Detecting S-wave velocity contrasts across BSRs using AVO

Christine Ecker

christin@sep.stanford.edu

ABSTRACT

Reservoir characterization of methane hydrate data requires a reasonable estimate of their S-wave velocity behavior. AVO analysis at the bottom simulating reflector (BSR), which is the reflection off the bottom of a stable hydrate structure, is a tool to extract this information from surface P-wave data. Using sonic and density logging data from a recent ODP (Ocean Drilling Program) cruise at the Blake Outer Ridge, offshore Florida and Georgia, I evaluate the effect of different S-wave velocity structures on the seismic BSR amplitudes by forward seismic modeling. I show that even for an angle coverage of 30 degrees, S-wave velocity contrasts of about $\pm$ 17 m/s across the BSR can still be uniquely distinguished in the case of ideal amplitudes. Introduction of random Gaussian noise of a S/N ratio of about 2:1 strongly reduces the ability to differentiate between such small contrasts. In that case, errors between $\pm$ 50 m/s and $\pm$ 100 m/s can be introduced into the estimation of the shear wave velocity contrast. A subsequent 2-D inversion of a model, that varies laterally only in S-wave velocity, results in a more stable resolution in the presence of noise.