What is the basis for the data on compressional (p-wave) sound speeds?
Although there are some direct measurements of compressional sound speeds for areas of the seabed, they are few and far between. dbSEABED reports any direct measurements in the EXT outputs, including the average of those data which recognize velocity anisotropy. One difficulty with sound speeds is that they do depend on the frequency, pressure and temperature of the measurement. The outputs make no allowance for these factors, though conditions of the original measurements are recorded in the underlying raw data of the database. Several users of dbSEABED requested that estimated sound speeds be provided wherever possible. This is done as follows. Where the material is consolidated and a measured porosity is available, then the time average equation is employed (Nafe and Drake (1960)). Consolidation is judged to be when the ShrStrs based on measurement or a parsing of compaction/cementation is > 50kPa, or the porosity is < 33%. These values for constants were adopted: solids and fluid d
