Report on the Office of Naval Research Shallow Water Acoustics Workshop, April 24-26, 1991

The results of an unclassified Workshop on Shallow Water Acoustics, sponsored by the Office of Naval Research Code 11250A, are presented. The workshop was held on April 24-26, 1991 at the Woods Hole Oceanographic Institution and included about forty-five scientists specializing in ocean acoustics, geology, geophysics, and physical oceanography. The goal of the workshop was to determine future directions for basic research in shallow water acoustics. This report summarizes the recommendations of the workshop and includes a synopsis of the deliberations of four working groups which focus on the following specific research issues: (1) the seabed, (2) the water column and surface/Arctic, (3) analytic and numerical modeling/ambient noise, and (4) laboratory and field experiments/signal processing.Funding was provided by the Office of Naval Research through Contract No. NOOOl4-91-J-1776

Estimation of three-dimensional water column sound speed profiles and sediment compressional wave speed and density profiles using a distributed network of buoys

Author Posting. © Acoustical Society of America, 2020. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 147(3), (2020): 1392-1403, doi:10.1121/10.0000794.Broadband data acquired during the Modal Mapping Experiment (MOMAX) V experiment are used to invert simultaneously for the three-dimensional (3D) water column sound speed profiles and the compressional wave speed and density profiles of the seabed in shallow waters off the coast of New Jersey. Linear Frequency Modulation sweep signals in the band 50–300 Hz are transmitted from a nearly stationary source at several discrete positions to a set of freely drifting receivers. Mode travel times are estimated from the signals acquired by the drifting buoys, and these are then used as input data in an inversion algorithm that estimates the acoustic properties of the water column and sediments. The resulting 3D compressional wave speed profiles in the seabed are generally consistent with the one-dimensional profile obtained during the narrowband component of MOMAX V, as well as the results from other experiments in the same area. The validity of the inversion results has also been assessed by the ability of the inverted model to predict the fields measured during the narrowband experiments.The authors gratefully recognize the support of the entire team that was responsible for the execution of MOMAX V. The work reported in this paper was done with the support of the Office of Naval Research under Grant Nos. N00014-09-1-0505 and N00014-12-1-0083 and the Space and Naval Warfare Systems Command through Naval Sea Systems Command Contract No. N00024-02-D-6604.2020-09-0

A technique for measuring the plane-wave reflection coefficient of the ocean bottom

Also published as: Journal of the Acoustical Society of America 68 (1980): 602-612A new technique for the measurement of the plane-wave reflection coefficient of a horizontally stratified ocean bottom is described. It is based on the exact Hankel transform relationship between the reflection coefficient and the bottom reflected field due to a point source. The method employs a new algorithm for the numerical evaluation of the Hankel transform which is based on the "projection-slice" theorem for the two-dimensional Fourier transform. The details of the algorithm are described in the companion paper. Although the algorithm is applied to the case of an isovelocity ocean, the general theory for measuring the plane-wave reflection coefficient in a refracting ocean is developed. The technique provides information about the reflection coefficient, not only for real incident angles, _but also for complex angles, thus potentially providing substantial additional structural information about the bottom. The method is shown to yield excellent results with synthetically generated data for the cases of a hard bottom and slow isovelocity bottom.Prepared for the Office of Naval Research under Contracts N00014-77-C-0196 and N00014-75-C-0951; NR 049-328

Bottom interaction of low-frequency acoustic signals at small grazing angles in the deep ocean

Also published as: Journal of the Acoustical Society of America 69 (1981): 84-94The results of a deep-ocean bottom interaction experiment are presented in which the effects of both bottom refraction and subbottom reflection were observed. Data were obtained in the Hatteras Abyssal Plain using a deep towed 220-Hz pulsed cw source and two receivers anchored near the bottom. For ranges between 1 and 6 km, corresponding to bottom grazing angles less than 13°, the quadrature components of the received signals were recorded digitally. The observed amplitude shows a strong spatial interference pattern which is composed of the direct and bottom interacting arrivals. It is shown that for small source-receiver separations, the bottom return is dominated by a strong subbollom reflection. With increasing separation, this arrival evolves into a refracted arrival due to the presence of a positive sound-speed gradient in the sediment overlying the subbottom. Because of the gradient, a caustic is formed, and corresponding high intensity regions are observed in the data at the expected ranges. Values of sediment layer thickness, sound-speed gradient, and sound-speed drop at the water-bollom interface are obtained from best fits to the data using ray theory, normal mode theory, and the parabolic equation method. These values are consistent with those obtained in nearby locations by other workers. The success of the parabolic equation method indicates that at small grazing angles, the bottom interaction process may be modeled as a propagation process combined with the effect of a perfect, soft subbollom reflector. A value of sediment attenuation, 0.0015 dB/mat 220Hz, is also inferred from the data and is among the lowest values reported to date in the literature.Prepared for the Office of Naval Research under Contract N00014-77-C-0196 administered through NORD

The contribution of normal modes in the bottom to the acoustic field in the ocean

Also published as: Journal of the Acoustical Society of America 68 (1980): 602-612The effects of normal modes in the bottom on the acoustic field in the ocean are examined. The ocean bottom model consists of a slow isovelocity layer overlying an isovelocity half-space to simulate the characteristic sound velocity drop at the water-bottom interface. Attention is focused on the perfectly trapped modes which are excited in the layer by inhomogeneous waves emitted by a point source in the water column. The relative normal mode contribution to the total acoustic field in the water is calculated analytically for a near-bottom source/receiver geometry and evaluated for representative ocean bottom examples. It is shown that, for combined source/receiver heights less than a wavelength, the field is dominated by the leaky mode contribution at short ranges ( $ 2 km) and the trapped mode contribution at long ranges ( ~ 2 km). For fixed bottom parameters, the trapped mode contribution increases exponentially with decreasing combined source/receiver height. It is also shown that, for a fixed layer wavenumber-thickness product and fixed layer sound speed, the leaky mode fields at different frequencies are approximately range-scaled versions of the same field.Prepared for the Office of Naval Research under Contract N000 14-77-C-0196

Geoacoustic inversion by mode amplitude perturbation

Author Posting. © Acoustical Society of America, 2008. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 123 (2008): 667-678, doi:10.1121/1.2821975.This paper introduces a perturbative inversion algorithm for determining sea floor acoustic properties, which uses modal amplitudes as input data. Perturbative inverse methods have been used in the past to estimate bottom acoustic properties in sediments, but up to this point these methods have used only the modal eigenvalues as input data. As with previous perturbative inversion methods, the one developed in this paper solves the nonlinear inverse problem using a series of approximate, linear steps. Examples of the method applied to synthetic and experimental data are provided to demonstrate the method's feasibility. Finally, it is shown that modal eigenvalue and amplitude perturbation can be combined into a single inversion algorithm that uses all of the potentially available modal data.Funding for the research presented here was provided by the Office of Naval Research, and the WHOI Academic Programs Office

Computation of the Hankel transform using projections

Also published as: Journal of the Acoustical Society of America 68 (1980): 523-529In this paper two new algorithms for computing an nth‐order Hankel transform are proposed. The algorithms are based on characterizing a circularly symmetric function and its two‐dimensional Fourier transform by a radial section and interpreting the Hankel transform as the relationship between the radial section in the two domains. By utilizing the property that the projection of a two‐dimensional function in one domain transforms to a radial section in the two‐dimensional Fourier transform or inverse Fourier transform domain, several efficient procedures for computing the Hankel transform exploiting the one‐dimensional FFT algorithm are suggested.Prepared for the Office of Naval Research under Contracts N00014-75-C-0951; NR 049-328 and N00014-77-C- 0196

IMET shipboard systems, operations and software user manual

This report has two parts. The first is a copy of the Operations and Software User Manual prepared for use with the IMET shipboard software distribution. It describes the programs used to acquire and record data from IMET systems installed on R/V Knorr and R/V Oceanus. The second part adds appendix material that contains the documentation pages for programs and subroutines used in the IMET shipboard software system. These items are available through network or diskette access. This report has been prepared to give this information broader visibility and circulation.Funding was provided by the National Science Foundation under Grant Nos. OCE-92-04034 and OCE-87-09614

Digital Signal Processing

Contains an introduction and reports on fourteen research projects.National Science Foundation FellowshipNational Science Foundation (Grant ECS84-07285)U.S. Navy - Office of Naval Research (Contract N00014-81-K-0742)Sanders Associates, Inc.U.S. Air Force - Office of Scientific Research (Contract F19628-85-K-0028)Advanced Television Research ProgramAmoco Foundation FellowshipHertz Foundation Fellowshi

Digital Signal Processing

Contains an introduction and reports on fifteen research projects.U.S. Navy - Office of Naval Research (Contract N00O14-81-K-0742)U.S. Navy - Office of Naval Research (Contract N00014-77-C-0266)National Science Foundation (Grant ECS80-07102)National Science Foundation (Grant ECS84-07285)Amoco Foundation FellowshipSanders Associates, Inc.Advanced Television Research ProgramM.I.T. Vinton Hayes FellowshipHertz Foundation Fellowshi