INVSP gathers of local earthquake seismograms: an approach for modelling the upper crustalPandSvelocity structure

Abstract

A new approach is presented here for obtaining INVSP gathers of local earthquake seismograms in regions with adequate seismogenic depth range. The inverse vertical seismic profiling (INVSP) geometry is in principle quite similar to the up-hole surveys of shallow seismic exploration in which shots fired at various depths in boreholes are recorded on the surface at a fixed offset. Similarly, local earthquake seismograms of ‘selected’ events at different hypocentral depths can be gathered into ‘Constant offsetINVSP’ sections for various stations of a mobile seismograph network. They can be modelled for P and S velocity structure to upper crustal depths at each of the stations in the network with the aid of synthetic seismogram computations using appropriate source mechanisms of the earthquakes used. An INVSP gather, obtained to a depth range of about 10 km at a station with 28 km constant offset, is modelled for the upper crustal P and S velocity structure in the Koyna-Warna seismic region in the western Indian shield to demonstrate this approach.