Characteristics of sound propagation in shallow water over an elastic seabed with a thin cap-rock layer.
Summary (2 min read)
Introduction
- In addition, the effect of a thin layer of harder cap-rock overlaying less consolidated sediments is considered.
- In such conditions, the effect of shear in the seabed can have a substantial effect on acoustic propagation in the overlying water column.
- They a)Author to whom correspondence should be addressed.
II. NUMERICAL MODELING OF LOW-FREQUENCY SOUND PROPAGATION
- The numerical predictions made in this section are based on the formulation given in Ellis and Chapman (1985) and the Wave Number Integration (WNI) transmission loss calculation method implemented in computer programs SCOOTER and FIELDS (Porter, 2007).
- The natural logarithm transformation of Eq. (2) gives 2cmH u ilnðjRjÞ ¼ pð2m 1Þ; (3) where u is the phase of the reflection coefficient.
- At its critical frequency and below, a mode is radiating into the seabed, and its contribution to the sound intensity in the far field in the water column is minor.
- The result is that the minimum transmission loss for a given mode occurs at a frequency slightly higher than the modal critical frequency.
- If the wavelength remains much larger than the top layer thickness, then the major effect of the cap rock is a rapid reduction of the reflection coefficient at the basement critical angle as the frequency increases (Fig. 5).
A. Experimental measurements
- Measurements of the transmission loss of airgun signals from an offshore seismic exploration survey were made in 2011 in the western part of Bass Strait as part of an 8-month sea noise monitoring and blue whale tracking program supported by Origin Energy.
- The easternmost inshore and offshore transects are shown in Fig.
- This result was obtained from the best fit to the ESD values of the airgun signals recorded by receivers 1 and 3.
- For the mean sea depth of approximately 115 m along the acoustic paths from the inshore seismic transect to the receiver array and the mean sound speed in water of 1509 m/s, the compressional wave speed derived from the critical frequencies of modes 1 and 2 at approximately 5 and 14 Hz, respectively, is expected to be within 2000–2100 m/s.
- The frequency band of the maximum intensity of mode 3 at about 24 Hz was accurately predicted by both numerical models; however, the range-independent scenario modeled by WNI resulted in noticeably lower transmission loss around the critical frequency of mode 3, while the PE method applied to the range-dependent bathymetry resulted in an accurate prediction of the transmission loss.
IV. CONCLUSIONS
- Low-frequency acoustic propagation over elastic seabeds with shear wave speeds less than the water column sound speed is characterized by relatively low transmission loss only in narrow frequency bands.
- Each of these bands occurs just above the critical frequency of a mode and is a result of two counteracting factors:.
- Moreover, the numerical model predicted that the group velocity should gradually decrease with increasing mode number, which was not observed in the experimental data.
- Understanding the propagation of low-frequency underwater sound over continental shelf seabeds is very important to the prediction of sound levels from all low-frequency sources but particularly for the prediction of levels due to offshore seismic surveys.
- These surveys utilize arrays of airguns that have source spectra similar to that shown in Fig. 10 and produce large amounts of low-frequency acoustic energy.
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3,064 citations
"Characteristics of sound propagatio..." refers methods in this paper
...Acoustic reflection from a layered elastic seabed was analyzed by Brekhovskikh (1960), and Ewing et al. (1957) considered acoustic propagation in the water column over an elastic seabed with an emphasis on interface waves....
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1,987 citations
"Characteristics of sound propagatio..." refers background or methods in this paper
...Acoustic reflection from a layered elastic seabed was analyzed by Brekhovskikh (1960), and Ewing et al. (1957) considered acoustic propagation in the water column over an elastic seabed with an emphasis on interface waves....
[...]
...In addition, the effect of a thin layer of harder cap-rock overlaying less consolidated sediments is considered....
[...]
376 citations
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...Sound propagation over fluid seabeds in shallow water has been thoroughly considered in many publications, from the pioneering work by Pekeris (1948) to the most recent book on shallow water acoustics by Katsnelson et al. (2012)....
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131 citations
"Characteristics of sound propagatio..." refers background in this paper
...Sound propagation over fluid seabeds in shallow water has been thoroughly considered in many publications, from the pioneering work by Pekeris (1948) to the most recent book on shallow water acoustics by Katsnelson et al. (2012)....
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38 citations
"Characteristics of sound propagatio..." refers background or methods in this paper
...II and numerical predictions for range-dependent bathymetry using an algorithm based on the parabolic approximation (Collis et al., 2008)....
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...8) and a parabolic equation (PE) solution recently developed by Collis et al. (2008)....
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...Exposure of calcium-carbonate rich marine sediments to fresh water from atmospheric precipitation resulted in the calcium carbonate in the top layer of sediment partly dissolving, penetrating deeper as a pore fluid and then re-crystallizing, cementing the remaining sediment grains together....
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