M. Landisman

Shield-like upper mantle velocity structure below the Indo-Gangetic Plains: Inferences drawn from long-period surface wave dispersion studies

Abstract Shield-like upper mantle velocity structure has been inferred to exist below the Indo-Gangetic Plains from a study of mantle Rayleigh and Love wave group and phase velocities for periods extending to 200 seconds. The investigations have been carried out for the path between the World Wide Standard Seismic Network Stations at Chiengmai (CHG), Thailand, and New Delhi (NDI), India, for an earthquake with its epicenter in the New Hebrides Islands. Application of improved frequency time analysis has permitted measurement of regional surface wave dispersion for periods extending to several minutes; previous results for this region were for periods no greater than about one minute.

VES dipole-dipole filter coefficients

The conversion of sampled Schlumberger and dipole‐dipole vertical electrical sounding (VES) apparent resistivity values into raised kernel function values is an important step in the interpretation of these data. This conversion involves the convolution of the sampled values, uniformly spaced on a log distance scale, with a set of filter coefficients. For Schlumberger and dipole‐dipole configurations, these coefficients can be calculated directly from the Schlumberger filter function. Oscillations of the filter coefficients at large indices (distances) can be minimized by proper selection of the sampling interval, as suggested by Koefoed. The choice of an optimal sampling interval has a more pronounced effect on the accuracy of the inverse transform (raised kernel function into apparent resistivity) than on that of the forward transform. The accuracy of the inverse transform can be improved for nonoptimal sampling intervals by “phase perturbation” of the filter function. Both transforms are sensitive to a...