Analysis and Interpretation of the Borehole Televiewer Log: Information on the State of Stress and the Lithostratigraphy at Hole 504B

Abstract

Hole 504B in the eastern equatorial Pacific has been the focus of several scientific expeditions during the past few years, where a series of re-entry, drilling, and coring operations has yielded important data regarding the structure of the upper kilometer of the oceanic lithosphere. As part of the extensive downhole experiments program planned for ODP Leg 111 at this site, a borehole televiewer (BHTV) log was obtained across a 355-m vertical section of crustal base­ ment, 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 according to the well-substantiated hypothesis that breakouts occur coincident with the direction of minimum principal stress (Sh) where compressive forces are at their greatest. This Sh orientation is found to be NI22.5°E, a direction that agrees closely with the results of a previous BHTV study of breakouts in the upper section of this same borehole and also with stress orientation estimates inferred from regional intraplate earthquakes. The distri­ bution of breakout frequency vs. azimuth shows the data to be bimodal and orthogonal to a degree that is statistically significant. The Sh direction is inferred from the primary mode. The orthogonal pattern indicates that a considerable number of wellbore enlargements were detected coincident with the direction of maximum principal stress (SH). These are interpreted as extensional fractures produced by localized tensional stresses parallel with the SH orientation. Consid­ eration of various hydraulic fracturing schemes and attendant analyses indicates that these fractures are likely to be in­ duced when the ratio of effective horizontal principal stresses {SH/Sh) is very large in a cylindrical wellbore or is some­ what smaller in a hole exhibiting a slight degree of ellipticity. This hypothesis is supported by the solutions to source mechanism studies associated with local earthquakes that indicate a strike-slip sense of faulting and a relative order of stress magnitude where SH » Sh. The bimodal, orthogonal distribution of wellbore enlargements also results in a slight correction to the method used for estimating the principal stress orientation, one based upon the population modes rather than the mean. Inspection of three-dimensi onal images generated from the BHTV record in the lower portion of the borehole re­ veals the surprising appearance of pillow lavas at a depth previously assumed to be composed of sheeted dikes. When the ophiolite analogy is applied to contemporary oceanic crust, 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 interlayering of lithologies that extends over a thicker vertical interval than previously thought.