Accurate RMS velocity determinations from CGP data by measuring residual NMO with an adaptive method

Abstract

Accurate root-mean-square (RMS) velocities are needed to stack a common-ground-point (CGP) gather properly, to calculate interval velocities, thickness, and dips of subsurface layers accurately, and to infer correct lithological information from the interval velocities. The RMS velocities are usually determined from a CGP gather by using a velocity-spectra method. The RMS velocity of a layer determined by the method is an approximate one because it is one of the velocities assumed in the velocity-spectra generation. The accuracy of the RMS velocity determined from the velocity spectra depends on the velocity increment by which the assumed RMS velocities are varied. Also, the accuracy depends on the sensitivity of the arrival time variation of reflected signals on the CGP gather. The sensitivity of the arrival time variation is higher for a layer which has a small RMS velocity, V, and zero offset time, t₀, than for a layer having a large v and t₀. Two new techniques of RMS velocity determination have been developed: one of the two is a velocity spectra method which generated the velocity spectra efficiently and the other is an adaptive method which determines the RMS velocities very accurately. The efficient velocity-spectra method generates the velocity spectra so that the RMS velocities can be determined more accurately for shallow layers which have smaller t₀, and V values than deep layers..