James A. Doutt

Shallow-water sound transmission measurements on the New Jersey continental shelf

A fuzzy logic controller for ship path control in restricted waters is developed and evaluated. The controller uses inputs of heading, yaw rate, and lateral offset from the nominal track to produce a commanded rudder angle. Input variable fuzzification, fuzzy associative memory rules, and output set defuzzification are described. Two maneuvering situations are evaluated: track keeping along a specified path where linearized regulator control is valid; and larger maneuvers onto a specified path where nonlinear modeling and control are required. For the track keeping assessment, the controller is benchmarked against a conventional linear quadratic Gaussian (LQG) optimal controller and Kalman filter control system. The Kalman filter is used to produce the input state variable estimates for the fuzzy controller as well. An initial startup transient and regulator control performance with an external hydrodynamic disturbance are evaluated using linear model simulations of a crude oil tanker. A fully nonlinear maneuvering model for a smaller product tanker is used to assess the larger maneuvers. The fuzzy controller exhibits promising performance and application flexibility.

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

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 subbottom 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–bottom 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 subbottom reflector. A value of sediment attenuation, 0.0015 dB/m at 220 Hz, is also inferred from the data and is among the lowest values reported to date in the literature.

Using GPS at sea to determine the range between a moving ship and a drifting buoy to centimeter-level accuracy

In many experiments at sea the measurement of distance is of great importance. This can be the distance between instruments mounted on a ship, where the ships superstructure makes direct measurement difficult if not impossible, or, for example, the distance between the ship and a drifting buoy. In March 1997, the MOMAX shallow-water acoustics experiment was performed off the coast of New Jersey. Single-frequency GPS receivers were fitted both to a ship and to drifting buoys so that the range between them could be determined. The raw receiver output of pseudorange and carrier phase was recorded to disk for post-processing with a differential, moving-baseline kinematic software package. Two receiving antennas were fixed on the ship; thus variation in the calculated distance between them could be examined over time and various sea states. In addition, they provided a fixed baseline for the computation of the triangle closure error between them and a remote buoy. The derived distance between the two ships antennas was 28.11 m, had a standard deviation of about one centimeter, showed a peak-to-peak scatter of a few centimeters, and was essentially independent of sea state. The triangle closure error between the two ship antennas and a buoy was generally better than two centimeters out to ranges of five kilometers. Single-frequency GPS is capable of obtaining centimeter-level range measurements at sea even when both the base station (ship) and the remote (buoys) are moving.

Inversion for the compressional wave speed profile of the bottom from synthetic aperture experiments conducted in the Hudson Canyon area

In September 1988, a series of acoustic propagation experiments were conducted in the Hudson Canyon area. These included synthetic aperture experiments in which a source transmitting a set of four pure tones was towed toward/away from a vertical array of 24 receivers. Data obtained at 50 Hz during one of the synthetic aperture experiments are used to obtain a model for the compressional wave speed profile in the bottom using a modal inverse method. This model is further refined using 175 Hz data. The ability of the inferred model to predict the field at 50 Hz and higher frequencies is examined.

Shallow‐water transmission measurements taken on the New Jersey Continental Shelf

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 u...

Determination of compressional wave and shear wave speed profiles in sea ice by crosshole tomography—Theory and experiment

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 alo...

The determination of geoacoustic models in shallow water

A technique for determining 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. 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. Comparisons are also made between the measured and theoretically computed pressure field magnitudes. The technique is demonstrated using experimental data at 140 Hz and 220 Hz.

Modal mapping in shallow water using synthetic aperture horizontal arrays

An experimental technique is described for mapping the wavenumber spectrum of the normal mode field as a function of position in a shallow water waveguide with three-dimensional variation in its acoustic properties. These modal maps provide a characterization of the modal properties of the waveguide, can be used as input data to inversion techniques for inferring the 3D geoacoustic properties of the bottom, and improve our ability to localize and track source. The experimental configuration consists of a source radiating one or more pure tones to a field of freely drifting buoys, each containing a hydrophone, GPS navigation, and radio telemetry. A key component of the method is the establishment of a local differential GPS system between the source ship and each buoy, thereby enabling the determination of the positions of the buoys relative to the ship with submeter accuracy. In this manner, the drifting buoys create 2D synthetic aperture horizontal arrays along which the modal evolution of the waveguide can be observed in the spatial domain, or after beam forming, in the horizontal wavenumber domain. Typical results from two modal mapping experiments (MOMAX) are presented in which fixed and moving source configurations were used to transmit pure tones in the band 50-300 Hz to several buoys at ranges up to 10 km. MOMAX I was conducted in about 70 m of water off the New Jersey coast in March, 1997, while MOMAX II was carried out in 50-150 m water depths in the Gulf of Mexico in February, 1999. A striking feature of these data is the remarkable stability and regularity of the phase, although the magnitude displays a complex multimodal interference pattern.

Geoacoustic models for the Icelandic Basin

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 ...

Experimental Horizontal Wavenumber Spectra and Implications for Full Field Processing

Acoustic pressure levels were recorded from a multifrequency source towed at mid-depth on three legs, both towards and away from a 24-element vertical hydrophone array. Horizontal wavenumber spectra were estimated using a synthetic aperture Hankel transform technique. Plots of modal amplitude as a function of depth and wavenumber reveal details of the propagation which are not readily apparent in plots of transmission loss. Two compressional speed profiles beneath the ocean bottom were derived for different legs from the modal wavenumbers at 50 Hz. Comparisons between the experimental data and SAFARI modelling using these profiles give excellent agreement both in amplitude and phase. KRAKEN and IFD/PE modelling using profiles based on nearby borehole data also give excellent agreement. Some aspects of the modal plots can be explained by lateral variability along with a double-ducted sound channel in the sediment below a depth of 75 meters. The multifrequency experimental acoustic data, measured sound speed profiles, and derived geoacoustic bottom profiles were used with variable success in matched field processing to obtain source localization. The high quality of this experimental data set makes it particularly useful for the testing of full field inversion techniques.