Groundwater Exploration Techniques

Introduction:
Groundwater is a precious gift given by nature for free present in aquifers and it is human psychology that we do not care about things that are available for free. Even the groundwater was not able to keep itself out of human exploitation. Groundwater was discovered many years ago. We can assume the level of groundwater as we know that at that time there were only dug wells or shallow wells. Later came into play deep tubewells after handpumps and other pumps. It clearly shows that the groundwater level is going down very fastly due to the over-exploitation. More than 60% of the global population thrives by using groundwater resources. Exploring the groundwater at deep levels is a major challenge to the geological scientists.

Renewable resource:
YES!!! Groundwater is a renewable resource as we can refill water into the ground using various methods. We can restore water and increase the water table by performing some jobs like installing a rainwater harvesting system in our houses and then separating the rainwater to go along with the wastewater into sewerage. But since many people don’t want to do so because of a lack of knowledge and also they want to save money. According to them, the government should install these systems but it is the responsibility of every citizen who uses groundwater to recharge water into the ground. If everyone will keep depending on others then the future scenario is just gonna become worse. You can think of the situation that’s our future generation going to face.

Heterogeneous distribution:
As you know that boring for groundwater extraction can’t be done anywhere as every surface doesn’t consist of groundwater.   That’s why well log is prepared before boring to know about the profile of soil.So this characteristic of groundwater makes the challenge to geologists more harder to determine the perfect place containing groundwater.

Exploration of groundwater:
The exploration is mainly the field of hydrogeologists and engineers. Groundwater exploration techniques have also advanced along with time and technology. Test drilling is a very successful and popular technique to determine the presence of groundwater below the surface. But it also has a major con that is its cost, which is quite high that, unable to make its popularity among more people. Several methods have been invented so far. They are classified into surface and subsurface methods.

Surface Methods:
As clear from the name these methods are performed on the surface itself, so comparatively easier to use. These require minimum facilities like maps, reports, topo-sheets, field measurements and interpretation of data in laboratories.The surface methods include:

• ·         Esoteric Methods
• ·         Geomorphologic methods
• ·         Geological & structural Methods
• ·         Soil and Micro-Biological Methods
• ·         Remote Sensing Techniques
• ·          Surface Geophysical Methods

Subsurface Methods:
The subsurface methods of groundwater exploration include much harder work as compared to the surface methods. It includes test drilling and Borehole Geophysical Logging Techniques. These methods are highly accurate and often used by the Government for investigations. Well log method is also a part of this method.

Surface Methods:

1.       Esoteric Method:

·        The oldest water divining methods practiced by ancient people for several centuries.

·         It was totally an assumption method with no scientific proof.

·         When a wet plant twig is moved above such zones, it tends to rotate the twig as well. Wet twigs of trees, husk-removed coconuts, watches, and other materials have been used as dowsing materials. The person handling the twig has some role of induction and hence it does not apply to everybody attempting to divine water.

·         All these methods have been practiced since the 17th century.

·         There is no scientific explanation available concerning these approaches.

·         The probability of success is 0.5.

·         These methods are called as water divining and water dowsing.

2.       Water Witching:

·         adopted by people to detect bore-well locations.

·         forked stick was used to locate water.

·         This method lacks any scientific justification.
Basically, this method was also a hit and trial method just like the previous one.

·         Commonly, the method consists of holding a forked stick in both hands and walking over the local area until the butt end is attracted downward-ostensibly by subsurface water.

3.       Geomorphological Methods:

·         The study of the physical features of the surface of the earth and their relation to its geological structures.

·         Geomorphology is indispensable in studying the occurrence of subsurface water in areas of late Pleistocene to recent deposits.

·     Satellite imagery and geomorphology map are studied followed by a field check.

4.       Study of Landforms

·         Landforms are the likely indicators to show the relatively permeable strata.

·         A landform is a feature on the Earth's surface that is part of the terrain. Mountains, hills, plateaus, and plains are the four major types of landforms. 
Minor landforms include buttes, canyons, valleys, and basins. The tectonic plate movement under the Earth can create landforms by pushing up mountains and hills.

5.       Topography and Drainage

·         Hydraulic gradients of groundwater systems will always follow the topographic gradients and slopes.

·         Such locations are also suitable for water collection and storage for recharge.

6.       Drainage density of stream network

·         Drainage density is the ratio between the total length of all streams and the area of watershed or river basin.

·  The resultant drainage density is used to indicate the the potentiality of groundwater.

·         If the drainage density is low, groundwater potentiality will be more.

·         If it is high, due to more streams, the runoff will be more.

7.       Geological Methods

·         A geologic procedure begins with the collection, analysis, and hydrogeologic interpretation of existing topographic maps, aerial photographs, geologic maps and logs, and other pertinent records.

·         This should be along with  hydrologic data on streamflow and springs, well yields, groundwater recharge, discharge, and levels and water quality.

·         The Zones having all the data about the groundwater are good and potential zones for groundwater exploration. 

8.       Structural methods

·        Along the sides of valleys that cut across the interface between different strata are suitable locations for groundwater.

·     Springs occurring on or near the base of hillsides, valley slopes, and local scarps are indicators of groundwater occurrence over hilly terrain.

9.       Well-inventory

·         Well-inventory is a method of analyzing the well-cuttings and inner surfaces of open dug wells to know about the subsurface geology, structures, seepage zones, and fluctuations of water levels, rate of recovery after pumping and the the geo-environmental setting of the wells in a region.

·         This method consists of collecting data from several wells from the surrounding area and then gives a combined conclusion for a resultant well.

·         The groundwater follows the predetermined path as made by other wells due to the well inventory.

10.   Soil and Micro-Biological Methods

·         Geo-botanical indicators are very helpful in groundwater exploration.

·         The growth of vegetation in a particular area shows the presence of groundwater in that area.

·         Presence of Halophytes, at the ground surface indicate the presence of shallow brackish or saline groundwater.

·         Xerophytes, the well-known desert plants, which can survive on very little water also show the presence of groundwater.

·         All these are tools in detecting the locations of groundwater zones.

11.   Moist depressions and seepages

·         Moist depressions, marshy environments, and seepages, the string of alkali flats or lakes, inactive drainage systems, salt precipitates, "burn out" patches in the soil, and vegetation  are good indicators for groundwater availability.

·         Depression springs, where land surface locally cuts the water table or the upper surface of the zone of saturation, Contact springs containing a permeable water-bearing strata are good locations.

·         The presence of artesian springs, valley slopes, and local scarps are very good indicators.

12.                    Geophysical methods

·                    Exploring the groundwater by the geophysical method is termed Groundwater geophysics.

·                    This method is used to explore groundwater by measuring certain parameters  like density, velocity, conductivity, resistivity, magnetic, electromagnetic & radioactive phenomena.

·                    Geophysical methods comprise of measurement of signals from natural or induced phenomena of physical properties of the subsurface formation.

·                    Geophysical methods detect the differences in physical properties within the earth's crust.

·                    The purpose of exploration is to detect false indicators and explore the direct indicators of groundwater.

·                    The main geophysical methods useful are the Electrical, Seismic, Gravity, and Magnetic methods.

13.               Gravity Method

·                    The gravity method has a significant name in the exploration of mineral resources and groundwater.

·                    Gravimeters are useful in finding out the density difference that helps in exploration.

·                 The method is expensive and because of differences in water content in subsurface strata seldom involve measurable differences in specific gravity at the surface, the gravity method has little application to groundwater exploring. 

14.               Magnetic Method

·                    The magnetic method involves detection of the magnetic fields of the earth which can be measured and mapped.

·                    Magnetometers are the types of equipment used to measure the magnetic fields and variations.

·                     Because magnetic methods are seldom associated with groundwater occurrence, the method has little trust for exploring groundwater.

·                    Indirect information pertinent to the groundwater studies, such as the presence of dikes that form aquifer boundaries or limits of a basaltic flow could be obtained with this method.

15.               Seismic Method

·                    Seismic methods are of two kinds as 

1.              Seismic refraction  
2.              Seismic reflection

·                    The seismic refraction method involves the creation of a small shock at the earth's surface either by the impact of a heavy instrument or by a small explosive charge and measuring the time required for the resulting sound, or shock, wave to travel known distances.

·                    Seismic waves follow the same laws of propagation as light rays and maybe reflected or refracted at an interface where a velocity change occurs.

·                    Seismic reflection methods provide information on geologic structure thousands of meters below the surface, whereas seismic refraction methods-of interest in groundwater studies-go only about 100 meters deep.

·                    The time taken for seismic waves is a function of the media type.

·                    The velocities are greatest in solid igneous rocks and least in unconsolidated materials.

16.               Analyzing Seismic velocities

·                    The study of seismic waves is important according to the geological survey.

·                    These velocities help to identify the nature of rock whether bedrock or alluvial.

·                    In coarse alluvial terrain, seismic velocity increases. This method can find the depth of groundwater with an accuracy of 10% in a homogeneous soil profile.

17.               Electrical resistivity method

·                    This method consists of finding the resistivity of the soil profile by passing current through soil bed with electrodes and measuring the voltages across it. This method's values can be varied according to water saturation, porosity, and other parameters.        

·           This technology has been used for decades in hydrogeological, mining and geotechnical investigations.

·                    Each electrical property is a function of the geophysical method.

·                    From the current (I) and voltage (V) values, an apparent resistivity (pa) value are calculated, using pa = k V / I, where k is the geometric factor which depends on the arrangement of the four electrodes.

18.               Vertical electrical sounding

·                    Vertical electrical sounding, VES, is used to determine the resistivity variation with depth.

·          This method is only applicable to the surfaces that can be assumed to be horizontal,.

·                     A resistivity meter is used to measure the apparent resistivity values.

·                    Resistivity meters normally give a resistance value, R = V/I, which leads to         pa = k R.where k is the geometric factor

·             The calculated value is not true since it shows the values of the surrounding aquifers.

·                    The relationship between the “apparent” resistivity and the “true” resistivity is a complex relationship.

·                    To determine the true subsurface resistivity, an inversion of the measured apparent resistivity values using a computer program is done.

19.               Electrical Profiling

·         Another The classical survey technique is this method. ·     The spacing between the electrodes remains fixed, but the entire array is moved along a straight line. ·          High-quality data is obtained using this method. ·         The major limitation of this method is that subsurface resistivity is varied with horizontal changes. ·         The resistivity can change rapidly over short distances. ·         The resistivity sounding method may not be sufficiently accurate. 20.      Photogeology ·       Photogeology is used for making aerial images of groundwater, minerals present beneath the surface. ·       The Photographs of the earth taken from the aircraft or satellites are useful for knowledge of groundwater conditions. ·   Black-and-white aerial photographs have gained importance in this field. ·       Various patterns obtained help in differentiating geology, soils, soil moisture vegetation, and land use. ·       Photogeology is helpful in the distribution of are based on rocks, groundwater, discharges, aquifers. ·       Maps showing areas as good, bad and poor can be prepared from the data collected. ·       Aerial photographs also show the fracture in rocks that directly leads to porosity, the permeability of the soil profile. ·       Thus this method provides data for the presence of groundwater and helps in its exploration. 21. Remote Sensing techniques ·       Remote sensing is the science of acquiring information about the Earth's surface without actually being in contact with it. ·       This is done by sensing, recording, processing, analyzing, and applying that information. ·      The process involves an interaction between incident radiation and the targets of interest. ·       Remote sensing shows an increasing role in the field of hydrology and water resources development. ·       Remote sensing provides multi-spectral, multi-temporal and multi-sensor data of the earth’s surface which are suitable for mineral explorations, water resources evaluation, environmental monitoring, and groundwater targeting. Conclusion ·       Thus there are several methods and techniques to explore the groundwater. ·       The success of groundwater exploration depends upon the technique used and with what technology and also on the field and soil conditions ·       Exploration is the first stage, followed by drilling, development and finally well completion, which is leading to more advancements in groundwater extraction. ·       The explored water should be managed properly. ·       Water is precious. Let us conserve.