Modelling of gravity data in order to construct the subsurface structure in terms of a density distribution is a time-tested technique of geophysical interpretation and is always relied upon to yield first order information, which is correlatable with observed geological knowledge. Nevertheless gravity models are plagued by the ambiguity of results, which are intrinsic to the mathematical treatment of potential field methods and suffers from increasing uncertainty and diminishing resolution with depth. Simultaneous three dimensional modelling of geoid data in conjunction with gravity data is an advanced interpretation technique, which provides added constraints and helps to reduce uncertainty, particularly in areas where there is a lack of data from other geophysical and/or geological studies.  

            In the Western Continental Margin of India, this method of interpretation, based on predominantly satellite derived potential field data, allows us to estimate the effects of the various regional scale geological units, viz. the crust, the upper mantle, the underplated magmatic material, low density crustal heterogeneities, etc. On subtraction of these effects from the observed anomalies, we are able to delineate the extent of the Deccan basalts, which was presumably associated with the episode of Reunion hotspot activity under the waters of the Arabian Sea, as far as 800 km from the coastline. Localised basins of low density material beneath the north western Deccan Traps are also resolved.  

Two dimensional spectral approach has been used to identify the regional gravity anomalies over Deccan Volcanic Province constraining from isostatic analysis. Gravity data are filtered for wavelength more than 250 km based on the results of isostatic analysis and also upward continued to 38 km (an average Moho depth found along  profiles) to delineate regional Bouguer anomalies in the region. Both analyses show similar trends and define the regional gravity anomalies in the region, which might be originating from Moho undulations.  Modelling of filtered ground magnetic data recorded at 1-2 km spacing along a E-W profile running from Kelsi (at west coast) to Diglur (end of the trap in the east) suggests that magnetic anomalies may be explained with a varying trap thickness having a susceptibility of 0.004 cgs units and NRM of .045 emu/cc in the direction I=-400 and D=3300.

            The study area falls in Central India and is covered by Deccan basalts overlying the Gondwana, Vindhyan and Bijawar sediments and the basement of Bundelkhand granite. Bundelkhand granite forms the high resistivity basement. Combined Schlumberger and equatorial direct current dipole soundings and collinear dipole dipole traversing were conducted along Sanwer–Borgaon profile across the Narmada–Son lineament primarily to identify the lithology and to locate the shallow faults and fractures. A fault at Chhegaon–Makhan has been delineated from the resistivity pseudo–section plot which corroborated the resistivity sounding data. 2–D model prepared from I–D inversion of sounding data revealed that the thickness of the Deccan trap varied from a few tens of metres to more than 1 km along the Sanwer–Borgaon traverse. Occurrence of abundant water within the subsurface formations in the Dulhar zone, and further south, one conductive thick zone presumably due to highly weathered Deccan trap or intratrappeans have been inferred. Sudden increase in depth of the basement north of the Narmada–Son lineament indicated that a major fault coincides with the Narmada river. Both resistivity sounding and profiling techniques proved to be very useful tools for understanding lithology and structure below the Deccan trap.

Abstract 

Petrophysical analysis was carried out for all the identified hydrocarbon intervals, from the four wells studied in the Abura Field using suites of geophysical well logs. From the analysis of the geological logs comprising gamma-ray, spontaneous potential, electrical resistivity, neutron and density logs, the total porosity in the hydrocarbon bearing zone was found to range from 18.0% to 28.0% and the water saturation range from 16.0 to 54.0%.  Good well-to-well lithologic correlation was established across the fields studied .The researcher found that the bulk of the hydrocarbon encountered in the Niger Delta basin was found to be within a depth range of 2510.0-3887.0 m. From the analysis of the lithology and fluid contents, wells 1, 2, 3, and 4 were found to have penetrated the Benin and Agbada formations.

The hydrocarbon reservoirs were found to be in the Agbada formation, which is in conformity with the geology of the Niger Delta.

            Application of aerogeophysical input constitutes an integral part of integrated approach in resolving geoscientific problems. The utility of different airborne geophysical data gets impetus if these techniques are to be employed in unravelling subsurface geology of a concealed terrain where the surface geology is either obscure or nonexistent owing to thick soil cover. An attempt has been made in this paper to interpret the aeromagnetic and spectrometric data qualitatively to decipher the subsurface geology over soil covered area in parts of Raichur district and to refine the existing geological map. 

            The study area forms a part of the Raichur granite-greenstone belt of the Eastern Block  of Dharwar Craton and exposes a complex terrain of schists and granitoids ranging in age from Archaean to early Proterozoic. The qualitative interpretation of the aeromagnetic map has helped in delineating the litho contact between the metasedimentary rocks of the schist belt and the adjoining granites/gneisses towards south of Raichur area where a considerable part is concealed under soil cover. The inherent inhomogenity evident in the Raichur granitoids is clearly brought out through the qualitative interpretation of spectrometric map which reveals several zones of high elemental concentration particularly among younger potassic granites. Low elemental concentration is quite conspicuous among schistose rocks and older gneisses while the dolerite dykes and the structural features are construed through the aeromagnetic linears and  discontinuities. 

Integrated geoelectric and geochemical investigation are carried out in the southern part of Sagar Island region to assess the prevailing groundwater condition and its chemical quality.  Geologically, the area  consists of alluvial and marine sediments of quaternary age, which is underlain by thick tertiary sediments. Landform assemblages like tidal flats, runnels, longshore bars, marshy land, sand dunes and criss-crossing tidal creeks are supportive of estuarine process under which they have been formed. Vertical electrical soundings (VES) in the study area show mostly five layers consisting of topsoil, saline and  brackish water bearing layers, clay layer underlain by a zone with fresh water.  The VES findings show potential freshwater bearing zone of appreciable thickness at depths from 180 m to 212 m under confined condition. The results of VES studies significantly correspond with the borehole data and a litho-resistivity relationship is established for this area. Chemically the fresh groundwater is Na-HCO3 type with TDS ranging from 470 mg/l to 645 mg/l, and is safe for drinking and domestic purposes but unsuitable for irrigation purposes (SAR values range from 8.22 to 13.2) and Na content is significantly higher than what is typically found in groundwater. The seawater contamination (SWC) values for these water samples are low (<0.5), except for one sample (T3), nearest to the sea, for which it is 0.72. The concentration of arsenic, iron, lead and mercury in the samples are below the recommended limit for drinking water of World Health Organization (WHO).