Jack A. Shooter

Ambient Noise Analysis of Deep-Ocean Measurements in the Northeast Pacific

During the late 1960s and throughout the 1970s, the U.S. Navy conducted a series of ocean acoustic measurement exercises to support development of systems and techniques to detect nuclear submarines. The exercises and most of the technical documentation were classified. In 2003, a project was sponsored by the U.S. Office of Naval Research (ONR, Arlington, VA) to declassify documentation and demonstrate the capability to recover acoustic data recorded on magnetic tape. One of the exercises, known as CHURCH OPAL, was selected for demonstration of acoustic data recovery. The record on magnetic tape spanned a period of ten days in September 1975 from a vertical assembly of hydrophones at a site midway between Hawaii and California. This paper presents selected excerpts from a key report (Wittenborn, 1976) on ambient noise that previously was unpublished and unavailable for general distribution. The earlier work is augmented with more complete and detailed analyses of the recovered digital data using modern analytical techniques. Data acquired from the hydrophones below critical depth enabled isolation of ambient noise due to distant shipping and local wind. The frequency band of the acoustic analyses was 30-500 Hz. The wind component of the ambient noise was evaluated at frequencies lower than reported by Wenz (1962).

Depth dependence of noise resulting from ship traffic and wind

Under conditions of distantly generated noise, the noise level is found to decrease with depth in the mid-northeastern Pacific. These data show a decrease in noise level greater than 25 dB between critical depth and the ocean bottom. A result of this decrease is that locally wind-generated noise can be detected on near-bottom receivers for wind speeds less than 10 kn. It is shown that the noise level generated form local sources such as wind and nearby shipping is almost independent of receiver depth. The differences in spectra shape between the distant shipping noise and wind-generated noise and the low noise levels detected near the ocean bottom allow the measurement in the frequency band at 200-500 Hz of local wind noise level for wind speeds less than 10 kn. >

Beamforming on seismic interface waves with an array of geophones on the shallow sea floor

This brief communication relates a generalization to and additional comments on the above editorial*. The present author generally supports the editorial’s recommendations.

Wind generated noise in the Parece Vela Basin

A 24‐h segment of noise level as a function of wind speed and frequency is presented for the frequency range 150–500 Hz. These data were measured in the Parece Vela Basin and represent one of the few (maybe the only) cases where the wind speed was measured in situ along with the noise level in the deep open ocean. The acoustic data show lower levels and a greater sensitivity to changes in wind speed than some other areas.

Effect of Inhomogeneous Sub-Bottom Layering on Broadband Acoustic Propagation

In this paper, the effects of range inhomogeneities in the seabed layering on low-frequency broadband sound propagation in a shallow-water ocean environment are examined. Acoustic measurements generated by an impulsive sound source were made on the New Jersey continental shelf over propagation paths where previous geophysical analyses provide information on the seabed layering structure. Additional information on the physical properties of the sediment layers, such as sound speed and attenuation, was obtained from previous analyses of continuous wave tow experiments. The seismic and the geophysical information, the inferred geoacoustic information for the sediment layers, and sparse water-column sound-speed measurements provide inputs for a finite element parabolic equation propagation model. For the three propagation paths considered in this study the seabed layering structure for two of the paths is range dependent and the other is approximately horizontally stratified. With this information modeled, time series are produced and are compared in the 35-265-Hz band to the measured received times series from impulsive sources deployed at ranges between about 70 and 350 water depths. Further, simulations of the received time series are performed for each of the three paths using modified sub-bottom layering structures for the purpose of quantifying the acoustic field effects associated with deviations of the seabed structure from horizontal stratification.

Underwater Acoustic Scattering from Lake Surfaces. I. Theory, Experiment, and Validation of the Data

A statistical analysis, based directly on ensemble averages, is made of reverberation data obtained from the wind‐driven surface of a fresh water lake. To produce the reverberation, two types of signals consisting of gated (linear “chirp”) FM or CW pulses, are transmitted with a carrier frequency of 110 kHz. In the CW cases the pulse durations are 0.1 and 1.0 msec. The bandwidth of the FM signals (pulse durations 1.25 msec) is 9.6 kHz, and operation in all instances is monostatic (R@T). In the CW experiment the grazing angle is 22°, and in the FM experiment the grazing angle is 0°. Particular care is taken to insure that a valid sample ensemble is obtained by testing each sample ensemble for independence (from a one‐sample runs test) and homogeneity (by a suitable Kolmogorov‐Smirnov test). A recently developed point scattering model is introduced and adapted to the present experimental geometry to provide a theoretical description of the reverberation and a basis for later comparisons with the data. From this model covariance functions and intensities of the scattered signal are determined and quantitative insight obtained regarding the nonstationarity of the reverberation process.

Frequency Response Measurements on Backscattering from a Shallow Sea Floor, Using a Parametric Source

Measurements are reported on the backscattering of sound from a sand bottom overlying the continental shelf near San Diego, California. Data were acquired in the 1–10 kHz band with a directive parametric source having a sidelobe-free beam, 4° in half-power width, at a grazing angle of 10°. The bottom was a flat layer of coarse grained sand and clay that contained many shell fragments, overlying a hard sub-bottom layer, some 5 m deep. The mean bottom backscattering coefficient at 1 kHz was -39 dB re 1 m2 and increased with frequency to the power of 0.5. Statistical data in the form of probability distributions show a Rayleigh distribution for ensembles of peak acoustic amplitudes at each frequency studied.

Geoacoustic inversion from moving ship of opportunity in deep‐water environment

This study examines the estimation of seabed geoacoustic parameters in a deep‐water environment using acoustic data generated from the noise of a moving surface ship. The received sound field was recorded on a vertical line array in the North Pacific in approximately 5000 m of water. These data, which were recorded in 1973 in an experiment called Church Opal, have recently been re‐digitized and made available to the underwater acoustics community. Spectrograms of time and frequency for hydrophones below the critical depth illustrate striation patterns associated with the inverse of the time interval between direct and bottom reflected rays paths. The cross‐correlation of signals between phones on the VLA is thus sensitive to the structure of the seabed. A cost function of the difference between cross‐phone spectra of modeled and measured received acoustic signals in the 10–600‐Hz band is minimized using a simulated annealing method. Parameters that are estimated include those associated with the ship trac...

Recovery and digitization of archival analog ocean acoustic data

In October 2002 the Office of Naval Research (ONR) sponsored a Convocation that reviewed the ocean acoustic studies done from 1967 to 1992 under the U.S. Navy’s Long Range Acoustic Propagation Project (LRAPP). Beginning in 1972 LRAPP fielded self‐contained assemblies of vertically distributed hydrophones as part of environmental acoustic exercises in a variety of oceanic regions. Analog signals were recorded in a submerged buoy on multi‐channel magnetic tape. Presentation of results from a 1975 measurements exercise in the Northeast Pacific stimulated interest in recovering and digitizing the 10‐day dataset from 13 hydrophones. ONR sponsored a pilot project to demonstrate the feasibility of recovering the data and setting up digital files while preserving absolute accuracy. The original magnetic tape was sticky. It had to be heat treated and cleaned before playback. A spectrum from the original analysis for near‐field passage of a ship was used to validate data recovery in the range of 10–500 Hz. A final ...

Underwater Acoustic Scattering from Lake Surfaces. II. Covariance Functions and Related Statistics

The construction and validation of sample ensembles of reverberation have been considered in Pt. I. These data have been generated by insonifying the rough moving surface of a fresh water lake by narrow‐band acoustic signals of comparatively short duration. Included also in Pt. I is a concise account of a basic theoretical model of this scattering process, appropriately modified to the conditions of these experiments. From the above validated data we now determine specifically the various second‐order statistics of the reverberation, which are the principal objects of our study. These include the covariance functions and mean intensities. The characteristic nonstationarity of the process appears as an asymmetry of the covariance functions. These experimentally derived statistics are also compared to the corresponding theoretical quantities obtained in Pt. I. Among the critical factors here are the system geometry, including side lobe structure of the beam patterns, and the frequency‐selective actions of both the transmitting and receiving apertures on the original, input “electronic” signal. When the assumptions of the model and conditions of the experiment are properly incorporated, good agreement between theory and experiment is obtained.