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Apr 1991

Volume 89, Issue 4B, pp. 1851-2015

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back to top Session 4OC: Acoustical Oceanography: Open Workshop on Remote Sensing of Sediment Properties by Measurements on or Near the Seafloor
Invited Papers
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Seafloor shear wave velocity structure determined from interface wave dispersion (A)

LeRoy M. Dorman, Anthony E. Schreiner, and L. D. Bibee

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1905-1905 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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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.]
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Sediment properties derived from shear and interface waves (A)

John I. Ewing, George H. Sutton, and Jerry A. Carter

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1906-1906 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Bottom‐mounted sources and receivers have been used to measure shear wave velocity and attenuation in sediments of the U.S. East Coast continent shelf. Experimental results are compared with borehole and other subseafloor geologic information. Shear sources were designed primarily for generating SH waves but also produced SV and P waves. Receiver nodes containing orthogonal geophone or accelerometer sensors, plus hydrophone, provided four‐component data, each component producing unique information on wave type, velocity/attenuation structure, scattering, lateral heterogeneity, and anisotropy. Measurements were made with two systems; one with a large source and 4–8 m sampling interval to a maximum range of 200 m, the other with a smaller source and 1‐m sampling to a range of 30 m. Velocity and attenuation are estimated by matching recorded data with full‐waveform synthetic seismograms. [Work supported by ONR.]
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Remote sensing of sediment properties using seafloor arrays (A)

Robert D. Stoll

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1906-1906 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The placement of both source and receiver on the seafloor yields significant advantages when measuring the geoacoustic properties of near‐bottom sediments. Moreover, by using multiple receivers in an array and specifically designed sources that focus energy into the bottom and minimize the water‐borne “noise,” it is possible to measure both p‐ and s‐wave properties in significant detail. In this paper a detailed example of this technique illustrating equipment design, data acquisition, and inversion of data to obtain a geoacoustic model is presented. Results show that the horizontal component of motion contains important information that cannot be derived easily from hydrophone or vertical motion data. In addition, both near‐field and far‐field data are shown to be complementary in determining near‐bottom, high‐resolution models.
Contributed Paper
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Application of remote sensing and in situ measurements of ocean sediment properties to the prediction of acoustic propagation loss (A)

T. Akal, A. Caiti, F. Ingenito, and A. Kristensen

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1906-1906 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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An experiment was conducted in the Adriatic Sea to evaluate the ability of geophysical models of the ocean bottom, constructed by the application of in situ and remote sensing measurement techniques, to predict acoustic propagation loss. Short‐range wide‐angle measurements of acoustic reflection from the bottom and measurements of the dispersive characteristics of seismic interface waves at the water‐sediment boundary are used to estimate the P‐ and S‐velocity profiles in the upper 30 to 50 m of the bottom sediment. Pointwise in situ and core measurements were taken at the same site. Simultaneously, broadband propagation loss measurements were made. Geophysical models were constructed from the bottom measurements and used as inputs to state‐of‐the‐art acoustic models to predict propagation loss. Satisfactory agreement between measured and predicted propagation loss was obtained, indicating the general validity of the procedure.
Invited Papers
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Experimental verifications of bottom shear modulus profiler (BSMP) method (A)

Tokuo Yamamoto and Mark Trevorrow

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1907-1907 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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By measuring the response of a seabed to the pressure forcing of traveling ocean waves, the seabed shear modulus profile can be extracted by solving a geophysical inverse problem [Yamamoto and Torii, Geophys. J. R. Astron. Soc. 85, 413–431 (1986)]. The experimental results as favorably compared with geological boreholes have been reported [Yamamoto et al., Geophys. J. Int. 98, 173–182 (1989)]. ONR multi‐institutional experiments were recently conducted to compare the various experimental methods for measuring the seabed shear properties at several sites on the continental shelf of the northeastern United States. Comparisons between the BSMP results and the SH wave refraction results by J. I. Ewing will be reported in this paper. Generally, good agreements between the two methods are obtained. BSMP method penetrates 200 m below seafloor (b.s.f.) at resolution of a few meters while the SH wave seismic method penetrates 50 m b.s.f. at approximately the same resolution. As an example application of the BSMP results to modeling, comparisons between model predictions (using BSMP data), and propagation experiments of seismic waves and acoustic waves in the ocean by W. Carey and the present authors are also made, resulting in favorable agreements. [Work sponsored by ONR.]
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A pseudo‐underway geophysical technique for quantifying seabed sediment properties (A)

Angela M. Davis

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1907-1907 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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A towed geophysical device has been developed that will allow the rapid quantification of seafloor sediment properties for engineering and other purposes. The seabed hardware consists of seismic sources, a focused electrode pad (all sledge mounted), and a string of gimballed triaxial geophones. The seismic sources are impulsive devices that are essentially electromagnetic hammers that can preferentially generate shear or compressional waves dependent on their mode of operation. Travel time relationships obtained for seismic waves propagating through the sediment body to the geophone array allow the velocity structure to be defined to a depth dictated by the maximum receiver offset. Of particular interest to the engineers are the velocity gradient effects that for shear waves are indicative of the seabed sediment rigidity. The device can be used to construct distribution maps of geophysically related physical properties of the sediment body, and their variation with depth. Surveys carried out with the device in well‐documented test‐bed sites have produced significant correlations between the geophysical parameters and the known sediment variability.
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Near surface measurements of sediment geoacoustic properties using in situ probes (A)

Michael D. Richardson

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1907-1907 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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In situ and laboratory measurements of sediment geoacoustic properties together with sediment physical property measurements were made over the range of sediment types commonly encountered on continental shelves. In situ compressional wave velocities ranged from 1464 m/s in soft silty‐clay sediments to as high as 1989 m/s in gravels. Velocities in sands were 1600–1700 m/s. Compressional wave attenuations (measured at 58 kHz of 4–12 dB/m were common in soft silty‐clay sediments with higher attenuations of near 30 dB/m measured at sandy sites. In situ shear wave velocities ranged from 16 m/s in soft silty‐clay sediments to 90 m/s in hard packed fine sands. Differences among in situ and laboratory values of sediment geoacoustic properties are discussed. Empirical relationships between sediment geoacoustic and physical properties are presented.
Contributed Papers
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Bottom backscattering modeling and model/data comparison for 100–5000 Hz (A)

Pierre D. Mourad, Peter H. Dahl, and Darrell R. Jackson

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1907-1908 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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A low‐frequency bottom backscatter model that includes both surface and (shallow) volume contributions is described. It is assumed that these different scattering mechanisms are uncorrelated. The surface model [Jackson et al., J. Acoust. Soc. Am. 79, 1410–1422 (1986)] consists of Kirchhoff theory for large grazing angle contributions, spliced into composite roughness theory for backscatter from mid and low grazing angles. This part of the model is applied to isotropic, Gaussian rough surfaces whose surface height spectra are described by a two‐parameter power law. These parameters are fixed by previous, high‐frequency applications of this surface roughness theory. The volume scattering contribution is based upon an analytic theory for backscatter from uncorrelated, uniformly distributed point scatterers embedded within an upward refracting, constant density, “fluid” sediment layer. For this part of the model, the water/sediment interface is smooth, with discontinuities in compressional sound speed and density. The model predictions are compared with backscatter observations that have supporting geoacoustical data. By using these geoacoustical measurements along with standard bounds in the literature, the model contains only one free parameter: a quantity proportional to the effective volume scattering cross section per unit volume of sediment. [Work supported by NAVOCEANO and APL.]
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High‐frequency acoustic penetration of sandy ocean sediments (A)

Nicholas P. Chotiros and Michael L. Ramaker

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 1908-1908 (1991); (1 page) | Cited 1 time

Online Publication Date: 14 Aug 2005

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High‐frequency acoustic signals, from 5 to 80 kHz, were projected from a point in the water to an array of hydrophones in a sandy sediment. The data were processed to yield acoustic wave speeds and directions. For normal and near‐normal incidence, the results are in good agreement with liquid‐viscoelastic solid propagation models. For grazing angles less than the critical value, the results cannot be explained in terms of a viscoelastic medium; a slow wave is observed that can only be explained in terms of Biot's theory. [Work supported by ONT under NOARL management.]
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