摘要:

Modal cutoff during up‐slope propagation in a wedge‐shaped ocean is studied using the parabolic equation model; theoretical results are compared with some model tank experiments.

ISSN:0001-4966    

DOI:10.1121/1.384330

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

This paper demonstrates, by theoretical means, that measured acoustic scattering data can be inverted to yield estimates of the ocean‐surface directional‐frequency spectrum. The inverse scattering technique developed is based on least‐squares inversion through the generalized inverse for underdetermined linear systems of equations. The procedure is based on a recently developed acoustic scattering model which is capable of samplingallfrequenciesanddirectionsof the gravity‐wave field in alocalizedarea of ocean surface. The technique is numerically tested on simulated data samples, constructed with the scattering model using pseudorandom numbers, for a Gaussian ocean surface. The estimation procedure is independent of the assumed form of the simulated directional‐frequency spectrum, and the results obtained show that estimation agrees with that spectrum.

ISSN:0001-4966    

DOI:10.1121/1.384331

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

An acoustic source, in conjunction with an array of acoustic receivers in a nearby isovelocity region, can be used as a probe to measure the ocean surface. F. M. Labianca [J. Acoust. Soc. Am. 67, 1567–1577 (1980)] has developed a procedure for estimating the ocean‐surface directional spectrum by this means. The problem he solves is typically called inverse scattering. This paper addresses the issue of what region of the ocean surface is being sampled in his procedure. It is desirable that the sampled region be sufficiently confined that the assumption of spatial homogeneity may be safely made, otherwise the directional spectrum has no meaning. It is proved in this paper that, for a given receiver, the region sampled by Labianca’s procedure is a small circle on the ocean surface, with center over the source and radius equal to the horizontal range between source and receiver. The proof is subject to a constraint on the wavelength of the source.

ISSN:0001-4966    

DOI:10.1121/1.384332

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

In many situations of low frequency acoustic propagation in the ocean, the acoustic properties of the bottom are an important factor. One quantity of which few measurements have been made or reported is the compressional wave attenuation profile. This paper reports attenuation measurements based upon measurements from explosive sources which encountered the ocean bottom several times. It was found that compressional wave attenuation coefficients at 100 m depth are approximately 0.015 dB/m‐kHz in terrigenous turbidite sediments and 0.03 dB/m‐kHz in calcareous sediments. These values are as low as, and lower than, the smallest previously reported estimates.

ISSN:0001-4966    

DOI:10.1121/1.384333

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A computer‐based system has been developed for the measurement of acoustic propagation loss in multipath arrivals of signals from explosive sources. The system involves both signal measurement operations and careful theoretical representations of the measured quantities. Initial applications have been for measurement of bottom‐reflection losses using signals which reflect many times from the bottom. It has been found that the levels of arrival pulses can be strongly affected by predictable surface‐reflection effects. Measurements demonstrate consistent bottom‐reflection loss values across homogeneous basin areas.

ISSN:0001-4966    

DOI:10.1121/1.384334

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Low‐frequency (<50 Hz) sound transmission studies were conducted off Mission Beach, California. Sound from both explosive and cw sources, as received on bottomed geophones and hydrophones cable connected to the NOSC Oceanographic Tower, were used to determine the transmission paths and propagation loss. Sound speed and other parameters of the sub‐bottom layers, as determined by divers, bore holes, and refraction shooting, were used as input to a FFP computer program to model the propagation loss. The computed propagation loss is less than that observed at short ranges (<2.5 Km) and greater than observed at longer ranges. The discrepancy is believed due to a slope reversal of the interface between the unconsolidated and the semiconsolidated layers. Propagation was best in the E–W direction, but the noise level was also higher. The buried hydrophones were quieter than the bottomed hydrophones and the S/N ratio of the E–W and vertical geophones was greater than on the hydrophones at 5 and 15 Hz. At these very low frequencies and short range the S/N ratio of the geophone recordings was greater than that on the hydrophone recordings.

ISSN:0001-4966    

DOI:10.1121/1.384335

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

An experiment to locate objects buried in seafloor sediment by using ultrasonic imaging has been conducted and shows potential for practical application. Attenuation measured in actual seafloor sediment indicated that 15 kHz could be used for detecting objects buried to about 4 m in the seafloor. Tests were conducted using both plane and focused transducers with elements arranged on a concave surface. Imaging of inclined objects by a focused transducer was found to be significantly better than by a plane transducer as long as the object’s gradient was within one‐half the transducer aperture angle. Scaled model tests were carried out at a frequency of 150 kHz and full‐scale tests were performed at 15 kHz. Two display formats were used—one for the vertical plane and the other for the horizontal plane. In the full‐scale test, a cylindrical iron tube and an iron plate were buried in a sand box 4 m long by 3 m wide by 2 m deep. Ultrasonic images were observed on a CRT.

ISSN:0001-4966    

DOI:10.1121/1.384201

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

The results of shallow‐water acoustic experiments performed off the coast of Corpus Christi, Texas, are compared with theoretical computations. The measured results include vertical amplitude distributions in the water column, transmission loss in one‐third octave bands, and dispersion analyses. Good agreement was obtained between the measured and theoretical results. Theoretical computations are based on a four‐layer, fluid, normal‐mode model.

ISSN:0001-4966    

DOI:10.1121/1.384336

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

Several theoretical results linking ray and normal‐mode quantities are derived by assuming Wentzel–Kramers–Brillouin (WKB) solutions for the depth dependence of the acoustic field in both water and bottom. Some of the results are not new but are extended here to the general case of reflection from a structured, lossy bottom: (1) Mode and ray cycle distances are equal if the ray cycle distance includes the beam displacement on reflection; (2) cycle distance is simply related to the normalization of the normal modes; (3) mode attenuation per cycle is equal to bottom loss per bounce of the equivalent ray; (4) a complete normal‐mode solution for water may be obtained from the plane‐wave reflection coefficient with no further knowledge of the bottom; and (5) the WKB solution for the bottom is equivalent to a calculation assuming interfering geometric ray paths.

ISSN:0001-4966    

DOI:10.1121/1.384283

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP

摘要:

A new method of ultrasonic frequency analysis is introduced which employs frequency‐tracked gated rf drive pulses rather than shock‐excited broadband spikes to generate the ultrasonic waveform. The new technique, a variation of the sampled‐continuous wave technique, eliminates problems associated with finite pulse widths of conventional methods. It is shown to yield correct ultrasonic wave velocity measurements of the sample irrespective of receiver gate width or position provided any portions of two successive echoes are gated simultaneously into the spectrum analyzer. The experimental observations are substantiated by a theoretical model based on the time‐frequency domain formulation of ultrasonic frequency analysis.

ISSN:0001-4966    

DOI:10.1121/1.384284

出版商:Acoustical Society of America    

年代:1980

数据来源: AIP