Seafloor shear wave velocity structure determined from interface wave dispersion

Abstract

The propagation of interface waves in the seafloor waveguide is primarily controlled by the sediment shear velocity, which increases rapidly with subbottom depth. This gradient causes the propagation velocity to be strongly frequency dependent and this dispersion allows one to infer the shear velocity structure from propagation velocity measurements. These waves were excited with small explosions on the seafloor and the dispersion was measured over distances of a few hundred meters, observing seafloor motion on ocean bottom seismographs (OBSs). It is common to observe these waves in the 0.5–5 Hz frequency range, even from explosions with bubble frequencies in the 100‐ to 400‐Hz range. Typically several modes are seen and shear velocity in the 1‐ to 30‐m depth range can be recovered. The attenuation is high (Q = 25) in the very near surface, consequently, modes with large energy just below the surface suffer severe attenuation, while modes with little energy in the highly absorbing regions (higher modes and all modes whose phase velocity is near that of water) show Q up to 500. [Work supported by ONR.]