Analysis of full waveform acoustic logging data at ODP Site 642 - outer Voring Plateau

Abstract

Formation compressional and shear-wave velocities were calculated from sonic full waveform logs in ODP Hole 642E. The data were recorded over a 710-m interval of a cyclic repetition of lava flows and volcaniclastic sediments which are intruded by tholeiitic dikes. Although compressional (p) and shear (s) velocities in the flows are above 5 km/s and 3 km/s, respectively, interval velocities are 4.14 km/s for p and 2.25 km/s for s, resulting in an average Vp/Vs of 1.84 for the entire logged interval. This is a consequence of the much lower velocities in the volcaniclastics. The velocity results are interpreted in conjunction with the companion logs (density, porosity, natural gamma, and electrical resistivity) run in the drill hole in an attempt to determine the mechanisms causing the characteristic velocity response. The results show that porosity and microstructure have the primary influence on velocities. The degree of alteration to clay minerals is a secondary effect that requires the existence of initial pore connectivity. -Authors