Observed anisotropy in low frequency acoustic signal loss at the sea floor.

Abstract

Anisotropy of 20%–30% in the low-frequency (10–120 Hz) acoustic bottom loss of marine sediments 1200-m deep off the coast of Oregon is reported here. Two nearly perpendicular acoustic profiles were acquired with air guns towed at the sea surface, crossing at a point directly over a four-component ocean bottom seismometer (OBS) on the seafloor. The area is a convergent margin, with a regional principal stress field perpendicular to broad bathymetric ridges and the shoreline, and numerous faults running parallel to the bathymetry. Amplitudes of sediment reflections are consistently 20%–30% less on the profile parallel to the bathymetry than on the profile perpendicular to the bathymetry. This is the opposite of what would be expected if the dominant attenuation mechanism were scattering from the fault planes. The anisotropy exists at angles for which acoustic energy penetrates sediments only a few hundred meters from the crossing point, where layering in both directions is laterally consistent. The direct arrival amplitudes, from the source to the OBS, are identical on both transects, suggesting that neither the acquisition technique nor the water column is responsible for the anisotropy. Possible explanations include seafloor bed forms and wavelength-scale roughness at the buried sediment interfaces.