Geophysical Signatures for Detection of Fresh Water and Saline Water Zones

Abstract

Physical and physicochemical backgrounds behind possible geophysical signatures for the detection of saline water and fresh water zones are highlighted. Coastal areas generally encounter this type of problems. Self-potential (SP), direct current resistivity sounding and traversing (DC, VES), induced polarisation sounding and traversing (IP), electromagnetic geometric and frequency sounding (EM), electromagnetic transients (TEM), audio frequency magnetotellurics (AMT), controlled-source audio frequency magnetotellurics (CSAMT), ground-penetrating radar (GPR), remote sensing with Geographical Information System (GIS) and Global Positioning System (GPS), seismic reflection, seismic refraction and high-resolution seismic reflection are the surface geophysical tools needed for studying this problem. Borehole geophysical tools needed for studying saline water and fresh water problems are SP, resistivity, natural gamma ray, neutron–thermal neutron, neutron-capture gamma ray, gamma–gamma density and sonic logging. High flight imagery, viz. remote sensing, has wide application in groundwater industries. Low flight imagery, viz. aerial photos, airborne geomorphology and aeroelectromagnetics, has application in this problem.

Combination of geophysical tools needed for studying saline water and fresh water problems and discussed in this short chapter are (i) joint resistivity and IP sounding, (ii) central frequency sounding (CFS) and dipole frequency sounding (DFS), (iii) transient electromagnetic sounding, (iv) GPR, (v) remote sensing with GIS and GPS, (vi) high-resolution seismic reflection and GPR, (vii) seismic reflection or refraction jointly with DC resistivity sounding or electromagnetic frequency sounding, (vii) combined natural gamma ray, SP and DC resistivity logging, (viii) combined neutron–thermal neutron and neutron-capture gamma ray logging and (ix) combined sonic density and neutron logging will be useful geophysical tool combinations for (a) estimation of secondary porosity in all types of rocks, especially carbonate and hard rocks, and (b) accurate estimation of acoustic impedance and bed boundaries.

Basic reasons for combinations of these geophysical tools are as follows: (i) Both saline water sand and clay/shale beds can show the same order of resistivity, but IP effect for clay/shale will be higher than saline water sands. Therefore, combined resistivity and IP sounding can resolve the saline water–fresh water zone in a more effective way. (ii) Since DC resistivity sounding may not respond properly in a hard-rock area, electromagnetic sounding (CFS or DFS) can join hands with seismic reflection and a couple of porosity logs to resolve the issue. (iii) In transient electromagnetic sounding or time domain electromagnetics, the response signals are recorded in the absence of the primary field. Therefore, the signals are better and can be used for better detection of saline water aquifers at greater depth. (iv) Combination of GPR and a high-resolution seismics makes a very effective tools for mapping the first 10–15 m of the earth’s surface with great details. Since dielectric constants of saline water and shale/clay are significantly different, GPR can differentiate between saline water sand and clay/shale. Electrical conductivity-dependent electromagnetic sounding may fail when the electrical conductivity is of the same order. (v) Since electrical conductivity of the water-saturated earth formation increases both with temperature as well as salinity, geoelectrical tools used for exploration of geothermal sources can also be used for exploration of deep-seated connate water pockets. Hence AMT, CSAMT, TEM, Long Offset Transient Electromagnetics (LOTEM) and deep EM and DC resistivity soundings can be used for detection of deep-seated saline water pockets. (vi) Remote sensors are instruments those measure electromagnetic signals sensing the surface or medium due to scattering or emission. Remote sensing is a very powerful tool for the detection of faults, fractures, fissures and lineaments in hard-rock areas. It is also widely used in soft-rock groundwater technology. It is an excellent tool for reconnaissance survey. (vii) Since shale and clay are better absorber of radionuclides, natural gamma ray logging along with SP and resistivity logs can differentiate between saline water sand and clay/shale. (ix) Nuclear capture cross-section of chlorine is much higher than that of common earth elements, viz. hydrogen, oxygen, silicon, etc.; therefore, saline water sand has higher capturing capacity of thermal neutrons than that of fresh water. As a result, combined neutron-thermal neutron and neutron-capture gamma ray logging can easily differentiate between saline water and fresh water. (x) SP reversal is an important qualitative diagnostics for identification of saline water and fresh water aquifers because one gets negative SP for saline water and positive SP for fresh water. Whether the salinity of the formation is more or less than the salinity of the borehole mud fluid dictates the direction of flow of SP current. This SP current is due to the formation of the liquid junction and membrane electrochemical cells and the direction of flow of current changes. (xi) Single-point resistance and normal and lateral resistivity logging tools along with SP are widely used geophysical tools for location of saline water-saturated beds. Appropriate geophysical tools best suited for different depth ranges are as follows: (i) GPR and high-resolution seismics will be for 1–10 m, (ii) DC resistivity, IP, CFS and DFS along with shallow seismic reflection or refraction are for 5–100 m or a little beyond, (iii) transient electromagnetic sounding, DC resistivity sounding/traversing using collinear dipole–dipole array and high-power low-frequency electromagnetic sounding jointly with seismic reflection sounding can go from 50 to 500 m, and (iv) deep resistivity sounding, AMT and CSAMT are for 100–2,000-m depths or a little beyond. Times are in (i) nanoseconds for GPR, (ii) microseconds to a few hundred milliseconds for TEM, (iii) milliseconds for time domain-induced polarisation and high-resolution seismics and (iv) fraction of a millisecond to 1 s for AMT, CSAMT, CFS, DFS and seismics. Longer-period signals go outside the periphery of groundwater geophysics. It is an important problem for the countries having a long coastline.