IVA-00-01

Joint analysis of surface-wave and refraction events from river-bottom sediments

Summary

An underwater seismic survey was performed in the Fraser River Delta area near Vancouver, B.C., Canada, using hydrophones.  Data were analyzed using multichannel analysis of surface waves (MASW), and refraction tomography inversion provided an accurate P-wave velocity (Vp) profile for riverbottom sediments in the upper 30 m.  To optimize the compressional wave velocity function in the 30 to 70 m depth interval, Vp information from a nearby well was used as a priori information.Using shear wave velocity profiles from surface-wave inversion as a priori information allows a fast and reliable inversion of the first arrival events interpreted on an underwater shot gather. The inverted Vp profile, strongly suggesting the presence of gas within water-bottom sediments, corroborates a nearby land well. This example demonstrates the usefulness of joint inversion and its potential for detecting near-surface anomalies. between measured and calculated first arrivals. The match is improved by iteratively changing the Vp model until a satisfactory solution has been reached.  As described previously, through joint analysis of this shallow marine data set a reliable Vp solution will be obtained for the shallow sediments independent of the measurements made in the nearby control well.

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