GEOACOUSTIC AND PHYSICAL-PROPERTIES OF CARBONATE SEDIMENTS OF THE LOWER FLORIDA KEYS

Near-surface sediment geoacoustic and physical properties were measure d from a variety of unconsolidated carbonate sediments in the Lower Fl orida Keys. Surficial values of compressional and shear speed correlat e with sediment physical properties and near-surface acoustic reflecti vity. Highest speeds (shear 125-150 m s(-1); compressional 1670-1725 m s(-1)) are from sandy sediments near Rebecca Shoal and lowest speeds ( shear 40-65 ms(-1); compressional 1520-1570 ms(-1)) are found in soft, silty sediments which collect in sediment ponds in the Southeast Chan nel of the Dry Tortugas. High compressional wave attenuation is attrib uted to scattering of acoustic waves from heterogeneity caused by accu mulation of abundant shell material and other impedance discontinuitie s rather than high intrinsic attenuation. Compared to siliciclastic se diments, carbonate sediment shear wave speed is high for comparable va lues of sediment physical properties. Sediment fabric, rather than cha nges due to the effects of biogeochemical processes, is responsible fo r these differences.