Calibrated acoustic transmission measurements were made under calm sea conditions on the New Jersey shelf near Amcor 6010, a surveyed area with known geophysical properties. The experiment was conducted in 73-m water with supporting measurements of salinity, temperature, and sound speed. These measurements were obtained with a vertical array of 24 equally spaced hydrophones at 2.5 m; one of which was on the bottom. A source towed at either 12- or 34-water depth transmitted one of two sets of four tones spaced between 50 and 600 Hz for each run to ranges of 4 and 26 km. The data were processed with Hankel transform and Doppler processing techniques to yield horizontal wave-number spectrum at several depths as well as mode shapes. Results were obtained along both a constant and a gradually depth varying radial. Similar modal interference patterns were observed at lower frequencies and critical angle bottom limited propagation at higher frequencies. The constant radial results were compared to calculations using several shallow-water propagation models employing both geoacoustic profiles derived from the geophysical parameters and Yamamoto's (1990) shear wave inversion. Predicted and measured levels generally agreed; however, differences in computed and measured modal interference patterns were observed. [Work sponsored by ONR 1125 OA and NUSC IR.]

Sea ice is a heterogeneous material whose acoustic properties are functions of time and space. Results of a crosshole tomography experiment conducted in multi-year ice with the objective of determining the spatial structure of the compressional and shear wave speeds from travel time measurements made with high-frequency pulses are presented here. The results of the experiment indicate that the wave speeds can be determined from such a crosshole experiment with good resolution. The compressional and shear wave speed contour maps indicate that the spatial variations of the wave speeds are complex with regions of low speed. Low-speed regions observed are likely caused by high brine volume content. Resolution and variance studies performed on the estimates are also presented. Material properties such as Poisson’s ratio, salinity, and elastic and shear moduli of sea ice are obtained from the estimates of compressional and shear wave speeds. By measuring the amplitude of the transmitted and received signals along specific paths, estimates of the attenuation coefficients at different depth intervals are obtained. Spatial variability observed in the estimates is believed to be due to scattering by inhomogeneities in the material.

Geoacoustic models inferred from amplitude versus range data at 220 Hz are presented for three locations in the Icelandic Basin. The data were obtained using a deep-towed pulsed cw source and two receivers anchored near the bottom. For ranges out to 4800 m, the data were analyzed using an iteration of forward models technique in which the parabolic equation method and a Hankel transform method were used sequentially to compute acoustic fields for different bottom parameters until best fits to the data were obtained. The inferred geoacoustic models consist of a sediment layer containing a positive, linear sound-speed gradient overlying an isovelocity sub-bottom. The geoacoustic parameters include the layer thickness, the sound-speed gradient, and the sound-speed discontinuities at the water–bottom and sub-bottom interfaces. The density and attenuation of the structures are also determined. The geoacoustic models are substantiated by other types of measurements, which include piston coring, 3.5-kHz seismic profiling, and in situ sediment profiling. These additional measurements, combined with the 220-Hz results, yield a consistent picture of the geoacoustic nature of the three areas. Specifically, the highly coherent character of the 220-Hz data, the clearly visible sub-bottom layering in the 3.5-kHz records, and the low values of attenuation are related to the fine-grained nature of the sediment at two of the sites. At the third site, the presence of coarse sediment provides an explanation for the major incoherent component in the 220-Hz data, the poorly discernible layering in the 3.5-kHz records, and an anomalously high value of attenuation.

A technique for determining the geoacoustic models in shallow water is described. For a horizontally stratified ocean and bottom, the method consists of measuring the magnitude and phase versus range of the pressure field due to a cw point source and numerically Hankel transforming these data to obtain the depth-dependent Green's function versus horizontal wavenumber. In shallow water, the Green's function contains prominent peaks at horizontal wavenumbers corresponding to the eigenvalues for any trapped and virtual modes excited in the waveguide. From the Green's function, one can obtain the geoacoustic model via either forward modeling or perturbative inverse techniques. In the forward modeling approach, a geoacoustic model for the bottom is obtained by computing the theoretical Green's function for various values of the bottom parameters and determining the parameter set which provides the best agreement with the experimental Green's function, particularly in the positions and relative magnitudes of the modal peaks. In the perturbative inverse technique, one uses the differences between the measured modal peaks and those predicted by a background model as input data to an integral equation, which is solved for the bottom geoacoustic parameters. These techniques are demonstrated using experimental data at 140 and 220 Hz. [Work supported by ONR.]

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