Volume 118, Issue 3, September 2005
Index of content:
- UNDERWATER SOUND 
118(2005); http://dx.doi.org/10.1121/1.1940447View Description Hide Description
A time-reversal mirror refocuses back at the original probe source position. A goal has been to refocus at different positions without model based calculations. A method to refocus at different ranges has already been developed using frequency shifting. Here we present a technique to refocus at different depths than the original probe source in a shallow ocean range-independent waveguide. The requirement is to collect data from various ranges at a single depth, as from a moving broadband radiator, over a distance sufficient to construct the relevant frequency-wave-number structure of the waveguide. With this information, it is then possible to focus at arbitrary depth at any of the ranges that the probe source data were taken. Experimental results confirm the theory.
118(2005); http://dx.doi.org/10.1121/1.1993130View Description Hide Description
In this paper a new method is derived for the computation of scattering from a finite, rough free surface. The free surface is infinite in extent but only a portion of it is rough. In order to reduce the amount of numerical computation for such a problem, it is desirable to restrict the computations to the interval of roughness, even for remote sources and receivers. This can be easily done in the case that the rough portion of the surface is only directed into the surrounding fluid medium. In this case, the use of the appropriate half-space Green’s function will restrict the integral equation to the interval of roughness only. However, for general deformations this Green’s function cannot be used. The use of truncated integral equations utilizing the free space Green’s function is discussed. An alternate approach is then described. A system of boundary conditions is derived for a finite curve containing the interval of roughness and a surrounding contour in the fluid half-space. The resulting equations are solved using the method of wave-field superposition. The derived method is also easily generalized to the case that the rough surface under consideration is the upper boundary of a waveguide.
118(2005); http://dx.doi.org/10.1121/1.1979503View Description Hide Description
The acoustic signal backscattered from a fish in water, , at a frequency, , differs from the incident signal in both magnitude, , and phase, , and it has been common practice for many years to use the backscatter magnitude from individual fish as an aid to species identification. However, very little use has been made of the phase of the backscattered signal relative to that of the incident acoustic pulse. If the gross phase changes due to propagation through water are compensated for, the residual phase signature is found to contain useful target-specific information. The phase signature can be characterized by estimating the rate at which the echophase changes, relative to the transmitted pulse, during the echo from a fish. Clear groups are produced when single fish targets from in situ data are plotted in complex target space (target strength versus target rate of change of phase) and this is explored here by computing the acoustic backscatter, in complex target space, from a series of simple large and small model fish targets, both with and without gas-filled swimbladders, using a Monte Carlo technique. It is shown that all the features found in the in situ data can be explained in terms of the size and attitude of the fish. The modeling was carried out at the frequency of 38 kHz, which pertained to the in situ data, and the specific results only apply to this frequency. However, the complex target approach is generally applicable to fish target strength analysis independently of frequency.
118(2005); http://dx.doi.org/10.1121/1.2000750View Description Hide Description
The relationship between array design and ambiguity surface characteristics is not straightforward for matched field processing(MFP). Because MFP makes use of multipath propagation, ambiguities are a function of the environment as well as the array. This paper develops a statistical approach that seeks to provide an analytic link between array design and metrics describing the MFP output. Approximate expressions are derived for the probability distribution of power output across conventional MFP ambiguity surfaces. The validity of the expressions is examined through numerical simulation. This approach can be used as a design tool for comparing the expected performance of different array geometries.
118(2005); http://dx.doi.org/10.1121/1.1984990View Description Hide Description
The concept of environmentally adaptive reverberation nulling using a time reversal mirror (TRM) recently has been described [Song et al. , J. Acoust. Soc. Am.116, 762–768 (Year: 2004)]. In this paper, monostatic reverberation nulling is demonstrated experimentally at 850 and using data from a shallow water experiment conducted off the west coast of Italy in April 2003. The active transmission of a seafloor spatial null from a vertical source array is shown to result in the attenuation by of prominent reverberation features with their levels being reduced to that of the more diffuse reverberation background.