Velocity Anisotropy in Calcareous Sediments from Leg 130, Ontong Java Plateau

Abstract

During Leg 130, an intensive program of P-wave velocity measurements was conducted on calcareous sediments of varying induration (ranging from ooze to limestone) that cover the Ontong Java Plateau (west equatorial Pacific). The high quantity and quality of shipboard acoustical property measurements seemed to be particularly appropriate for gaining a better understanding of acoustic anisotropy in calcareous sediments. Major results can be summarized as follows:1. In calcareous oozes, acoustic anisotropy exists even at shallow depths of burial. Values are low, ranging from -0.5% to 3.5% and averaging 1.2%. Anisotropy profiles are affected by coring disturbances, but they still roughly correlate between holes cored at a same site. There is no general increase of anisotropy with depth of burial in ooze sections. These observations imply that most of the anisotropy measured in oozes is controlled by original changes in sediment composition rather than by reorientation of anisotropic minerals or flattening of pores during burial. Given the data available, bedding lamination seems to be the most satisfactory explanation for acoustic anisotropy in calcareous oozes of Ontong Java Plateau. However, bioturbation affects the entire ooze sections cored and much of the original depositional features may have been removed. Further analysis such as scanning-electron-microscope (SEM) observations of epoxy impregnated ooze samples are necessary to test the "bedding lamination" hypothesis.2. Ooze to chalk transition is marked by a tremendous change in the anisotropy profiles. In chalk sections the anisotropy values are higher (reaching 20%) than in ooze sections. Most of the anisotropies are negative (horizontal P-wave velocities lower than vertical velocities), whereas positive values are expected. Weakly cemented chalk samples are affected by cracking when compressed between the Hamilton Frame transducers for P-wave measurement. Cracks occur along bedding planes during horizontal P-wave velocity measurements, thus leading to anomalously low horizontal P-wave velocities and negative anisotropies.

Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

“…Vp velocities most frequently show a girdle distribution of maxima variably dipping to the plane of shear, recalling Type A and Type B distributions. In general, the Vp and Vs velocities overlap with those calculated for our samples, a part for those samples from very shallow depth (<1000 m) [114,115] where only after reducing porosity and increasing cementation the Vp values go from low (<5 km/s) to higher values, more similar to those here measured and calculated (>6 km/s). These observations support the general idea that carbonate rocks anisotropy also contributes to the overall anisotropy of the crust [7,22,82], at any depths.…”

“…A large literature is available for sedimentary rocks, though the large majority is dedicated to shales and sandstones (see complete review [7]). Similar Vp and Vs values have been measured and calculated for calcite [13,22,82,114,115] at different temperature and depth conditions. Vp velocities most frequently show a girdle distribution of maxima variably dipping to the plane of shear, recalling Type A and Type B distributions.…”

Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

“…Vp velocities most frequently show a girdle distribution of maxima variably dipping to the plane of shear, recalling Type A and Type B distributions. In general, the Vp and Vs velocities overlap with those calculated for our samples, a part for those samples from very shallow depth (<1000 m) [114,115] where only after reducing porosity and increasing cementation the Vp values go from low (<5 km/s) to higher values, more similar to those here measured and calculated (>6 km/s). These observations support the general idea that carbonate rocks anisotropy also contributes to the overall anisotropy of the crust [7,22,82], at any depths.…”

“…A large literature is available for sedimentary rocks, though the large majority is dedicated to shales and sandstones (see complete review [7]). Similar Vp and Vs values have been measured and calculated for calcite [13,22,82,114,115] at different temperature and depth conditions. Vp velocities most frequently show a girdle distribution of maxima variably dipping to the plane of shear, recalling Type A and Type B distributions.…”

“…The calculated values from texture analysis [68,84] as well as those reported in the literature, differ by about 10% from these calculated values. These values are generally closer to measured seismic velocities taken orthogonal to the maximum shear direction (i.e., lineation) [53] or in the more isotropic samples or very shallow sample, dominated by low porosity (<10%) [115]. Consequently, these Vp and Vs can be used as a reference for the isotropic component of carbonate rocks, to investigate the amount of anisotropy to be expected when observing real seismic data for similar rock composition.…”