In this study, we investigate the transition zone discontinuities beneath the northwestern Deccan volcanic province of India through analysis of ~1000 high-quality receiver functions abstracted from three-component teleseismic waveforms from 428 earthquakes recorded by six broadband stations in the northwestern Deccan volcanic province. Our analysis reveals that the P 410 s and P 660 s time lags are delayed by ~1 s relative to those predicted by the IASP91 model. A largely unperturbed mantle transition zone, revealed by the transition zone time lag (t P660s-t P410s) of 23.83 s, implies that the observed delays are primarily associated with reduced shear velocities in the upper mantle above the 410 km discontinuity. The velocity reduction is likely to be associated with lithospheric thinning coupled with compositional and reduced thermal variations in the shallow upper mantle. Our results contrast with the normal shield-like velocity structure imaged beneath the south-central Deccan volcanic province in an earlier receiver function study. For comparison, a revised composite receiver function plot for the south-central Deccan volcanic province was constructed by employing identical receiver function processing techniques on an updated high-quality data set of 1400 receiver functions from 11 stations, which reaffi rms, with better precision, the earlier results. We propose that the relative differences in the lithospheric thicknesses beneath the northwestern and south-central parts of the Deccan volcanic province possibly governed the melting and fl ow patterns of the upwelling mantle material, resulting in the contrasting seismic signatures. The lithospheric architecture of the northwestern Deccan volcanic province, coupled with the reactivation of preexisting rift systems, appears to have facilitated the eruption of the Deccan basalts, for which source signatures are still retained in the upper mantle.

SummaryIt is important to choose an acquisition technique and geometry, which produces minimum footprints. Uniform distribution of fold, offset and azimuth for all the bins will reduce the footprints to a great extent but it is not achievable in any 3D practical geometry. Achieving the uniform nominal fold and minimizing the variation of offset and azimuth sampling across the bins is also the prime objective of the designer in designing the 3D survey geometry so that the geometry creates minimum footprints.The Slant geometry, which provides better offset distribution but narrow azimuth, is widely used in acquisition of 3D seismic data by Geophysical Crews of ONGC. In all the investigations carried out with Slant Geometry in acquiring 3D seismic data, the active spread for all the shots of salvo had been kept same. But the variation of Xmin, Xmax provided by the slant geometry as used in ONGC is more. It has been analyzed and found that by keeping the near offsets same for all the shot points of the salvo will provide uniform fold, equally good unique foldage, offset and azimuth but with minimum variation of Xmin, Xmax and Xavg across the bins. Hence, this suggested option of slant geometry will minimize the acquisition footprints. The analysis of the two options is compared in detail and it is shown that new options will have minimum acquisition footprint

The importance of characterizing the site effects in urban areas, especially Mumbai, the commercial capital of India, with a quarter of land below sea level, is well realized. Mumbai is built on a cluster of seven basaltic islands that were merged together through reclamation of land from the Arabian Sea. Due to rapid urbanization, the demarcation between reclaimed areas and original islands is blurred. A pilot study is undertaken to investigate and characterize the local site effects at 27 locations in Mumbai. The Nakamura technique is used to estimate the fundamental frequency of soft soils at each site, characterized by the ratio (H/V) of the Fourier spectra of the horizontal and vertical components of ambient noise measurements made with a 3-component short period (1 Hz) seismograph. Validation of peak frequency was done using both pre-event and event data. The peak amplification was also validated through measurements at a 10-m exposed soil section and over a soil dump. Overall, the site responses correlate well with the local geology and the lithologs obtained at 40 boreholes at 8 locations The reclaimed areas are characterized by resonance frequencies ranging from 3.3 to 4.6 Hz with significant peak amplification ([4) in contrast to hard rock sites that do not exhibit peak amplification. The hard rock sites with soil cover exhibit peak amplification in the frequency range 3.3-10.5 Hz indicating large variations in soil thickness. The H/V curves at most sites exhibit clear single peaks with large amplitude, which could be associated with sharp discontinuities corresponding to a uni-model of a single layer with large velocity contrast overlying the basement. The low resonance frequencies together with large amplification of site responses enable identifying and demarcating reclaimed areas that form important inputs in the seismic microzonation of Mumbai.

S U M M A R YOver 800 receiver functions (RFs) from three broadband and five short period stations deployed in the northern segment of the Western Ghats of India, were analysed to examine the crustal constraints on the rift flank uplift mechanism of the Ghats. Estimates of Moho depths, mean shear velocities and Poisson's ratios were determined and used as constraints to model the RFs. The study reveals that the crust beneath the Ghats varies in thickness between 31 and 39 km and is neither uniformly thin nor thick as required by existing hypotheses for uplift mechanism. The Poisson's ratio is 0.26 and the mean shear velocity is 3.65 km s −1 . The uncertainties in the estimates of the Moho depths and Poisson's ratios are ±2 km and ±0.01, respectively. The crust beneath the Ghats is 3-4 km thinner than that beneath the adjoining low-lying coastal plains suggesting block tectonics. Modelling the RFs using crustal constraints reveals sub-Moho low-velocity zones (LVZs) with velocity reductions of 0.5-0.65 km s −1 which are confined to shallow depths of about 50 km beneath the Ghats. The persistent relief of the Ghats may be attributed to the buoyancy forces of the possibly rift related shallow upper mantle LVZs beneath the Ghats.

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