Seabed sediment attenuation profiles from a movable sub-bottom acoustic vertical array

Abstract

The acoustic lance ͓S. S. Fu, M.Sc. Thesis, University of Hawaii at Manoa ͑1994͒; Fu et al., J. Acoust Soc. Am. 99, 234-241 ͑1996͔͒ consists of a linear array of acoustic receivers below an acoustic source, all mounted on the outside of a core barrel or independent probe which is embedded in the seafloor. In earlier studies lance travel time data were processed to give in situ compressional wave sound speed as a function of depth. In this study lance waveforms are processed to extract compressional wave attenuation AϭQ Ϫ1 as a function of depth. The processing technique is unusual because the L 1 norm is used instead of the usual L 2 norm, the model space of attenuation profiles is exhaustively searched within the limits of discretization, and the marginal posterior probability density function of attenuation is computed explicitly at each depth. The technique is described in terms of Bayesian inverse theory using the notation of Tarantola.

In situ acoustic and laboratory ultrasonic sound speed and attenuation measured in heterogeneous soft seabed sediments: Eel River shelf, California

“…Higher amplitudes of the self-normalized acoustic signal were noticed when emitted in the water column than in the sea£oor (C, top). Water reference signals obtained at Site S90 (after Frazer et al, 1999) revealed a subtle di¡erence in frequency response of the individual transducers and di¡erent waveforms when recorded in sediments (C, bottom).…”

“…2C, top). Waveforms from this calibration allowed us to determine the exact spacing between the acoustic centers of the individual receiver elements, and then to calculate attenuation (Frazer et al, 1999). After the water calibration, the corer freefell to the sea£oor and penetrated the subbottom to a depth that depended primarily on the shear strength of the sediment.…”

“…However, in nearly every experimental geometry attenuation is di⁄cult to measure (Jannsen et al, 1985;Tonn, 1989;Bromirski et al, 1995;Bowles, 1997;Richardson and Briggs, 1997). The processing technique by Frazer et al (1999), which we applied to our data, combines the standard spectral ratio method (e.g., Tonn, 1989) and Bayesian inverse theory (e.g., Tarantola, 1987).…”

“…Acoustic Lance con¢guration used for the present study (A). Frequency spectra of Lance signals at Site S90 (after Frazer et al, 1999) showed little noise contamination and a center frequency of approximately 7.5 kHz (B). Higher amplitudes of the self-normalized acoustic signal were noticed when emitted in the water column than in the sea£oor (C, top).…”

“…2A) measures in situ interval sound speed and e¡ective attenuation pro¢les using full-waveform records (Fu et al, 1996a;Frazer et al, 1999) and has been successfully deployed in a variety of unconsolidated sediments. Mid-Atlantic Ridge pelagic oozes showed low velocities corresponding to low attenuation (Fu et al, 1996a).…”