University of Texas at Austin, United States

We examined 3-component seismic data from four ocean bottom seismometers (OBS) deployed along a 94 km long E-W line in the Sigsbee plain of the Gulf of Mexico. Each OBS had 3 components; one vertical and 2 horizontal. We first constructed a 2-D P-wave velocity model consisting of six principal layers (a water layer, three sedimentary layers, an igneous crust, and upper mantle), using a standard ray-based inversion technique. Then, we attempted to identify events associated with P-S mode conversions in the radial component OBS records. We assumed a depth-invariant Poisson's ratio within each layer and performed trial-and-error, forward ray-trace modeling. We were able to identify mode-converted events originating from the bottom of three sedimentary layers with high confidence, because of the relatively simple geologic structure of the area. This information yielded an S-wave velocity model of the sedimentary layers. According to previous results obtained from laboratory high-frequency sonic measurements, the trend between V (sub p) /V (sub s) and P-wave velocities of these sediments is suggestive of the following lithologic change with depth: poorly consolidated shale at the top, compacted sandstone or sandy shale in the middle, and carbonate rocks at the bottom.