VELOCITY DISPERSION - A TOOL FOR CHARACTERIZING RESERVOIR ROCKS

Apparent discrepancies between velocity measurements made with differe nt frequencies in a formation at the Gypsy test site are explained in terms of elastic scattering and intrinsic attenuation. The elastic sca ttering component of the dispersion (38%) in a marine interval above t he Gypsy sandstone is estimated via simple models constructed from wel l log information. Any dispersion above the predicted value for elasti c scattering in this interval is assigned to intrinsic attenuation (62 %). Using the vertical measurements in the well, the marine interval d irectly above the Gypsy sandstone has an estimated intrinsic Q(I) = 51 and an effective Q because of the scattering of Q(SC) = 85. The total Q of the combined mechanisms is 32. The dispersion of the vertical me asurements through the heterogeneous sands and shales of the Gypsy for mation can be explained using an intrinsic Q(I) = 30 and neglecting th e effects of scattering. The horizontal observations require a more de tailed modeling effort to unravel the relative roles of path and volum e effects, elastic scattering, attenuation, and intrinsic anisotropy. Thin layers barely resolvable on the sonic logs play a significant rol e in modifying the crosswell response. Potentially, the dispersion can be a key to mapping reservoir properties using crosswell and surface seismic data.