Abstract:
Deep-sea carbonates are known to exhibit a significant degree of acoustic anisotropy, and because of their widespread occurrence, this phenomenon may affect the propagation of acoustic waves in the uppermost oceanic crust. Possible causes of anisotropy are (1) alignment of cracks and pores, (2) preferred orientation of mineral grains, and (3) compositional bedding laminations. Based on a study of the compositions, textures and compressional-wave velocities of 41 calcareous sediment samples from DSDP Site 516F, we find the following. (1) These sediments are transversely isotropic, with velocities parallel to bedding higher than velocities normal to bedding by as much as 0.43 km/sec, and measured anisotropy ranging from 1 to 13 percent. (2) The difference between horizontal and vertical velocities does not decrease with increasing confining pressure, suggesting that alignment of cracks is not the cause of acoustic anisotropy. (3) Although a very strong concentration of calcite c-axes normal to bedding would account for the observed anisotropy, X-ray pole figure goniometry reveals that preferred orientations are not sufficiently strong to account for the observations. (4) Anisotropy in visibly bedded samples increases markedly with depth and is inversely proportional to carbonate content. By contrast, unbedded samples have low anisotropies (2-3 percent), with no depth dependence. Compositional bedding is the principal cause of velocity anisotropy in calcareous deep-sea sediments. Calcareous deep-sea sediments are known to exhibit acoustic anisotropy, and have been assumed to be transversely isotropic with compressional-wave velocities parallel to bedding exceeding velocities normal to bedding by as much as 0.5 km/sec. Because of the widespread occurrence of calcareous sediments, their anisotropy may significantly influence propagation of acoustic energy in the uppermost oceanic crust.