Undoing wavefield interference for AVAZ measurements

David C. Henley, Faranak Mahmoudian

There is a lot of interest in measuring the amplitudes of reflections from anisotropic rock layers in the earth. Such measurements can be used to estimate a rock layer's anisotropic elastic parameters. In the case of fracture-induced anisotropy, fracture orientation and intensity can be determined from these parameters. Most methods for estimating anisotropic elastic parameters require a set of reflection amplitude measurements as a function of both source-receiver offset and survey azimuth (AVAZ). A large problem with many of these measurements, however, is the interference between the targeted reflection event and other, shallower seismic events, or 'noise'. This interference can result in large amplitude disturbances of the target event, making accurate amplitude measurements impossible.

We show here how to remove most of this interference using radial trace (RT) filtering techniques. The RT filter method is attractive because it estimates and subtracts coherent components along various dip directions directly observed on the input trace gathers. Careful parameter selection ensures that amplitudes in the frequency band of the target reflection are unaffected by the noise subtraction. The resulting amplitude trend on each trace gather is much smoother, and a better fit to the theoretical trend. We demonstrate on a set of AVAZ survey data acquired at the CREWES physical modeling facility. Elastic parameters estimated from these measurements were verified elsewhere by an independent technique.