Compressional wave attenuation in muddy sediments at the New England Mud Patch

A new method for measuring in situ compressional wave attenuation exploiting the spectral decay of Bragg resonances is applied to sediments at the New England Mud Patch. Measurements of layer-averaged attenuation in a 10.3 m mud layer yield 0.04 {0.03 0.055} dB/mi kHz (braces indicate outer bounds); the attenuation is twice as large at a site with 3.2 m mud thickness. Both results are strongly influenced by a ∼1 m sand-mud transition interval, created by geological and biological processes which mix sand (at the base of the mud) into the mud. Above the transition interval, homogeneous mud exhibits an attenuation 0.01–0.02 dB/m kHz, lower than that in the sand-mud transition interval by a factor of 10. Informed by these and additional observations, mud attenuation in and above the transition interval appears to be roughly spatially invariant across the area, explaining the factor of two in attenuation between the two sites by simple depth scaling. Further, the ubiquity of the processes that form the transition interval suggests that the scaling may be broadly applied to other muddy continental shelves. In principle, attenuation predictions in shallow water could be substantively improved with a modest amount of geologic and biologic information. [Research supported by ONR Ocean Acoustics.]