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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Additional excerpts
...In this environment, the intra-modal frequency dispersion is stronger and has an opposite sign than that in a Pekeris waveguide (Duncan et al., 2013)....
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...ranges thousands of kilometres from the source when sound travels down the continental slope (McCauley et al., 2008; Duncan et al., 2013)....
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...Further technical details on underwater soundpropagation are included in Supplementary Material A and can be found elsewhere (McCauley et al., 2003a; Duncan and McCauley, 2008; McCauley et al., 2008; Duncan et al., 2013)....
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...…tens of kilometres due to upslope propagation from a seismic source in shallow water over a low reflectivity seabed, whereas other signals may be detectable at ranges thousands of kilometres from the source when sound travels down the continental slope (McCauley et al., 2008; Duncan et al., 2013)....
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Cites methods from "Characteristics of sound propagatio..."
...In acoustic modelling studies, specific characteristics of sound propagation over Australia’s continental shelf have been investigated with the help of CHORUS [35]....
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References
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"Characteristics of sound propagatio..." refers background or methods in this paper
...Victor et al. (1965) theoretically modeled impulsive sound propagation in a fluid layer overlying a layered solid whereas Tolstoy and Clay (1966) considered the dispersive characteristics of normal modes propagating in water over an elastic basement....
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...Low-frequency transmission loss data collected from an offshore seismic survey in Bass Strait on the southern Australian continental shelf are analyzed and shown to be in broad agreement with the numerical predictions based on the theoretical analysis and modeling using an elastic parabolic…...
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