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Groundwater exploration-Geophysical Methods.

Unit-II

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•Exploration geophysics is an applied branch of geophysics, which uses physical methods 

(such as seismic, gravitational, magnetic, electrical and electromagnetic) at the surface of the Earth to measure the physical properties of the subsurface, along with the anomalies in those properties.

Traditional solution:Drill several boreholes & install wells:

Expensive Labor IntensiveTime

If site is contaminated:Disposal of contaminated coreClean equipment

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Geophysical Investigations•Geophysical investigations involve simple

methods of study made on the surface with the aim of ascertaining subsurface detail. This is achieved by measuring certain physical properties and interpreting them mainly in terms of subsurface geology.

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Importance of Geophysical Investigations•These investigations are carried out

quickly. •This means large area can be investigated

in a reasonable short period and hence time is saved.

•The geophysical instruments used in the field are simple, portable and can be operated easily.

•This means fieldwork is not laborious.

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Importance of Geophysical Investigations•Different inferences to suit different

purposes can be drawn from the same field data, for example electric resistivity data can be interpreted for knowing subsurface of rock types, geological structures, groundwater conditions, ore deposits depth to the bed rock, etc. Hence the investigations are multipurpose.

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ELECTRICAL RESISTIVITY METHOD 

•The electrical resistivity method involves the measurement of the apparent resistivity of soils and rock as a function of depth or position.

•It is calculated by dividing the measured potential difference by the input current and multiplying by a geometric factor specific to the array being used and electrode spacing.

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Electrical Resistivity Method•All geological formations have a property

called electrical resistivity which determines the ease with which electric current flows through them. This resistivity is expressed in the units of Ώm ohms meter and is indicated by the symbol ᵖ

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Electrical Methods ELECTRICAL RESISTIVITY METHOD• The electrical resistivity of a rock

formation limits the amount of current passing through the formation when an electrical potential is applied.

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♦Electrical resistivity is the intrinsic property of any material, it is the resistance of a volume of material to the flow of electrical current.

♦ Current is introduced into the ground through a pair of current electrodes

♦ Resulting potential difference is measured between another pair of potential electrodes

♦ Apparent resistivity is then calculated as:

IVa2a

V is the measured Potential difference (in Volts) and I is the current introduced (in Amperes).

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Resistance and Resistivity

The amount of current that flows as a result of a given applied voltage potential in various materials is known as the resistance of the material.

Resistance is measured in ohms and in general is inversely proportional to the cross-sectional area of the material that is carrying the applied current, and is proportional to the resistivity of the subject material.

Ohm’s Law states that the current flowing in any given circuit (or material) is directly proportional to the applied voltage (e.m.f.) and inversely proportional to the resistance. Therefore, the resistance of a circuit is equal to the applied voltage divided by the resultant current.

R = V/I

For instance, if a voltage potential of 10 volts were applied across a material, and a current of .001 amperes was measured flowing through the material (circuit), the resistance of the material would be 10/.001, or 10,000 ohms.

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Resistivity is a bulk property of material describing how well that material inhibits current flow. This is slightly different from resistance, which is not a physical property. If we consider current flowing through a unit cube of material, resistivity is defined as the voltage measured across the unit cube's length (V/m) divided by the current flowing through the unit cube's cross sectional area (I/m2). This results in units of Ohm-m.

Resistivity(rho) = R A/LWhere;R = resistance of material along its lengthL = lengthA = cross sectional area

Solid annealed Copper has a Resistivity of 1.72 ohm-cm x 10-6

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Electrical Resistivity method

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Electrical Resistivity method

•Profiling and Sounding are two types of resistivity investigations. Profiling is done to detect lateral changes in resistivity. This study reveals the changes in the subsurface lithology or structure from place to place.

•Sounding is done to determine the vertical changes in resistivity, this study reveals changes in lithology, at a particular place with increasing depth.

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•Representative values of resistivity. The values are expressed in units of 103 ohm-cm(after Peck et al, 1974)Material Resistivity (ohm-cm)

•Clay and saturated silt 0 – 10•Sandy clay and wet silty sand 10 – 25•Clayey sand and saturated sand 25 – 50•Sand 50 – 150•Gravel 150 – 500•Weathered rock 100 – 200•Sound rock 150 – 4000

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The Wenner Method• This was developed by Wenner in 1915. In this

configuration, the outer electrode C1 and C2, are used to send current into the ground and the inner electrode P1 and P2 are used to measure the potential. The important feature of this set-up is that the distance between any two successive electrodes is equal. The apparent resistivity measured with the wenner array is given by pa= 2 ∏a[∆v] /I ,

IVa2a

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The Schlumberger Method• This was developed by

Schlumberger in 1916.. For this purpose, potential electrodes are kept at smaller separation compared to the current .Here A and B are the current electrode and M and N are the potential electrodes.

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The Schlumberger configuration

IV

b2/b2/L 22

a

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Sounding• This method is popularly known as vertical electrical

sounding. It is also described as ‘depth probing’. ‘electrical coring’ etc.

• In this method, Resistivity value ‘pa’ is measured at the same place by increasing the distance between the current electrodes each time after taking reading.

• This kind of successive increasing in distance makes the current penetrate more and more deeply. Hence the change in pa value measured indicate the vertical variations in the subsurface at the investigated point.

• Thus the sounding technique is useful in investigating only horizontal or gently inclined structures. This is so because, under such conditions only pa value shows variation when the successive readings are taken.

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Inverse Slope Method

Electrode spacing, a

Inverse slope method

• This semi-empirical method is mostly used in drought prone areas

where the water table is very deep. It is simple to operate and gives

good results.

• In this method Plotting are made by electrode space ‘a’ in X axis

and electrode separation divided by apparent resistivity values in

Y axis. The point of intercepts give depth of various interfaces.

a/p

a

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SEISMIC METHOD

•What are seismic waves?• Useful property: Waves travel at different velocities

True of different phases (e.g. solid, liquid, and gas) True of different Earth materials:

Dry soil (vadose zone) ~ 1000 ft/sec Wet soil (phreatic zone) ~ 5000 ft/sec Shale = 7000-9000 ft/sec Sandstone = 8000-12,000 ft/sec Granite > 17,000 ft/sec

• If we can measure the velocity of seismic waves versus depth, we can define the water table and infer the stratigraphy at depth without intrusive methods.

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Reflection/refraction seismology  is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves.

The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a seismic vibrator, commonly known by the trademark name Vibroseis. Reflection seismology is similar to sonar and echolocation. 

Seismic waves are mechanical perturbations that travel in the Earth at a speed governed by the acoustic impedance of the medium in which they are travelling. The acoustic (or seismic) impedance, Z, is defined by the equation:

Z = Vp

where V is the seismic wave velocity and ρ (Greek rho) is the density of the rock

The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity

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When a seismic wave travelling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interfacethe seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers (or geophones) at the surface.

 Knowing the travel times from the source to various receivers, and the velocity of the seismic waves, a geophysicist then attempts to reconstruct the pathways of the waves in order to build up an image of the subsurface.

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How to measure the composition of a 1-layer, homogeneous site

•Place a geophone (mini-seismometer) at a known distance from a seismic source

•Generate a seismic wave •Measure the time it takes the P-Wave to

move from the source to the geophone•Calculate and interpret the velocity:

TimeDistanceVelocity

How to address 2-layered site using seismic refraction•Step 1:

Use multiple geophones (an array) typically spaced equal distances apart (e.g. 10 feet)

•Step 2: Generate a seismic wave

•Interpret results

“Bison Digital Instantaneous Floating Point Signal Stacking Seismograph”

Make an artificial seismic wave

Borehole explosion

Thumper truck

Shotgun blast

Sledge hammer

Sledge hammer

How seismic waves refract•Time 1:

▫Direct wave travels in all directions from source▫Vibrations from Layer 1 arrive at closest geophone (1)

1 2 3 4 6 7 8 1095

Soil

Bedrock

How seismic waves refract•Time 2:

▫Wave front reaches layer boundary ▫Vibrations from Layer 1 recorded by next closest

geophone (2)1 2 3 4 6 7 8 1095

Soil

Bedrock

How seismic waves refract•Time 3:

▫ Wave front travels faster through Layer 2 (bedrock) than Layer 1 (soil).

▫ This causes the interface between the layers to vibrate in advance of the wave front in Layer 1 (seismic refraction).

▫ Waves generated by the interface travel back to surface. 1 2 3 4 6 7 8 1095

Soil

Bedrock

Refracted Wave

How seismic waves refract•Time 4:

▫Refracted waves reach geophones ahead of direct waves from source.

1 2 3 4 6 7 8 1095

Soil

Bedrock

Refracted Wave

Make a data tableDistance from Source Arrival Time

(Feet) (Milliseconds)10 920 15.530 1840 2050 22.560 2370 2380 25.590 26.5

100 26110 27120 28.5

Refraction Results SUNY Oneonta

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5

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0 20 40 60 80 100 120

Distance (feet)

P-W

ave

Arr

ival

Tim

e (m

illis

econ

ds)

0

5

10

15

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0 20 40 60 80 100 120P-W

ave

Arr

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Tim

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Distance (feet)

Refraction Results SUNY Oneonta

v1=1000 ft/secv2=5000 ft/sec

v3=12,000 ft/sec

Raw data Seismic Refraction data

Geo

phon

e N

umbe

r

P-Wave Arrival Time (milliseconds)

Data analysisStep 1:

Pick P-Wave arrival times (first deviation)Step 2:

Graph arrival time versus distance

Pit P-wave arrival times Seismic Refraction data

Geo

phon

e N

umbe

r

P-Wave Arrival Time (milliseconds)

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Degree of separation C Vp (m/s) RQD

No joints 0.65-1 > 4500 very good

Few joints 0.45-0.65 4000-4500 good

Jointed rock 0.3-0.45 3500-4000 moderate

Numerous joints 0.15-0.3 3000-3500 bad

Strongly jointed rock 0.00-0.15 < 3000 very bad

Seismic velocity and rock type

Seismic velocities are dependent upon rock type but also upon factors like porosity, water content, and degrees of fracturing and jointing. In  environments where the rock type is uniform, variations in the  P-wave velocity may sometimes be used to infer how these factors vary with locationjointing factor (C), seismic velocity (Vp), and the rock quality designation (RQP).

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• In seismic method of prospecting, artificial explosions are made and elastic deformation are induced in rock present in the ground. The propagation of such seismic(elastic) waves through the geological formation is studied.

Seismic Refraction Method

Geophone

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• Depending upon whether reflected waves or refracted waves are used in the investigation, there are two types of methods, namely, seismic reflection method and seismic refraction method.

Seismic Refraction Method

• The basic components of seismic instruments are meant to take into account the chief functions involved in prospecting. These chief functions are (i) detection of signal, (ii) differentiation of signal followed by its amplification, (iii) recording of that signal. These functions are performed by the geophones, amplifier and galvanometer.

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Instruments Used in Seismic Studies• The geophone (detector), which is planted in the

ground, picks up the signal and, depending on the consequent ground displacement velocity (i.e. the intensity of disturbances suffered by the geophones), gives out a proportional voltage.

• This Voltage, after undergoing unwanted frequency filtration, is amplified in electrical circuit (i.e. signal). This is done by amplifier.

• The voltage is fed to the seismic galvanometer which has the mechanism to give out a trace of the signal on photographic paper. This photograph is the seismic record which is used for interpretation.

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Seismic Refraction Method

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Travel-Time Curves

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•Seismic refraction is a useful tool in hydrologic investigations:▫Identifies stratigraphy▫Identifies the depth to water table▫Non-intrusive: No contaminated soil to

dispose of or equipment to clean•Inexpensive and time saving compared to

borehole drilling