Analysis of the borehole televiewer log from DSDP hole 395A: Results from the Dianaut Program

Abstract

The French wireline re‐entry expedition known as DIANAUT culminated in the successful completion of a series of downhole measurements in three Deep Sea Drilling Project (DSDP) holes in the north‐central Atlantic Ocean. Among these measurements was a borehole televiewer (BHTV) log obtained in Hole 395A from bottom of casing at 112 meters below seafloor (mbsf) to a depth of 605 mbsf. In contrast with a BHTV survey previously conducted in this well during DSDP Leg 78B in 1981, televiewer data were not degraded by random cable oscillations due to ship heave and the quality of this log is excellent. These latest data were digitized and processed in terms of acoustic travel time and amplitude to compute hole size and shape, evaluate the structural integrity of the surrounding rock, and characterize intersecting fractures. The acoustic caliper and reflectivity logs correlate well with the lithologic column, particularly at the boundaries between major units which often are marked by breccias. Fractures in the lower part of the hole appear to be effectively sealed, in contrast with the open fractures identified in the upper sections. This supports a systematic pattern derived from complementary geophysical logs of increasing density, electrical resistivity, and elastic‐wave velocity, and decreasing in situ permeability as a function of depth. Fractures intersecting the well exhibit a wide range of orientations marked by some clustering of dip azimuths approximately east‐west. The presence of moderately dipping fractures striking subparallel to the ridge axis is consistent with both ridge‐parallel topographic elongations observed in this area and with azimuthal seismic anisotropy reported elsewhere, suggesting that fractures such as those identified from the BHTV record contribute to these effects.