Groundwater prospecting is a means of searching for groundwater. It encompasses the way to find the places where groundwater is available as well as its approximate quantity and quality. Groundwater prospecting is very important when there is the intention to dig tubewells for water supply or when there is a need to investigate whether any groundwater presence would interrupt an underground construction process. Before we delve into the processes/methods of groundwater prospecting, it would be important to consider factors that contribute to groundwater occurrence.
Factors that Affect Groundwater Occurrence
Climate
Climate is popularly defined as the average weather condition of a place over a long period of time. The rainfall, temperature, humidity, and evaporation are the components of climate. These factors affect the occurrence and stability of groundwater balance on Earth.
The annual rainfall of the region, seasonal variation, and daily fluctuation of temperature and humidity are crucial for deciding the infiltration of rainwater and rate of evaporation respectively. Heavy to moderate rainfall favours more infiltration through soils and pervious formations. The occurrence of rainfall in many areas is uncertain, the variables of such uncertainty being changes in aggregated rainfall, the dates of onset and cessation of monsoon, the number of rainy days, and the frequency of dry spells.
High temperature during summer leads to prolonged and intense evaporation which depletes the water levels in surface reservoirs and also in the groundwater from shallow deposits. Evapotranspiration which occurs through plants further enhances the process of depletion of the water table.
Topography
Topography is the arrangement of natural and artificial features of an area. It is another factor that decides the occurrence of groundwater. Mountainous topography though receives heavy to moderate rainfall, the rainwater is carried away in the form of high surface runoff; as a result, such areas experience a shortage of water in a few months after the cessation of monsoon. The groundwater in such areas occurs as a perched water table or emerges in the form of springs. The surface runoff transports the sediment load, derived from physical and chemical weathering processes, downslope leaving behind erosional landforms. Depending upon the intensity and periodicity of flowing streams, the sediment load either accumulates at the foot-hill areas or is carried further down and gets spread along the banks of middle or higher-order streams. The depositional landforms are thus developed due to the accumulation of colluvium at the foot-hill areas and alluvium along the stream or river courses.
Erosional landforms which occur at hilly topography are generally unsuitable for the occurrence of groundwater whereas depositional landforms which occur at flat topography permit rainwater to percolate vertically downward until obstructed by impervious bedrock. The groundwater, thus, stored ultimately follows the topography and meets the streams or rivers making their banks suitable for sinking groundwater structures such as wells.
In summary, hilly topography exhibits high runoff, low order streams and perched water table and springs; undulatory topography which comprises the interface of erosional and depositional landforms always exhibits moderate runoff, middle order streams, and discontinuous water table whereas flat or lowly inclined topography shows low runoff, high order streams, and continuous water table and is the most suitable for well development.
Geology
The geological features such as folds and faults, clay beds, intrusions like dykes and pegmatites, etc exert control over the groundwater regime. During folding and faulting, major fractures and associated joints are developed. These fractures and joints enhance the groundwater recharge. The clay beds being impervious in nature, cause hindrances to groundwater recharge and movement. The dykes and pegmatites, depending upon the presence or absence of joints act as a carrier or a barrier to groundwater respectively. The dispositions of these features are thus crucial in groundwater occurrence.
The hydrogeological properties of rocks such as porosity and permeability decide the storativity and transmissivity of an aquifer. The primary porosity in rocks like basalts and granites is only up to 3 percent and hence they are regarded as low-permeable rocks. Some hydrogeologists have attempted to classify rocks, on the basis of their processes of formation, into ‘hard’ and ‘soft’ types. The rocks formed by the process of sedimentation being more pervious, are grouped into the soft rock category. The rocks formed by effusion of magma being less pervious, are grouped into the hard rock category.
The secondary processes such as tectonic disturbances produce fractures and joints’, cooling of magma gives shrinkage to joints whereas physical and chemical processes enhance the weathering and opening of incipient joints. The voids, cracks and cleavages, joints, and fractures thus impart secondary porosity on the soft and hard rocks making them suitable for storage and transmission of groundwater.
Groundwater Investigation
An investigation to find out the location, quantity, and quality of groundwater is very important, especially in the arid regions of the world. In such regions, if surveys are not carried out prior to the digging of wells, it could be an effort in futility if no groundwater or poor groundwater is discovered at the end of the excavation. The main aim of the investigation is to find out the location of recharge, storage, and discharge zones of groundwater, and understand the function of the groundwater regime with respect to the environment in which it occurs viz. the lateral and vertical extent of the aquifer system (location), its storage and transmission properties (quantity) and the chemistry of waters (quality)
Groundwater investigation studies help us to achieve the following:
- Understanding the occurrence and availability of groundwater,
- Selecting methods to be adopted in the exploitation of groundwater, and
- Deciding management policies and effective conservation of this vital resource.
Methods/Techniques of Groundwater Prospecting
Investigation for the occurrence of groundwater can be divided into two major techniques:
Laboratory techniques, and
Field techniques
Laboratory Techniques
Laboratory techniques comprise the study of toposheets such as qualitative aspects, quantitative aspects, controls over drainage, development of the basin, and aerial photographs such as lithology, relief, and other indicators. The laboratory techniques are important because the landforms present on the surface of the earth are the result of the action of various endogenic and exogenic forces operating on the earth’s crust. These landforms can be studied initially in the laboratory using toposheets and aerial photographs. The study brings out some elements and their characteristics that directly or indirectly throw light on subsurface shallow groundwater regimes. With information on the hydrogeological features obtained from the toposheets and aerial photographs, the next step should be to verify these features in the field.
Field Techniques
Field techniques comprise two surveys: preliminary surveys and field surveys. Preliminary surveys usually precede detailed surveys for groundwater prospecting because they give an indication of the likely presence of underground water. For instance, in arid regions of the world, if plants such as phreatophytes and vegetation are present at a site, it gives an indication of the presence of underground water because the blossoming of such plants in an arid environment shows that they are really obtaining water from a source which invariably would be the underground water source. Different types of plants serve similar purposes in different regions of the world.
The detailed survey consists of geological investigations, geochemical investigations (water quality and vegetation), geophysical investigations (electrical resistivity, seismic refraction and borehole logging), and hydrological investigations (well inventory and pumping tests).
Steps in Groundwater Prospecting
Groundwater prospecting can be carried out in the following steps:
Step 1: Laboratory investigations that comprise the review of related literature pertaining to the drainage basin under study, qualitative and quantitative interpretation of data with the help of toposheets and aerial photographs as well as preparation of photo-hydrogeological map.
Step 2: Verification of good information obtained in Step 1 through field traverses in which the drainage basin area under study is divided into potential, moderately potential, and non-potential categories.
Step 3: Hydrological method of the inventory of dug, bore, and tube wells to obtain information on groundwater regimes at shallow and deep depths and to obtain specific yield. Hydrochemical investigation to find out information on vegetal cover and quality of water should also be carried out here.
Step 4: Geophysical methods such as seismic refraction, electrical resistivity, or borehole logging will be used to get in-depth information on groundwater regimes at different depths.
Step 5: Based on the data obtained from the geophysical survey, there may be a need to make changes to the boundaries of potential, moderately potential, and non-potential categories of a drainage basin.
Explanation of Core Techniques of Groundwater Prospecting
Toposheets (Laboratory Technique)
Toposheets provide a wealth of information about the nature of the terrain (whether rugged, undulatory, or flat), landforms and type of drainage, and controls over their development. A look at toposheets gives a wealth of qualitative information as regards to disposition of contours (steep, mild, or rolling terrain), and drainage characteristics (drainage density, stream length, and drainage pattern). Information on the drainage characteristics can be obtained through the establishment of stream orders, bifurcation ratio, and hypsometric analysis.
Aerial Photographs (Laboratory Technique)
Aerial photographs are an aspect of remote sensing that deals with the gathering and recording of information on many aspects of natural phenomena from a distance without actual contact with the object. Other useful aspects of remote sensing are visual photography, multispectral imagery, infrared imagery, Radar imagery, etc. Aerial photographs are considered more suitable for groundwater investigations.
Aerial photography consists of taking photographs at regular time intervals with overlapping of about 60 percent from an aeroplane flying along definite lines at a certain flight altitude above the ground. An assembly and matching of aerial photographs called a mosaic, provides a continuous photographic representation of the drainage basin under study. Aerial photographs are available in various scales from 1: 5000 to over 1: 40,000. Large scale aerial photographs upto 1: 20,000 scale, are most useful for yielding information on various hydrogeological indicators. Aerial photographs taken in different seasons yield different details. The appropriate period of photography for hydrogeological purposes is the summer season because of the absence of crops and grass in the field, drying of streams, lowering of water table, etc. The potential areas can be delineated with the help of hydrogeological indicators such as springs, large diameter or dug wells, moisture conditions, etc. on large-scale photographs. Photographs taken during the rainy season are of very little significance in groundwater investigations.
Well Inventory (Hydrological Investigation)
This consists of dividing wells existing in an area in a map based on lithological features i.e. the wells are divided into groups based on the rocks from which they are tapping. Afterward, the reduced levels of the ground surface of the wells are established with the use of suitable instruments such as altimeters.
A day is then chosen to measure the depths of the static groundwater levels with reference to the reduced ground levels of the respective wells, once before the advent of rains, a second time within a month after the rains have stopped (i.e. after a steady state of groundwater levels is reached) and a third time after irrigation requirements of irrigation seasons are met with. On the day of measurement, one is to ensure that water is not drawn from the wells and that the groundwater has reached its approximate static water level position for that part of the season. Other details such as the diameter of the well, its depth, the nature of lining materials used for construction of the well, the horse power of the pump used, etc., are also noted. The details provided by the farmer about the ability of the well to irrigate the land in terms of hectares during different seasons of the year are also to be recorded.
The groundwater depths are converted into reduced groundwater levels with reference to the earlier determined reduced ground surface levels of the wells. These are noted in the map against the respective wells. With the help of these data, the ground water contour map at one-metre contour intervals is prepared and grouped based on the horizontal distances between the groundwater level contours.
Out of the wells equipped with pumps occurring in each of the groups, wells that have penetrated the entire aquifer are selected for pumping and recuperation tests during different seasons of the year. The results obtained for each category are taken as yields and recuperation times for the respective seasons of the year.
Vegetation (Geochemical Investigation)
A study of plants in relation to the hydrologic environment i.e., ‘hydrobotanical investigation has its own importance in groundwater prospecting. Proper identification of vegetation and plant association helps to treat some flora as indicators of potential groundwater regime, as well as brackish and saline ground waters.
On the basis of this relationship, plants can be classified into:
- phreatophytes – plants that grow on water from subsurface zone of saturation,
- halophytes – salt-resistant plant, and
- xerophytes – drought-resistant plants.
The hydrogeobotanical observations are based on the fact that water is an essential ingredient for plant life. Plants get nutrition through water in several ways. Some plants penetrate their roots deep below the subsurface to suck water from the water table, others reach the capillary level, and yet others receive water on account of internal evaporation and condensation. It is thus, clear that not all plants are linked with ground water.
Phreatophytic plants are of great significance for groundwater investigations because they are in direct and intimate contact with the groundwater. It is customary to classify phreatophytic plants into ‘significant’ and ‘insignificant’ types because not all are of equal significance in ascertaining favourable groundwater conditions. The ‘significant’ type of phreatophytic flora shows luxuriant growth where the storativity of an aquifer is high. Significant types of phreatophytic flora commonly found in literature include: Ficus glomerala, Phonix sylvestix, Pongamia glabra, Prosopis spicigera, Eugenia jambdana, Aegle marmelos, etc., being the most commonly mentioned plants. These plants have a tendency to grow mainly along streams and rivers, in areas where the water table is shallow, around lakes, reservoirs, springs, etc.
Electricity Resistivity Method (Surface Geophysical Methods – SGM)
This method was first used for practical purposes by Schlumberger in France in 1912. Since then it has proved to be the most effective means of subsurface groundwater investigations.
The electrical Resistivity method is based on the variations in resistivity of subsurface layers depending on density, chemical composition, and moisture content. The survey makes use of the fact that water increases the conductivity of the rocks, thereby decreasing their resistivity. Hence, if it can be established geologically, that the same rock formation exists for a certain depth, say 100 m, and by electrical testing, it is found that the resistivity is decreasing below say 60 m depth, then it can be easily concluded that the water is present below 60 m depth.
The resistivity of rocks at various depths can be calculated on the following principle: if electrodes (usually 4) are inserted in the ground and connected in a circuit to a source of electrical energy, the current will flow from one electrode, pass through the ground, and finally leave through the other electrode. The potential difference, V, between the two electrodes is measured and converted to earth resistivity [R = (V/I)]. The measured resistivity is converted to the apparent resistivity, Rho (ρa) by applying the proper geometrical factor. It should be noted that the arrangement (or configuration) of electrodes affects the measured earth resistivity. Two electrode arrangements: Wenner configuration and Schlumberger configuration commonly apply.
In the Wenner configuration, the electrodes are spaced at equal distances (A) and the apparent resistivity is given by the expression below:
In the Schlumberger configuration, the distance ‘b’ between the two inner potential electrodes is kept constant for sometime and the distance between the current electrodes ‘L’ is varied. The apparent resistivity can be determined by the expression below:
The depth to which this current penetrates the ground depends upon the distance between the two other electrodes (generally, it is the order of 1/4th the distance between electrodes). Thus, it is possible to send the current deeper into the ground by simply increasing the distance between the electrodes. Hence, it is possible to determine the resistivity of the given rock formations by measuring the passing current in the potentiometer circuit; and at different depths, by repeating the experiments with different electrode spacings. The field curve in the Wenner arrangement is plotted on a semi-log paper (ρa versus A), ρa being in ohm-metres on a logarithmic scale and A in metres on an arithmetic scale. Likewise, the field curve of the Schlumberger configuration is plotted on a log-log paper as ρa versus (L/2), ρa being in ohm-m as usual and (L/2) in metres. The variations can be studied along with geological or stratigraphical knowledge of existing rock formations.
Seismic Refraction Method (SGM)
In this method, shock waves or seismic waves produced by artificial explosions have also been used these days, to detect the presence of groundwater and its depth of occurrence. The principle underlying these seismic methods is that the velocity of the elastic waves (caused by artificial shocks) is different in moist deposits than in dry formations of the same composition. The shock or sound waves are generated by setting off a small explosion at a depth of about 1 metre or more or by a sledgehammer striking a metal plate on the ground. The waves thus propagated travel downwards into various rock layers and are refracted back to the surface from the interface between those layers. The detectors called geophones, are placed on the ground surface. These geophones are connected with cables to a central oscillograph or other device for recording the arrival time of the first wave after detonation or striking of a sledgehammer on a metal plate. The arrival times of different waves at different distances from the shot point are utilised for calculating the velocity of propagation of the wave through each rock layer. The velocities are characteristic of particular rocks in particular conditions, i.e., whether the rock is dry, saturated with water, weathered, or jointed. The refracted waves arrive at the surface only if the velocity of the propagation in the underlying layer is higher than that of the overlying layer. Since the velocity of the shock waves depends on the type of formation and the presence of water, it may be possible to detect and estimate the depth of the water table from the differences in the indicated velocities recorded at several geophones.
Vertical Electrical Sounding and Profiling (SGM)
In this method, two operations usually apply which depend on the area of coverage and the nature of geological formation. The first operation known as vertical electrical sounding (VES) involves the exploration of the depths, thicknesses, and true resistivities of the layers. The operation is more suitable for horizontal or nearly horizontal layers. The second operation known as resistivity profiling (or traversing) involves the horizontal exploration of the ground in search of lateral inhomogeneities caused due to faulting, intrusions, variation in weathering, jointing, and other factors.
Beyond the operation phase of VES +, the interpretation of the results presents another challenge. The interpretation of the results is usually carried out by two techniques:
- Empirical or rule of thumb technique, and
- Master curve comparison technique
The empirical technique lies in the recognition of certain discontinuities or breaks noticeable in the depth-sounding curves while the master curve method relies on matching curves obtained from the field data with that of master curves to get information about the thickness and resistivity values of various layers. Master curves are curves prepared that show the apparent resistivity versus depth for a variety of two-, three-, or four-layered systems with different resistivity values for each layer. The master curve method is more popular when compared to the empirical method because they are supported by a sound theoretical background and gives quantitative treatment to the data. The master curve method is more accurate for the two-layered case. For multi-layered cases, a number of uncertainties and variables affecting field data make the interpretations of the lithology a difficult task, hence, accurate interpretation of field curves can be achieved if additional information about depth and type of aquifer, is available from well logs.
According to Shishaye and Abdi (2016), this method is gaining more popularity for groundwater investigations because it is simple. During tests, ArcGIS 10.2 can be used to locate the VES points and well locations. After tests, computer-assisted iterative interpretation packages like IPI2 and Win RESIST software of Schlumberger analysis can be used to interpret the resistivity data gathered from the field, that is to identify the thickness and depth of each hydro-stratigraphic layer in the area.
Gravity Surveys
Gravity surveys measure differences in density on the earth’s surface which is a reflection of the subsurface geologic environment. As gravity surveys are relatively insensitive to small changes in geology, these are normally used in groundwater studies to map only large buried valleys. In this method, a gravimeter is used to measure the direct effects of the pull of gravity on a mass suspended by a delicate spring.
Magnetic Surveys
These are employed in basaltic and granitic terrains where vertical or nearly vertical dykes are present. An anomaly in the magnetic fields can be observed if dykes possess different mineral composition and texture than the rocks in which they are emplaced. A magnetometer is used to measure the intensity and duration of the magnetic forces.
Self-potential Logging (Subsurface Geophysical Methods – SbGM)
In this method, a log of self or spontaneous potential (SP) is obtained by recording the naturally occurring voltage difference between an electrode that is placed in the surface soil near the borehole and another electrode that is lowered into the hole. The hole must be uncased and filled with drilling fluid when the SP log is to be obtained. Variations in the recorded voltage difference will occur as the hole electrode passes through different formations. These variations are due to electrochemical effects between dissimilar layers, different streaming potentials, and other electrokinetic effects associated with the movement of water through various layers. The resulting recorder trace thus serves as a fairly accurate indicator of the depth of discontinuities and types of materials existing.
Resistivity Logging (SbGM)
To obtain a log of the apparent resistivity of formations an alternating current is applied to two electrodes and the potential difference between these electrodes or two other electrodes is measured with a recording potentiometer. The total current is measured with an ammeter. Various electrode arrangements are used. The simplest is the single-point electrode where one current electrode is placed in the surface soil near the well and the other is lowered into the borehole. The hole must be uncased and is usually still filled with drilling mud. The potential difference is measured between the current electrode in the borehole and a potential electrode placed in the soil near the well (singly-point device).
The resistivity of its water-bearing formation primarily depends on the salt content of the water and the porosity of the material. Interpretation of resistivity logs is most successful when used in conjunction with sell-potential logs. Resistivity values are highest for dense, solid rock and lowest for clay arid shale layers. Medium resistivities in combination with negative sell-potentials indicate sand aquifers. If the formation material and porosity are known, the measured resistivity may furnish estimates of the salt content of the formation water.
Gamma Ray Logging (SbGM)
Logs of the natural gamma-ray emission of various soil/rock strata are obtained by lowering a gamma-ray detector into the well and recording its output (counts per second). Since gamma rays pass through metals, the technique can also be used in cased holes. Gamma-ray logs, however, are mainly used to distinguish between clay and non-clay materials. Clays and shales contain much more of the gamma-emitting elements (for example, daughter products of uranium and thorium) than limestones and sands. In this way, gamma logs enhance the usefulness of the interpretation of electrical logs.
Neutron Logging (SbGM)
Neutron logs are obtained by lowering a probe with a fast-neutron source, e.g., 3 mCi of americium-beryllium, into the borehole and recording the intensity of the slow neutrons caused by backscatter and attenuation of the fast neutrons by hydrogen in the surrounding formation. The intensity of the slow neutrons, which is measured with a detector in the same probe, can then be related to the water content of the formation material around the probe. The method can be used on both, cased or uncased holes. Neutron logs yield information about water content and, if the formation is saturated, about the porosity of the material around the well. Measured changes in water content may be helpful in locating water tables.
Gamma-gamma Logging (SbGM)
Gamma-gamma logs are obtained by lowering a probe with a gamma-radiation source, e.g., 10 to 35 mCi of 60c, into a borehole and measuring the intensity of the backscattered and attenuated gamma rays with a detector in the same probe. This intensity is related to the density of the surrounding material so that bulk density and porosity of formation material around the probe can be detected.
Acoustic Logging (SbGM)
Acoustic logging is also called sonic logging. The method utilizes many frequencies not audible to the human ear. Basically, all acoustic logging devices contain one or two transmitters that convert electrical energy to acoustic energy. The energy is transmitted through the bore well and the rock surrounding. The receiver converts the acoustic energy to electric energy. The tool is constructed so that the shortest path for the acoustic wave is through the rock surrounding the well and then refracted along the borehole well. The method is used to locate the positions of the inflow of water in the hole.
Caliper Logging (SbGM)
The caliper log is a record of the average diameter of drill-hole. Most caliper sondes consist of one to four pads, or bow springs, which follow the wall of the hole. Caliper logs are utilized for the identification of lithology and stratigraphic correlation, for the location of fractures and other openings, as a guide to well construction and to correct the interpretation of other logs for hole-diameter effect. Lithological factors that will affect the hole diameter include type and degree of cementation or compaction, porosity aid permeability, bed thickness and vertical distance to adjacent hard beds, size, spacing, and orientation of fractures, and swelling of clay.
Temperature Logging (SbGM)
Temperature logs are continuous records of the temperature of the environment immediately surrounding a sensor in a borehole. They can provide information on the source and movement of water and the thermal conductivity of rocks. Basically, temperature logging sondes contain a thermistor whose internal electrical resistance changes in response to temperature changes. In general geothermal gradient is steeper in rocks with low permeability than in rocks with high permeability, possibly because of groundwater flow.
Exploratory Borewells Programme
An exploratory borewells programme involves drilling boreholes on an exploratory basis. This method becomes suitable when other methods such as interpretation of toposheets and aerial photographs and data obtained in hydrogeological, geochemical, and geophysical surveys do not give reliable information about the actual groundwater regimes used to observe deviations of features indicated and inferred from groundwater regimes that were obtained. This method is particularly more suited in virgin areas which do not possess any hydrogeological information. The borewells provide adequate data on subsurface geology and aquifers and also help in delineating vertical and lateral limits of groundwater regime and qualitative and quantitative variations, if any, due to changes in rock types. Information obtained from exploratory borewells is useful in deciding the number of wells to be drilled, their optimum depth, and the safe distance between them. The quality of water can also be tested through exploratory borewells. It is, thus, evident that exploratory borewell operations are of great significance in groundwater exploration programmes.
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References
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