Natalia A. Sidorovskaia

Assessing the Deepwater Horizon oil spill impact on marine mammal population through acoustics: Endangered sperm whales

Long-term monitoring of endangered species abundance based on acoustic recordings has not yet been pursued. This paper reports the first attempt to use multi-year passive acoustic data to study the impact of the Deepwater Horizon oil spill on the population of endangered sperm whales. Prior to the spill the Littoral Acoustic Demonstration Center (LADC) collected acoustic recordings near the spill site in 2007. These baseline data now provide a unique opportunity to better understand how the oil spill affected marine mammals in the Gulf of Mexico. In September 2010, LADC redeployed recording buoys at previously used locations 9, 25, and 50 miles away from the incident site. A statistical methodology that provides point and interval estimates of the abundance of the sperm whale population at the two nearest sites is presented. A comparison of the 2007 and the 2010 recordings shows a decrease in acoustic activity and abundance of sperm whales at the 9-mile site by a factor of 2, whereas acoustic activity and abundance at the 25-mile site has clearly increased. This indicates that some sperm whales may have relocated farther away from the spill. Follow-up experiments will be important for understanding long-term impact.

Three-dimensional seismic array characterization study: experiment and modeling.

In the summer of 2003, the Littoral Acoustic Demonstration Center conducted an acoustic characterization experiment for a 21-element marine seismic exploration airgun array of total volume of 0.0588 m(3) (3590 in.(3)). Two Environmental Acoustic Recording System buoys, one with a desensitized hydrophone, were deployed at a depth of 758 m in a water depth of 990 m, near Greens Canyon in the Gulf of Mexico. Shots over a grid were recorded and calibrated to produce absolute broadband (up to 25 kHz) pressure-time dependencies for a wide range of offsets and arrival angles in the water column. Experimental data are analyzed to obtain maximum received zero-to-peak pressure levels, maximum received sound exposure levels, and pressure levels in 13-octave frequency bands for each shot. Experimental data are quantitatively modeled by using an upgraded version of an underwater acoustic propagation model and seismic source modeling packages for a variety of ranges and arrival angles. Experimental and modeled data show good agreement in absolute pressure amplitudes and frequency interference patterns for frequencies up to 1000 Hz. The analysis is important for investigating the potential impact on marine mammals and fish and predicting the exposure levels for newly planned seismic surveys in other geographic areas.

Broadband pulse signals and the characterization of shallow water oceanic properties

The new normal-mode method SWAMP (shallow water acoustical mode propagation) is used to investigate the high-frequency sound propagation in oceanic waveguides with pronounced ducts. Both the monochromatic and pulse signals are considered. It is shown that the time domain maps representing the arrived signal intensity relative to time and depth contain information to reconstruct the vertical sound velocity profile, source location, and bottom properties.

Modeling the three‐dimensional field of a seismic airgun array and comparison to 2003 measured data.

The full three‐dimensional field of a seismic airgun array is modeled using an enhanced parabolic equation run on a parallel cluster computer system, which is part of the Louisiana Optical Network Initiative network cluster. Source signatures are calculated using GUNDALF and NUCLEUS softwares. The calculated results are compared to available measurements collected in Green Canyon in the northern Gulf of Mexico in 2003. Three‐dimensional maps showing angular variation (both emission and azimuthal angles) and range dependence are generated, which show peak pressures, sound exposure levels, total shot energy spectra, and one‐third octave band analyses. [Research supported by the International Association of Oil and Gas Producers and the International Association of Geophysical Contractors.]

Unified acoustic model for simulating propagation and scattering effects in oceanic waveguides

The paper addresses the recent developments of Shallow Water Acoustic Modal Propagation (SWAMP) model (http://www.ucs.louisiana.edu/∼nxs7560/swamp.html) to account for the scattering events along the acoustic signal propagation path. The theoretical and numerical aspects of SWAMP extension to model the acoustic pulse scattering by a spherical elastic shell in an inhomogeneous oceanic waveguide within the T‐matrix approach are discussed. The algorithm utilizes the incident modal functions obtained by SWAMP for an empty waveguide in the spherical representation and the free‐field T‐matrix. The theoretical foundation for the approach is based on the work by Hackman and Sammelmann [J. Acoust. Soc. Am. 80, 1447–1458 (1986)]. The T‐matrix calculations are implemented in MATLAB software. The interpretation of the scattering event as the modal transformation through the T‐matrix is attempted. The numerical studies based on the new model are presented. [Work supported by the Louisiana Board of Regents Support Fund...

Spectrogram analysis of low to mid frequency marine mammal clicks

Previous investigators have proposed explanations for some sperm whale click structure and pointed out that the separation of individual pulses within the click might be used to determine approximately the size of the sperm whales. Recently, Mohl et al. [J. Acoust. Soc. Am. 114, 1124–1154 (2003)] have shown that echo‐location click structure is highly dependent on the received angle. In data measured by the Littoral Acoustic Demonstration Center using bottom‐moored hydrophones in the northern Gulf of Mexico in the summers of 2001 and 2002, rich click structures were observed in the spectrograms of many click trains, some of which exhibit strikingly consistent spectral nulls across the train. Although this structure in the spectra could be due to propagation effects, investigations to date suggest this possibility is highly unlikely, as discussed in the next abstract. Therefore it is at least plausible that the structure could be used to identify individual animals. This is known to be a difficult problem ...

Analysis of lethal and sublethal impacts of environmental disasters on sperm whales using stochastic modeling

Mathematical models are essential for combining data from multiple sources to quantify population endpoints. This is especially true for species, such as marine mammals, for which data on vital rates are difficult to obtain. Since the effects of an environmental disaster are not fixed, we develop time-varying (nonautonomous) matrix population models that account for the eventual recovery of the environment to the pre-disaster state. We use these models to investigate how lethal and sublethal impacts (in the form of reductions in the survival and fecundity, respectively) affect the population’s recovery process. We explore two scenarios of the environmental recovery process and include the effect of demographic stochasticity. Our results provide insights into the relationship between the magnitude of the disaster, the duration of the disaster, and the probability that the population recovers to pre-disaster levels or a biologically relevant threshold level. To illustrate this modeling methodology, we provide an application to a sperm whale population. This application was motivated by the 2010 Deepwater Horizon oil rig explosion in the Gulf of Mexico that has impacted a wide variety of species populations including oysters, fish, corals, and whales.

Localization to verify the identification of individual sperm whales using click properties.

Previous research to identify individual sperm and beaked whales from the properties of their echolocation and coda clicks using cluster analysis has been reported. Although reasonably consistent and robust results showing distinct classes (each corresponding to an individual) have been obtained with both self‐organizing maps and K‐means, no independent verification of these identifications has previously been available. However, data from a July 2007 experiment in the Gulf of Mexico should provide enough geometry information to verify some identifications. The Littoral Acoustic Demonstration Center deployed six environmental acoustic recording system buoys, measuring to 96 kHz for 9 days, to record sperm and beaked whale clicks in the northern Gulf of Mexico. Three buoys were in close proximity to enable multisensor detections of single clicks for possible localization, tracking, or bearing estimation sufficient to confirm the identification of individuals from cluster analysis. Displays of cluster class...

Source characterization study 2007: The three‐dimensional primary field of a seismic airgun array.

During September 2007 the Littoral Acoustic Demonstration Center collected acoustic and related data from three moored arrays and ship‐deployed hydrophones spanning the full water column to measure the 3‐D acoustic field of a seismic airgun array. A seismic source vessel shot a series of lines to give detailed angle and range information concerning the field of the primary arrival. The data were collected in the western Gulf of Mexico between the East Break and Alamos Canyon regions. Peak pressures, sound exposure levels, total energy spectra, one‐third octave band, and source directivity analyses are measures used to characterize the field. Three‐dimensional maps of these quantities are generated to show dependence on emission and azimuthal angles and range. Three‐dimensional visualizations are being developed using a visualization cave and software for 2‐D cave emulation. [Research supported by the Joint Industry Programme through the International Association of Oil and Gas Producers.]

The source characterization study 2007: A detailed three dimensional acoustic field measurement of a seismic airgun array.

In September 2007 the Littoral Acoustic Demonstration Center (LADC) collected acoustic and related data from three moored arrays and ship‐deployed hydrophones spanning the full water column to measure the three‐dimensional acoustic field of a seismic airgun array. The seismic source vessel shot a series of lines to give a detailed angle and range information concerning the field. The data were collected in the western Gulf of Mexico between the East Break and Alamos Canyon regions. Peak pressures, sound exposure levels, total shot energy spectra, and one‐third octave band analyses are measures used to characterize the field. Three dimensional maps of these quantities are generated to show dependence on emission and azimuthal angles and range. Both the direct and indirect fields are characterized. Moveout analysis is done to delineate arrivals and to detect ducted and interface waves. [Research supported by the International Association of Oil and Gas Producers.]