Experimental study and hypotheses testing on a sandy ocean sediment to determine the underlying physical processes in the penetration of sound into sandy ocean sediments, particularly at shallow grazing angles, and the scattering of sound from the sediment.
The two processes of particular interest are penetration into sediments and scattering. Although a number of measurements have been made in the past, including both in-situ and laboratory experiments, and a number of hypotheses have been advanced, the underlying physical processes could not be determined from them with any confidence. The hypotheses may be roughly divided into two groups, one in which the sediment is approximated as a fluid [1], and the other in which the sediment is treated as a poro-elastic solid according to Biot’s theory [2].
A. Penetration: There are two competing hypotheses for the penetration path at shallow grazing angles: (1) Biot slow wave refraction and (2) scattering by surface and/or volume inhomogeneities. Within each there are a number of interconnected possibilities. The Biot slow wave path is applicable to a uniform sediment with a flat surface, but it may be enhanced by surface roughness and volume inhomogeneities through energy conversion between the slow and fast waves. The scattering path requires either surface roughness and/or volume inhomogeneities, but it is not known if the process may be adequately represented by a fluid bottom approximation, or if it is necessary to resort to a Biot representation.
B. Scattering: At the most fundamental level, it is desirable to determine if the scattering process is (1) single or (2) multiple scattering. At the physical level, the scatterers need to be identified, and the likely candidates include (a) surface roughness, (b) volume inhomogeneities, and (c) the sand grains. The acoustic path by which the scatterers are insonified is covered under the penetration study. Tests are needed to distinguish between the different scattering mechanisms.
For best results, in-situ experimentation is required owing to the near impossibility of reproducing realistic ocean sediment conditions in the laboratory, particularly the arrangement of grains under natural sediment deposition conditions. To test the above hypotheses, a number of new methods are proposed. These methods are designed to eliminate the shortcomings in previous studies and provide the necessary discrimination between the candidate hypotheses. They will be realized in an experiment involving a mobile sound source carried on a remotely operated vehicle (ROV) in combination with tilted, rigidly supported, buried acoustic line arrays.
A. Tilted buried line arrays, on rigid supports, will be used instead of the vertical buried line arrays of past experiments. The unit will consist of one or more tilted line arrays, attached to a support frame resting on the sand surface, down range of the sound source. The tilted geometry was chosen because it has been argued that possible scattering artifacts at the water–sand interface caused by the insertion of vertical arrays could have had the appearance of a refracted slow wave. In the tilted geometry, the sediment directly above the acoustic sensing elements is undisturbed, eliminating the possibility of scattering artifacts from within a cone of angles about the vertical.
The tilted buried line arrays will be sampling only the acoustic waves traveling through the undisturbed part of the sediment. The sensing elements will be acoustically isolated from the support rods by sound absorbing materials. Scattering artifacts at the points of entry and from the support frame will be greatly attenuated owing to the obliqueness of the paths, and also may be rejected by time gating.
The array unit will be self-burying. A small water jet at the lower tip of each line array, driven by a portable, battery operated pump, liquefies a small volume of sediment directly ahead of the array and allows the unit to slip into the sediment. Only a small volume of sediment around each line array is disturbed by the action of the water jet. Otherwise, the sediment is undisturbed. Preliminary tests, with a single line array in a Gulf of Mexico site, indicate that it is feasible to bury rigid line arrays of up to 2 meters in length.
The improved positioning accuracy due to rigid supports will allow coherent processing up to 200 kHz, which is an essential requirement for distinguishing between a refracted wave, which tends to be coherent, and scattered sound energy, which is incoherent.
B. Broad band signals, made possible by new transducer materials, will be used in order to detect frequency dependent trends in both penetration and scattering, which will provide important clues to the underlying physical mechanisms. Of particular interest are the attenuation, transmission and scattering coefficients as a function of frequency. Existing empirical models [3] assume that attenuation is linearly proportional to frequency, but older [4] and more recent [5] laboratory experiments show significant deviations. On a practical level, broad band signals allow sparse arrays to be used in the estimation of direction and speed of coherent waves, and phase coherence across a broad band is a good indicator of a refracted wave, as opposed to a scattered wave.
C. Using an ROV as the platform for the sound source and backscatter receiver will provide several advantages that were previously unavailable.
(a) The vertical mobility provided by an ROV allows the buried array to be insonified over a continuous range of grazing angles, and the resulting changes in the penetrating sound waves observed.
(b) The horizontal mobility will allow the buried array to be insonified from a wide range of azimuth angles. This will be particularly useful for discriminating between scattering and refraction processes. The refraction process is expected to be deterministic, but the scattering processes are expected to be random with a definite spatial correlation function. Using short, broad band acoustic pulses, the scattered signal is expected to have a long correlation length in the down range direction, making it difficult to distinguish between the refraction and scattering processes. However, owing to the relatively broad sound beam, the scattering processes are expected to have a short correlation length in the cross range direction, and the horizontal mobility of the ROV will allow the cross range correlation function to be measured, and thus provide a method for distinguishing refraction and scattering processes.
(c) It will be possible to observe the dependence of bottom backscattering strength on height above bottom, which is an indicator of single or multiple scattering. Such a dependence was first observed in a recent experiment [6].
(d) It will be possible to obtain an ensemble of backscattering measurements from a large area, as a function of grazing angle and bearing, providing enough independent data points to construct a detailed frequency distribution curve.
There is a complication associated with using a moving sound source: the position of the sound source, at each ping, must be determined relative to the receiving array. This will be done by triangulation and time of flight measurement, using the three hydrophones on the support structure at the sediment surface.
D. Using a new inversion algorithm, the most difficult Biot parameters, i.e. grain bulk modulus, frame bulk and shear moduli, will be inverted from acoustic measurements, including reflection loss at normal incidence (Steve Schock, FAU, or Doug Lambert, NRL) and p- and s-wave speeds (M. Richardson, NRL), given the usual geoacoustic measurements of porosity, density, grainsize (K. Briggs, NRL). The inversion is a non-linear one, but it has proved to be stable and rapidly convergent.
E. The frequency dependence of the Q-factor of the scattered signal will be computed as another independent indicator of single or multiple scattering. A Q-factor that is increasing with frequency is indicative of a multiple scattering process.
F. A gas free sediment is preferred, and for verification, a diver operated gas catcher will be used to estimate approximate concentration and the identity of the gas, if any.
[1]. Jackson, D. R.; Winebrenner, S. P.; Ishimaru, A. Application of composite roughness model to high-frequency bottom backscattering. J. Acoust. Soc. Am. 79(5), 1410–1422, May, 1986
[2]. Chotiros, N. P. Biot model of sound propagation in water-saturated sand. J. Acoust. Soc. Am. 97(1), 199–214, January 1995
[3]. Hamilton, E. L. Geoacoustic Modeling of the Sea Floor. J. Acoust. Soc. Am. 68, 1313–1340, 1980
[4]. Nolle, A. W.; Hoyer, W. A.; Mifsud, J. F.; Runyan, W. R.; Ward, M. B. Acoustical properties of water-filled sands. J. Acoust. Soc. Am. 35(9), 1394–1408, Sept., 1963
[5]. Simpson, Harry J.; Houston, Brian H. Measurement of acoustic wave speeds in a fluid-saturated sandy bottom. J. Acoust. Soc. Am. 100(4), Pt. 2, 2766, October 1996
[6]. Chotiros, Nicholas P.; Altenburg, Robert A.; Piper, James. Analysis of acoustic backscatter in the vicinity of the Dry Tortugas. Geomarine Letters 17: 325–334, 1997
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