Sverre Planke2 and Herve Cambray3


Ocean Drilling Program Hole 917A penetrated 779 m of subaerially emplaced basalts and dacites near the landward edge of the seaward-dipping reflector sequences on the southeast Greenland volcanic margin. Wireline logs were recorded in a 430-m interval covering four very thin sediment units and 48 lava units with mean and maximum thickness of 8.9 m and 53 m, respectively. Seismic compressional-, shear-, and tube-wave velocities were obtained by slowness-time coherency inversion of sonic waveform data. The compressional-wave velocity log shows asymmetric cyclic variations, with velocities of 2.5 to 5.5 km/s in the brecciated and vesicular flow tops, and high velocities, 5 to 6 km/s, in the massive and fractured central and lower part of the lavas. The variations in velocity are attributed to systematic changes in total porosity, pore geometry, and alteration. The shear-wave velocity log recorded in high-velocity (Vs > 1.5 km/s) intervals correlate well with the compressional-wave velocity log. High shear-wave amplitudes identified near numerous high-impedance boundaries are related to mode-conversion within the lava pile. Vp/Vs ratios of 1.8 to 2.0 are recorded throughout the sequence, with no systematic variations. The normalized tube-wave energy log is broadly inversely proportional to the velocity logs, with low energy values in fractured massive intervals and high energy values near unit boundaries and within the top part of the lavas. Comparison of conventional logs and the Formation MicroScanner image gives high confidence to the usefulness of conventional logs in terms of recording downhole textural and lithologic variations in flood basalt terrains. The average velocity in the lava pile is 4.17 km/s, corresponding to 4.05 km/s obtained from the interpretation of seismic reflection data, and is primarily a function of the average lava unit thickness.

1Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program).
2Department of Geology, University of Oslo, Box 1047 Blindern, 0316 Oslo, Norway. planke@geology.uio.no
3Laboratoire de Mesures en Forage, Institut Méditerranéen de Technologie, 13451 Marseille Cedex 20, France.