Estimating shallow shear velocities with marine multicomponent seismic data

Abstract

Accurate models of shear velocities in the shallow subsurface (<300 m depth beneath the sea floor) would help to focus images of structural discontinuities constructed, for example, with P to S converted phases in marine environments. Although multicomponent marine seismic data hold a wealth of information about shear velocities from the sea floor to depths of hundreds of meters, this information remains largely unexploited in oil and gas exploration. We present a method, called the multiwave inversion (MWI) method, designed to use a wide variety of information in marine seismic data. As presented here, MWI jointly uses the observed traveltimes of P and S refracted waves, the group and phase velocities of fundamental mode and first overtone interface waves, and the group velocities of guided waves to infer shear velocities and Vp/Vsratios. We show how to obtain measurements of the traveltimes of these diverse and, in some cases, dispersive waves and how they are used in the MWI method to estimate shallow shear velocities. We illuminate the method with synthetic and real multicomponent marine data and apply MWI to some real data to obtain a model of Vswith uncertainty estimates to a depth of 225 m and Vp/Vsto about 100‐m depth. We conclude by discussing the design of offshore surveys necessary to provide information about shallow shear‐velocity structures, with particular emphasis on the height of the acoustic source above the sea floor.