Shima H. Abadi

Blind deconvolution for robust signal estimation and approximate source localization.

Synthetic time reversal (STR) is a technique for blind deconvolution in an unknown multipath environment that relies on generic features (rays or modes) of multipath sound propagation. This paper describes how ray-based STR signal estimates may be improved and how ray-based STR sound-channel impulse-response estimates may be exploited for approximate source localization in underwater environments. Findings are based on simulations and underwater experiments involving source-array ranges from 100 m to 1 km in 60 -m-deep water and chirp signals with a bandwidth of 1.5-4.0 kHz. Signal estimation performance is quantified by the correlation coefficient between the source-broadcast and the STR-estimated signals for a variable number N of array elements, 2 ≤ N ≤ 32, and a range of signal-to-noise ratio (SNR), -5 dB ≤ SNR ≤ 30 dB. At high SNR, STR-estimated signals are found to have cross-correlation coefficients of ∼90% with as few as four array elements, and similar performance may be achieved at a SNR of nearly 0 dB with 32 array elements. When the broadband STR-estimated impulse response is used for source localization via a simple ray-based backpropagation scheme, the results are less ambiguous than those obtained from conventional broadband matched field processing.

Ranging bowhead whale calls in a shallow-water dispersive waveguide

This paper presents the performance of three methods for estimating the range of broadband (50-500 Hz) bowhead whale calls in a nominally 55-m-deep waveguide: Conventional mode filtering (CMF), synthetic time reversal (STR), and triangulation. The first two methods use a linear vertical array to exploit dispersive propagation effects in the underwater sound channel. The triangulation technique used here, while requiring no knowledge about the propagation environment, relies on a distributed array of directional autonomous seafloor acoustics recorders (DASARs) arranged in triangular grid with 7 km spacing. This study uses simulations and acoustic data collected in 2010 from coastal waters near Kaktovik, Alaska. At that time, a 12-element vertical array, spanning the bottom 63% of the water column, was deployed alongside a distributed array of seven DASARs. The estimated call location-to-array ranges determined from CMF and STR are compared with DASAR triangulation results for 19 whale calls. The vertical-array ranging results are generally within ±10% of the DASAR results with the STR results providing slightly better agreement. The results also indicate that the vertical array can range calls over larger ranges and with greater precision than the particular distributed array discussed here, whenever the call locations are beyond the distributed array boundaries.

Broadband sparse-array blind deconvolution using frequency-difference beamforming

Synthetic time reversal (STR) is a technique for blind deconvolution of receiving-array recordings of sound from an unknown source in an unknown multipath environment. It relies on generic features of multipath sound propagation. In prior studies, the pivotal ingredient for STR, an estimate of the source-signals phase (as a function of frequency ω), was generated from conventional beamforming of the received-signal Fourier transforms, P(j)(ω), 1 ≤ j ≤ N, where N is the number of array elements. This paper describes how STR is implemented even when the receiving-array elements are many wavelengths apart and conventional beamforming is inadequate. Here, the source-signals phase is estimated by beamforming P(j)(*)(ω(1))P(j)(ω(2)) at the difference frequency ω(2) - ω(1). This extension of STR is tested with broadband signal pulses (11-19 kHz) and a vertical 16-element receiving array having a 3.75-m-spacing between elements using simple propagation simulations and measured results from the FAF06 experiment involving 2.2 km of down slope propagation from 46 to 92 m water depth. The cross-correlation coefficient between the source-broadcast and STR-reconstructed-signal waveforms for the simulations and experiments are 98% and 91%-92%, respectively. In addition, frequency-difference beamforming can be used to determine signal-path-arrival angles that conventional beamforming cannot.

Estimating the location of baleen whale calls using dual streamers to support mitigation procedures in seismic reflection surveys

In order to mitigate against possible impacts of seismic surveys on baleen whales it is important to know as much as possible about the presence of whales within the vicinity of seismic operations. This study expands on previous work that analyzes single seismic streamer data to locate nearby calling baleen whales with a grid search method that utilizes the propagation angles and relative arrival times of received signals along the streamer. Three dimensional seismic reflection surveys use multiple towed hydrophone arrays for imaging the structure beneath the seafloor, providing an opportunity to significantly improve the uncertainty associated with streamer-generated call locations. All seismic surveys utilizing airguns conduct visual marine mammal monitoring surveys concurrent with the experiment, with powering-down of seismic source if a marine mammal is observed within the exposure zone. This study utilizes data from power-down periods of a seismic experiment conducted with two 8-km long seismic hydrophone arrays by the R/V Marcus G. Langseth near Alaska in summer 2011. Simulated and experiment data demonstrate that a single streamer can be utilized to resolve left-right ambiguity because the streamer is rarely perfectly straight in a field setting, but dual streamers provides significantly improved locations. Both methods represent a dramatic improvement over the existing Passive Acoustic Monitoring (PAM) system for detecting low frequency baleen whale calls, with ~60 calls detected utilizing the seismic streamers, zero of which were detected using the current R/V Langseth PAM system. Furthermore, this method has the potential to be utilized not only for improving mitigation processes, but also for studying baleen whale behavior within the vicinity of seismic operations.

Frequency-sum beamforming in an inhomogeneous environment

The arrival directions of ray paths between a sound source and a receiving array can be determined by beamforming the array-recorded signals. And, when the array and the signal are well matched, directional resolution increases with increasing signal frequency. However, when the environment between the source and the receivers is inhomogeneous, the recorded signal may be distorted and beamforming results may be increasingly degraded with increasing signal frequency. However, this sensitivity to inhomogeneities may be altered through use of an unconventional beamforming technique that manufactures higher frequency information by summing frequencies from lower-frequency signal components via a quadratic (or higher) product of complex signal amplitudes. This presentation will describe frequency-sum beamforming, and then illustrate it with simulation results and near-field acoustic experiments made with and without a thin plastic barrier between the source and the receiving array. The experiments were conduct...

Sound source localization technique using a seismic streamer and its extension for whale localization during seismic surveys

Marine seismic surveys are under increasing scrutiny because of concern that they may disturb or otherwise harm marine mammals and impede their communications. Most of the energy from seismic surveys is low frequency, so concerns are particularly focused on baleen whales. Extensive mitigation efforts accompany seismic surveys, including visual and acoustic monitoring, but the possibility remains that not all animals in an area can be observed and located. One potential way to improve mitigation efforts is to utilize the seismic hydrophone streamer to detect and locate calling baleen whales. This study describes a method to localize low frequency sound sources with data recoded by a streamer. Beamforming is used to estimate the angle of arriving energy relative to sub-arrays of the streamer which constrains the horizontal propagation velocity to each sub-array for a given trial location. A grid search method is then used to minimize the time residual for relative arrival times along the streamer estimated by cross correlation. Results from both simulation and experiment are shown and data from the marine mammal observers and the passive acoustic monitoring conducted simultaneously with the seismic survey are used to verify the analysis.

Comparing frequency-difference beamforming and delay-and-sum beamforming as an estimator of direction-of-arrival

Direction-of-arrival estimation (DOA) is an important and long-studied problem in array signal processing. Existing algorithms such as conventional delay-and-sum beamforming are limited in performance and become inadequate for a high-frequency broadband source and when the receiving linear array is sparse. Abadi, Song, and Dowling (2012), however, showed that DOAS estimation is possible in this case with an algorithm they introduced called frequency-difference beamforming. This was demonstrated empirically in their previous work using a real dataset. In addition, the resemblance between frequency-difference beamforming and delay-and-sum beamforming was briefly discussed. The purpose of this study is to analyze this relationship further and determine how well frequency-difference beamforming compares to delay-and-sum beamforming as an estimator of DOA. The approach taken in this study is to explicitly formulate the estimator equation for frequency-difference beamforming and compare it mathematically to the...

Baleen whale localization using a dual-line towed hydrophone array during seismic reflection surveys

Three dimensional seismic reflection surveys use multiple towed hydrophone arrays for imaging the structure beneath the seafloor. Since most of the energy from seismic reflection surveys is low frequency, their impact on Baleen whales may be particularly significant. To better mitigate against this potential impact, safety radii are established based on the criteria defined by the National Marine Fisheries Service. Marine mammal observers use visual and acoustic techniques to monitor safety radii during each experiment. However, additional acoustic monitoring, in particular locating marine mammals, could demonstrate the effectiveness of the observations, and improve knowledge of animal responses to seismic experiments. In a previous study (Abadi et al., 2014), data from a single towed seismic array was used to locate Baleen whales during a seismic survey. Here, this method is expanded to a pair of towed arrays and the locations are compared with an alternative method. The experimental data utilized in thi...

Studying underwater sound level caused by bridge traffic in Lake Washington

Underwater noise pollution due to human activities has greatly increased in recent years. There are several studies on high-intensity impulsive noises such as pile driving and seismic exploration. However, less is known about the effects of long-term exposure to low-intensity noises such as those due to bridge traffic. To characterize such noises, we have designed an underwater recording package. This package is equipped with a hydrophone to measure noise, a Raspberry-Pi to control recordings, and a cellular modem to stream near real-time data to the cloud. Data is collected in Lake Washington at two locations: (1) close to the Evergreen Point floating bridge across Lake Washington that connects Seattle to its eastern suburbs and (2) far from the bridge in the middle of the lake for comparison. The Evergreen Point floating bridge, the longest floating bridge in the world, is made of 77 large water-tight concrete pontoons and capable of carrying over 70,000 vehicles per day. The noise level measured close to this bridge is correlated with the public data on traffic load and wind speed available by the Washington State Department of Transportation.Underwater noise pollution due to human activities has greatly increased in recent years. There are several studies on high-intensity impulsive noises such as pile driving and seismic exploration. However, less is known about the effects of long-term exposure to low-intensity noises such as those due to bridge traffic. To characterize such noises, we have designed an underwater recording package. This package is equipped with a hydrophone to measure noise, a Raspberry-Pi to control recordings, and a cellular modem to stream near real-time data to the cloud. Data is collected in Lake Washington at two locations: (1) close to the Evergreen Point floating bridge across Lake Washington that connects Seattle to its eastern suburbs and (2) far from the bridge in the middle of the lake for comparison. The Evergreen Point floating bridge, the longest floating bridge in the world, is made of 77 large water-tight concrete pontoons and capable of carrying over 70,000 vehicles per day. The noise level measured close ...

Applying concepts and tools from signal processing on graphs (SPG) to problems in array signal processing

Signal processing on graphs (SPG) is an emerging area of research that extends well-established data analysis concepts and tools to support a special type of signal where data samples are defined on the vertices of a graph. Since SPG emerged in 2013, fundamental operations such as filtering, the Fourier transform, and modulation have been formally defined that uniquely consider and take advantage of the underlying complex and irregular relationship between data elements which is captured mathematically by a graph. The purpose of this study is to analyze the applicability of SPG to array signal processing. We show that signals defined on a graph, or graph signals for short, are natural models for data collected over a line array of sensors. We also apply existing SPG processing algorithms to array signal data and investigate and probe whether SPG can help increase array gain.