S U M M A R Y Deep seismic sounding (DSS) studies have been carried out in the north Cambay and Sanchor sedimentary basins in western India along three lines covering about 350 km. Seismic refraction and wide angle reflection data, pertinent to the sedimentary basin as well as the deep crustal section, have been recorded from 41 shot points using a 60 channel DFS-V digital recording system with 200 m geophone spacing and 4ms data sampling. Extensive modelling and interpretation of a large number of seismic record sections reveal four sub-basins in the sedimentary section along these lines. Maximum depth to the granitic/Proterozoic basement (P-wave velocity 5.9-6.0kms-l) is about 5000m in the north Sanchor and the Patan sub-basins and about 5600 m in the south Sanchor sub-basin. The deepest part of the sedimentary basin is delineated within the Gandhinagar sub-basin where the basement depth reaches 7700m. The Deccan Traps (P-wave velocity 4.3-4.8 km s-l) form the base of the Tertiary sediments, almost in the entire study area except the extreme northern part. There is also some indication of the presence of sub-Trappean Mesozoic sediments along this profile. Within the sedimentary basin two horst features, one near Diyodar (the Diyodar ridge) and the other northwest of Mehsana (the Unhawa ridge), are indicated by the seismic data consistent with the tectonics of the region.The thickness of the upper crust in this region does not exceed 15 km (P-wave velocity reaching 6.3 km s-'). A prominent low-velocity zone (velocity 5.5 km s-') occurs in the depth range from 10.5 to 12.5 km. The lower crust consists of two layers of velocities 6.6-6.9 km sC1 and 7.3-7.4 km S K I , the discontinuity between them occurring at 23-25 km depth. The Moho discontinuity (PM velocity 8.0 km s-') lies at a depth of 31-33 km. The high-velocity (7.3-7.4 km s-') lower crustal layer represents underplating of the crust due to mantle upwelling and rifting with large-scale extrusion of the Deccan volcanics. The large thickness of the Tertiary sediments in the Cambay basin and a relatively thin crust in the region suggest further rifting during the Tertiary.

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The 2-D crustal velocity model along the Hirapur-Mandla DSS profile across the Narmada-Son lineament in central India (MURTY et al., 1998) has been updated based on the analysis of some short and discontinuous seismic wide-angle reflection phases. Three layers, with seismic velocities of 6.5-6.7, 6.35-6.40 and 6.8 km s )1 , and upper boundaries located approximately at 8, 17 and 22 km depth respectively, have been identified between the basement (velocity 5.9 km s )1 ) and the uppermost mantle (velocity 7.8 km s )1 ). The layer with 6.5-6.7 km s )1 velocity is thin and is less than 2-km deep between the Narmada north (at Katangi) and south (at Jabalpur) faults. The upper crust shows a horst feature between these faults, which indicates that the Narmada zone acts as a ridge between two pockets of mafic intrusion in the upper crust. The Moho boundary, at 40-44 km depth and the intra-crustal layers exhibit an upwarp suggesting that the Narmada faults have deep origins, involving deep-seated tectonics. A smaller intrusive thickness between the Narmada faults, as compared to those beyond these faults, suggests that the intrusive activities on the two sides are independent. This further suggests that the two Narmada faults may have been active at different geological times. The seismic model is constrained by 2-D gravity modeling. The gravity highs on either side of the Narmada zone are due to the effect of the high velocity/high density mafic intrusion at upper crustal level.

Abst ractFirst arrival refraction data does not normally provide any indication of the velocity inversion problem. However, under certain favourable circumstances, when the low-velocity layer (LVL) is considerably thicker than the overlying higher-velocity layer (HVL), the velocity inversion can be seen in the form of a traveltime skip. Model Studies show that in such cases the length of the HVL traveltime branch can be used to determine the thickness of the HVL and the magnitude of the traveltime skip in order to determine the thickness of the LVL. This is also applicable in the case of field data.

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