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. ¿ American Geophysical Union 1992