We compared in situ and laboratory velocity and attenuation values measured in seafloor sediments from the shallow water delta of the Eel River, California. This region receives a substantial volume of fluvial sediment that is discharged annually onto the shelf. Additionally, a high input of fluvial sediments during storms generates flood deposits that are characterized by thin beds of variable grain-sizes between the 40-and 90-m isobaths. The main objectives of this study were (1) to investigate signatures of seafloor processes on geoacoustic and physical properties, and (2) to evaluate differences between geoacoustic parameters measured in situ at acoustic (7.5 kHz) and in the laboratory at ultrasonic (400 kHz) frequencies. The in situ acoustic measurements were conducted between 60 and 100 m of water depth. Wet-bulk density and porosity profiles were obtained to 1.15 m below seafloor (m bsf) using gravity cores of the mostly cohesive fine-grained sediments across-and along-shelf. Physical and geoacoustic properties from six selected sites obtained on the Eel margin revealed the following. (1) Sound speed and wet-bulk density strongly correlated in most cases. (2) Sediment compaction with depth generally led to increased sound speed and density, while porosity and in situ attenuation values decreased. (3) Sound speed was higher in coarser-than in finer-grained sediments, on a maximum average by 80 m s 31 . (4) In coarse-grained sediments sound speed was higher in the laboratory (1560 m s 31 ) than in situ (1520 m s 31 ). In contrast, average ultrasonic and in situ sound speed in finegrained sediments showed only little differences (both approximately 1480 m s 31 ). (5) Greater attenuation was commonly measured in the laboratory (0.4 and 0.8 dB m 31 kHz 31 ) than in situ (0.02 and 0.65 dB m 31 kHz 31 ), and remained almost constant below 0.4 m bsf. We attributed discrepancies between laboratory ultrasonic and in situ acoustic measurements to a frequency dependence of velocity and attenuation. In addition, laboratory attenuation was most likely enhanced due to scattering of sound waves at heterogeneities that were on the scale of
The acoustic lance is an instrument developed to obtain in situ compressional wave velocity and attenuation ͑Q Ϫ1 ͒ profiles for a sedimentary layer of several meters thickness at the sedimentseawater interface. The self-contained instrument consists of ten independent recording channels with a linear array of receivers embedded in the seafloor below a broadband acoustic source. It provides in situ recording of full waveforms to determine interval velocity and attenuation. The system can be attached to a gravity corer or to a specially designed probe. A comprehensive experiment was carried out in Mid-Atlantic Ridge sediment ponds where the lance made in situ measurements, and core samples were recovered. Core data agree well with in situ data in one location, but disagree in other locations. Lance data indicate that the sediment ponds have similar in situ velocity distributions, with an acoustic channel much thinner than that predicted by earlier investigators.
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.
An in situ experimental study of variations of compressional wave speed and attenuation with depth in natural coral sands has been made offshore of Oahu, Hawaii. In situ data were collected at a center frequency of 7.5 kHz. Compressional wave speed averages around 1620 m/s and attenuation ͑expressed as Q p Ϫ1 , the reciprocal of the quality factor͒ decreases from 0.04 at the seafloor to 0.01 at 2 m depth. Very little change in compressional wave speed is seen to 9 m below the seafloor. Coral sand sound speeds are lower than those reported elsewhere for quartz sand. Waveforms recorded over the upper 9 m below the seafloor exhibit virtually no peak broadening, suggesting that scattering contributes little to the in situ attenuation. The relationship of attenuation to frequency in the coral sands agrees with Hamilton's observations of attenuation in other sediments, although the coral sand attenuation is slightly higher than in other sediments. The coral sand relationship between attenuation and porosity also agrees with Hamilton's when the volume of intraparticle voids is deducted from the total porosity.
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