Hydrogeology and Groundwater Part 1

Hydrogeology and Groundwater WFM 5103 Hydrogeology and Groundwater Subsurface environment Water bearing properties of rocks and soils Principles of groundwater movement Recharge Groundwater withdrawal Groundwater Quality Groundwater in Coastal zones Hydrogeological mapping Groundwater management Conjunctive use Groundwater Models Groundwater development in Bangladesh

Groundwater Movement

RECAP piezometer Observation well 4

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Aquifer properties/parameters Water transmitting parameter Permeability or Hydraulic Conductivity Mean pore velocity: 6

Pores, Porosity and Permeability Pores: The spaces between particles within geological material (rock or sediment) occupied by water and/or air. Porosity: is defined as the ratio of the volume of voids to the volume of aquifer material. It refers to the degree to which the aquifer material possesses pores or cavities which contain air or water. Permeability: The capacity of a porous rock, sediment, or soil to transmit ground water. It is a measure of the inter-connectedness of a material's pore spaces and the relative ease of fluid flow under unequal pressure. 7

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Perched Aquifers An aquifer in which a ground water body is separated from the main ground water below it by an impermeable layer (which is relatively small laterally) and an unsaturated zone. Water moving downward through the unsaturated zone will be intercepted and accumulate on top of the lens before it moves laterally to the edge of the lens and seeps downward to the regional water table or forms a spring on the side of a hillslope.

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Specific yield Specific retention -Water that will drain under the influence of gravity Specific retention -Water that is retained as a film on rock surfaces and in very small openings. The physical forces that control specific retention are the same forces involved in the thickness and moisture content of capillary fringe

Groundwater exploration Groundwater withdrawal Geologic methods Groundwater withdrawal Groundwater exploration Geologic methods 16

Water transmitting Parameter….contd. Relation between K and grain-size distribution Water transmitting Parameter….contd. (a) General relationship (b) Empirical formulas (i) Hazen A = 1.0 for K in [cm/sec] and d10 in [mm] (ii) Krumbein and Monk dg= geometric mean grain diameter [mm]; k in [mm]; (i) Kozeny-Carman 17

Water transmitting Parameter….contd. 18

Transmissivity Water transmitting Parameter….contd. T = Kb 19

Storage parameter Unconfined aquifer Confined aquifer Specific yield -Water that will drain under the influence of gravity Confined aquifer Storage coefficient/storativity -Water that is released or taken into storage per unit surface area of aquifer per unit change in head  = bulk modulus of compression of matrix  = bulk modulus of compression of water 20

Cone of Depression Head gradient From surrounding Convergent flow Pumping Decline in WL in well Head gradient From surrounding aquifer to well Convergent flow into the well 21

Cone of Depression Unconfined aquifer Confined aquifer Cone of depression expands very slowly (drainage through gravity) Increased drawdown in wells and in aquifer (dewatering of aquifer) Confined aquifer Cone of depression expands very rapidly (why??) No dewatering takes place Mutual interference of expanding cones around adjacent wells occurs more rapidly in confined aquifers 22

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1. 1 Exploration of groundwater Objective: to locate aquifers capable of yielding water of suitable quality, in economic quantities, for drinking, irrigation, agricultural and industrial purposes, by employing, as required, geological, geophysical, drilling and other techniques. Assessments of ground water resources range in scope and complexity from simple, qualitative, and relatively inexpensive approaches to rigorous, quantitative, and costly assessments. Tradeoffs must be carefully considered among the competing influences of the cost of an assessment, the scientific defensibility, and the amount of acceptable uncertainty in meeting the objectives of the water-resource decision maker. 25

Exploration of Groundwater Groundwater exploration Exploration of Groundwater 1.1.1 Surface exploration - “non-invasive" ways to map the subsurface. -less costly than subsurface investigations 1. Geologic methods 2. Remote Sensing 3. 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 Radar and others 26

Exploration of Groundwater Groundwater exploration Exploration of Groundwater 1.1.2 Subsurface exploration 1. Test drilling geologic log drilling time log Water level measurement 2. Geophysical logging/borehole geophysics Resistivity logging Spontaneous potential logging Radiation logging Temperature logging Caliper Logging Fluid Conductivity logging Fluid velocity logging 3. Tracer tests and others 27

Exploration of Groundwater Groundwater exploration Exploration of Groundwater 1.1.1 Surface exploration - “non-invasive" ways to map the subsurface. -less costly than subsurface investigations 1. Geologic methods 2. Remote Sensing 3. 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 Radar and others 1.1.2 Subsurface exploration 1. Test drilling geologic log drilling time log Water level measurement 2. Geophysical logging/borehole geophysics Resistivity logging Spontaneous potential logging Radiation logging Temperature logging Caliper Logging Fluid Conductivity logging Fluid velocity logging 3. Tracer tests and others 28

1.1.1.1 Geologic Methods Groundwater withdrawal Groundwater exploration Geologic methods Groundwater exploration Geologic methods 1.1.1.1 Geologic Methods an important first step in any groundwater investigation involves collection, analysis and hydrogeologic interpretation of existing geologic data/maps, topographic maps, aerial photographs and other pertinent records. should be supplemented, when possible, by geologic field reconnaissance and by evaluation of available hydrologic data on stream flow and springs, well yields, groundwater recharge and discharge, groundwater levels and quality. - nature and thickness of overlying beds as well as the dip of water bearing formations will enable estimates of drilling depths to be made. 31

Groundwater withdrawal Groundwater exploration Geologic methods Groundwater exploration Geologic methods Relationship between geology and groundwater  The type of rock formation will suggest the magnitude of water yield to be expected.  it is the perviousness or permeability and not porosity which is significant in water yielding capacity of rocks.  Igneous rocks have a porosity of 1% and may yield all water while some clays have a pososity as high as 50% but are practically impervious.  Porosity = f (grainsize, shape, grading, sorting, amount and distribution of cementing materials)  Permeability = f (interconnectedness, fissures, joints, bedding planes, faults, shear zones and cleavages, vesicles ) 32

Groundwater exploration alluvial aquifers : 90% of all developed Geologic methods alluvial aquifers : 90% of all developed Aquifers are alluvial aquifers, consisting of unconsolidated alluvial deposits, chiefly gravels and sands. Limestone aquifer varies in density, porosity and permeability depending on degree of consolidation and development of permeable zones after deposition. Original rock materials offer important aquifers. Volcanic rock can form highly permeable aquifers. Basalts form a good source of water; easily susceptible to weathering. Sandstones are cemented forms of sands and gravels; yields are reduced by the cements. Some may form good aquifers depending on shape and arrangement of constituent particles and cementation and compaction. Igneous and metamorphic rocks, in solid state, are relatively impermeable and hence serve as poor aquifers. Under weathered conditions, however, the presence of joints, fractures, cleavages and faults form good water bearing zones, and small wells may be developed in these zones for domestic water supply. Relationship between geology and groundwater 33

Selection of site for a well Factors to be considered are: (i) Topography: Valley regions are more favorable than the slopes and the top of the hillocks. (ii) Climate (annual rainfall, sunlight intensity, max. temperature, humidity): heavy to moderate rainfall -- more deep percolation – good aquifer. Intense summer weather -- evaporates and depletes GW through direct evaporation from shallow depths and evapotranspiration through plants. 34

Selection of site for a well Groundwater exploration Geologic methods (iii) Vegetation: can flourish where GW is available at shallow depths. Phreatophytes, plants that draw the required water directly from the zone of saturation indicate large storage of groundwater at shallow 35

Selection of site for a well Groundwater exploration Geologic methods (iii) Vegetation: can flourish where GW is available at shallow depths. Phreatophytes, plants that draw the required water directly from the zone of saturation indicate large storage of groundwater at shallow depths. Xerophytes, plants that exist under arid conditions by absorbing the soil moisture (intermediate or vadose water), indicate the scarcity of groundwater at shallow depths. 36

Selection of site for a well Groundwater exploration Geologic methods (iii) Vegetation: can flourish where GW is available at shallow depths. Phreatophytes, plants that draw the required water directly from the zone of saturation indicate large storage of groundwater at shallow depths. Xerophytes, plants that exist under arid conditions by absorbing the soil moisture (intermediate or vadose water), indicate the scarcity of groundwater at shallow depths. Halophytes, plants with a high tolerance of soluble salts, and white efflorescence of salt at ground surface indicate the presence of shallow brackish or saline groundwater. 37

Selection of site for a well Groundwater exploration Geologic methods (iv) Geology of the area: thick soil or alluvium cover, highly weathered, fractured, jointed or sheared and porous rocks indicate good storage of groundwater, whereas massive igneous and metamorphic rocks or impermeable shales indicate paucity of groundwater. (v) Porosity, permeability: highly porous, permeable zones of dense rocks encourage storage of groundwater. Massive rocks do not permit the water to sink. (vi) Joints and faults in rocks: Wells sunk into rocks with interconnected joints, fractures, fissures and cracks yield copious supply of water. (vii) Proximity of rivers: Streams and rivers serve as sources of recharge and water is stored in the pervious layers. 38

1.1.1.2 Remote sensing Groundwater exploration Remote sensing Source Processor Sensor A physical quantity (screen) (light/radiation) (eyes) (records data and interprets information) signal 39

Remote sensing Groundwater exploration an increasingly valuable tool for understanding GW conditions. information on an object on the earth is acquired by remote registration/ sensing from aircraft or satellite at various wavelengths of the electromagnetic energy reflected and emitted. difference in reflectance properties of objects produce varying signatures on the photos or images, which can be interpreted for a variety of purposes of which application of hydrogeology is one. stereoscopic airphotos (color, black and white, infrared), oblique air photos and high resolution satellite imageries taken from GMS, APT, NOAA, AVHRR, SPOT and Landsat, ERS-SAR, RADARSAT, open up new possibilities for the assessment of groundwater resources. - observable patterns, colors, and relief makes it possible to distinguish differences in geology, soils, soil moisture, vegetation and land-use (hence areas of groundwater recharge and discharge). 40

forest cover mapping and monitoring; Groundwater exploration Remote sensing RS applications forest cover mapping and monitoring; land use and land cover mapping;  mapping of water resources; Others: agriculture; fisheries; coastal zone; marine environment. 41

Identify data needs Land cover 42

Advantages of remote sensing technique in general: Groundwater exploration Remote sensing Advantages of remote sensing technique in general: - speed of operation - survey of inaccessible areas - possibility of repetitive coverage of changing landform, land use, vegetal cover, water spread in reservoirs, soil salinity, water logged areas, etc. - permits mapping and preliminary evaluation at lesser cost. “The remote sensing technique is only an additional tool in the quest of groundwater and not a substitute for other methods. For a meaningful interpretation, there should be adequate ground check in the field”. 43

1.1.1.3. Surface Geophysical Methods Groundwater withdrawal Groundwater exploration Surface geophysical methods 1.1.1.3. Surface Geophysical Methods - scientific measurement of physical properties and parameters of the earth’s subsurface formations and contained fluids by instruments located on the surface for investigation of mineral deposits or geologic structure. provide only indirect indication of groundwater -success depends on how best the physical parameters are interpreted in terms of hydrogeological language. - Accurate interpretation requires supplemental data from subsurface investigations to substantiate surface findings. 44

1.1.1.3 (a) Electric Resistivity Method Groundwater withdrawal Groundwater exploration Surface geophysical methods Groundwater exploration Surface geophysical methods Electric resistivity 1.1.1.3 (a) Electric Resistivity Method ♦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: V is the measured Potential difference (in Volts) and I is the current introduced (in Amperes). 45

1.1.1.3 (a) Electric Resistivity Method Groundwater exploration Surface geophysical methods Electric resistivity 1.1.1.3 (a) Electric Resistivity Method Wenner arrangement Schlumber configuration 46

Groundwater exploration Surface geophysical methods Electric resistivity  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. 47

Hydrogeology and Groundwater Part 2

Hydrogeology and Groundwater WFM 5103 Hydrogeology and Groundwater Subsurface environment Water bearing properties of rocks and soils Principles of groundwater movement Recharge Groundwater withdrawal Groundwater Quality Groundwater in Coastal zones Hydrogeological mapping Groundwater management Conjunctive use Groundwater Models Groundwater development in Bangladesh

Exploration of Groundwater Groundwater exploration Exploration of Groundwater 1.1.1 Surface exploration - “non-invasive" ways to map the subsurface. -less costly than subsurface investigations 1. Geologic methods 2. Remote Sensing 3. 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 Radar and others 1.1.2 Subsurface exploration 1. Test drilling geologic log drilling time log Water level measurement 2. Geophysical logging/borehole geophysics Resistivity logging Spontaneous potential logging Radiation logging Temperature logging Caliper Logging Fluid Conductivity logging Fluid velocity logging 3. Tracer tests and others

1.1.1.3 (a) Electric Resistivity Method Groundwater withdrawal Groundwater exploration Surface geophysical methods Groundwater exploration Surface geophysical methods Electric resistivity 1.1.1.3 (a) Electric Resistivity Method ♦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: V is the measured Potential difference (in Volts) and I is the current introduced (in Amperes).

1.1.1.3 (a) Electric Resistivity Method Groundwater exploration Surface geophysical methods Electric resistivity 1.1.1.3 (a) Electric Resistivity Method Wenner arrangement Schlumber configuration

Groundwater exploration Surface geophysical methods Electric resistivity  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 traversing Electrode 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 Qualitative: from shape of the curves, decipher number of layers and resistivity relationships Quantitative: (i) Analytical methods (mass curves or type curves)

Interpretation Qualitative: from shape of the curves, decipher number of layers and resistivity relationships Quantitative: (i) Analytical methods (mass curves or type curves) (ii) Empirical methods - Cumulative resistivity method - Inverse slope method Electrode spacing (a) Semi-log paper (log scale) Cumulative resistivity method Electrode spacing, a Inverse slope method

Groundwater withdrawal Groundwater exploration Surface geophysical methods Groundwater exploration Surface geophysical methods Electric resistivity 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.

Groundwater withdrawal Groundwater exploration Surface geophysical methods Groundwater exploration Surface geophysical methods Electric resistivity 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 Groundwater withdrawal Surface geophysical methods Seismic refraction 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 withdrawal 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 Groundwater withdrawal Surface geophysical methods Seismic refraction  A series of seismic receivers, geophones (right) are laid out along the survey line at regular intervals and receive the reflected wave energy.  

Groundwater exploration Surface geophysical methods Seismic refraction Vo For horizontal two-layer case: For horizontal three-layer case: V1 V2 Seismic refraction methods are effective in formations with definite boundaries between strata and where density increases with each successive lower layer.  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.   Travel-time versus distance graphs are then constructed and velocities calculated for the refractor layers

O. Travel time of waves depend on media (greatest in igneous, i. e 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.

Exploration of Groundwater Groundwater exploration Exploration of Groundwater 1.1.1 Surface exploration - “non-invasive" ways to map the subsurface. -less costly than subsurface investigations 1. Geologic methods 2. Remote Sensing 3. 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 Radar and others 1.1.2 Subsurface exploration 1. Test drilling geologic log drilling time log Water level measurement 2. Geophysical logging/borehole geophysics Resistivity logging Spontaneous potential logging Radiation logging Temperature logging Caliper Logging Fluid Conductivity logging Fluid velocity logging 3. Tracer tests and others

SUBSURFACE EXPLORATION Groundwater exploration Subsurface geophysical methods Test drilling SUBSURFACE EXPLORATION 1.1.2.1 Test 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 Geologic Logging Groundwater withdrawal Groundwater exploration Subsurface geophysical methods Geologic 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 (b) Drilling time log - useful supplement to test drilling Groundwater exploration Subsurface geophysical methods Geologic 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.

1.1.2.2 Geophysical logging/ Borehole geophysics Groundwater withdrawal Groundwater exploration Subsurface geophysical methods Borhole geophysics 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.

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 log Groundwater withdrawal Groundwater exploration Subsurface geophysical methods Resistivity logging 1.1.2.2(a) Resistivity log

Factors that influence formation resistivity: Nature of water Temperature of water Rock structure

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

Can be made only in the uncased portion of drill holes. Groundwater withdrawal Groundwater exploration Subsurface geophysical methods Resistivity logging Can be made only in the uncased portion of drill holes.  Must also contain drilling mud or water Uses of Resistivity Logs interpretation 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

1.1.2.2(b) Spontaneous potential (SP) logging Groundwater exploration Subsurface geophysical methods Spontaneous potential logging 1.1.2.2(b) Spontaneous potential (SP) 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.

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

1.1.2.2(c) Radiation logging Generally two types: Groundwater exploration Subsurface geophysical methods Radiation logging 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 Natural-gamma logging Gamma-gamma logging Neutron logging

Natural-Gamma logging Groundwater exploration Subsurface geophysical methods Radiation 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)

Groundwater exploration Subsurface geophysical methods Radiation logging 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, industrial domestic wells; Unconsolidated and consolidated formations