Analysis of multicomponent seismic data from the Hydrate Ridge, offshore Oregon

Abstract

Multicomponent seismic data can be used to derive P- and S-wave velocity structures of the subsurface, which can be used further to estimate rock and reservoir properties. Most seismic analysis methods and algorithms assume that the earth is isotropic. In many geologic situations, however, sedimentary rocks exhibit anisotropic behavior, and the isotropic assumption will introduce errors in the estimates of the elastic properties of the subsurface. With the goal of analyzing multicomponent seismic data from complex regions (which may show anisotropic behavior), I have developed new algorithms for 1) seismic modeling based on a ray-Born approximation and 2) traveltime computation in tilted transversely isotropic media based on Fermat’s principle. This new traveltime computation algorithm is tested on prestack depth migration of a physical model dataset. Such algorithms are essential for estimating subsurface rock properties in complex areas such as the Hydrate Ridge area, offshore Oregon. viii I participated in the acquisition of multicomponent seismic data (summer 2002), at the Hydrate Ridge of the Cascadia convergent margin. The primary goal of the experiment was to map the gas hydrates and free gas, and to understand the mechanism of fluid migration. Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules. Gas hydrates and free-gas are generally detectable with seismic methods because the seismic velocity increases in the presence of gas hydrates, and it decreases in the presence of free-gas. My analysis results in estimates of P- and S-wave interval velocities and anisotropic parameters with the final goal of relating these parameters to the presence and quantification of gas hydrate and free gas. I performed interval velocity analysis in the τ-p (intercept time - ray parameters) domain following three main steps: 1) P-wave velocity analysis, 2) P- to S-wave (converted PS-wave) event correlation, and 3) S-wave velocity analysis. P- to Swave event correlation is done using synthetic seismograms and traveltime tables. Seismic velocities are correlated to gas hydrate and free gas saturation using a Modified Wood equation. I find that Hydrate Ridge is heterogeneous and is weakly anisotropic (maximum of 10%) in some regions caused possibly by the hydrate veins. The P-wave velocity is more sensitive to the saturation of gas hydrates (maximum of 7% of rock volume) and free gas than the S-wave velocity. The S-wave velocity does not show an anomalous increase in the hydrate-bearing sediments. Thus, I conclude that hydrate does not cement sediment grains enough to affect shear properties. It is more likely that the hydrates are formed within the pore space in this region.