The effects of grain size and density on compressional wave speed and attenuation are investigated for a clastic silica sand from a seabed study site south of Panama City, Florida, using an automated core logging device that allows for highly accurate, non-destructive, fine-scale measurements to be taken on unopened core sections. Measurements were conducted on relatively undisturbed cores obtained using a large-diameter gravity corer, as well as on reconstituted sections containing sediment segregated into narrow grain size ranges. Findings indicate that whereas density is the primary physical sediment attribute controlling speed, attenuation at 500 kHz is primarily a function of grain size and grain structure. Sandy sediments, particularly those with narrow sorting, are susceptible to liquefaction, which can reduce attenuation dramatically. ¿ 2001 American Geophysical Union