Abstract

Microearthquake signals from widely distributed geographical regions have been noticed repeatedly to be strongly affected by local site effects. The corresponding seismograms contain important information on the individual propagation path. A single station inversion method is presented which allows the imaging of the subsurface impedance structure from locally recorded SH waves records.

The technique is based on the Kunetz-Claerbout equation which in its original form states the relationship between the transmission response and the reflection response for P waves in a horizontally layered medium and vertical incidence (Claerbout 1968). By adapting Claerbout's formulation to the transmission problem for SH waves under oblique incidence, microearthquake recordings can be used to calculate corresponding pseudo-reflection seismograms which in turn can be inverted for the impedance structure by exploiting Levinson recursion.

Limitations in the high-frequency content of the signals due to the combined effects of absorption, recording system, and bandlimited source signals, as well as the influence of noise, drastically reduce the resolutional power of the inversion procedure. A number of different deconvolution techniques have been tested with synthetic seismograms for the applicability to reduce these effects. Good results have been obtained with the blind deconvolution technique of Scherbaum and Stoll (1985), utilizing the minimum phase property of the Green's function.

From the analysis of synthetic data, it can be expected that for realistic source-receiver situations and short-period recording systems, the main reflectors down to a depth of several hundreds of meters could be resolved.

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