Borehole Televiewer Data Analysis of Hole 504B from Legs 137 and 140

Abstract

Hole 504B, about 200 km south of the Costa Rica Rift in the eastern equatorial Pacific, has been drilled over a succession of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legs to a total depth of 2000.4 meters below seafloor (mbsf). Borehole televiewer (BHTV) measurements were recorded over the intervals 275-497 mbsf on Leg 137, and 1497-1715 mbsf and 1868-1990 mbsf on Leg 140. Detailed investigation of the BHTV data revealed stress-induced wellbore breakouts in all depth intervals with variable occurrence. The statistical analysis of the wellbore breakouts shows that the orientation of the maximum horizontal stress (S Hmax ) varies from N53°E ± 28° in the interval 410-490 mbsf, to N21 °E ± 35° in the interval 1497-1715 mbsf, and N82°E ± 38 in the interval 1868-1990 mbsf. Whereas the intermediate depth interval shows a mean azimuth that is consistent with previous observations from borehole breakout analysis between 700 and 1500 mbsf, the upperand lowermost breakout intervals are rotated clockwise by 22° and 61°, respectively. As none of the observed intersecting structures correlates with the observed rotation ofS Hmwc , it is assumed that the rotation is affected by non-intersecting structures or structures beyond the wellbore head. The onset of the breakout interval from 410 to 490 mbsf correlates well with the Layer 2A/2B boundary and corresponds to a seismic anisotropy observed in the upper 500 m of the crust. For breakouts to occur in shallow depths, high S Hma jS hmin ratios are required or rock strengths must locally vary (to values <IOO MPa) to allow the initiation of breakouts. Analytical results also propose a drastic reduction of pore pressure (20%) for breakout initiation, which is, however, far beyond present-day observations. The breakouts below 1497 mbsf occur within the sheeted dikes and close to the sheeted dike/gabbro transition zone (1868-1990 mbsf). The observation of breakouts below 1497 mbsf and the lack of vertical fractures in this depth allow us to estimate 131 MPa for S Hmax and 96 MPa for S lmm , which agree with stress estimates deduced with Byerlee's law for a TF/SS regime.

Crustal fissuring on the crest of the southern East Pacific Rise at 17°15′–40′S

“…An initial caveat is that these models are primarily two dimensional and assume that oceanic crust is uniform, isotropic, and elastic. Seismic velocity, electrical resistivity or deep borehole investigations (such as the recent borehole televiewer studies of Krammer et al [1995] and Ito [1995]) may eventually be adequate to determine the actual depth distribution of cracks within the study area. However, until in situ determinations or a rigorous three‐dimensional analytical solution for anisotropic, heterogeneous ocean crust become available, one must rely on a two‐dimensional solution, using reasonable values for the linear elasticity and density of the rock.…”