KIM, SANGMYUNG and SEIICHI NAGIHARA, Department of Geosciences, University of Houston, Houston, TX; and YOSIO NAKAMURA, Institute for Geophysics, University of Texas at Austin, Austin, TX
We examined 3-component seismic data from 5 ocean bottom seismometers (OBS) deployed along a 94-km long E-W line in the Sigsbee plain of the Gulf of Mexico. Every OBS had 3 components: one vertical and 2 horizontal. After constructing a 2-D P-wave velocity model consisting of six principle layers (a water layer, three sedimentary layers, an igneous crust, and the upper mantle), using a standard ray-based inversion technique, we attempted to identify in the radial-component OBS records events associated with P-S mode conversions. We assumed a constant Poisson's ratio within each of the layers and performed trial-and-error, forward ray-trace modeling. We were able to identify mode-converted events coming from the 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. Vp/Vs and Poisson's ratio of the sedimentary layers varied from 3.3, 0.45 (top), 1.7, 0.27 (middle) to 1.8, 0.29 (bottom): respectively. According to previous results obtained from laboratory high-frequency sonic measurements, the trend between Vp/Vs and P-wave velocities of these sediments is suggestive of lithologic change with depth; unconsolidated shale for the top, compacted shale for the second, and carbonates for the third layer.
KIM, SANGMYUNG, SEIICHI NAGIHARA, and YOSIO NAKAMURA
AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas