Alec J. Duncan

In situ source levels of mulloway (Argyrosomus japonicus) calls

Mulloway (Argyrosomus japonicus) in Mosman Bay, Western Australia produce three call categories associated with spawning behavior. The determination of call source levels and their contribution to overall recorded sound pressure levels is a significant step towards estimating numbers of calling fish within the detection range of a hydrophone. The source levels and ambient noise also provide significant information on the impacts anthropogenic activity may have on the detection of A. japonicus calls. An array of four hydrophones was deployed to record and locate individual fish from call arrival-time differences. Successive A. japonicus calls produced samples at various ranges between 1 and 100 m from one of the array hydrophones. The three-dimensional localization of calls, together with removal of ambient noise, allowed the determination of source levels for each call category using observed trends in propagation losses and interference. Mean source levels (at 1 m from the hydrophone) of the three call categories were calculated as 163 ± 16 dB re 1 μPa for Category 1 calls (short call of 2-5 pulses); 172 ± 4 dB re 1 μPa for Category 2 calls (long calls of 11-32 pulses); and 157 ± 5 dB re 1 μPa for Category 3 calls (series of successive calls of 1-4 pulses, increasing in call rate).

A critical review of the potential impacts of marine seismic surveys on fish & invertebrates

Marine seismic surveys produce high intensity, low-frequency impulsive sounds at regular intervals, with most sound produced between 10 and 300Hz. Offshore seismic surveys have long been considered to be disruptive to fisheries, but there are few ecological studies that target commercially important species, particularly invertebrates. This review aims to summarise scientific studies investigating the impacts of low-frequency sound on marine fish and invertebrates, as well as to critically evaluate how such studies may apply to field populations exposed to seismic operations. We focus on marine seismic surveys due to their associated unique sound properties (i.e. acute, low-frequency, mobile source locations), as well as fish and invertebrates due to the commercial value of many species in these groups. The main challenges of seismic impact research are the translation of laboratory results to field populations over a range of sound exposure scenarios and the lack of sound exposure standardisation which hinders the identification of response thresholds. An integrated multidisciplinary approach to manipulative and in situ studies is the most effective way to establish impact thresholds in the context of realistic exposure levels, but if that is not practical the limitations of each approach must be carefully considered.

Characteristics of sound propagation in shallow water over an elastic seabed with a thin cap-rock layer

Measurements of low-frequency sound propagation over the areas of the Australian continental shelf, where the bottom sediments consist primarily of calcarenite, have revealed that acoustic transmission losses are generally much higher than those observed over other continental shelves and remain relatively low only in a few narrow frequency bands. This paper considers this phenomenon and provides a physical interpretation in terms of normal modes in shallow water over a layered elastic seabed with a shear wave speed comparable to but lower than the water-column sound speed. A theoretical analysis and numerical modeling show that, in such environments, low attenuation of underwater sound is expected only in narrow frequency bands just above the modal critical frequencies which in turn are governed primarily by the water depth and compressional wave speed in the seabed. In addition, the effect of a thin layer of harder cap-rock overlaying less consolidated sediments is considered. 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 equation solution for range-dependent bathymetry.

Acoustic coupled fluid–structure interactions using a unified fast multipole boundary element method

This paper presents a numerical model for the acoustic coupled fluid-structure interaction (FSI) of a submerged finite elastic body using the fast multipole boundary element method (FMBEM). The Helmholtz and elastodynamic boundary integral equations (BIEs) are, respectively, employed to model the exterior fluid and interior solid domains, and the pressure and displacement unknowns are coupled between conforming meshes at the shared boundary interface to achieve the acoustic FSI. The low frequency FMBEM is applied to both BIEs to reduce the algorithmic complexity of the iterative solution from O(N(2)) to O(N(1.5)) operations per matrix-vector product for N boundary unknowns. Numerical examples are presented to demonstrate the algorithmic and memory complexity of the method, which are shown to be in good agreement with the theoretical estimates, while the solution accuracy is comparable to that achieved by a conventional finite element-boundary element FSI model.

Modelling acoustic reflection loss at the ocean surface for small angles of incidence

This paper investigates the performance of available models of acoustic surface loss for small grazing angles of incidence, and for lower wind speeds for which bubble formation is less likely to play a role. In order to investigate the expected importance of shadowing for these scenarios, a wave-type model has been run with a deterministic simulation of the surface profile, for a number of situations for which transmission has been confined to small angle arrivals. This modelling includes, implicitly, the effects of diffraction into the shadowed regions, and the effects of the varying levels of intensity of insonification in accord with the different surface slopes. The surface loss values inferred from these simulations are compared with those from the other models, and to the extent possible, with the apparent effects observed with available at sea data. In this work, attention has been paid to the angles of incidence at the surface which dominate the loss effects. To the extent possible, conclusions are drawn on the apparent accuracy of the various models, and the need for further work to ascertain their appropriateness at small grazing angles.

Underwater sound of rigid-hulled inflatable boats

Underwater sound of rigid-hulled inflatable boats was recorded 142 times in total, over 3 sites: 2 in southern British Columbia, Canada, and 1 off Western Australia. Underwater sound peaked between 70 and 400 Hz, exhibiting strong tones in this frequency range related to engine and propeller rotation. Sound propagation models were applied to compute monopole source levels, with the source assumed 1 m below the sea surface. Broadband source levels (10-48 000 Hz) increased from 134 to 171 dB re 1 μPa @ 1 m with speed from 3 to 16 m/s (10-56 km/h). Source power spectral density percentile levels and 1/3 octave band levels are given for use in predictive modeling of underwater sound of these boats as part of environmental impact assessments.

Regional Variations and Trends in Ambient Noise: Examples from Australian Waters.

Studies of ambient noise south of Australia show higher levels at low frequencies in the deep water off the continental shelf compared with locations on the shelf. The difference arises because of differences in transmission loss. Marine animals would experience significantly different noise levels and directionality in the two regions and while crossing the boundary, provide positional information. Opportunities for long-range, low-frequency communication by animals would be significantly limited by the higher background noise in the open ocean. Measures of long-term sea noise trends highlight the influence of biological sources and the importance of local sound transmission regimens.

Compressed sensing based channel estimation and impulsive noise cancelation in underwater acoustic OFDM systems

Impulsive noise occurs frequently in underwater acoustic (UA) channels and can significantly degrade the performance of UA orthogonal frequency-division multiplexing (OFDM) systems. In this paper, we propose a novel compressed sensing based algorithm for joint channel estimation and impulsive noise cancelation in UA OFDM systems. The proposed algorithm jointly estimates the channel impulse response and the impulsive noise by utilizing the pilot subcarriers. The estimated impulsive noise is then converted to the time domain and removed from the received signals. The proposed algorithm is applied to process the data collected during the UA communication experiment conducted in December 2015 in the estuary of the Swan River, Western Australia. The results show that the proposed approach improves the performance of both channel estimation and impulsive noise mitigation.

Channel estimation based on compressed sensing in high-speed underwater acoustic communication

The underwater acoustic (UA) channel is dispersive in both time and frequency with severe frequency-dependent signal attenuation. Efficient channel estimation and tracking are crucial to coherent high-rate UA communication. In this paper, we propose a new compressed sensing (CS) based channel estimation method with block-by-block channel tracking for UA communication. Compared with conventional channel estimation algorithms, the proposed method efficiently exploits the sparsity of the UA channel, and improves the channel tracking capability of UA communication system. The proposed algorithm was tested during our UA communication experiment conducted in December 2012 in the Indian Ocean off Rottnest Island, Western Australia. At a data rate of 8 kbps (QPSK constellations), average uncoded bit-error-rates (BERs) of 3% and 14% have been achieved over 1 km and 6 km ranges, respectively, using MMSE equalization based on the proposed channel estimation and tracking method.

Predicting the Environmental Impact of Active Sonar

The effect of active sonar on marine animals, particularly mammals, has become a hot topic in recent times. The Australian Environmental Protection and Biodiversity Conservation Act 1999 obligates Defence to avoid significant environmental impacts from Navy activities including those which produce underwater sound such as active sonar. It is in the interests of all parties that these effects be modeled accurately to facilitate both the quantitative evaluation of the consequences of any proposed sonar trials, and the identification of suitable mitigation procedures. This paper discusses the received signal parameters that are of importance when predicting the effect of sonar systems on marine animals and techniques for modeling both the expected values of these parameters and their statistical fluctuations.