Three‐dimensional determination of structure and velocity by seismic tomography

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Abstract
A computerized seismic tomographic method was developed to obtain body‐wave velocities and three‐dimensional (3-D) structure of interfaces from reflection data simultaneously. The medium consists of layers with continuous arbitrary 3-D curved interfaces separating homogeneous material with different acoustic properties. The interface is defined by a polynomial surface. The elastic waves are assumed to be transmitted or reflected at curved interfaces in which the raypaths satisfy Snell's law. The ray tracing for each source‐receiver pair is determined by solving a system of nonlinear equations. This method of 3-D ray tracing is effective in computing many seismic rays, including converted phases and multiples. A damped least‐squares inversion scheme is formulated to reconstruct the interval velocity and 3-D structure of the interface by minimizing the difference between observed traveltimes and computed traveltimes. The results from a synthetic model indicate that the solutions converge quickly to the true model. The seismic tomographic method was applied to a Vibroseis seismic section obtained in 1984 on Vancouver Island as part of PROJECT LITHOPROBE. The method was found to be useful for imaging the 3-D subsurface of subducting plates by taking advantage of crooked lines in the nominally two‐dimensional seismic reflection survey.

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