20. INTEGRATION OF VERTICAL SEISMIC PROFILING, LOGGING, AND SEISMIC DATA IN THE VICINITY OF THE DÉCOLLEMENT, NORTHERN BARBADOS RIDGE ACCRETIONARY PRISM ¹

G.F. Moore,² Z. Zhao,² and T.H. Shipley ³

ABSTRACT

Vertical seismic profiling (VSP) and downhole logging carried out on Ocean Drilling Program Leg 156 provide ties to the regional three-dimensional seismic reflection data as well as velocity data near the décollement. Two thin, low-density, high-porosity zones appear on logs within the décollement zone at Site 948, but the seismic signature of the zone is positive. The low-density zones are shown to be below the resolution of surface seismic data. The low-density zones therefore must increase in thickness along strike to yield the typical negative-polarity seismic signature of the décollement. The VSPs document the location the screened sections of casing at Sites 948 and 949 and confirm that they were emplaced over the décollement zone. VSP velocities measured in the accretionary prism above the décollement zone are higher than well-logging sonic velocities through the same section.

¹ Shipley, T.H., Ogawa, Y., Blum, P., and Bahr, J.M. (Eds.), 1997. Proc. ODP, Sci. Results,156: College Station, TX (Ocean Drilling Program).
² Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822, U.S.A. gmoore@hawaii.edu
³ Institute for Geophysics, University of Texas at Austin, Austin, TX 78759-8345, U.S.A.