The Deccan trap covers about 500,000 sq Km of the region in India covering
part of Gujarat, Maharastra and Madhya Pradesh. It is believed that the Deccan
trap is formed due to volcanic eruption, which took place about 65 Ma years
ago during upper Cretaceous-Eocene. The sub-surface trap layer in the Deccan
trap region causes very strong seismic reflections. Therefore, the deeper
subsurface structure of the Deccan trap region is not known by the conventional
seismic method. Apart from deep seismic sounding (DSS) method, the magnetotelluric
(MT) method is another alternative for deeper subsurface investigation. In frequency-
-domain, the depth of penetration in a homogeneous medium is directly proportional to
resistivity of the media and period of the penetrating signal. Therefore, MT investigation
covering a wide period range (10-1-104 sec) are generally carried out to elicit a very
reliable information about the spatial distribution of resistivity within the deeper Earth.
The MT data have been recorded along the three profiles across the Deccan trap and
in the Cambay graben lying in the western part of the Deccan trap region. The Ahmedabad-
-Bhopal profile runs west to east across the Deccan trap and other two profiles, Navsari-
-Ahmedabad and Ahmedabad-Dhanera, run in south-north direction in the Cambay basin
where Deccan trap formation underlies sedimentary formations of the graben. The
one-dimensional resistivity sections have been deduced from these three profiles from
the magnitude and phase of surface impedance in the period range 10-1-3.6x103 seconds.
The field data along these profiles have been inverted for one-dimensional structure using
the algorithm based on the non-linear least-square approach. The east-west Ahmedabad-
-Bhopal profile shows a three to four layered resistivity structure. Discontinuous
subsurface / mid-crustal conductive layers have been found along this profile. The
two-dimensional resistivity section of Ahmedabad-Dhanera profile shows synclinal
shape structure across the basin axis. The sedimentary formation lies up to 4 Km depth.
The thickness of sedimentary formation is maximum in the central part of the profile. In
the northern part of the profile, a very high resistive region is seen to arise due to Aravallis
formations which is seen to extend up to great depth. The Navsari-Ahmedabad profile
runs parallel to the axis of the lower half of Cambay graben. The aim of MT investigations
was to map the Deccan formation underlying the thick sedimentary formation and to map
the existence of conductive formation (Mesozoic sediments) below the Deccan trap. The
varying thickness of Deccan trap formation between 0.8 to 4.0 Km depth has been found
. MT data has also revealed a thick and conductive formation below the resistive Deccan
formation. The thickness of conductive formation at some locations has not been delineated
due to the skin effect in the region. The one-dimensional resistivity section obtained along Navsari-Ahmedabad profile does not match with the existing borehole information which
were obtained by Oil and Natural Gas Commission. The deviation in the geo-electrical
parameters have been found. These deviations seem to arise due to coast effect of
conductive Arabian sea which lies nearly 20 km away from the profile. The Geomagnetic
Coast Effect (GCE) has been analyzed by two-dimensional finite-different algorithm of
Brewitt-Taylor and Weaver (19760 over four probable geophysical (geo-electrical) models
representative of the western coast region of India. These models represent the different
geological regions along and around the western coast. It has been found that the
E-polarization mode (electric current parallel to the strike) is least affected by the coast.
The effect of the coast distorts the magnetotelluric field which have been used to compute
resistivity and variation of phase of the surface impedance in the entire period range for
the models. The transfer function deduced in the Magneto-Variational (MV) studies at
longer periods for the coast models have been obtained. The result shows a pronounced
coast effect up to 100 km distance from the coast. The one-dimensional inversion of MT
data shows a fictitious conductive layer within the resistive formation. The qualitative
estimation of layered parameters of the fictitious conductive layer have been carried out
for models representative of the western coast region. It has also been observed that the
layer parameters of the existing conductive formation have been changed significantly
due to the coast effect. Quantitative estimation of the layer parameters of the existing
conductor have been made to analyze the coast effect in the conducting formation.
Deccan trap region is considered to be seismically stable region inspite of number of
minor and few major earthquakes recorded during last 30 years. The mechanism of the
origins of earthquakes have been conflicting due to the lack of deeper geophysical
information of the region. In some part of Deccan trap, a conductive formation has been
found in the forma of continuous and discontinuous layer. The DSS results obtained in
the Deccan trap region have been compared with the one-dimensional resistivity section
obtained by the MT data. At many stations, the depth of the conductive formation is found
to be of the same order as of the low-velocity layer. The presence of low resistivity and
low velocity is attributed to the presence of fluid in the crust. The presence of fluid in the
Deccan trap region has been discussed in light of present seismicity of the region.