Frisk George V.

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George V.

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  • Technical Report
    Bottom interaction of low-frequency acoustic signals at small grazing angles in the deep ocean
    (Woods Hole Oceanographic Institution, 1981-07) Frisk, George V. ; Doutt, James A. ; Hays, Earl E.
    The 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.
  • Article
    Estimation of three-dimensional water column sound speed profiles and sediment compressional wave speed and density profiles using a distributed network of buoys
    (Acoustical Society of America, 2020-03-02) Rajan, Subramaniam D. ; Frisk, George V.
    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.
  • Technical Report
    Report on the Office of Naval Research Shallow Water Acoustics Workshop, April 24-26, 1991
    (Woods Hole Oceanographic Institution, 1992-01) Frisk, George V.
    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.
  • Technical Report
    The contribution of normal modes in the bottom to the acoustic field in the ocean
    (Woods Hole Oceanographic Institution, 1981-04) Macpherson, Mark K. ; Frisk, George V.
    The 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.
  • Article
    An International Quiet Ocean Experiment
    (Oceanography Society, 2011-06) Boyd, Ian L. ; Frisk, George V. ; Urban, Edward ; Tyack, Peter L. ; Ausubel, Jesse ; Seeyave, Sphie ; Cato, Doug ; Southall, Brandon L. ; Weise, Michael ; Andrew, Rex K. ; Akamatsu, Tomonari ; Dekeling, Rene ; Erbe, Christine ; Farmer, David M. ; Gentry, Roger ; Gross, Thomas F. ; Hawkins, Anthony D. ; Li, Fenghua ; Metcalf, Kathy ; Miller, James H. ; Moretti, David J. ; Rodrigo, Cristian ; Shinke, Tomio
    The effect of noise on marine life is one of the big unknowns of current marine science. Considerable evidence exists that the human contribution to ocean noise has increased during the past few decades: human noise has become the dominant component of marine noise in some regions, and noise is directly correlated with the increasing industrialization of the ocean. Sound is an important factor in the lives of many marine organisms, and theory and increasing observations suggest that human noise could be approaching levels at which negative effects on marine life may be occurring. Certain species already show symptoms of the effects of sound. Although some of these effects are acute and rare, chronic sublethal effects may be more prevalent, but are difficult to measure. We need to identify the thresholds of such effects for different species and be in a position to predict how increasing anthropogenic sound will add to the effects. To achieve such predictive capabilities, the Scientific Committee on Oceanic Research (SCOR) and the Partnership for Observation of the Global Oceans (POGO) are developing an International Quiet Ocean Experiment (IQOE), with the objective of coordinating the international research community to both quantify the ocean soundscape and examine the functional relationship between sound and the viability of key marine organisms. SCOR and POGO will convene an open science meeting to gather community input on the important research, observations, and modeling activities that should be included in IQOE.
  • Technical Report
    Computation of the Hankel transform using projections
    (Woods Hole Oceanographic Institution, 1981-04) Oppenheim, Alan V. ; Frisk, George V. ; Martinez, David R.
    In 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.
  • Article
    Geoacoustic inversion by mode amplitude perturbation
    (Acoustical Society of America, 2008-02) Poole, Travis L. ; Frisk, George V. ; Lynch, James F. ; Pierce, Allan D.
    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.
  • Technical Report
    A technique for measuring the plane-wave reflection coefficient of the ocean bottom
    (Woods Hole Oceanographic Institution, 1981-04) Frisk, George V. ; Oppenheim, Alan V. ; Martinez, David R.
    A 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.