensive and provide

information only at discrete locations.

- Geophysical surveying provides a relatively rapid and effective means of deriving

distributed information on subsurface geology.

Solid Earth Geophysics

Global or pure Geophysics Applied Geophysics

Hydro-Geophysics Mining Geophysics Engineering Exploration Environmental Glacio-geophysics

( Geophysics in ( geophysics for Geophysics Geophysics Geophysics (geophysics in

Water investigation) mineral glaciology)

Exploration) Archaeo-

Geophysics

(in archaeology)

History of Geophysics

The beginning of geophysics has been started since:

a- Gilbert's discovery which stated that the earth

behaves as a great and irregular magnet.

b- Newton 's theory of gravitation .

* The initial step in the application of geophysics to

the search for minerals was taken in 1843 by von

warde which used the magnetic theodolite of Lamont

to discover magnetic ore bodies.

* in 1879 a book by Robert Thalen was published

entitled" on the examination of iron ore deposits by

magnetic methods".

* At that time, the first magnetometer called Thalen-

Tiberg magnetometer was manufactured in Sweden.

* During the past seventy years, geophysics was used

greatly in oil and gas exploration and many

geophysical techniques have been developed for the

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detection and mapping of unseen deposits and

structures.

* Advances have been rapid during the past decade

because of the development of new electronic devices

for field equipment and the widespread applications

of the digital computer in the interpretation of

geophysical data.

* Several of the devices now used by geophysicists

were developed from methods used for locating guns,

submarines and aircraft during the two world wars

Examples:

1- Guns were located in France during the First

World War by measuring the arrival times of the

elastic waves generated in the earth by the recoil

of the guns. This lead to the refraction methd of

seismic prospecting

2- Submarines were located by transmitting sound

pulses underwater and measuring the interval

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between the emission and return of the pulses;

knowing the velocity of sound in water, the

distance to the reflecting object could be

determined.

3- Radar, which was developed during the Second

World War, utilizes radio pulses in similar

manner. A modified from of radar has been

widely used for navigation purposes in marine and

airborne geophysical surveys.

4- Ships, submarines and mines were also detected in

both wars by their magnetic properties.

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:nd GeophysicsRelation between Geology a

* GEOLOGY

It involves the study of the earth by direct

observations on rocks either from surface exposures

or from boreholes and the deduction of its structures,

composition and historical evolution by analysis of

such observations.

GEOPHYSICS

It involves the study of the inaccessible earth by

means of physical measurements, usually on or above

the ground surface. It also includes interpretation of

the measurements in terms of subsurface structures

and phenomena.

* Geophysical studies are quantitative and tangible,

whereas geological studies are qualitative and descriptive.

Example(1)

* In exploration Geophysics for oil, the petroleum

geologists extract quantitative information from

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Geophysical data (such as seismic records, well

logs,…).

* On the other hand, Geophysicists who are

concerned with measurements of physical

phenomena are incorporating more geology in

order to increase the reliability of the conclusions.

Example(2)

* The information gained about the sea floor

spreading and plate tectonics is due to integrating

geophysical and geological information.

* Every earth scientist, especially the geologist, should

be familiar with the methods of geophysics. This

familiarity should enable one to know:

a – which of the geophysical methods can (or

cannot) be of help in a given geological

situation.

b- The limitations of the geophysical methods.

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* The incorporation of the available geophysical in-

formation in interpretation of geophysical

measurements is very important

PHYSICAL PROPERTIES OF ROCKS

* The physical properties of rocks that are most

commonly utilized in geophysical investigations are:

- Density

- Magnetic susceptibility

- Elasticity

- Electrical resistively or conductivity

- Radioactivity

- Thermal conductivity

* These properties have been used to devise geophysical

methods, which are:

- Gravity method

- Magnetic method

- Seismic method

- Electrical and electromagnetic methods

- Radiometric method

- Geothermal method

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1- Rock Densities:

* Any Geologic condition that result in a horizontal

variation in density will cause a horizontal variation in

gravity or a gravity anomaly.

• It is therefore the significant parameter in gravity

Exploration (i. e. the anomaly source is a local

variation in density).

* Two problems are faced in connection with this

parameter (i. e. density).

1- The maximum density variation between

different rocks and between rocks and minerals is

approximately (2). This is a very small change

compared to the range of magnetic susceptibility

(≈105) and electrical conductivity (≈1010 ).

Examples:

1.6

2.3

Sand

Dolomite

1.95

2.11

Gravels

limestone

1.7

2.24

Clay

sandstone

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2- It is not possible to measure density in situ. A

density borehole logger has been used to a limited

extent in oil exploration

* It is necessary to make density measurements in the

laboratory on small samples of outcrops or drill cores.

• In this case, the laboratory results do not necessarily

give the true bulk density of the formation, since the

samples may be weathered, fragmented or dehydrated.

* Sedimentary rocks have lower densities than igneous

and metamorphic rocks.

• Their densities depend on: composition, porosity and

pore fluids, their age and depth below surface (i.e. the

density increases with depth and time because the rock

becomes compacted and consolidated).

• For that, the laboratory density measurements should

be made, if possible, with the sample in the same

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conditions as those prevailing in the formation from

which it was removed.

* Igneous rocks are denser than sedimentary rocks.

* Basic igneous rocks have larger densities than acidic

forms.

* Porosity is of minor significance in igneous and

metamorphic rocks, unless they are highly fractured.

Examples:

Rock Density Rock Density

Granite 2.64 Basalt 2.99

Gabbro 3.03 Acidic igneous rocks 2.61

Basic igneous rocks 2.79

* Density of metamorphic rocks increases with the degree of

metamorphism since this process tend to fill pore spaces and

recrystallize the rock in a denser form.

Density Metamorphic

form

Density Rock

2.75

2.79

2.6

2.8

Marble

Slate

Quartzite

Gneiss

2.55

2.4

2.35

2.64

Limestone

Shale

Sandstone

Granite

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* The density of metamorphic rock increases as the

acidity decreases.

* Non – metallic minerals are of lower density than the

average of rocks.

*Metallic minerals are heavier than this average.

Copper 8.7 silver 10.5 Galena

7.5

Laboratory estimation of density :

* Density can be determined by direct measurements on

rock samples as follows:

-The sample is weighted in air and in water. The

difference in weights gives the volume of the sample.

- Dry density = weight / volume

* The density of a rock is quite variable. For that, it is

necessary to measure several tens of samples of each rock

in order to obtain a reliable mean density.

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2- Magnetic susceptibility of rock and minerals (K)

* When a magnetizable body is subjected to an external

magnetizing field (H), it acquires a magnetization that is

lost when the applied field (H) is removed.

* Such a magnetization ( ji ) is said to be induced by the

applied magnetizing field (H).

* ( Ji ) is parallel and proportional to the applied field (H)

Ji = KH

* (K) is called the magnetic susceptibility.

* A substance is called Diamagnetic if the (K) is negative;

it is called Paramagnetic and Ferromagnetic if (K) is

positive.

• Magnetic susceptibility is the significant variable in

magnetic, playing the same role as density in gravity.

• Sedimentary rocks have the lowest average

susceptibility and basic igneous rocks have the highest.

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Examples:

Rock K Rock K

Limestone 10x 106 shale 50x106

Granite 200x106 Quartzite 350x106

• The susceptibility depends upon the amount of

ferromagnetic minerals present (i.e. magnetite, ilmenite

or pyrrhotite).

Laboratory determination of ( k) :

• The simplest method involves a comparison of the

sample with a standard by using a laboratory instrument.

• It is to compare the deflection produced on a

tangent magnetometer by a prepared sample (a drill core

or powered rock, Ks ) with that of a standard sample of

magnetic (Kstd, FeCI3 powder in a test tube). "K" is then

given by the following equation:

Ks= Kstd (ds/d std)

Where ds is the deflection of the sample, dstd is the

deflection of the standard.

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3- Elastic properties of rocks ( Elasticity):

* The seismic method utilizes the propagation of waves

through the earth. This propagation depends upon the

elastic properties of rocks.

* The size and shape of a solid body can be changed by

applying forces (stresses) to the external surface of the

body.

* These external forces (stresses) are opposed by internal

forces (stain) which resist the changes in size and shape.

* As a result the body tends to return to its original

condition when the external forces are removed. This

property is called Elasticity,

• The theory of elasticity relates the forces which are

applied to the external surface of a body to the resulting

changes in size and shapes.

• The relations between the applied forces and the

deformations are expressed in terms of Stress and Strain

• Stress is a measure of the forces (F) per unit area

across a surface element (A) within the material.

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S=F/A

• when (F) is perpendicular to the area element, the

stress is called Normal Stress

• Normal stress can be classified into Tensile stress if

the force is directed away from the material or

Compressive stress if the force is directed into the

material .

Compressive Stress Tensile Stress

* When (F) is tangential to the area element, the stress is

a Shearing stress

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* Strain is a measure of the relative deformation

(expressed per unit length or per unit volume) of a body

when it is subjected to a stress.

* It is the change in size or shape.

* A change in shape with no change in volume is called a

Shear strain or distortion

* A change in volume without change in shape is called

a dilatation or contraction .

* Strains that are associated with relative change in

length in the direction of stresses are called Normal

strains.

Elastic properties of materials (Elastic or elastic

constants)

* The elastic properties of a material are described by

certain elastic moduli or elastic constants which specify

the relationships between different types of stress and

strain.

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1- Young's Modulus (E):

* If a load (w) is hung on the end of a wire of length (L)

and cross- sectional area (A), the wire is elongated by a

small length (∆L) in vertical direction (Z).

* Young's Modulus (E) represents the tensile stress (Pz)

tensile strain (ez) proportionality constant:

Pz α ez

Pz = E ez

E= pz/ ez = ( W/A) / ( ∆L/L)

)K(Bulk Modulus -2

* If a body of volume (V) is subjected to a uniform

compression stress (P), its volume will be decreased by

an amount (∆v).

* Bulk modulus (K) is defined as the ratio of the

pressure to the fractional change in volume.

K= P/ (∆V/V)

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):µ, Rigidity( Shear modulus -3

* It is a measure of the stress/ strain ratio in the case

of a simple tangential stress (Shear), without change

of volume.

Example:

* A pile of cards can be sheared without affecting

total volume of cards.

µ= (F/A) / ф

* The strain in this case is expressed by the angle of

deformation.

4- Poisson's ratio ( σ ):

* It is a measure of the geometrical change in the

shape of an elastic body.

Example:

* A cylinder of a length (L) and diameter (D) when

subjected to a tensile stress parallel to (L), the length

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will be elongated by (∆L) and the diameter will be

shortened by (∆D). The opposite will occur if it is

subjected to a compressional stress, the length will be

shortened by (∆L) and the diameter will be increased

by (∆D). In either case,

σ = (∆D/D)/ (∆L/L)

* For most rock, the value of (σ) is about 0.25, for

liquids the value of (σ) attains its maximum possible

value of (0.5) as the liquids have no rigidity (µ = 0).

* The relations between the elastic moduli are given

by the following formulas:

E = 9 µ K / (µ + 3K)

K= E/3 (1-2σ)

µ = E/2 ( 1+σ)

σ = (3K-2µ) / (6K+2 µ)

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4- Electrical properties of rock :

* Several electrical properties of rock are significant

in electrical prospecting which are:

A- Natural electrical potential.

B- Electrical conductivity (or the inverse, electrical

resistivity).

C- Dielectric constant.

* Electrical conductivity is the most important while

the others are of minor significance.

A- Natural ( Spontaneous) potentials:

* These potential are associated with:

- weathering of sulphide mineral bodies.

- Variation in mineral content at geologic contacts.

- Bioelectric activity of organic material (i.e. in

plant roots).

- Metal corrosion of underground pipes, cables, …..

- Thermal gradient in underground fluids.

٢٧

* There are four principal mechanisms producing

these potentials.

a- Electrokinetic or streaming potential :

* It is of mechanical origin, observed when a solution

of electrical resistively (ρ) and viscosity (η) is forced

through a porous medium.

b- Liquid – Junction (diffusion) potential:

* It is of a chemical origin, due to the difference in

nobilities of various ions in solutions of different

concentrations.

* When two identical metal electrodes are immersed

in a homogeneous solution, There is no potential

difference between them, If the concentrations at the

two electrodes are different, There is a potential

difference

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C –Shale potential :

* It is of chemical origin, occurring when the

concentrations at the two electrodes are different.

* The combined diffusion and shale potentials are

know as the electrochemical or static self potential.

d- Mineralization potential

* It is of a chemical origin, occurring when two

dissimilar metal electrodes are immersed in a

homogeneous solution.

* These potentials are especially pronounced in zones

containing sulphides, graphite and magnetite and

have larger values than the other potentials.

* The presence of metallic conductors in appreciable

concentrations is necessary to produce mineralization

potential

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B: Electrical conductivity (or the inverse,

electrical resistivities)

* Electrical current is propagated in rocks and minerals

depending on the electrical resistivities of these

materials.

* The electrical resistance of a material is expressed in

terms of its resistivity.

Example:

If the resistance between opposite faces of a

conducting cylinder of length (L) and cross sectional

area (A) is (R), the resistively (ρ) is given by:

ρ = R A / I

If "A" is in meters2 "L" in meter, "R" in ohm, "ρ"

will be in ohm- meter.

• If these dimensions are in cm ,"ρ" will be ohm –

centimeter, where: 1 Ωm - 100 Ω cm.

• 'R' is given in terms of the voltage (V) applied

across the ends of the cylinder and the resultant

current (I) flowing through it , by ohm's law:

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R= V/I

"R" in ohm "v" in volt and "I" in ampere.

• The conductivity (σ ) is the reciprocal of

resistivity:

σ = I / ρ = L/RA= (I/A) / (V/L) = J/E mhos/m or

mhos/ cm, where J = current density

(ampere/m2), E= electric field (volt/m).

• Most mineral grains are insulators except

metallic ores and clay minerals.

Sample

∆ V

I

A

٣١

• Electric conduction in these mineral grains being

through interstitial water in pores and fissures.

• The conductivity of a porous rock varies with the

volume and arrangement of the pores and the

conductivity and amount of contained water.

• Hard rocks are bad conductors of electricity, but

conduction may take place along cracks and

fissures.

• In porous sedimentary formations, the degree of

saturation and the nature of the pore electrolytes

govern the resistivity.

5 – Radioactivity of rocks:

* Radioactivity of rocks and minerals are attributed

to traces of uranium, thorium and the isotope of

potassium (K40) and their radioactive decay

products.

٣٢

* Among the earth's rocks, granites and shale show

the largest radioactivity.

* In general, the radioactivity in sedimentary rocks

and metamorphosed sediments is higher than that

in igneous and other metamorphic types, with the

exception of potassium – rich granites.

6- Thermal properties of rocks:

* It is a fact that the temperature increases with

depth. Therefore, heat must be flowing upward in

the earth.

* The amount of heat flow depends on the thermal

conductivity of the rocks.

* The thermal conductivity is a measure of how easily

heat flows through a material.

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GENERAL REVIEW OF GEOPHYSICAL

METHODS

* The physical properties of rocks have been used to

devise geophysical methods that are essential in the

search for minerals, oil and gas and other

geological and environmental problems.

* These methods are:

1- Gravity method

3 - Seismic method

5- Electromagnetic method

7- Geothermal method

2- Magnetic method

4- Electrical method

6- Radiometric method

* Geophysical methods respond to the physical

properties of the subsurface media (rocks,

sediments, water, voids, etc.. ) and can be used

Successfully when one region differs sufficiently

from another in some physical property.

• These methods can be classified into two distinct

types:

٣٤

1- Passive methods:

Which detect variations within the natural fields

associated with the earth, like the gravitational and

magnetic fields, such as gravit, magnetic, some

electric and some electromagnetic methods,

radioactive and geothermal methods.

2- Active motheds:

* These artificially generated signals transmitted into

the ground and then modify the received signals in

ways that are characteristic of the materials

through which they travel. Examples of these

methods are seismic and some electrical methods.

* Generally, natural field methods (passive methods)

can provide information on earth properties to

greater depths and are simpler to carry out than

artificial source methods (active methods).

Moreover, the artificial source methods are

capable of producing a more detailed and better

resolved picture of the subsurface geology.

٣٥

* Geophysical methods may from part of a larger

survey and thus geophysicists must be in contact

with the whole survey team and particularly to the

client.

* Few, if any geophysical methods provide a unique

solution to a particular geological situation. It is

possible to obtain a very large number of

geophysical solutions to some problems, some of

which may be geologically non-sensical. It is

necessary, therefore, always to ask the question:

"Is the geophysical model geologically plausible?.

If it is not, then the geophysical model has to be

rejected and a new one developed which does

provide a reasonable geological solution.

• Conversely, if the geological model proves to be

inconsistent with the geophysical interpretation,

then it may require the geological information to

be re-evaluated.

٣٦

• It is important that geophysical data are

interpreted within geological framework.

• The various geophysical methods depend on

different physical properties. For example:

gravity methods are sensitive to density contrasts

within the sub-surface geology and so are ideal

for exploring for major sedimentary basins

where there is a large density contrast between

the lighter sediments and the denser underlying

rocks.

• It would be inappropriate to use gravity methods

to search for ground water where there is a

negligible density contrast between the saturated

and unsaturated rocks.

• If the physical principles upon which a method is

based are understood, then it is less likely that

the technique will be misapplied or the resultant

data misinterpreted.

٣٧

• The basic geophysical methods are listed below

with the physical properties to which they relate

and their main uses.

Geophysical methods and their main applications

Applications

1 2 3 4 5 6 7 8 9 10

Physical property Method

s s s s p p Density Gravity ـــ ـــ s ــ

s s p p Susceptibility Magnetic ــm ـــ p p ـ

s s p mp p Elastic moduli, density Seismic refraction ـــ ـــ ــ ــ

ms s mp p Elastic moduli, density Seismic reflection ـــ ـــ ــ ــ

mp s p p p p p mmResistivity Resistivity

ــ m m mp mp - - Potential differences Spontaneous ــ

potential

mmm m ms mp mmResistivity Induced

polarization

mp p p p p p p p s Conductance, inductance Electromagnetic

mm s s s mp mmConductance, Inductance EM - VLF ــ

p p p s p p p m- - Conductivity EM–ground radar

mmp p s Resistivity Magneto - telluric ــ ـــ ـــ ــ ــ

P = primary method S = secondary method

m= may be used but not necessarily the best

approach, or has not been developed for this

application, - = unsuitable

٣٨

Applications :-

1-Hydrocarbon exploration (coal, gas, oil)

2-Regional geological studies (over areas of 100s of

km2 )

3- Exploration of mineral deposits.

4- Engineering site investigation.

5- Hydrogeological investigation .

6- Detection of subsurface cavities .

7- Mapping of leachate and contaminant plumes.

8- Location and definition of buried metallic objects.

9- Archaeo-geophysics .

10- Forensic geophysics .

* Several geophysical surveying methods can be used

at sea ( marine geophysics ) or in the air (aero-

geophysics )

* Reconnaissance surveys are often carried out from

the air because of the high speed of operation.

• In such cases the electrical or seismic methods are

not applicable, since these require physical

٣٩

contact with the ground for the direct input of

energy.

• Geophysical methods are often used in

combination.

Example: The search for metalliferous mineral

deposits often utilizes airborne magnetic and

electromagnetic surveying.

- prospecting for oil usually includes gravity,

magnetic and seismic surveying

• The importance of such combination appears in

the interpretation stage, ambiguity arising from

the results of one survey method may be

removed by consideration of results from a

second survey method.

٤٠

Airborne versus ground geophysical methods:

• Airborne geophysical methods are used in

reconnaissance work, but the ground methods

are used in more detailed investigations.

• They are fast and are relatively inexpensive per

unit area.

• Several kinds of surveys can be done at once.

• They can provide a more objective coverage than

ground surveys in many kinds of terrains.

• For example: several hundred line kilometers of

airborne electromagnetic surveying can be done

in a day compared with three to five line

kilometers per crew in a ground EM survey .

• The cost of an airborne electromagnetic survey,

with magnetic and radiometric data included is

likely to be 1/4 to 1/5 the cost of an equivalent

ground EM survey

٤١

• Airborne survey patterns are reasonably

uniform and complete because they do not have

the access and traverse problems of ground

survey in swamps, dense brush and rugged

topography.

• An airborne survey will give more accuracy than

a ground survey in some areas, but it will seldom

provide such detail or such sharp signals as a

ground survey .

1- Gravity method:

• It is mainly used for oil exploration. Sometimes in

mineral and ground water prospecting.

• Gravity prospecting involves the measurement of

variations in the gravitational field of the earth

(i.e. minute variations in the pull of gravity from

rock within the first few miles of the earth's

surface).

٤٢

• Different types of rock have different densities and

the denser rocks have the greater gravitational

attraction.

• If the higher–density formations are arched

upward in a structural high, such as an anticline,

the earth's gravitational field will be greater over

the axis of the structure than along its flanks.

Gravity anomaly over an anticline

Gravity

Anticline

٤٣

* A salt dome which is generally less dense than the

rock into which it is intruded, can be detected

from the low value of gravity recorded gravity

recorded above it compared with that measured

on either side.

Gravity Anomaly Over a salt dome

* Anomalies in gravity which are sought in oil

exploration may represent only one - millionth or

even one - ten - millionth of the earth's total field.

Salt dome

٤٤

* For this reason, gravity instruments are designed to

measure variations in the force of gravity from one

place to another than the absolute force itself.

* The gravity method is useful wherever the

formations of interest have densities which are

appreciably different from those of surrounding

formations.

* Gravity is an effective means of mapping

sedimentary basins where the basement rocks

have a higher density than the sediments.

* Gravity is also suitable for locating and mapping

salt bodies because of the low density of salt

compared with that of surrounding formations.

* Gravity can be used for direct detection of heavy

minerals such as chromite

٤٥

Magnetic method:

* Magnetic method deals with variations in the

magnetic field of the earth which are related to

changes of structures or magnetic susceptibility in

certain near surface rocks.

* Magnetic surveys are designed to map structure on

or inside the basement rocks or to detect magnetic

mineral directly.

* In mining exploration, magnetic methods are

employed for direct location of ores containing

magnetic minerals such as magnetite.

* Intrusive bodies such as dikes can often be

distinguished on the basis of magnetic observations

alone.

Electrical methods:

* Electrical prospecting uses many techniques, based

on different electrical properties of the earth's

materials such as:

٤٦

- The resistively method is designed to give

information about the electrical conductivity of the

earth's rocks.

- In resistivity method the current is driven through

the ground using a pair of electrodes and the

resulting distribution of the potential in the ground

is mapped by using another pair of electrodes

connected to a sensitive voltmeter.

- The resistivity method has been used to map

boundaries between layers having different

conductivities.

- It is employed in engineering geophysics to map

bedrock.

- It is used in groundwater studies to determine

salinity.

- The induced polarization (IP) makes use ionic

exchanges on the surfaces of metallic grains

(disseminated sulphides).

٤٧

- Telluric current and magneto-telluric methods use

natural earth currents and anomalies are sought in

the passage of such currents through earth

materials.

- The self potential method is used to detect the

presence of certain minerals which react with

electrolytes in the earth to generate electrochemical

potentials.

- Electromagnetic methods detect anomalies in the

inductive properties of the earth's subsurface

rocks.

- The method involves the propagation of time

varying, low frequency electromagnetic fields in

and over the earth.

- An alternating voltage is introduced into the earth

by induction from transmitting coils and the

amplitude and phase shift of the induced potential

٤٨

generated in the subsurface are measured by

detecting coils and recorded.

- Electromagnetic methods are used to detect metallic

ore bodies.

Seismic methods:

* There are two main seismic methods, reflection and

refraction:

1- seismic reflection method :

* This method is used to map the structure of

subsurface formations by measuring the times

required for a seismic wave, generated in the earth

by a near surface exploration of dynamite,

mechanical impact or vibration, to return to the

surface after reflection from interface between

formations having different physical properties.

٤٩

* The reflections are recorded by detecting

interments which are called geophones responsive

to ground motion.

* Variations in the reflection times from place to

place on the surface indicate structural features in

the strata below.

* Depths to reflecting can be determined from the

times using seismic velocity information.

* Reflections from depths as great as 20,000 feet can

be observed from a single explosion, so that in most

areas, geologic structures can be determined

throughout the sedimentary section.

S.P. G

Reflected Ray

Layer 1, V1

Layer 2, V2

Reflector

٥٠

* With reflection method one can locate and map

such features as anticlines, faults, salt domes and

reefs. Many of these are associated with the

accumulation of oil and gas.

Seismic refraction method:

* In refraction method, the detecting instruments

recorded the arrival times of the seismic waves when

refracted from the surface of discontinuity.

* These times give information on the velocities and

depths of the subsurface formations along which

they propagate.

Refracted Ray

Refractor Layer 1, V1

Layer 2, V2

S.P. G

٥١

* Refraction method makes it possible to cover a

given area in a shorter time and more economically

than with the reflection method.

Radioactive Method :

* This method is used to detect radioactive minerals

such as uranium and thorium.

Well logging method:

* This involves probing the earth with instruments

which give continues readings recorded at the

surface as they are lowered into boreholes.

* The rock properties which are covered by well

logging techniques are electrical resistivity, self

potential, gamma ray generation density, magnetic

susceptibility and acoustic velocity.

* Well logging is one of the most widely used of all

geophysical techniques

٥٢

GEOPHYSICAL ANOMALIES

* It is the local variation in a measured parameter,

relative to some normal background variation is

attributed to a localized subsurface zone of

distinctive physical property and possible

geological importance.

* A local variation of this type is known as a

geophysical anomaly.

Example:

* The Earth's gravitational field after the application

of certain corrections would everywhere be

constant if the subsurface were of uniform density.

* Any lateral density variation associated with a

change of subsurface geology results in a local

deviation in the gravitational field

٥٣

* This local deviation from the otherwise constant

gravitational field is referred to as a gravity

anomaly .

* It may be positive (high anomaly) or negative (low

anomaly).

10 20

30 10

30

20

Positive (high anomaly) Negative (low anomaly)

٥٤

AMBIGUITY IN THE INTERPRETATION OF

GEOPHYSICAL ANOMALIES

* In studying the Earth's hidden features, most

problems are of an Inverse type (i.e. deducing the

source from the observed anomaly).

* The measured physical effect ( e.g. surface

variations in gravity, magnetic or electrical fields)

can not be interpreted in terms of a unique source

occurring at a particular depth inside the earth (

i.e. the same anomaly gives more than one

interpretation).

* This is because a variety of sources with varying

parameters at different depths can theoretically

produce the same affect.

* A combination of several geophysical methods and

the different geological information often yields

more information that can help reduce the

ambiguity by narrowing down the range of possible

solutions.

٥٥

NOISE IN THE INTERPRETATION OF

THE GEOPHYSICAL DATA

* Noises are undesired readings recorded during

geophysical measurements and make the

interpretation more difficult.

* The reliability of geophysical mapping is strongly

dependent upon the quality of the field records.

* We use the term signal to denote any event on the

geophysical record from which we wish to obtain

valuable information. Everything else is called

noise.

* The signal / noise ratio, is the ratio of the signal

energy in a specified position of the geophysical

record to the total noise energy in the same

portion.

* Poor geophysical records result whenever the

signal/ noise ratio is small.

٥٦

* When signal / noise ratio is less then unity, the

record quality is usually marginal and deteriorates

rapidly as the ratio decrease further.

* Some noise can be anticipated on the basis of

existing information possible sources of terrain

noise (swamps, conductive overburden) may be

identified.

* Sources of cultural noise- mines, pip lines, and

abandoned town sites may be known.

* Noise can be attenuated by applying some

processing and treatment techniques to the

geophysical field data to increase the signal/ noise

ratio.

٥٧

FIELD GEOPHYSICAL SURVEYING

* The field surveying in geophysics can be carried out

in the form of profiles or traverses.

* These profiles must be, as possible as it can

perpendicular to the strike of the causative body.

* The distance (interval) between the measuring

points ( e.g. stations) depends up the purpose of

the surveying ( e.g. regional or detailed studies)

Example:

1- In oil exploration, we look for oil traps ( geologic

structures) which may be extended for several

hundreds of meters or even several kilometers . In

this case, the station interval may be of as large as 1

to 2 kilometers.

2- In mineral exploration, we look for mineralized

zone of few tens of meters.

٥٨

* In this case, the station interval should be as small

as possible to cover the target body with enough

number of measuring points

* The field geophysical measurements can be carried

out in more than one profile, parallel to each other

to form what is called Grid pattern system.

GLOSSERY

Anomaly : An irregularity in observed or theoretically

calculated geophysical effect caused by a

significant change in some physical

property ( e.g. density, magnetization ,

seismic velocity) of rocks.

Aquifer: A permeable rock formation that stores and

transmits groundwater to wells.

Disseminated ore: An ore body in which metal is

distributed in small amounts throughout the

rock.

٥٩

Geomagnetic reversal : A reversal of the polarity of

the earth's magnetic field.

Hydrothermal activity: Any process involving high

temperature groundwater.

Isostasy : the concept that areas of the crust are in

gravitational balance by a mechanism

which compensates for the broad

topographic variations .

Magnetic epoch : A period of the order of one million

years during which the earth's magnetic

field was predominantly of one polarity.

Magnetic event: A short period within a magnetic

epoch during which the earth's field had a

polarity opposite to that of the epoch.

Prospecting: Exploration of an area with the aim of

locating minerals, oil gas, water, …….etc

٦٠

REPRESENTATION OF GEOPHYSICAL

MEASUREMENTS

* The geophysical data can be represented in TWO

forms:

Profiles: when the measurements are taken along a

single traverse, the measured parameter is

plotted on the "Y" axis and the measuring

points on the "X" axis.

* The measurements can also be potted on parallel

profiles, called stacked profiles.

Value

Stations

٦١

b- contour maps: when the measurements are

recorded on a grid pattern system they can be

contoured in the form of maps.

PROCESSING OF GEOPHYSICAL DATA * The field geophysical data are affected by

interference from undesired sources (e.g. noises).

* This data must be subjected to different correction

and processing techniques before being interpreted.

* Rapid advances in digital computer technology

made extensive calculations for this purpose are

available.

٦٢

Examples:

* Gravity field measurements are affected by

latitudes, terrains, drifts and should be corrected for

these effects before interpretation .

* Magnetic measurements are usually affected by

daily variations in the earth's magnetic field and must

be corrected.

* Some electromagnetic methods are affected by

variations in topography and must be corrected

before interpretation.

INTERPRETATION OF GEOPHYSICAL DATA

* Interpretation of geophysical field measurements

means that the transformation of digital data into

understandable geological forms (e.g. structures,

groundwater occurrences, mineral deposits, …..)

* It can be divided into Qualitative and Quantitative.

٦٣

Qualitative interpretation * A first step towards interpretation is the

preparation of a contour map on which the intensity

values at different stations are plotted and on which

the contours of equal values are drawn at suitable

intervals.

* Contouring of geophysical maps is nowadays often

done on automatic plotters using computer programs

for interpretation.

* Qualitative interpretation means general inspection

of the contour map or profile without making any

calculations.

* Most geophysical anomaly maps are colored using

suitable color schemes and color gradations for the

areas enclosed between successive contours.

* Coloring is a very valuable aid in the qualitative

interpretation of a geophysical map in general .

٦٤

* Many features of geological interest is first

discernible when a map is suitably colored.

* An important point in considering the anomalies

in an area is the zero level, that is the reading of the

instrument at points where the field is the normal

undisturbed field.

* The qualitative interpretation of geophysical map

begins with a visual inspection of the shape and

trend of the major anomalies.

* Each contour pattern should have its geological

counterpart.

* After delineation of the structural trends, a closer

examination of the characteristic features of each

individual anomaly is made. These features are:

a- The relative locations and amplitudes of the

positive and negative parts of the anomaly.

٦٥

b- The elongation and areal extent of the contours

which suggests the strike of the corresponding

geological feature.

c- The sharpness of the anomaly as seen by the

spacing of contours (e.g. high horizontal

anomaly gradients are often associated with

contacts between rocks and with bodies at

shallow at depths).

d- Circular patterns of contours are associated with

circular bodies such as ore body.

e- Long narrow patterns are due to long narrow

bodies such as dike, tectonic shear zones,

isoclinally folded strata.

f- Dislocations, when one part of an anomaly

pattern is displaced with respect to the other

part, are indicative of geological faults.

٦٦

Quantitative interpretation

* After completing qualitative study it is important to

extract some quantitative information (e.g. the

important parameter to be estimated is the depth to

the anomalous structures).

* From the relative spreads of the maxima and

minima of the anomaly, the approximate location

and horizontal extent of the causative body may be

determined.

* From the from of the anomaly, the other parameter

of the body, its shape and depth may be

determined.

* The usual procedure in quantitative interpretation

is to guess a body of suitable from, calculate its field

at the points of observation and compare it with the

measured values.

* It is then possible to adjust the depth and

dimensional parameters of the body by trial and

٦٧

error or by automatic optimizing methods until a

satisfactory agreement is achieved between the

calculated and observed values.

* The geometrical parameters must then be

translated into structural terms. In the light of

know geology.

THE PLACE OF GEOPHYSICS IN SOLVING

GEOLOGICAL AND ENVIRONMENTAL

PROBLEMS

1- In hydrocarbon ( petroleum) exploration:

* Petroleum, when in an accumulation, forms only a

small proportion of the total fluids present in a

rock section, and none of its properties differs

sufficiently from those of the salt water.

* Since rocks can vary considerably in their physical

properties such as densities, magnetic properties,

electrical conductivities, and the seismic velocities.

It has proved possible to use these variations in

٦٨

rock properties to assist in the location of

subsurface structures which are favorable for the

accumulation of petroleum.

* All the geophysical methods concentrate on the

discovery of anomalies in the rock which overlie or

surround possible petroleum accumulations.

* Nowadays geophysical surveys are generally

considered to be standard pre- requisites before an

exploration drilling program.

* Geophysics was first applied to petroleum

exploration in the U.S.A in the early 1920's.

* Hydrocarbons (oil and gas) are normally found in

association with thick sedimentary sequences in

major sedimentary basins.

* The hydrocarbons are accumulated in commercial

quantitative in suitable geological environments

called traps.

٦٩

* There are many types of traps including tectonic

structures such as anticlines, tilted fault blocks, salt

domes and stratigraphic traps such as local sand

bodies surrounded by clay envelops or local reef

developments in limestone sequences.

* Geophysical exploration for hydrocarbons normally

employs an indirect approach, searching for the

traps, depending on the great variations in the

physical properties of the earth's rock such as

density, magnetic properties , electrical

conductivities and seismic wave velocities.

* The only techniques which are believed to be

directly related to the properties of petroleum itself

are the geochemical and radioactive surveys.

* All the other geophysical techniques concentrate on

the discovery of anomalies in the rocks which

overlie or surround possible petroleum

accumulations.

٧٠

* Exploration is usually carried out in several phases:

A-In cases where the subsurface geology is unknown

(unexplored areas), the initial reconnaissance may

involve gravity and/ or aeromagnetic surveying.

* Gravity surveying is capable of identifying areas of

thick sediments by their relatively low densities and

the large scale negative Bouguer anomalies.

* Gravity is also used to determine the subsurface

structures by the lateral changes in density. It is

employed as a preliminary to the seismic survey

enabling areas of maximum interest to be

delineated.

٧١

* Gravity method is an ideal technique for detecting

the salt domes often associated with oil

accumulations, because the density of the salt is low

compared with the surrounding sediments.

* Gravity "highs" are usually due to buried anticlines.

Gravity

Anticline

Salt dome

٧٢

* Aeromagnetic surveying can be used to estimate

variations of depth to an igneous or metamorphic

basement underlying a sedimentary sequence ( i.e.

thickness of sediments) and hence to determine

indirectly the areas of main sediment accumulation.

* Aeromagnetic measurements depends mainly on the

great difference in magnetic susceptibility between

the sedimentary rocks and the underlying

basement rocks.

* The aeromagnetic survey is usually used in

petroleum exploration more than ground survey

for the following reasons:

- The speed of the survey.

- The possibility of reaching inaccessible area.

- Local influences which would affect the accuracy

of the ground instrument are avoided.

- The aeromagnetic survey provides a rapid and

effective method of estimating the depth and

shape of the crystalline basement and hence

٧٣

approximate thickness of the overlying

sedimentary material.

*The presence of oilfields may sometimes be directly

indicated by the results of aeromagnetic surveys

which detect the presence of concentrations of

diagenetic magnetite. These concentrations are

produced by the reduction of hydrated iron oxides

and/or hematite as a direct result of micro –

seepage from buried oil accumulations.

* Once a prospective sedimentary basin environment

has been identified, further geophysical surveying

normally carried out using seismic methods,

especially reflection profiling .

* Reconnaissance seismic exploration surveying

involves measurements along widely spaced profile

lines covering large areas in order to detect

regional structural elements.

٧٤

* Detailed refection seismic surveying involves closely

spaced, intersected profile lines in more restricted

areas containing the main prospective targets, to

delineate the most promising structures.

* The main job of seismic interpretation is to involve

structural mapping in the search for the structural

closures that may contain oil or gas. Geochemical

investigations may help to differentiate between

those which are hydrocarbon – bearing and those

which are barren.

* The petroleum geologist must be able to relate the

resultant sections and maps to the surface

geological evidence and the subsurface data

furnished by well samples and cores.

* Exploration boreholes are normally sited on

seismic profile lines so that the borehole logs can

correlated directly with the local seismic section.

٧٥

* Exploration boreholes are normally sited on seismic

profile lines so that the borehole logs can be

correlated directly with the local seismic section.

*Seismic stratigraphic provides additional criteria

on which to select areas for detailed study, for

example, the definition of local deltaic of reef faces

with an associated high reservoir potential.

*Geochemical investigations may help to differentiate

between those which are hydrocarbon bearing and

those which are barren.

*Radioactive survey is a method of surface

exploration for oil. It is based on the hypothesis

that most crude oils contain radioactive material,

some of which notably dissolved radium salts or

radon gas, may be carried to the surface by

percolation and thus are areas under which oil may

lie.

٧٦

* Tests for radioactivity may be made on gas samples

drawn from shallow surface holes or on soil

samples.

* These samples are collected along closely spaced

profiles or grids covering the area under test, and

the relative radioactivity of each sample is then

measured and plotted against its map position.

* By such measurements radioactivity haloes could be

defined over oil fields.

2- The place of geophysics in mineral exploration:

* Geophysical methods are extensively used in the

search for economically valuable mineral deposits,

including non- metallic deposits such as sand,

gravel and limestone and metallic deposits such as

massive and disseminated sulphides and iron ores.

* These deposits differ significantly from their host

rocks in their physical properties and consequently

give rise to geophysical anomalies of various types.

٧٧

* The initial aim of a geophysical survey for ore

deposits is to locate mineralized areas of potential

interest.

* Airborne magnetic and electromagnetic techniques

are suitable since large areas can be surveyed

rapidly at relatively low cost .

* Once possible target area are determined, further

information on causative bodies within the

anomalous zones is obtained by ground surveys

which enable the prospector to determine whether

the anomalous bodies are of economic importance.

* If ore bodies are present, the ground geophysical

data will provide information on their depths,

extent and attitude and consequently control the

location of exploratory boreholes or trenches.

* The return - ratio is very important in geophysical

surveys. It is the ratio of the estimated value of the

٧٨

ore to the cost of the geophysical work. This ratio

must be several hundred to one.

A – Massive sulphide ores:

* They are considered to be a single mass with a cross

sectional area of at least 100m2 comprising 50% or

more of metallic sulphides.

* Such ore may contain magnetic minerals pyrrhotite

and magnetite. If these minerals are present in

reasonable quantities, the ore will produce large

magnetic anomalies.

* The electrical conductivity of massive sulphides is

very high, in the range 102 – 104 S.m-1.

* The geophysical methods applicable to the search

for such ores those responding to very dense

material (gravity), high magnetic (magnetic) and

conductive materials (electrical and

electromagnetic).

٧٩

* Airborne prospecting techniques for massive

sulphides usually exploit the property of high

conductivity ( i.e. electromagnetic).

* Airborne prospecting techniques for massive

sulphides usually exploit the property of high

conductivity ( i.e. electromagnetic methods are

used).

* The survey aircraft usually also carries a

magnetometer to provide additional information

at little extra cost.

* Ground geophysical surveys employ electrical and

electromagnetic methods. Self potential methods

are cheap and effective if the correct subsurface

conditions exist and the ore body lies at a depth of

less than a bout 30m.

* Gravity surveying is a secondary ground

exploration tool because of the high cost and

ambiguities in interpretation.

٨٠

* It provides accurate estimates of ore tonnage on the

basis of the total mass anomaly.

* Although electrical and electromagnetic methods

are the major exploration techniques, they suffer

from drawback that anomalies may result from

non- economic sources such as graphite or water

filled shear zones.

* It is possible to eliminate such non – economic

sources by using a combinations of electrical,

magnetic and gravity methods.

B- Disseminated sulphides ores :

* Disseminated sulphide deposits are those bodies in

which sulphides are scattered as specks and

veinlets throughout the rock and constitute not

more than 20% of the total volume.

٨١

* Magnetic method is not an effective tool in the

exploration for disseminated sulphides because

their magnetic susceptibility is low.

* The electrical and electromagnetic methods appear

to be the most suitable survey techniques.

* The conductivity of a disseminated sulphide ore

body is highly variable because of the irregular

dispersion of the sulphides throughout the host.

Consequently, Resistivity and electromagnetic

anomalies are encountered.

* Since electrical conduction through the metallic

sulphides is not electronic, but electrolytic through

the host rock, disseminated sulphides produce

strong induced polarization anomalies. So that, the

induced polarization method is the most

appropriate to detect such bodies.

* The physical properties of economically important

sulphides such as chalcopyrite ores are not great

٨٢

different from zones of disseminated uneconomic

minerals such as pyrite. Hence, the economic

importance of a deposit cannot be judged solely

from its IP response and further geological and

geochemical surveying need to be executed prior to

any costly drilling program.

Iron ores:

* The most widely exploited physical property of iron

ores in geophysical exploration is their magnetic

susceptibility.

* The ratio of magnetite to haematite must be high

for the ore to produce significant magnetic

anomalies, as haematite is non – magnetic.

C- Geophysical in Hydrogeology:

* Many geophysical methods find application in

locating and defining subsurface water resources.

* The magnetic method is rarely used, but it can be

used to locate faults and shear zones which could

٨٣

affect the pattern of ground water flow and

determine the basement configuration underlying

the alluvial deposits.

* The gravity method is widely used in regional

reconnaissance surveys to delineate the from and

extent of porous sedimentary deposits such as

buried valley–fill, determining the configuration of

the bedrock surface over an area of recent surface

cover.

* The gravity method has also been used to determine

groundwater volumes from anomalous mass

calculations.

Observed Gravity

Anomaly

٨٤

* Seismic refraction method is widely used in hydro

geological investigations. They provide direct

information on the level of the water table since an

increase in water content causes a significant

increase of seismic velocity.

* The technique of fan – shooting may be adapted to

the location of buried channels and gravel filled

valleys which are important sources of

groundwater in regions of largely impermeable

bedrock.

* The most widely used geophysical methods in

hydrogeology are the electrical techniques.

Buried Valley Shot

point

Impermeable rock

Impermeable rock

Fan shooting

٨٥

* Resistivity surveys are routinely employed in

ground water exploration to locate zones of high

conductivity corresponding to saturated strata at

depths down to 400 ms.

* Resistivity surveys may also provide indications of

ground water quality.

* The Resistivity of the rock is controlled by the

volume of water present and will decrease as the

salinity of the water increases.

* In a homogeneous aquifer, it is possible to

distinguish fresh from saline ground water and

even to trace the subsurface flow of contaminated

ground water resulting from polluted water has a

distinctive resistivity.

D- Geophysics in engineering geology:

* Geophysical methods are frequently used in an

initial site investigation to determine subsurface

٨٦

ground conditions prior to excavation and

construction work.

* Both seismic refraction and vertical electrical

soundings are routinely employed in the

determination of overburden thickness for

foundation purposes.

* Magnetic surveys are occasionally used to delineate

zones of faulting in bedrock, and may be employed

in the location of buried , metallic, man made

structures such as pipelines or old mine working.

* Micro-gravimetric method may be used to detect

subsurface cavities, buried valleys, faults within

bedrock, underground workings and various

archaeological features.

* Resistivity method is used to detect the presence of

the subsurface voids which constitutes highly

resistive zones.

٨٧

* A recent ground based radar transmitter can be

used successfully to detect the subsurface voides. It

provides a shallow penetration continuous profile

of the subsurface similar to a seismic section.

* It may be noted that all the above survey

techniques find application in archaeological

investigations, where they may be used in the

delineation of buried buildings, walls, tombs and

other artifacts .

* Geophysical techniques have a major role in

offshore engineering activities such as: the

construction of harbors, tidal barrages and

offshore platforms, the laying of submarine

pipelines, and dredging.

* Such offshore constructions usually require detailed

information on the nature of the sea bed and the

thickness of any unconsolidated sediment layers.

٨٨

* Dredging , which may be carried out either to

establish and maintain a navigation channel in the

approaches to a harbour or to extract sand or

gravel from offshore banks, requires information

on the thickness and distribution of sediment

layers.

E- Geophysics in the investigation of the

Earth's crust.

* The crust is defined as that part of the Earth lying

above the Mohorovici discontinuity ( i.e. Moho,

after a Yugoslavian seismologist Andrijia

Solid Crust

5-65 Km Mantle

2900 Km

Solid core

2400

Km

Fluid Core

1100 Km

٨٩

Mohorovicic 1909) which is the boundary between

the crust and the mantle, below which the velocity

of compressional seismic body waves increases

abruptly to about 8.0 km/s.

* It is composed of a series of lithospheric plates in

relative of rocks.

* Large scale seismic refraction surveys, using

explosions as seismic sources, have been carried

out to study crustal structure in most continental

areas.

* Such surveys show that continental crust is typically

30-40 Km. Thick and it is internally layered. These

layers are:

Continental Crust

Oceanic Crust

Sedimentary layer

Moho

Upper mantle

٩٠

* Upper crust : which has seismic velocities in the

range 5.8to 6.3 km/s and may represent mainly

granitic or granodioritic rocks.

* Lower crust : which has seismic velocities in the

range 6.5 to 7 km/s and may represent igneous and

metamorphic rocks, including gabbro and basic

granulite.

* Marine seismic refraction surveys show that the

thickness of the ocean crust is 6 to 8 km, composed

of three layers with different seismic velocities

which are:

Layer Thickness

(km)

Velocity

(km/s)

Rock type

1 0 : 1.0 1.6 : 2.5 Sediments

2 1: 2 4 : 6 Pillow lava

3 4.5 : 5.5 6.5 : 7.0 Dolerite dykes and

gabbro

* Gravity surveying estimated regional variations of

crust thickness on the basis of variations in the level

of the Bouguer anomaly field.

٩١

F- Geophysics in the investigation of the

Earth's interior

* Most of the our methods for studying the interior of

the Earth are geophysical in nature.

* Our knowledge about the earth's interior is gained

from large earthquakes whose waves pass through

the entire earth.

* These knowledge comes from the behavior of these

waves as they travel through the earth ( i.e.

Moho

Surface

Lithosphere

Astenosphere Astenosphere

Zone of low

S-Wave velocity

Zone of slowly increased

S-Wave velocity

Zone of rapidly increased

S-Wave velocity

Mesosphere

70 Km

400 Km

٩٢

includes the changes in the velocities and paths of

the waves as they travel through different kind of

rocks and from solids to fluids).

* The key to our understanding of the earth's interior

is the knowledge of seismic wave velocities, because

from this we learn what kind of materials lie at

depth and how these materials are distributed.

* Earthquakes produce compression waves ( P- waves)

and shear waves or secondary ( S- waves) , together

called body waves because they travel through the

earth.

* P- waves vibrate in the direction of wave

propagation, and S- waves vibrate at right angles to

the direction of propagation.

* P- waves travel faster than S- waves and therefore

at recording station first.

٩٣

* From the focus of an earthquake, P and S waves

spread outward in all directions.

* The velocity of waves depends on both the elasticity

and the density of rocks through which the waves

travel.

* Elasticity is a measure of the degree to which a rock

deforms when subjected to stress. It generally

increases with depth.

* Density also increases with depth.

* Greater elasticity allows seismic waves to travel

faster, greater density slow them down.

٩٤

Layers of the Earth

1- Crust:

* In crust, there is a general increase

٩٥

* The Moho discontinuity is a boundary between

different types of rocks and is marked by sharp

increase in the velocities of both P and S waves.

2-Mantle:

* It has the greatest share of the earth's volume,

extending from a depth of about 20 Km. to 2900

Km.

* The mantle can be subdivided, based on seismic

wave behavior, into a number of layers:

a- lithosphere:

* It is the most important in the theory of plate

tectonics.

* It comprises the top part of the mantle and all the

crust, about 70 to 100 Km. thick composed of

strong, brittle rocks.

* The lithosphere is broken up into about two dozen

sections called plates, which are shifting position

٩٦

with respect to one another over the earth's

surface.

b- Astenosphere:

* It lies below the lithosphere and extends from about

70 Km. under the oceans and 100 Km. under the

continents to a depth of about 700 km.

* It is a weak material in contrast to the stronger

lithosphere.

* It is characterized by low seismic velocities,

particularly in the top part from 70 km (100 Km.

under continents) to about 400 km.

* In this section, S-wave velocities decrease from 4.7

km/s to 4.2 km/s. P-wave velocities also decrease.

This zone is called the low velocity zone.

* The decease in velocities is interpreted to mean that

partial melting occurs in the low velocity zone such

٩٧

decrease is called attenuation. A weak material has

greater attenuation than a strong material.

_____________________________

Example:

* The vibration of a bell: A good bronze bell low

attenuation and will vibrate when struck because

bronze is a strong material. But, a ball made of

lead is weak and has a high attenuation when

struck.

* The low velocities and high attenuation of seismic

waves in the astenosphere indicate that the

astenosphere is not nearly as rigid as the overlying

lithosphere.

* The significance of the partially molten

astenosphere is that the lithosphere can slide over

it. This movement is a vital part of the theory of

plate tectonics.

Amplitude Attenuation

٩٨

C- Mesosphere:

* It is be the lower part of the mantle (400 to 2900

km). In it, rocks are dense and highly elastic and

seismic wave velocities increase.

* At its base lies a thin transition zone in which S

waves die out quite rapidly.

3- core:

* It lies below the mesosphere and the transition zone,

little is known about the core.

* It plays no role in the movement of lithospheric

plates, but it is the source of the earth's magnetic

field.

* The P- seismic waves show a sharp drop in velocity

when they reach the core and that their velocities

increases as they travel through it, out with slower

velocities than in the mantle. Scientists conclude

٩٩

that the core is of much greater density than the

mantle.

* There is a discontinuity at about 5100 km. velocities

increase there, the wave behavior has led

seismologists to postulate a solid inner core.

* S- waves cannot travel through fluids because they

act to change the shape of a body. Water and air

can have their volumes changed by contraction or

expansion, but they cannot have their shapes

changed.

* S- waves are not refracted down into the core but

die out at the core-mantle boundary. This

convincing evidence for a molten outer core.

Continental crust versus oceanic crust

* The structure and composition of oceanic crust is

relatively simple compared with continental crust.

* The igneous part of the oceanic crust consists of

basalt, rich in iron and magnesium.

١٠٠

* The oceanic crust is uniform in thickness, being

about 10 km thick.

* Continental crust may be thin as 20 km and as thick

as 70 km under mountain ranges. It averages about

35 km thick.

* The rock in the upper 10 to 20 km. Have the

average composition of the igneous rock

granodiorite. Downward, the continent is composed

of the common metamorphic rock gneiss( i.e.

metamorphic equivalent of granodiorite).

* Continental and oceanic crust also differs in density.

Basalt is denser than granodiorite. For that, the

average density for continental crust is 2.7 gm/cc

and for oceanic crust is 3.0 gm/cc.

* About 65% of the earth's surface is underlain by

oceanic crust and about 35% by continental crust.

Although the oceans cover approximately 71% of

١٠١

the earth's surface, part of the ocean waters lie over

the edges of the continents and thus over

continental crust.

• Oceanic crust is relatively young, not greater

than 200 million years. Continental crust range

to as old as 3.8 billion years.

PLANNING AND COORDINATING

GEOPHSICAL WORK

* There is a special need to coordinate geophysical

work with geological investigations because they

are so interdependent.

* A geophysicist chooses field methods and traverses

on the basis of interpreted geology.

* A geologist uses geophysical information in making

an interpretation.

١٠٢

Preparing for geophysical surveys

A – preliminary considerations

1- Geophysical exploration models:

* These models depend on the information gained

from:

a- Geology of the area.

b- Contrasts between physical properties.

c- Probable range in depth of occurrence.

2- Objectives:

* It is important that the objectives of a geophysical

survey should be clear at the beginning.

* The geophysical survey produced poor results for

the following reasons:

1- Inadequate and / or bad planning of the

survey.

2- Incorrect choice or specification of technique.

3- Insufficient experienced personnel conducting

the investigation.

١٠٣

* For cost effective, experienced geophysical

consultants are employed for survey design, site

supervision and final reporting.

١٠٤

* The objective will be to do the work within the best

of some sequences such as:

a- Limits in cost.

b- Time.

c- Scheduling.

Survey constraints (limitations):

1- Finance: how much is the survey going to cost

and how money is available?

* The cost of the survey will depend on:

• Where the survey is to take place?

• How accessible the proposed field site is?

• What scale the survey is to operate?

• The more complex the survey in terms of

equipments and logistics, the greater the cost is

likely to be.

• It is important to remember that the geophysics

component of a survey is a part if an exploration

program and thus the costs of the geophysics

١٠٥

should be viewed in relation to those of the whole

project.

• The factors that influence the various components

of a budget also vary from country to country

and form job to job.

• Some of the basic elements of a survey budget are

given in the following table:

Staffing Management, technical, support,

administration, etc.

Operating costs Including logistics

Cash flow Assets versus usable cash

Equipment For data acquisition and data reduction

analysis – computers and software

whether or not to hire or buy.

Insurance To include liability insurance as

appropriate

Overheads Administration, consumables, etc.

Development

costs

Skills, software, etc.

Contingencies Something is bound to go wrong at some

time, usually when it is most convenient.

١٠٦

• The main people to be involved in a survey are:

Geologists, Geophysicists, Surveyors.

• Vehicles and equipments will need maintaining ,

so skilled technicians and mechanics may be

required.

• Everybody has to eat and it is surprising how

much better people work when they are provided

with well prepared food: a good cook at base

camp can be a real asset.

• Due considerations should be paid to health and

safety and any survey team should have staff

trained in First Aid.

• Local labor (workers) may be needed as porters

(carriers), guides, translators, guards.

• In some countries, access to a survey site in dry

season may be possible whereas during the rains

١٠٧

of the wet season, roads may be totally

impossible.

• Also access to land for survey work can be

severely hampered during the growing season

with some crops reaching 2-3 meters high.

• some survey such as seismic refraction and

reflection may cause a limited amount of

damage for which financial compensation may

be sought.

• Consideration has to be given to the transport

of the geophysical and other equipments.

• It may even be necessary to make provision (

arrangement ) for a bulldozer to excavate a

rough road to provide access for vehicles.

• Other constraints (limitations) are those

associated with politics, society, and

religion:

١٠٨

Political limitations:

• This means gaining permission form land

owners tenants ( ]^_`abcdefا) for access to land

and communications with clients which

often requires great diplomacy.

• It is important to have a permission from

the appropriate authority to carry out

geophysical field work.

Examples: permissions from a local council if

survey work along a major road is being

considered. Permissions from the local

harbour master in case of marine surveys to

safe other shipping.

Social limitations:

• In designing the geophysical survey, the

questions must be asked" Is the survey

technique socially and environmentally

acceptable?

١٠٩

• It is always best to keep on good terms with

the local people. Treating people with

respect will always bring dividends (hiاjk).

• Each survey should be socially and

environmentally acceptable and not cause a

nuisance (problems). An example is in not

choosing to use explosives as a seismic

source for reflection profiling through

urban areas or night . Instead, the seismic

vibrator technique should be used.

• Another example: an explosive source for

marine reflection profiling would be

inappropriate in area associated with a

fishing industry because of possibly

unacceptable high fish kill.

Religious limitations:

• Religious traditions must be respected to

avoid difficulties. The survey should take

into account local social customs such as:

١١٠

• Muslims like to go to their mosques on

Friday afternoon and are thus unavailable

for work then.

• Similarly , Christian workers do not like to

work on Sundays or Jews on Saturdays.

Geophysical survey design:

A- Target identification:

• Geophysical methods locate boundaries

across which there is a marked contrast in

physical properties. Such a contrast gives

rise to geophysical anomaly which indicates

variations in physical properties relative to

some background value.

١١١

١١٢

o The physical source of each anomaly is termed

the geophysical target. Some examples of targets

are trap structures for oil and gas, mineshafts,

pipelines, ore bodies, cavities , groundwater,

buried rock valleys.

o In designing a geophysical survey, the type of

target is of great importance. Each type will

dictate (tell) to a large extent the appropriate

geophysical method (s) to be used.

١١٣

Examples:

- Consider the situation where saline water intrudes

into a near- surface aquifer, saline water has a high

conductivity ( low resistivity ) in comparison with

fresh water and so is best detected using electrical

resistivity or electromagnetic conductivity methods.

- Gravity method would be in–appropriate in this

case because there would be no density contrast

between the saline and fresh water.

- Similarly, seismic methods would not work as there

is no significant difference in seismic wave

velocities between the two saturated zones.

- Also, the shape and size of the target is important to

know. In the case of a metallic ore body, a mining

company might need to known its lateral and

vertical extent. This comes from the amplitude of

the anomaly (I .e. its maximum peak-to–peak

value).

١١٤

B- optimum line configuration:

• There is an important question in this case:"

How are the data to be collected in order to

define the geophysical anomaly ? Two concepts

need to be introduced, namely: profiling and

mapping.

a- profiling :

* It is a mean of measuring the variation in a

physical parameter along the surface of a two

dimensional cross section.

* The best orientation of a profile is normally at right

angles to the strike of the target. Indication of

١١٥

geological strike may be obtained from existing

geological maps, mining records, etc..

• The length of the profile should be greater than

the width of the expected geophysical anomaly to

define a background value.

• Data values from a series of parallel lines or from

a grid can be contoured to produce a map on

which all points of equal values are joined by

isoclines

• A great care has to be taken over the methods of

contouring or else the resultant map can be

misleading.

١١٦

C- Selection of station intervals:

* The point at which a geophysical measurement is

made is called a station and the distances between

successive measurements are station intervals.

* The success of a geophysical survey depends on the

correct choice of station intervals. It is a waste of

time and money to record too many data and

equally wasteful if too few are collected.

* How is a reasonable choice of station intervals to be

made?. This requires some idea of the nature and

size of the geological target.

* Any geophysical anomaly found will always be

larger than the feature causing it. For example: to

find a mineshaft with a diameter of two meter, an

anomaly with a width of at least twice this might be

expected. Therefore, it is necessary to choose a

station interval that is sufficiently small to be able

to resolve the anomaly.

١١٧

* Reconnaissance survey tend to have coarser station

intervals in order to cover a larger area quickly

and to indicate zones over which a more detailed

survey should be conducted with a reduced station

interval and a more closely spaced set of profiles.

١١٨

* The figure (A) shows a typical electromagnetic

anomaly for a buried gas pipe. The whole anomaly

is 8m wide. If a 10m sampling interval is chosen,

then it is possible either to clip the anomaly as

shown in figure (B) or to miss it entirely (fig. C).

* The resultant profile with 2 m and 1m sampling

intervals are shown in figures (D) and (E)

respectively.

* The smaller the sampling interval, the better the

approximation is to the actual anomaly.

* The loss of high- frequency information, as in

figures (B) and(C), is a phenomenon known as

know as spatial aliasing .

* Another from of spatial aliasing may occur when

gridded data are contoured, particularly by

computer software. For example figure 1.8 A

shows a hypothetical aeromagnetic survey. This

١١٩

map was complied from contouring the original

data at line spacing of 150m.

* Figures (B) and (C) were contoured with line

spacing of 300m and 600m respectively.

* The difference between the three maps is very

marked, with a significant loss of information

between figures (A) and (C).

* The higher frequency anomalies have been aliased

out, leaving only the longer wavelength (lower

frequency) features.

١٢٠

* In addition, the orientation of the major anomalies

has been distorted by the crude contouring in

figure (C).

* The spatial aliasing can be removed or reduced

using mathematical functions, which provide

١٢١

means of developing a better gridding scheme for

profile line- based survey.

* Similar aliasing problems associated with

contouring can arise from radial survey lines

and/or too few data points as shown in figure 1.9.

١٢٢

* Figures (A) and (B) both have 64 data points over

the same area. In figure (A) the orientation of the

contours follows that of the line of data points to

the top left – hand corner, whereas the orientation

is more north- south in figure(B).

* Figure (C) shows the inadequacy of the number of

data points, which is based on only 13 data values,

forming concentric rounded contours. On the

other hand, figure (D) has been compiled on the

bases of 255 data points and exposes the observed

anomalies much more realistically.

D- Noises

* When a field survey is being designed it is

important to consider what extraneous data

(noise) may be recorded.

* There are various sources of noises:

a- Man made sources (cultural noise): such as

electric cables, vehicles, pipes drains.

١٢٣

b- Natural sources: such as wind and rain, waves,

and electric and magnetic storms.

• Electrical resistivity survey should not be

conducted close to or parallel to metal pipes, nor

parallel to cables as power lines will induce

unwanted voltages in the survey wires.

• Before a survey start, it is always advisable to

consult with public utility companies to provide

maps of their underground and overhead

facilities.

* It is important to check on the location of water

mains, sewers ( underground pipes for carrying

off sewage or rainwater), gas pipes, electricity

cables, telephone cables and cable-television

wires.

* Such utilities may mask any anomalies caused by

deeper- seated natural bodies.

١٢٤

* It is also worth checking on the type of fencing

around the survey area. Wire mesh and metal

sheds can affect on the electromagnetic and

magnetic surveys.

* Cultural and unnecessary natural noise can often be

avoided or reduced significantly by careful survey

design.

* Modern technology can help to increase the signal –

to – noise ratio, so that even when there is a degree

of noise present, the important geophysical signals

can be enhanced above the background noise levels.

١٢٥

E – data analysis

• As automatic data logging and computer analysis

are becoming more common, it is increasingly

important to standerdise the format in which the

data are recorded to ease the portability of

information transfer between computer systems.

• This also makes it easier to download the survey

results into data processing software packages.

• To make computer analysis much simpler it

helps to plan the survey well before going into

the field to ensure that the collection of data and

the survey design are appropriate for the type of

analyses anticipated .

١٢٦

• To get a reliable analysis, the following questions

must be considered:

1- How reliable is the software?

2- Has it been calibrated against proven manual

methods, if appropriate?

3- What are the assumptions on which the software

is based and under what conditions are these no

longer valid, and when will the software fail to

cope ( succeed) and then to produce erroneous

results?

* Unfortunately, there are no guidelines are accepted

standards for much geophysical software apart

from those for the major seismic data processing

systems.

* However, the judicious (having sound judgment)

use of computers and of automatic data – logging

methods can produce excellent results.

١٢٧

F- Procedure:

* One or more organizations will be capable of doing

the job. In order for then to set up a tentative

procedure and offer their services, the following

conditions must be taken into account:

a- Size of the area.

b- Degree of detail needed.

c- Orientation of survey lines and spacing of survey

stations.

d- Type of coverage needed (complete or partial).

e- Sensitivity required in each proposed method.

f- The format of the data to be delivered (raw data,

contoured data, interpreted data).

g- Scheduling of the job.

h- Kind of terrain involved, seasonal

characteristics, and field base facilities.

١٢٨

B- Preparations for geophysical work.

1- Before the job gets underway, the geologist and

geophysicist will design a specific program in

which the following points should be covered:

a- Geological conditions :

• Using existing geologic maps and prior

geophysical surveys to indicate discontinuities

and lithologic contrasts, the geologic pattern

will be related in detail to physical properties

such as density, conductivity and magnetic

susceptibility.

b- Sources of noise:

• Possible sources of terrain noise are swamps

and conductive overburden.

• Sources of cultural noise are mines, pipelines

and town sites.

c- Access:

• The geologist should have some information on

physical conditions of access such as roads,

terrain and weather.

١٢٩

• There will be some legal conditions of access. For

example, the geophysicist may need formal

permission to enter the land, permits to bring

geophysical equipment into the country and

work permits for personnel.

d- facilities:

* If the geologist or geophysicist is going to work in

an area, something must be known about the

facilities such as: supplies, campsites maintenance

and repair

2-Scheduling:

• The season, the time allowable for completion of

the job and possible delay and extensions are

taken into account.

a- Season:

• Weather conditions may decide the best

flying season and the best season for ground

access.

١٣٠

• In tropical areas man soon rains may make

geophysical work impossible during certain

months.

• In arctic areas: winter weather and

darkness will restrict certain types of

geophysical work.

• The best season for geophysics in a

particular region will also be the busiest for

contractors and there may be a shortage of

available crews.

b- Delays:

• It is almost impossible to avoid some delay due

to weather, equipment malfunction, magnetic

storms and unexpected problem with land,

govemment and people.

• The scheduling should therefore be flexible

enough to permit alternative geophysical

methods and traverses.

١٣١

3- Extension:

• Extra geophysical traverses may be needed

while the work is in progress. Survey lines

may have to be extended into nearby areas

(sometimes into areas not yet controlled by

permits or claims).

4-Sampling and orientation:

• Samples for the laboratory determination of

geophysical parameters can be furnished by

the geologist.

• The geologist and geophysicist may take an

orientation tour of the most significant

outcrops, across a known ore body to identify

a representative ore body in the area or in

some analogous area.

5-Survey control:

* Existing maps and aerial photographs will need

to be studied.

١٣٢

6- Subsurface information :

* Key information from stratigraphic sequences,

samples from depth and dimensions from

profiles are important to geophysical work.

* Drill holes may be planned to obtain

information in the most critical locations.

* In some instances, a few extra meters of

drilling to intersect a significant boundary in

physical characteristics or an inexpensive

noncore hole to the base of overburden may be

worthwhile with respect to geophysics.

* Down hole geophysical information is directly

applicable to surface geophysics.

* Certain drill holes may therefore be filled with

heavy mud or lined with plastic casing and

kept open for geophysical logging.

١٣٣

C- Coordination work during a geophysical

survey.

1- Sorting of apparent anomalies:

Some specific work may be needed to strengthen or

verify preliminary interpretations.

2- Key drilling and trenching:

Subsurface information may be needed for depth

control points.

3- Providing for extended coverage:

Earlier ideas on the limits of an exploration target

may be changed by the geophysical data.

Additional work may be needed.

D- Follow–up work:

* After the job has been completed, the geophysicist

will interpret the data and additional work may

be needed to confirm geophysical interpretations,

appropriate targets will be drilled.

١٣٤

* Geophysical surveys are keys to the depth

dimension but they are delicate keys.

* A geophysical survey is a job for specialists and the

interpretation of geophysical data is a job for

experts.

* Both specialists and experts know that their work

would be of limited value if it had no geologic

guidelines.

* Geologic specialists and experts know that their

work would be severely limited without geophysical

information.

References

1-Reynolds J.M. (1997): An introduction to applied

and environmental geophysics. John Wiley and

sons Ltd, England.

2-Telford W.M., Geld art L.P., sheriff R.E. and Keys

D.A (1990): Applied geophysics, 2nd ed. Cambridge.

١٣٥