Standard surface-recorded compressional and horizontally polarized shear wave seismic data are used to detect lateral variations in lithology and pore fluid within a stratigraphic unit. The analysis technique employed requires determining the horizontal shear and compressional wave interval transit time through a formation using measurements along coincident CDP shear and compressional lines. In addition, at each surface position along these seismic lines, the ratio of the shear-to-compressional transit time across the target interval is calculated. A strong correlation between observed lateral variations in the measured shear and compressional wave interval transit times and their ratio with known changes in the geologic cross section demonstrates the effectiveness of th technique to infer changes in the physical properties of a formation.

The formation chosen for this case study is the Lower Pennsylvanian Morrow formation, which produces gas from isolated channel sand deposits. Detailed geologic cross sections of the producing horizon have been mapped along two nearly perpendicular seismic profiles. Horizontal shear and compressional Vibroseis (FOOTNOTE *) surveys were conducted along these surface profiles using data acquisition parameters designed to produce similar signal-to-noise ratios and seismic resolution in both sets of field data. The profiles were located to insure that each line crossed productive and nonproductive regions of the gas field. After standard data processing, the horizontal shear and compressional wave transit times through the Morrow Formation showed significant variation in going from shaly n nproductive regions to productive channel sand areas. These variations, along with diagnostic changes in their ratio, are shown to be reliable indicators of lateral changes in the pore fluid content and the sand-shale ratio within the Morrow Formation.