Acoustic and geotechnical properties of carbonate sediments near the Marquesas Keys and the Dry Tortugas, Florida Keys.

摘要

Carbonate sediments recovered from the Marquesas Keys and the Dry Tortugas, Florida Keys, were examined to determine significant physical parameters that control the velocity of sound and characterize the variability of sediment properties in this environment. High-resolution records of P-wave velocity and wet-bulk density profile have been obtained from nondestructive whole-round core measurements. The physical properties of sediments for the two study areas show different distribution patterns. Relationship between P-wave velocity and wet-bulk density occurred only weakly due to considerable void volume within skeletal grains.;Abundant whole shells and shell fragments presented in this study show significant effects in controlling the velocity of sound. Sediment classification based on grain size analyses generally agrees with the sediment type in accordance with acoustic impedance values.;Comparison of seismic reflection, acoustic impedance, and sedimentary properties reveals some agreement between them. This comparison shows that changes in acoustic impedance are reflected in the change of sediment grain size.;Acoustic anisotropy was calculated based on major and minor axes determined by a best-fit ellipse equation from up to 12 measurements around the circumference of each core. In general, the sediments are acoustic anisotropic. High acoustic anisotropy is associated with high velocity zones caused by whole shells and shell fragments, or sediments with low velocity and low wet-bulk density caused by periodic storms.;Based on carbonate content and carbonate mineral data from X-ray diffraction analysis, aragonite was found to be more abundant rather than calcite in this environment. Non-carbonate minerals are present in these sediments in less than 10%.;By using the theoretical Wood equation and the XRD data, it is possible to predict P-wave velocity and determine important parameters controlling the velocity of sound. The incompressibility (bulk modulus) of carbonate sediments appears to be dependent on particle size. The P-wave velocity also relates to the bulk modulus. The shear (rigidity) modulus and the frame bulk modulus caused by the contacts between granules are found to be significant factors in controlling the sound velocity.