Philip R. Staal

Low‐frequency acoustic propagation loss in shallow water over hard‐rock seabeds covered by a thin layer of elastic–solid sediment

Shallow‐water seabeds are often varied and complex, and are known to have a strong effect on acoustic propagation. Some of these seabeds can be modeled successfully as fluid or solid half‐spaces. However, unexpectedly high propagation loss with respect to these models has been measured in several regions with rough, partially exposed, hard‐rock seabeds. It is shown that the high propagation loss in these areas can be modeled successfully by introducing a thin layer of elastic–solid sediment over the hard‐rock substrate. Propagation loss predictions using the safari fast‐field program exhibit bands of high loss at regularly spaced frequencies. Normal‐mode calculations show resonance phenomena, with large peaks in the modal attenuation coefficients at these same frequencies, and with rapid changes in the mode wavenumbers. Bottom reflection loss calculations indicate that the high propagation loss is due to absorption of shear waves in the sediment layer.

Underwater acoustic ambient noise levels on the eastern Canadian continental shelf

This paper reports the analysis of shallow‐water ambient noise levels collected by Defence Research Establishment Atlantic during 14 cruises over the period 1972 to 1985. A weighted average is formed to de‐bias the samples, with the aim of answering the question: ‘‘If one were to pick a site randomly on the eastern Canadian continental shelf at a random time, and perform an ambient noise measurement, what would be the expected noise level?’’ The samples are also grouped according to whether they were taken on the Scotian Shelf, the Grand Banks, or the Flemish Cap, and according to season. The frequency range covered is 30 to 900 Hz. The weighted mean and standard deviation of the noise levels are presented, as well as the correlation coefficient between the noise levels and wind speed. The results show that the eastern Canadian continental shelf as a whole presents levels that are characteristic of areas with high shipping density and good acoustic propagation, with the Scotian Shelf showing generally hig...

A Summary of DREA Observations of Interface Waves at the Seabed

During the past ten years or so, DREA scientists have observed interface waves in shallow water in the frequency range 2 Hz - 20 Hz over a variety of seabed types: sedimentary rock (chalk), unconsolidated sediments (sand and gravel), and crystalline bedrock (granite). In this paper we summarize our interface wave experiments and discuss how we used the observations to improve our understanding of low frequency acoustic propagation in shallow water. We have found that (a) the analysis of interface wave propagation is a good means of estimating seabed geo-acoustic parameters, including shear speed; and (b) over crystalline bedrock seabeds, interface waves in the frequency band 3 Hz - 6 Hz can be the dominant mode of propagation to ranges of 40 km or more.

Modular Digital Hydrophone Array

Defence Research Establishment Atlantic (DREA) has developed a linear hydrophone array for studying sound propagation in shallow water. The array consists of in-line hydrophones, with electronics connected by robust and interchangeable cable sections, making changes in array geometry easy to implement. These cable sections form a digital bus, controlled at one end by a microprocessor, which in turn is controlled via radio link by a shipboard computer. Through this system, many array characteristics including gain, bandpass, sampling rate and element activity are easily changed by the operator. The maximum hydrophone separation is one kilometer, and the frequency response covers one Hertz to three kiloHertz. The array is easy to handle, since with the exception of floats and weights, it is deployed, recovered and stored on a winch. The array has been successfully used in a number of different configurations, and its flexibility adequately demonstrated.

The Effect of Variable Roughness of a Granite Seabed on Low-Frequency Shallow-Water Acoustic Propagation

Previously (Staal and Chapman, 1985), we reported unexplained high loss in acoustic propagation in a shallow water area with a rough granite seabed, in the frequency range 10 Hz — 100 Hz. We had a chart of seabed roughness for this area prepared for us. The chart divides the area into four roughness provinces: 0 m to 2 m, 1 m to 6 m, 4 m to 8 m, and 8 m to 20 m. We performed a second propagation experiment, placing explosive sources at 65 m depth along a circle of 13 km radius centred on a receiving location near a boundary between rough and smooth provinces. In this way, the radial propagation paths from different sources to the same receiver covered areas on the seabed which varied in roughness from path to path. The propagation data collected show correlation between the roughness and the propagation loss. However, the main high-loss feature was not removed by choosing a propagation path with a smooth bottom. We conclude that either bottom roughness alone is not responsible for this main feature, or that our measure of bottom roughness is not appropriate.

Observations of Interface Waves and Low-Frequency Acoustic Propagation Over a Rough Granite Seabed

Experimental measurements of hydrophone signals from explosive sources in shallow water over a rough granite seabed have revealed several interesting seismo-acoustic phenomena. The pressure versus time waveforms from sources at 2 km to 40 km range show strong interface waves in the vicinity of 4 Hz arriving at a group speed of about 1200 m/s. The frequency-time dispersion of these arrivals is consistent with a simple dispersion model, including shear wave effects, based on a uniform water layer overlying a granite half-space. The propagation loss data show a notch of high loss in a band between 10 Hz and 100 Hz, centred at 30 Hz. A propagation loss model using the same geo-acoustic input does not reproduce these high losses but agrees well with the data in the range 2 Hz to 6 Hz. There are two hypotheses that may explain the discrepancy: either the seabed is made up of two or more layers having different acoustic properties giving rise to the observed propagation loss, or scattering at the rough interface (not included in the model) causes the extra loss. We favour the latter interpretation.

Shallow water acoustics on the Canadian East Coast Continental Shelf

The Canadian East Coast Continental Shelf is a large area (roughly 600 000 km2) of complex bottom types geologically related to the glacial cover and low water level of the recent Ice Age. The bottom types include thickly sedimented sand banks under less than 100 m of water, thickly sedimented silt/clay basins under more than 200 m of water, thinly covered or bare glacial till, and outcropped rock. This paper describes DREA measurements of underwater acoustic propagation and ambient noise in the 1‐ to 1000‐Hz frequency band, and attempts to explain these measurements. Acoustic propagation over thickly sedimented seabeds can be explained by models that assume “fluid” sediment layers (i.e., no shear waves). However, over thinly sedimented rocky seabeds the acoustic models need to include shear waves to explain the measured acoustic propagation, in particular the high losses between 10 and 100 Hz. Ambient noise is affected by the seabed, mainly by the difference in acoustic propagation over unlike seabed typ...

Experimental observations of interface wave and acoustic propagation over a rough granite seabed

Experimental measurements of explosive sound propagation in a shallow ocean area over a rough granite seabed have revealed several interesting features concerning low‐frequency acoustic and seismic transmission phenomena. The data were collected using DREAs shallow water research array Hydra, which was deployed as a 38‐m vertical line array consisting of eight hydrophones. The time series data from explosives at distances from 2 to 40 km show a strong 5‐Hz interface wave (or Scholte wave) arriving at a group speed of around 1200 m/s after the high‐frequency arrivals. The dispersion of this wave is consistent with a simple theoretical model consisting of a uniform water layer over a semi‐infinite elastic solid having a shear speed higher than the sound speed in the water. The propagation loss data show a notch of very high loss between 10 and 100 Hz centered at 30 Hz. A commonly used transmission loss model using the same geoacoustic input does not predict these high losses, but agrees at seismic frequenc...