Velocity variabilities and other physical properties of marine sediments measured by crosswell acoustic tomography

High-resolution images of the compressional wave velocity structure within various bottom sediments have been determined from high-frequency crosswell acoustic tomography. The three-dimensional wave-number spectra of the velocity variabilities were determined for each measured velocity image. The sediment velocity variability spectra are anisotropic in general. The variability frequency in the vertical direction is much higher than in the horizontal direction. The aspect ratio of the two is found to be approximately 5.0. In addition, the major and minor axes of the anisotropy are often tilted from the vertical and the horizontal direction, respectively. The angle of tilt called dip is found to be as much as 33 deg. The measured velocity power spectra approximate a power law. The spectral strength depends on the sediment type and the location. Based on the rock mechanics transformation procedure by Yamamoto et al. [Geophysics 59, 1530–1541 (1994)], the sediment porosity, density, permeability, and shear strength images have been transformed from the velocity images. These sediment properties, determined from the acoustic crosswell tomography, have been supported by direct measurements on cores and boreholes. The density variabilities of the sediment are found to be similar to the velocity variabilities. Unlike in ocean water, the relative density variabilities in the sediments are much larger than the relative velocity variabilities.