Abstract:
The hydraulic structure of the seabed (porosity, permeability and shear strength) is important with regard to many physical processes including the storage and transport of pore fluids, the consolidation and subsidence of man-made islands, the liquefaction of foundations, and the occurrence of catastrophic earthquakes and tsunamis. Propagation of acoustic waves through sediments is affected by the hydraulic properties of the sediments which may be modeled by poroelastic theories by Biot (1956), Dvolkin et al. (1993), and Chesnokov et al. (1991). Crosswell tomography measurements enable one to image the structure of sound speed and attenuation within the earth, from which the hydraulic structure is found based on the poroelastic theories. The hydraulic structures of the seabed revealed from four case studies will be presented and their implications to the earth processes of storage and transport of pore fluids, consolidation, liquefaction, and earthquake will be discussed. Acoustic waves are scatted from the seabed. It will be shown that the seabed structure can be extracted from measured acoustic bottom scatter. It will be also shown that the disturbances made in the seabed for instrumentation can act as vertical waveguides which may be mistaken as a false Biot slow wave. [Work sponsored by ONR, NSF and KSC.]