WFM 5103 Hydrogeology and Groundwater Lectures 5-6.

WFM 5103Hydrogeology and

Groundwater

Lectures 5-6

Subsurface environmentWater bearing properties of rocks and soilsPrinciples of groundwater movementRechargeGroundwater withdrawalGroundwater QualityGroundwater in Coastal zonesHydrogeological mappingGroundwater managementConjunctive useGroundwater ModelsGroundwater development in Bangladesh

WFM 5103Hydrogeology and Groundwater

1.1.1 Surface exploration

- “non-invasive" ways to map the subsurface.-less costly than subsurface investigations

1. Geologic methods2. Remote Sensing3. Surface Geophysical Methods

(a) Electric Resistivity Method(b) Seismic Refraction Method(c) Seismic Reflection Method(d) Gravimetric Method(e) Magnetic Method(f) Electromagnetic Method(g) Ground Penetrating Radarand others

1.1.2 Subsurface exploration

1. Test drillinggeologic logdrilling time logWater level measurement

2. Geophysical logging/borehole geophysics

Resistivity loggingSpontaneous potential loggingRadiation loggingTemperature loggingCaliper LoggingFluid Conductivity loggingFluid velocity logging

3. Tracer testsand others

Exploration of GroundwaterGroundwater exploration

♦Electrical resistivity 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:

I

Va2a

1.1.1.3 (a) Electric Resistivity Method

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

Groundwater explorationSurface geophysical methodsElectric resistivity

Groundwater withdrawalGroundwater explorationSurface geophysical methods

1.1.1.3 (a) Electric Resistivity Method Groundwater explorationSurface geophysical methodsElectric resistivity

Wenner arrangement

I

Va2a

Schlumber configuration

I

V

b

2/b2/L 22

a

The measured potential difference is a weighted value over a subsurface region controlled by the shape of the region, and yields an apparent resistivity over an unspecified depth.


Vertical electrical Sounding (VES)Changing the spacing of electrodes changes the depth of penetration of the current. So it is possible to obtain field curve of apparent resistivity vs depth.For a single homogeneous, isotropic layer of infinite thickness, resistivity curve will be a straight line.

True/actual resistivity - if formation is homogeneous and isotropic.Apparent resistivity if formation is anisotropic consisting of two or more layers of different materials.

Profiling or lateral traversingElectrode separation is kept constant for 2 to 3 values (say a=10 m, 15 m, 20 m) and the center of electrode spacing is moved from one station to another (grid points) to have the same constant electrode separations.

Used to detect subsurface changes in horizontal direction.

Wenner method is most convenient for this.

Horizontal and vertical measurements allow to produce an iso-resistivity contour diagram.

Interpretation

1. Qualitative: from shape of the curves, decipher number of layers and resistivity relationships

2. Quantitative:(i) Analytical methods (mass curves or type curves)

Interpretation

1. Qualitative: from shape of the curves, decipher number of layers and resistivity relationships

2. Quantitative:(i) Analytical methods (mass curves or type curves)(ii) Empirical methods

- Cumulative resistivity method- Inverse slope method

Ele

ctro

de s

paci

ng (

a)

a

Semi-log paper

(log scale)Cumulative resistivity method

Electrode spacing, a

R2

1

Inverse slope method

Usefulness

valuable in determining

-the depth and thickness of groundwater aquifers

- the depth to groundwater, the depth and thickness of clay layers

-in some cases the depth to bedrock, and accordingly limit the depth of well drilling.

Resistivities vary over a wide range, depending on density, porosity, pore size and shape, water content and quality, and temperature.



In relatively porous formations, resistivity is controlled more by water content and quality within the formation than by the rock resistivity.

For aquifers composed of unconsolidated materials, the resistivity decreases with the degree of saturation and the salinity of the groundwater.

Clay minerals conduct electric current through their matrix, therefore, clayey formations tend to display lower resistivities than do permeable alluvial aquifers.



1.1.1.3 (b) Seismic refraction method

based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity.

Seismic energy is provided by a source (hammer, weight drop or small explosive charge) located on the surface.

Groundwater withdrawalSurface geophysical methodsSeismic refraction

The seismic waves travel through the subsurface at a velocity dependent on the density of the soil/rock.

When the seismic wave front encounters an interface where seismic velocity drastically increases, a portion of the wave critically refracts at the interface, traveling laterally along higher velocity layers.

Due to compressional stresses along the interface boundary, a portion of the wave front returns to the surface

A series of seismic receivers, geophones (right) are laid out along the survey line at regular intervals and receive the reflected wave energy.

Groundwater withdrawalSurface geophysical methodsSeismic refraction

Travel-time versus distance graphs are then constructed and velocities calculated for the refractor layers

Data are recorded on a seismograph and later downloaded to computer for analysis of the first-arrival times to the geophones from each shot position.

Groundwater explorationSurface geophysical methodsSeismic refraction

12

121 VV

VV

2

XZ

21

22

1221

VV

VV

2

tZ

2x22

122

121 V

Xt

VV

VVZ

13

21

23

13

22

23

232 VV

VVZ

2

t

VV

VVZ

For horizontal two-layer case:

For horizontal three-layer case:

Seismic refraction methods are effective in formations with definite boundaries between strata and where density increases with each successive lower layer.

Vo

V1V2

O Travel time of waves depend on media (greatest in igneous, i.e. consolidated rocks, and least in unconsolidated rocks)

O Seismic velocity increases markedly from unsaturaded to saturated zone.

O The acoustic velocity of a medium saturated with water is greatly increased in comparison with velocities in the vadose zone. Thus, the refraction method is applicable in determining the depth to the water table in unconsolidated sediments.

1.1.1 Surface exploration

- “non-invasive" ways to map the subsurface.-less costly than subsurface investigations

1. Geologic methods2. Remote Sensing3. Surface Geophysical Methods

(a) Electric Resistivity Method(b) Seismic Refraction Method(c) Seismic Reflection Method(d) Gravimetric Method(e) Magnetic Method(f) Electromagnetic Method(g) Ground Penetrating Radarand others

1.1.2 Subsurface exploration

1. Test drillinggeologic logdrilling time logWater level measurement

2. Geophysical logging/borehole geophysics

Resistivity loggingSpontaneous potential loggingRadiation loggingTemperature loggingCaliper LoggingFluid Conductivity loggingFluid velocity logging

3. Tracer testsand others

Exploration of GroundwaterGroundwater exploration

Groundwater explorationSubsurface geophysical methodsTest drilling

-drilling small diameter holes that furnish information on substrata in a vertical line from the subsurface.- useful in

(i) verifying other means of investigation(ii) confirm groundwater conditions prior to well drilling(iii) serving as observation wells for measuring

groundwater levels and for conducting pumping tests.-If found fruitful, many a times the test holes are redrilled or enlarged to form pumping or production wells.

1.1.2.1 Test drilling

SUBSURFACE EXPLORATION

Groundwater withdrawalGroundwater explorationSubsurface geophysical methodsGeologic logging

constructed from the drill-cutting samples collected at frequent intervals during the drilling of a well. (samples are utilized for laboratory determinations of their hydrologic properties)

Furnishes a description of the geologic character and the thickness of each stratum encountered as a function of depth, thereby enabling aquifers to be delineated.

Preparation of proper geologic map may be difficult because of interference of drilling fluid with fine particles, and lack of proper knowledge to interpret the findings.

1.1.2.1 Geologic Logging

Groundwater explorationSubsurface geophysical methodsGeologic logging

- useful supplement to test drilling- consists of an accurate record of the time, in minutes and seconds, required to drill each unit of depth of the hole.- Because the texture of a stratum being penetrated largely governs the drilling rate, a drilling time log may be readily interpreted in terms of formation types and depths.

1.1.2.1 (b) Drilling time log

1.1.2.2 Geophysical logging/ Borehole geophysics

What is Borehole Geophysical Logging?Borehole geophysical logging is a procedure to collect and transmit specific information about the geologic formations penetrated by a well by raising and lowering a set of probes or sondes that contain water-tight instruments in the well

The data collected can be used to determine general formation geology, fracture distribution, vertical borehole flow, and water-yielding capabilities.

Groundwater withdrawalGroundwater explorationSubsurface geophysical methodsBorhole geophysics

1.1.2.2 Geophysical logging/ Borehole geophysics

The application of geophysical logging to groundwater hydrology lags far behind its comparable use in petroleum exploration.

The primary reason for this is cost. Most water wells are shallow, small-diameter holes for domestic water supply; logging costs would be relatively large and unnecessary.

But for deeper and more expensive wells, such as municipal, irrigation or injection purposes, logging can be economically justified in terms of improved well construction and performance.

Interpretation of many geophysical logs is more of an art than a science

1.1.2.2(a) Resistivity logGroundwater withdrawalGroundwater explorationSubsurface geophysical methodsResistivity logging

Factors that influence formation resistivity:(i)Nature of water(ii)Temperature of water(iii)Rock structure

m

wt

RR

m

1F

wt FRR Rt = formation resistivity Rw = water resistivity θ = porosity fraction m = cementation factor F = formation resistivity factor

Also, m

iR

RF

Multi-electrode method is most commonly employed (minimizes the effects of drilling fluid and well diameter.

Spacing between the elctrodes, and their arrangement, determine the radius of investigation.

Short normal – records app. Resistivity of the mud-invaded zone (useful for locating boundaries of formations)

Long-normal – records app. Resistivity beyond the invaded zone (useful for obtaining information on fluids in thick permeable formations)

Lateral – actual formation resistivity beyond the mud-invaded zone

Uses of Resistivity Logsinterpretation and identification of rock types

identification of the position of the water table

determination of bed contacts and bed thickness

determination of aquifer parameters

evaluation of quality of formation water

determination of depth of casing

Can be made only in the uncased portion of drill holes. Must also contain drilling mud or water

Groundwater withdrawalGroundwater explorationSubsurface geophysical methodsResistivity logging

measures differences in the voltages of an electrode at the land surface and an electrode in the borehole

potentials are primarily produced by electrochemical cells formed by the electric conductivity differences of drilling mud and groundwater where boundaries of permeable zones intersect a borehole.

Groundwater explorationSubsurface geophysical methodsSpontaneous potential logging1.1.2.2(b) Spontaneous potential

(SP) logging

Deflections of the SP curve occurs due to the development of a liquid junction potential, i.e. potential difference across the junction from mud filtrate to formation water.

The potential read for shales normally varies very little with depth. SP is measured relative to this base line zero called the shale line..

If water in permeable bed is more saline than drilling mud, SP is generally more –ve in the permeable bed than in the adjacent clay & vice versa.

useful in determining water quality

The right hand boundary generally indicates impermeable beds (e.g. clay, shale, and bedrock)

Left-hand boundary indicates sand and other permeable layers

http://en.wikipedia.org/wiki/Image:ShaleUSGOV.jpg

1.1.2.2(c) Radiation logging

Generally two types:(i) measures the natural radioactivity(ii) detects radiation reflected from or induced in the formation from an

artificial source

1.Natural-gamma logging2.Gamma-gamma logging3.Neutron logging

Groundwater explorationSubsurface geophysical methodsRadiation logging

Natural-Gamma logging

all rocks emit natural gamma radiation originating from unstable isotopes (potassium, uranium, and thorium)

Clayey formations (shale, clay) emit more rays than gravels and sands.

Can be used to differentiate between sand, clay and gravel (this is identifying lithology, the primary application)


Gamma-Gamma logging

Gamma rays from a source in the probe (cobalt-60 or cesium-137) are scattered and diffused through formation.

Part of the scattered rays re-enter the hole and are remeasured.

The higher the bulk density of formation, the smaller the number of gamma-gamma rays that reach the detector.

Primary applications: (i) identifying lithology (ii) measurement of bulk density and porosity of rocks.


Neutron logging

Useful in determining the porosity of formations

A fast neutron source is used to bombard the rock

When any individual neutron collides with a hydrogen ion (of a water molecule), some of the neutron’s energy is lost and it slows down.

A large number of slow neutrons, as recorded by a slow neutron counter, indicates a large number of fluid (i.e. high porosity)

Results are influenced by hole size. Therefore, in large uncased holes, information on hole diameter is required for proper interpretation.

1.1.2.2 (d) Temperature logging

A vertical measurement of groundwater temperature in a well by a resistance thermometer

Normally Temperature will increase according to geothermal gradients (roughly 3oC for each 100 m depth)

Departures from this normal gradient may provide information on circulation (hydrologic Cycle) or geologic conditions in the well.

Applications:(i) identify aquifers contributing water to a well.(ii) Provide data on the source of water(iii) identify rock types(iv) calculate fluid viscosity and specific conductivity from fluid resistivity logs(v) distinguish moving and stagnant water

Abnormally cold water may indicate recharge from ground surface ( in deep well)Abnormally warm water may indicate water of deep-seated origin. Geothermal gradient is usually steeper in rocks with low permeability

1.1.2.2 (e) Caliper Logging

Provides a record of average hole diameter of a borehole

Hole diameter will be equal to drilling bit when a hard sandstone is traversed.

Diameter becomes larger for shales/clays as they become wet with mud fluid, slough off and cave into the hole.

Applications:(i) identification of lithology and stratigraphic correlation(ii) Locating fractures and other rock openings(iii) Correcting other logs for hole-diameter effects

Groundwater withdrawal

- Groundwater exploration

- Well construction methods

- Well design (to get optimum quantity of water economically from a given geological condition to meet the requirement for a particular scheme)

- Well completion (placement and cementing of casing, screens)

- Well development (increase specific capacity, obtain maximum economic well life)

Types of wells and methods of construction

• Factors important in choosing the type of wells • Quantity of water required• Economic consideration• Hydro-geologic consideration• Depth of water table

Well Construction Methods

Dug wells Bored wells Driven wells Jetted wells

Percussion (Cable tool) method Hydraulic Rotary method Air Rotary method

shallow depths;domestic wells;unconsolidated formations

municipal, industrialdomestic wells;Unconsolidated and consolidated formations

WFM 5103 Hydrogeology and Groundwater Lectures 5-6.

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Transcript of WFM 5103 Hydrogeology and Groundwater Lectures 5-6.