Coincident P-wave and S-wave CDP lines were shot across the Willow Slough and Putah Sink fields, Yolo County, California, by the 1977-78 Conoco P-Wave/Shear-Wave Group Shoot. The fields produce gas from pay sands in the Cretaceous Starkey and Winters formations. Several of the thicker pay sands correlate with amplitude anomalies on the P-wave sections, and these amplitude anomalies are true seismic "bright spots." The equivalent events on the S-wave sections are much lower in relative amplitude when the overall gains of the P and S sections are balanced. The difference in the P and S responses is consistent with laboratory experiments which show that introducing gas into the pore space of a liquid-saturated rock dramatically lowers P velocity but minimally affects S veloc ty. The experimental lines demonstrate that comparison between the amplitudes of P and S is a diagnostic technique that can be sued to distinguish gas-liquid contacts from lithologic interfaces. An S-wave section validates a P-wave bright spot attributed to gas saturation when there is no anomalous amplitude at the equivalent S-wave event.

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