Seismic velocity, anisotropy, and fluid pressure in the Barbados accretionary wedge from an offset vertical seismic profile with seabed sources

Abstract

The state of compaction and fluid pressure in the Barbados accretionary wedge near its toe, at Ocean Drilling Program Site 949, were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array. The model, constrained by a three‐dimensional seismic survey and well logs, shows (1) a velocity gradient of about 1–1.25 s −1 in the uppermost 180–230 m of the wedge; (2) a zone of variable, but no net change in, velocity between 230 and 350 m depth; (3) a low‐velocity zone 40–50 m thick just above the décollement at 391 m; and (4) a displacement of the low‐velocity zone by thrust faults. Pore fluid pressure sections derived from P wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured. The ∼160 m thick, underconsolidated basal zone shows anisotropy, which increases downward. The lowest 40–50 m has velocity varying (1) azimuthally (3%), being fastest in the direction of plate convergence, and (2) in the vertical plane (2–5%), horizontal faster than vertical. After correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio λ does not exceed 0.9 and in the lowest 40–50 m of the basal zone, is between 0.71 and 0.82, with λ* [(fluid pressure − hydrostatic)/(lithostatic pressure − hydrostatic)] between 0.5 and 0.65, in accordance with in situ measurements of fluid pressure in the décollement zone beneath. These indicate that the accretionary wedge is stronger and less overpressured than was previously supposed.