Explain the various Geological, Geophysical, Hydrological methods?

Geological, Geophysical, and Hydrological investigations are used to estimate the physical characteristics of the subsurface by measuring, analyzing, and interpreting seismic, electromagnetic, gravitational, and magnetic fields estimated at the ground surface.

Following are the different methods that are used for groundwater exploration.

1. Geological Investigations
2. Geophysical Investigations
3. Hydrological Investigations

Geological Investigations

Geological investigation is the most important method in groundwater exploration. It includes a brief study of a given area from different angles to predict the extent of groundwater occurrence in that region. Geological investigations also involve the study of the following.

Rock Types

The study of rock types refers to the occurrence of outcrops of rock types and their thickness, attitude, porosity, and permeability features. The attitude (i.e., dip direction) of bidding planes affects the groundwater movement below the area.

Topography

Topographic study relates to the existence of highlands(hills) and lowlands(valleys). The water table is parallel to the surface as it occurs at a higher level in hilly regions and in lowlands, it occurs at a depth below valleys. But the rise or fall of the water table is always moderate. Moreover, the flow of groundwater moves from places with a higher water table to a lower water table. In an exploration of groundwater, places of the lowest elevation are because the depth of actual occurrence of the water table is less when compared to the depth of the water table in hilly areas and the groundwater comes nearer to that point from various sides.

Weathering

• They have weathered zones
• Joints and fractures which may exist on the surface become less important and disappear at depth, the reason the porosity will be less, and therefore there is no scope for the existence of plenty of groundwater.
• The valleys and low-lying places, on the other hand, are appropriate for tapping groundwater because they are the places of natural accumulation of sediments. They are also the places where the occurrence of in-situ rock weathering is more common.

Geological structures

The study of geological structures refers to the attitude of joints, cracks, faults, shear zones, folds, etc., which may,

• Contribute to secondary porosity
• Act as channel ways (or)
• Guide the direction of groundwater movement. Synclinal folding occurring with relatively lithology guides to the artesian condition. In groundwater exploration, these conditions are sought after because they normally yield an abundant water supply.

Intrusive Rocks

The study of intrusive rocks refers to the occurrence, trend, and size of dolerite dukes, quartz veins, etc., which are relatively hard and resistant. There may act as barriers to the groundwater movement and result in the deposition of groundwater on one side of the intrusive only. In some places, groundwater may be plenty in a specific area whereas the groundwater may be insufficient in a nearby area.

Geological Mapping

This is very important as it exhibits most of the preceding field data. In geological mapping, the actual occurrence of rocks along with the related geological structures and intrusives are indicated over the contour map of that area.

Geophysical Investigations

Geophysical investigations are mainly made on the surface by using simple portable instruments. Following are the different methods of geophysical investigations by which groundwater studies can be made.

• Electrical resistivity method (used in more than 80% of cases)
• Seismic refraction method (used in the remaining cases)

Electrical resistivity method

This method is mostly adopted because of its successful implementation and accurate details, the electrical resistivity method helps in,

1. Determining the aquifer zone.
2. Knowing fresh water and saltwater contacts in shore regions.

Depending on the necessity, the electrical resistivity method is divided into 3 types,

1. Profiling
2. Sounding
3. The fieldwork of the electrical resistivity method is done, according to the “Winner method” Or “Schlumberger method”. With the help of the curve matching method” Or “Inverse slope method”, the interpretation of the electrical resistivity method is accomplished. The subsurface resistivity is affected by the rock type, porosity, moisture content, and salinity of moisture. This is the principle, involved in resistivity investigation. Hence, resistivity studies show information about all the preceding factors and radial sounding also shows the direction of the flow of groundwater.

Seismic refraction method

The principle on which the seismic investigations depend, is the velocity of shock waves which rely on the fracturing of the medium. The shock waves in the compact formations have a greater velocity when compared to their fractured equivalents. Seismic investigations are helpful in recognizing subsurface fault zones, shear zones etc., which are the possible places for the occurrence of groundwater. The seismic study shows whether fractured zones exist in an area or not but it does not show the occurrence of groundwater in them or not.

In hard rock areas, both the methods (electrical resistivity method and seismic refraction method) are helpful in calculating the thickness of soil and fractured zones. These methods also help in finding dolerite dukes, buried river valleys etc., which have a significant beating on the moment of ground water.

Hydrological Investigations

Hydrological investigations are very important in the evaluation of groundwater potentially in any area. It also involves the study of the following,

Water Table

In any area, the study and evaluation of the depth of the static water level in a number of wells and bore Wells offer useful information about the relative place, nature of the water table slope, and the expected direction of groundwater flow, which is subjected to the hydraulic gradient, in the area connected.

Surface water bodies

This study refers to the existence of networks of streams, ponds, lakes, reservoirs etc. in the area. The presence of surface water bodies is desirable as they may act as sources of percolating or groundwater recharge.

Springs and Seepages

The occurrence of natural seepages and springs in an area reveals a high water table position and rich groundwater conditions. Any placement of springs reveals faults, which contributes to rich groundwater occurrence.

Quality of water

It is essential to identify whether groundwater is potable or not. Normally, the groundwater Tate’s good when drawn from granite but the groundwater is blackish when drawn creating health problems(such as fluorosis)when drawn from rocks such as pegmatites, which are rich in fluorine-bearing minerals(mica and apatite).

Rainfall, Climate, etc.

Areas having good rainfall and a humid climate will have a good ground water potential whereas the areas with little rainfall and arid climate will have low ground water potential.

Pumping Tests

Pumping tests are very important for identifying the yield features of the aquifer concerned. The rates of drawdown records during pumping indicate the required information.

Conclusion

This article explains the methods and techniques of geophysical surveys used in the hydrogeological study. These are the determination of the geometrical shape of the hydrogeological structures and assessment of the hydrogeological characteristics required in investigations of mineral waters; contamination and protection of ground-water resources; and soil improvement. Geophysical methods play a vital role in all stages of exploration, development, and protection of these waters. Direct geophysical methods are selected based on the physical properties of the waters and the specific character of the regions studied.

Frequently Asked Questions

What is meant by Geological investigations?

Geological investigations are refered to as the groundwater explorations, which are involved in the exploration through.

Explain the rock types.

Rock types are explained to be as the structure of Rock type, their permeability nature, and the thickness of rock porosity.

What is meant by Geophysical investigations?

Geophysical investigations are done through the instruments and it declares a boy the study of the water table.

What is meant by hydrological investigations?

It is the evaluation of groundwater in any region.

How the electrical resistivity method is useful?

It is used as follows below;

For sounding, profiling, and aquifer zone of the surface.

What does the geological mapping refer to?

It results in the type of geological structures and intrusive of the region based.