Analysis and interpretation of the borehole televiewer log: information on the state of stress and the lithostratigraphy at Hole 504B

Abstract

Log was obtained across a 355-m vertical section of crustal basement, extending upward from 1531 to 1176 m below the seafloor. The BHTV record was analyzed in terms of both acoustic amplitude and traveltime in order to accurately identify zones of structural failure in the surrounding basement rock. A subsequent examination of breakout frequency vs. azimuth helped identify the orientation of in-situ horizontal principal stresses. This Sh orientation is found to be N122.5°E. A considerable number of wellbore enlargements were detected coincident with the direction of maximum principal sites (SH. These are interpreted as extensional fractures produced by localized tensional stresses parallel with the SH orientation. Three-dimensional images generated from the record in the lower portion of the borehole reveals the surprising appearance of pillow lavas at a depth previously assumed to be composed of sheeted dikes. The transitional unit between the upper pillow lavas and minor flows and the variably metamorphosed diabase dikes below seems to be a complex, gradual, and gradational interlaying of lithologies that extends over a thicker vertical interval than previously thought. -from Authors