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Geophysical Studies
The role of geophysics in groundwater exploration is vital to understand subsurface conditions accurately and adequately. Geophysical investigations help assess the presence of aquifers in geologic formations, estimate weathered zone thickness or bed rock topography and fractures, and assess quality (in terms of salinity) of groundwater. Geophysically, it is possible to precise the three-dimensional geometry and disposition of aquifers.
Application of geophysical methods for aquifer characterization has been in practice in India since the 1930s, mostly through electrical resistivity sounding and profiling and geophysical logging of boreholes.
In view of mapping the entire aquifer systems in India within a short period of time, it is necessary to apply geophysical methodologies comprising a combination of techniques that give fast coverage and yield the desired information as precisely and adequately as possible. With this objective, NGRI, the agency in charge of geophysical investigations, applies both airborne and ground based (surface and borehole) state of the art techniques in the pilot project areas. All of these play a significant role in aquifer characterization at various scales in different hydrogeological terrains. The most appropriate geophysical methodologies are then identified for the National Aquifer Mapping Programme.
A consultancy has been carried out to review best global practices for aquifer mapping using geophysical methods and modeling tools and to make recommendations for their application in pilot project areas (Shah, 2012, Overview of Global Applications of Geophysical Techniques for Aquifer Mapping: Recommendations for Use to Aid Groundwater Management in India).
In the pilot study, the following methods are used:
Airborne: heliborne time-domain electromagnetic and magnetic
Ground-based: resistivity and induced polarization sounding and resistivity profiling, resistivity imaging, time-domain electromagnetic sounding, and
Geophysical logging of boreholes
The aquifer geometry mapping by these geophysical methods and techniques helps identify the aquifers, their lateral extents and vertical grouping through generation of cross-sections and contour maps. The hydrogeological studies in conjunction with geophysical parameters also help identify the areas for artificial recharge or areas requiring aquifer protection.
The flow chart below shows the approach adopted for geophysical investigations under the pilot study.
The geophysical surveys start with development of a conceptual model on the basis of existing data and hydrogeological studies conducted in the area.
To study the efficacy of integrating the surface geophysical surveys in aquifer mapping and standardize the methodology, 50% of the targeted ground geophysical surveys are conducted at the commencement of the project. Accordingly, 70% of the drilling sites are also pinpointed and boreholes drilled. This helps defining the measurement parameters for the heliborne surveys to be taken up subsequently.
The rest 50% of surface geophysical surveys and 30% exploratory drilling is taken up after the heliborne surveys are conducted to validate the heliborne survey results and assess their applicability.
Finally, the geophysical results are integrated, calibrated and standardized with aquifer parameters obtained from drilling and hydrogeological studies. Hydrogeological interpretation of geophysical data is then conducted to produce different geophysical maps on aquifers.
Outcomes of geophysical investigations in the pilot study
The expected outcomes of the geophysical investigations in the six pilot areas are as follows:
Aquifer-geometry maps up to 200 m depth in hard rocks and 300 m depth in sedimentary areas.
Establishment of the efficacy of various geophysical techniques under different hydrogeological conditions.
Establishment of a methodology for geophysical investigations to be up scaled up for the entire country.
Besides these, geophysical investigations will give information on the suitability of near surface layers for artificial recharge as well as help identify recharge zones. Also, geophysical investigations bring out the aquifer boundaries required for mathematical modeling.