Interface inversion using broadside seismic refraction data and three‐dimensional travel time calculations

A procedure has been developed to interpret densely sampled broadside seismic refraction data recorded from a large air gun array. First arrival travel times are inverted to find the structure on an interface beneath the shot line. Travel times are calculated for three‐dimensional velocity models using a rapid finite difference algorithm, adapted to allow variable sampling of the model and the determination of rays. A simple inversion parameterization eliminates the need for matrix inversion. The complete inversion procedure is computationally rapid yet allows the determination of detailed three‐dimensional structure. Broadside refraction data recorded in the Queen Charlotte Basin, offshore western Canada, during a multichannel reflection experiment are used to demonstrate the procedure. The data are inverted for the basement interface beneath the shot line, defining a rapidly varying thickness of sedimentary basin fill. The results of the inversion stimulate a reinterpretation of the reflection data and identify a new major basement fault. Structure out of the plane of the reflection section is determined, including the strike of the fault and other nearby features.