WAVE-EQUATION VELOCITY ANALYSIS

Velocity-sensitive (wide propagation angle) seismic data do not comply with the rms small propagation angle approximation. A hyperbolic velocity spectrum and Dix's equation cannot use wide-angle arrivals to estimate interval velocity accurately. The linear moveout method measures interval velocity exactly in stratified media. Snell midpoint corrdinates are constructed to image the data before velocity estimation. Energy focuses at the arrival coordinates of a fixed reference Snell wavefront, as required by the linear moveout method. The image of a common-midpoint seismic gather in Snell midpoint coordinates, for a nonvertical reference Snell wave, defines the wave-equation velocity spectrum. Approximations of the wave equation in Snell midpoint coordinates satisfactorily image wide-angle energy. To compute the velocity spectrum the authors use the 15-degree finite-difference wave equation in the frequency continuation velocity. This formulation resolves multivalued, wide velocity spectrum data using inhomogeneous, offset and depth dependent, downward continuation velocity.