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The demand of groundwater for drinking water and other requirements of the man are expected to increase tremendously with rise in population. Hence, the engineer has to equip himself with latest technologies available for exploration and exploitation of this resource. And, as in other fields, good progress has been made for quicker and more reliable identification of aquifers over larger areas using satellite imageries.
Prospecting for groundwater is a complicated process. It involves a detailed geological study of the area followed by more elaborate geophysical investigations on the one hand and a very careful analysis and interpretation of the data obtained from different sources, on the other hand.
Geological Investigations for Groundwater:
These studies involve:
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(i) Geological mapping of the area for an accurate information regarding the lithological character, mode of origin and petrological features of the rocks, both on surface and with depth;
(ii) Study of the aquifer structure, with special stress on its exposure and relation with other material.
Geological Maps:
As regards the geological maps, these are generally available for most of the areas from the Geological Survey Department of a country. In the groundwater exploration work, the job of engineer/ geologist may be to identify the formations shown on the maps and trace them on the ground.
From the contoured geological maps, sections can be prepared from which fairly accurate details will be available for subsurface geology of the area. The hydrogeologist then can broadly identify the potential water bearing formations from those which are known to be impervious and dry. He may then plan actual drilling programmes for test wells or observation wells through the formations so identified.
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The basic geological map, when supplemented with information about the water-bearing properties of the formation as determined from the field observation becomes a valuable document and is best called a Hydrogeologic Map. Sometimes a geologic map is made to contain mainly full information on chemical properties and characteristic of groundwater available in the area. Such maps are often referred as Geohydrochemical Maps.
Areal Photographs:
During the recent past great studies have been made in the use of areal photography and remote sensing techniques for location of potential aquifers on the globe. The true areal photographs are taken during areal surveys aboard planes from near the ground surface. The satellite picture, called Landsat Imagery, however, is obtained from satellites and requires specialized training in interpretation of the data obtained.
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In the areal photography and satellite imagery stress is laid to delineate faults, fracture systems, extensive outwash plains, landforms, drainage patterns and major recharge and discharge zones. These photographs provide invaluable aids to hydrogeologists in making them concentrate their surface investigations on really potential aquifers.
Test Drilling:
It is a general practice to dig test-wells in the area of investigations after ascertaining the broader geological features from hydrogeological maps.
These test wells are meant to provide accurate information on:
(i) Exact nature of the geological materials with depth;
(ii) Capacity of various formations to yield water;
(iii) Quality of the water available.
The drilling records are maintained in the form of logs, which indicate the petrological character of the material as encountered at increasing depth intervals. The material is obtained from below at regular intervals in the form of cuttings or cores depending on the type of the drilling machine. Among the common methods for drilling test-wells may be mentioned direct rotary method, auger drilling and cable tool drilling.
Geophysical Investigations for Groundwater:
Geophysical explorations for groundwater are based on the fact that rock formations differ in their gravitational, magnetic, seismic and radioactive behaviour. Further, and this is more important, the behaviour of water bearing formations towards tests involving these properties is generally different than of water free rocks of the same type. The contrast in physico-chemical characters is, therefore, the basis of usefulness of geophysical investigations.
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Geophysical explorations for groundwater reserves involve both surface and subsurface testing techniques; a test bore hole is required in the second case. Among the surface geophysical methods are included electrical, magnetic, electro-magnetic, gravimetric and seismic methods.
Similarly, the subsurface (borehole) geophysical methods involve recording variations in temperature, resistivity, conductivity, spontaneous potential and response to gamma radiation and acoustic waves, with increasing depth. Observations in almost all of these tests are recorded in the form of logs. We shall mention below only the most commonly used geophysical methods for groundwater exploration. These are the resistivity method and the seismic reflection and refraction method.
i. Electrical Resistivity Method:
Resistivity, or the inverse of electrical conductivity, is an important measure of the capacity of a rock to allow an electric current through it. Dry rocks and sediments, dense compact and pore-less rocks will offer a greater resistance to the electric current compared to loose, porous and wet, saturated samples of these very materials. In other words, they will have lower resistivity to an electrical current.
This simple fact is used in the field to determine the nature of rock at certain depths with the help of an induced electric current. There are two variations of the resistivity method.
ii. Electrical Sounding:
This technique is used to determine variation in the nature of the subsurface materials with increasing depths.
iii. Electrical Profiling:
It is used to determine the areal extent of various formations upto the same depth.
iv. The Wenner Configuration:
The test procedure in this method consists of inserting two electrodes into the ground at a specific distance from each other. Direct current (or low frequency alternating current) is to be introduced through these electrodes. Hence these are called Current Electrodes.
As the current is introduced, it travels from one electrode, passes through the material and leaves the ground at the other current electrode. In between, it (the current) will meet resistance from the material and there will be a drop in its potential. This drop in potential is measured through two more electrodes called the Potential Electrodes that are inserted in between (Fig. 18.10).
The resistivity measured by these potential electrodes is recorded as “apparent resistivity”, r, and is given by the relationship:
where a = spacing constant; ΔV = Potential difference; I = Current
The depth to which the current penetrates is of the order of 1/4 th of the distance between the two current electrodes. Hence, it is possible to find out the electrical character of the rocks upto a certain depth by varying a-spacing from original 3 meters to 6 meters, 9 meters, 12 meters, and 15 meters and so on, upto a depth of 50 meters or so.
The electrical resistivity methods have been found quite successful in locating buried valleys, outwash plains, sand beds rich in water and also the bed rock.
v. Seismic Methods:
These methods based on the observation of velocity of elastic waves (called the shock waves or seismic waves) through the subsurface formations have also been used with good success in the location of ground water supplies. There are two variations in seismic surveys – refraction method and reflection method.
The seismic waves in both the methods are created artificially by using hammer drops, or, explosive charge. The shock waves so created are of three types – the compressional or P waves, the transverse or shear S waves and the surface or L waves.
The P-waves are the fastest and after passing through the ground to considerable depth are recorded first on the recording stations. Their velocity is, however, considerably influenced by the nature, density and water content of the medium through which they travel. It is the record of travel of P-waves that is used primarily for identification of aquifers at certain depths below the surface.
The seismic-waves travel between two points on the ground (the explosive point, E and the recording point, R) by three different routes:
(i) Direct – travelling exactly along the surface without penetrating much below into the ground;
(ii) Refracted – in which seismic waves get to the boundary of other layer(s) located at certain depth(s) and return to the ground after getting refracted;
(iii) Reflected – in which case some of these waves suffer reflection and are bounced back on to the surface.
In the refraction survey, a number of geophones are placed at regular intervals from the point of explosive charge or hammer drop. The time taken by seismic refracted waves to reach these stations after the shock is recorded accurately. From such observations, distance-time curves are obtained that is used to calculate the velocity of the refracted waves.
The seismic velocity-versus- depth information, as available from such surveys, is then interpreted to give an idea of various geological formations that are encountered by these waves. In general, the seismic-waves travel faster in denser and elastic rocks; their velocity gets considerably reduced when they encounter unconsolidated, poorly consolidated, porous or saturated rock formations.
In the reflected surveys, it is the velocity data of reflected seismic waves which is recorded and interpreted. Their recording presents some practical difficulties due to interference from other types of waves created during the shock in the commonly used recording techniques. Once these difficulties are removed by using refining techniques the reflection surveys are also as reliable as the refracted surveys.
Gravimetric Surveys:
The density of subsurface sediments or formations has a pronounced influence on the gravitational attraction of the Earth at a particular location. When gravity measurements are taken along different directions in a given area, normally one should expect a uniform gravity value if it is the same material that is existing below.
But, if during such surveys, exceptionally high or exceptionally low values are observed along a certain direction, a gravity anomaly is discovered. If it is on the higher side than normal, a denser material buried below is indicated. It may be an ore of some metal. If the anomally is on the lower side, a lighter material is indicated, which among other things, may be an aquifer, a porous rock saturated with water.
The gravity measurements are taken by gravimeters based on spring-balance principle, which are extremely delicate and sensitive. Gravimetric surveys are very useful for quickly demarcating such areas where further thorough and detailed geological and geophysical investigations should be carried out. These surveys are particularly useful in indicating buried valleys in glaciated terrains. Gal and milligal are the units of acceleration used with gravity measurements.
Electromagnetic Surveys:
Among the other methods that have been used for groundwater explorations mention must be made of electromagnetic surveys. Electromagnetic waves at very high frequency (above 100 MHz) are made to propagate through the ground for recording such properties as magnetic permeability, electrical conductivity and dielectric constant.
The instruments used for such high frequency electromagnetic surveys are called Ground Penetrating Radars and consist of highly delicate and sophisticated assembly that is virtually required to emit and record radar pulses at the velocity of light. The effective depth of penetration for high frequency electromagnetic surveys is usually limited to the range of 14-15 meters. In the low frequency variation, the depth and volume of material that can be scanned is much bigger. Instruments used for that purpose are called “Ground Conductivity Meters”.
A Note on Logs of Boreholes:
Logs are records of subsurface investigations which provide useful information regarding the nature and set of properties of materials occurring upto a certain depth below the surface. The records may be in the form of mere tables or graphic plots with symbolic descriptions. The data on the basis of which these records are prepared may be obtained by any one or more of many available methods. Accordingly, there are many types of bore logs.
Given below are only few important examples:
Geological Log:
It is a record of petrological types of soil and rocks as penetrated by a borehole upto a certain depth. It is often referred as a Core-log and is a very useful tool for evaluating any variation in character and quality of the strata with depth. (Fig. 18.11 A).
When core logs as obtained from a number of boreholes judiciously located in a certain area are interpreted properly, they can reveal sufficiently accurate details regarding petrological characters, structural disposition and groundwater characteristics of an aquifer along with its depth and thickness.
Resistivity Logs:
These are obtained by using electrical methods for subsurface explorations. Resistivity of rocks depends on their composition, texture (especially porosity) and electrolyte content of the water that may be present in the pores. Thus, resistivity values may be considerably different in porous but dry sandstone, porous and saturated sandstone of the same type and saturated sandstones containing fresh water and salt water.
The dry rock will have very high resistivity values compared to the same rock in saturated conditions because moisture reduces the resistivity. Again, salt-water rich rocks will show still lower resistivity values compared to fresh-water-rich stones (Fig. 18.11B).
A variation in resistivity value of rocks encountered in a borehole is, therefore, a useful indication of the presence or otherwise of an aquifer up to depth of borehole.
Average resistivity values of some common rocks are given in the Table 18.4:
Radiometric Logs:
These include a variety of borehole records that are related to radiological characters of rocks as determined for different depths.
These may be based on:
(i) Measurement of natural gamma-radiation of a rock; in that case, they are called gamma- ray logs;
(ii) Induced and absorbed radiation of the rock; these are known as gamma-gamma ray logs;
(iii) Absorption of neutrons by the rocks; these are called neutron logs.
The usefulness of gamma ray logs- is based on the fact that potassium-40, a content of clays, micas and felspars is partly radioactive whereas quartz sands are almost free from such emissions. Hence detection of quartz sands at depths is indicated when exceptionally low counts of gamma emissions are recorded in the probes at particular depths.
Similarly, the gamma-gamma ray measurements are based on the relationship of density of the rock and its absorption capacity for the gamma rays. Here, a source of gamma rays (say cobalt 60) is lowered in the well along with a detector; they are placed at suitable distance from each other.
The amount of back-scattered radiation as recorded by the detector is a measure of electron density of the material around the borehole. Proper interpretation of gamma-gamma radio logs gives clear idea about porosity and bulk density of rock formation and hence about their water bearing qualities.
Neutron logs are also used to measure the porosity of the formations, and especially the water content of the porous formations. When rock formations around the borehole are highly saturated, a great amount of neutrons emitted from the source are absorbed by these formations and only a small part may be reflected for the detector. Hence greater the loss of neutrons, higher is the prospects for the formations being rich in water.