ASA 124th Meeting New Orleans 1992 October

1pAO11. Interpretation of in situ logging p-wave velocities in sediments over very young crust.

Dawn Lavoie

Kathleen Fischer

Code 361, NRL-Det., Stennis Space Center, MS 39529

Laboratory and downhole logging measurements on sediments in the Lau back arc basin display none of the expected trends in physical and acoustic properties as a function of depth. In situ, the sediments are underconsolidated, although in the lab the sediments can consolidate normally, as shown by measuring both compressional and shear wave velocity in a controlled laboratory triaxial setting. Both compressional and shear wave velocity, measured in the triaxial cell after consolidation to estimated in situ pressures, are significantly higher than those measured in situ. The primary reason that sediments are not consolidating in situ, and therefore, compressional and shear wave velocity are not decreasing downhole, is that water lost during the consolidation process is rapidly replaced by fluids circulating through the system convectively. The undisturbed microfabrics of these sediments were examined by scanning electron microscopy equipped with energy dispersive x-ray spectral analysis and image analysis systems. This allowed quantitative measurement of pore space and degree (and type) of cementation. In general, compressional and shear wave velocity did not show consistent correlation with grain size distribution. Instead, variations in velocity within lithological units were related to the distribution of pore space and digenetic state of the sediment.