Volume 108, Issue 3, September 2000
Index of content:
- UNDERWATER SOUND 
108(2000); http://dx.doi.org/10.1121/1.1288662View Description Hide Description
A space- and time-dependent internal wavemodel was developed for a shallow water area on the New Jersey continental shelf and combined with a propagation algorithm to perform numerical simulations of acoustic field variability. This data-constrained environmental model links the oceanographic field, dominated by internal waves, to the random sound speed distribution that drives acoustic field fluctuations in this region. Working with a suite of environmental measurements along a 42-km track, a parameter set was developed that characterized the influence of the internal wave field on sound speed perturbations in the water column. The acoustic propagation environment was reconstructed from this set in conjunction with bottom parameters extracted by use of acoustic inversion techniques. The resulting space- and time-varying sound speed field was synthesized from an internal wave field composed of both a spatially diffuse (linear) contribution and a spatially localized (nonlinear) component, the latter consisting of solitary waves propagating with the internal tide. Acoustic simulation results at 224 and 400 Hz were obtained from a solution to an elastic parabolic equation and are presented as examples of propagation through this evolving environment. Modal decomposition of the acoustic field received at a vertical line array was used to clarify the effects of both internal wave contributions to the complex structure of the received signals.
108(2000); http://dx.doi.org/10.1121/1.1286221View Description Hide Description
This article discusses inversions for bottom geoacoustic properties using broadband acoustic signals obtained from explosive sources. The experimental data used for the inversions are SUS charge explosions acquired on a vertical hydrophone array during the Shelf Break Primer Experiment conducted south of New England in the Middle Atlantic Bight in August 1996. The SUS signals were analyzed for their time-frequency behavior using wavelets. The group speed dispersion curves were obtained from the wavelet scalogram of the SUS signals. A genetic algorithm (GA) was used for the inversion of sound speeds in the water column and compressional wave speeds in the sediment layers. The variations in the sound speeds in the water column were represented using empirical orthogonal functions (EOFs). A range-independent normal mode routine was used to construct the replica fields corresponding to the parameters. Comparison of group speeds for modes 1 to 9 and for a range of frequencies 8 to 200 Hz was used to arrive at the best parameter fit. An efficient hybrid optimization scheme using the GA and a Levenberg–Marquardt algorithm is presented. Linear perturbation methods were also used to “fine tune” the inversions and to obtain resolution and variance estimates. Analysis was also done to compute the degree of convergence of each of the parameters by explicitly calculating the Hessian matrices numerically. A posteriori estimation of mean and covariance was also done to obtain error estimates. Group speeds for the inverted sound speed fields provide an excellent match to the experimental data. The inverted sediment compressional speed profile compares well with in situ measurements.
108(2000); http://dx.doi.org/10.1121/1.1285953View Description Hide Description
Understanding the basic physics of sound penetration into ocean sediments is essential for the design of sonar systems that can detect, localize, classify, and identify buried objects. In this regard the sound speed of the sediment is a crucial parameter as the ratio of sound speed at the water-sediment interface determines the critical angle. Sediment sound speed is typically measured from core samples using high frequency (100’s of kHz) pulsed travel time measurements. Earlier experimental work on subcritical penetration into sandy sediments has suggested that the effective sound speed in the 2–20 kHz range is significantly lower than the core measurement results. Simulations using Biot theory for propagation in porous media confirmed that sandy sediments may be highly dispersive in the range 1–100 kHz for the type of sand in which the experiments were performed. Here it is shown that a direct and robust estimate of the critical angle, and therefore the sediment sound speed, at the lower frequencies can be achieved by analyzing the grazing angle dependence of the phase delays observed on a buried array. A parametric source with secondary frequencies in the 2–16 kHz range was directed toward a sandy bottom similar to the one investigated in the earlier study. An array of 14 hydrophones was used to measure penetrated field. The critical angle was estimated by analyzing the variations of signal arrival times versus frequency, burial depth, and grazing angle. Matching the results with classical transmission theory yielded a sound speed estimate in the sand of 1626 m/s in the frequency range 2–5 kHz, again significantly lower the 1720 m/s estimated from the cores at 200 kHz. However, as described here, this dispersion is consistent with the predictions of the Biot theory for this type of sand.
108(2000); http://dx.doi.org/10.1121/1.1287021View Description Hide Description
Sonar performance predictions of reverberation in shallow water rely upon good estimates of the bottom-scattering strength. However, little is understood about bottom scattering in shallow water in the frequency range 400–4000 Hz, particularly its dependency upon frequency and its relationship to the physical properties of the seafloor. In order to address these issues, a new measurement technique has been developed to probe the frequency and angular dependency of bottom-scattering strength. The experimental technique is described which employs either coherent or incoherent sources (lightbulbs). In addition, measurement and modeling results for two diverse shallow water sites are presented. At one site, the scattering appears to arise at or near the water–sediment interface. At the other site, scattering from a 23-m sub-bottom horizon is clearly apparent in the data at and below 1800 Hz. The fact that our measurement technique can directly reveal the presence of sub-bottom scattering is a significant advance in the development of methods to explore the physical mechanisms that control bottom scattering.
108(2000); http://dx.doi.org/10.1121/1.1288664View Description Hide Description
Numerical wavefield modeling, based on time domain finite difference solutions to the full elastic wave equation, is used to quantitatively predict the sensitivity of monostatic backscatter to variations in geological properties of the seabed. This article addresses the hypothesis that observed backscatter signals from the seafloor are produced by a combination of seafloor (interface) and subseafloor (volume) scattering from structures having variations at scale lengths comparable to the wavelength of the insonifying acoustic field. Wavelength-scale seafloor roughness and subseafloor volume heterogeneity parameters are defined using stochastic spatial probability functions having Gaussian autocorrelations. The range of the variations in these parameters is constrained to realistic values based on estimates derived from seafloor bathymetry and other geologic data. Modeling results show that backscattering from rough, basaltic (hard) bottoms, in the absence of large-scale seafloor slope, is dominated by rough surface scattering with little contribution from volume scattering. Contrary to this, sediment (soft) bottoms with subseafloor volume heterogeneity, with or without seafloor roughness, produce significant backscattering signals compared to a homogeneous sediment bottom.