Littoral Acoustic Demonstration Center Research Articles

Marine compressed air source array acoustic field characterization from at-sea measurements: Long-range propagationa).

In 2007, the Littoral Acoustic Demonstration Center conducted a comprehensive experiment in the northern Gulf of Mexico to measure the three-dimensional acoustic field of a standard marine compressed-air source array used in seismic exploration. This study aims to enhance understanding of long-range acoustic propagation of the array signals, with focus on variations in received sound pressure levels and sound exposure levels (SELs) at various ranges from the source. These variations are influenced by factors, such as receiver depth, array orientation, and propagation conditions. The long-range measurements show that received peak pressure levels and SELs exhibit non-monotonic (oscillatory) behavior with range leading up to 10 dB increase in received levels at longer ranges. At ranges beyond 20 km, acoustic levels at the shallowest hydrophone consistently surpassed those at deeper ones by 3-10 dB, suggesting the impact of surface duct propagation effects. The results demonstrate that range-independent bathymetry leads to approximately 4 dB higher received acoustic levels than range-dependent propagation conditions. The measured long-range propagation acoustic metrics from controlled experiment provide a unique and critical dataset for validating both source and propagation model accuracy in predicting received sound pressure and SEL in the far-field of the source array.

Littoral Acoustic Demonstration Center passive acoustic localization and tracking of marine mammals in the Northern Gulf of Mexico

The Littoral Acoustic Demonstration Center (LADC) consortium, founded in 2000, has collected underwater acoustic data in the northern Gulf of Mexico since 2001 using Environmental Acoustic Recording Systems (EARS) buoys. These hydrophone systems were developed by the U.S. Navy, and originally LADC could record underwater signals only up to 11.7 kHz. Since then the EARS have been upgraded several times and currently record up to 192 kHz, thus allowing recordings of not only sperm whale clicks and codas, but also signals of beaked whales and dolphins. One goal of LADC is to develop an acoustic method for identification of individual animals, with the goal of an “acoustic catalog” similar to the existing photographic catalog of individual whale flukes. Localization and tracking is an important component for acoustic identification. Some localization and tracking techniques and procedures developed and used by LADC will be presented and discussed.

George Ioup’s contribution to the Gulf of Mexico acoustic research: paving the path into the future

By the end of the 1990s, researchers and regulators recognized the need for understanding how anthropogenic activities impact cetacean’s populations in the Gulf of Mexico. In 2000, George Ioup was one of the founders of the Littoral Acoustic Demonstration Center (LADC), a consortium of Gulf Coast scientists, with the long-term goal of studying the anthropogenic soundscapes of the Gulf of Mexico and their impact on marine mammals. In 2001, LADC was the first team to collect long-term acoustic data rich in sperm whale phonations from bottom-moored autonomous buoys, technology developed by NAVOCEANO and adapted for LADC needs. The first step in establishing a baseline database was taken. At the same time, George sparked the interest of bioacousticians with ideas on how to employ the differences in sperm whale phonations to identify individuals similar to how humans recognize voices. LADC endeavors continued through designing acoustic surveys to characterize the soundscapes of seismic exploration arrays and being the first team to record beaked whales in the Gulf of Mexico in 2007. Recent advances in understanding marine mammal habitats, studying the oil spill impact on these animals, and introducing new acoustic technologies would not be possible without the seminal work of George Ioup.

Bottom characterization in the northern Gulf of Mexico using chirp sonar and ship noise data

As a part of Prof. George Ioup’s multi-institutional research initiative, chirp sonar bottom surveys were conducted near the Mississippi Canyon in the northern Gulf of Mexico during the Littoral Acoustic Demonstration Center (LADC) 2001 experiment. Bottom geoacoustic properties were estimated using a shallow-towed 2–12 kHz chirp sonar that provided sub-meter resolution and 60 m penetration. The inversion results showed that the experimental site is mainly covered with clayey sediment. In 2010, a second bottom characterization experiment was conducted at ∼25 miles NE of the LADC 2001 experiment site using a deep-towed chirp sonar. Deep-towed chirp sonar provided geoacoustic property estimations as well as low-grazing-angle bottom-loss measurements. As a source of opportunity, own-ship noise data were also used for geoacoustic parameter estimation and bottom-loss measurements. Geoacoustic parameter estimations and bottom loss measurements from the chirp-sonar and ship-noise data provided similar results that were also in agreement with those from the LADC 2001 experiment. [Work supported by ONR.]

Species-level classification and clustering of beaked whale echolocation recordings

The Littoral Acoustic Demonstration Center—Gulf Ecological Monitoring and Modeling (LADC-GEMM) consortium has collected passive acoustic monitoring data in the northern Gulf of Mexico since 2001. Recordings were made in 2007 near the Deepwater Horizon oil spill, which have provided a baseline for an extensive study of regional marine mammal populations in response to the disaster. Beaked whales are of particular interest as they remain one of the least understood groups of marine mammals, and relatively few abundance estimates exist. Efficient classification and clustering algorithms are demanded for mining the long-term passive acoustic data. Three algorithms using k-means, self-organizing maps, and spectral clustering are tested with various features of detected echolocation transients. Several methods are observed to effectively isolate recorded echolocation clicks of regional beaked whale species. The waveform fractal dimension is introduced as a feature for marine biosonar classification and shown to improve accuracy. A feedforward neural network classifier is also evaluated and shows performance with high accuracy under various noise conditions. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Passive acoustic localization and tracking using synthetic data for the Northern Gulf of Mexico

Passive acoustic localization and tracking of marine mammals shows potential for future acoustic monitoring efforts. The Littoral Acoustic Demonstration Center—Gulf Ecological Monitoring and Modeling (LADC-GEMM) project collected underwater acoustic data in the northern Gulf of Mexico during the summer of 2015 using Environmental Acoustic Recording Systems (EARS) buoys, returning to sites previously surveyed by LADC. The localization and tracking method developed for the EARS hydrophones uses Monte-Carlo based simulations based on this data. Synthetic whale clicks at random times along a sinusoidal path were propagated to the EARS receivers and the location of the synthetic source was tracked. Several whale clicks in a series determine the most probable path of the animal. The localization/tracking method and relative localization are adaptable for variables such as site location, mooring locations, number of moorings, depth, etc. Localization of two synthetic moving whales in three-dimensional space will be demonstrated. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Calculating sperm whale lengths in the Northern Gulf of Mexico

Sperm whales (Physeter macrocephalus) produce short duration acoustical clicks while diving to search for food, each click composed of several pulses. The time interval between consecutive pulses, the interpulse interval (IPI), can be used to estimate the length of the whale. IPIs have been estimated from whales of unknown orientation by processing several hundred clicks and averaging the results. In this work, discrete wavelet transforms and autocorrelation-based methods are used to obtain IPI estimates with improved consistency. The Littoral Acoustic Demonstration Center—Gulf Ecological Monitoring and Modeling (LADC-GEMM) project collected underwater acoustic data in the northern Gulf of Mexico during the summer of 2015. Results presented here are produced using data recorded by the LADC Environmental Acoustic Recording Systems (EARS) at the site closest to the BP oil spill. Calculated lengths can be compared to lengths from previous measurements as well as to future measurements in order to determine changes in average whale size, useful in determining population health. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Chaos theory and topology for classification of marine mammals

The Littoral Acoustic Demonstration Center–Gulf Ecological Monitoring and Modeling (LADC-GEMM) project collected underwater acoustic data in the northern Gulf of Mexico during the summer of 2015, with passive acoustic data recorded by the LADC-GEMM Environmental Acoustic Recording Systems (EARS). Such data have been widely studied in the past by Fourier/spectral analysis. Although highly useful, there are, however, novel perspectives that may be obtained with techniques from chaos theory and topology. Using a method known as time-delay embedding, a time-series manifold can be constructed from measured data. This manifold will be classified in terms of a topological score (measures of connectedness, compactness, etc.), in conjunction with its fractal and correlation dimensions. These combined features can potentially be used to produce a more robust classification means for the identification of marine mammals heard by the EARS buoys in the LADC-GEMM data. [This research was made possible in part by a grant from The Gulf of Mexico Research Initiative, and in part by an Internal Grant from the University of New Orleans Office of Research and Sponsored Programs. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Sound speed profile calculations for the Northern Gulf of Mexico

Many sound speed calculations for the Gulf of Mexico use an average sound speed over the entire Gulf. In a variety of applications, such as localization or tracking of marine mammals, a more precise sound speed profile is helpful to make more accurate calculations. Using a reasonable approximation to Del Grosso’s equation, the sound speed profile may be computed as a function of temperature, pressure, and salinity. The Littoral Acoustic Demonstration Center—Gulf Ecological Monitoring and Modeling (LADC-GEMM) project collected underwater acoustic data in the northern Gulf of Mexico during the summer of 2015 using Environmental Acoustic Recording Systems (EARS), returning to sites previously surveyed by LADC. Oceanographic data were also collected at those sites and can be used for more accurate sound speed profiles at the specific locations and specific times of the acoustic data recordings, increasing the accuracy of subsequent calculations such as real-time tracking and other applications. Sound speed profiles for sites in the northern Gulf of Mexico will be presented. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Engaging high school students in studying marine mammals observed near the BP oil spill

The Littoral Acoustic Demonstration Center—Gulf Ecological Monitoring and Modeling (LADC-GEMM) project partnered with Warren Easton Charter High School to analyze underwater acoustic data collected in the northern Gulf of Mexico during the summer of 2015, returning to sites previously surveyed by LADC. Results presented here are produced using data recorded by Environmental Acoustic Recording Systems (EARS) at the site closest to the BP oil spill. The aim of the outreach project described here is for high school students to learn to work independently using scholarly works to analyze and interpret data, as well as to communicate and collaborate in a professional environment. Various marine mammals have been detected, including sperm whales, beaked whales (Cuvier, Blainville, Gervais, and BWG), and dolphins (bottlenose, Rizzo). The numbers and types of marine mammals are possible indicators of the health of the area and the marine mammals around the oil spill. Changes will indicate possible future problems. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Population densities of dolphins in the Northern Gulf of Mexico

The Littoral Acoustic Demonstration Center–Gulf Ecological Monitoring and Modeling (LADC-GEMM) consortium collected underwater acoustic data in the northern Gulf of Mexico during the summer of 2015, returning to sites previously surveyed by LADC (2007, 2009, and 2010). Results presented here are produced using data recorded by the LADC-GEMM Environmental Acoustic Recording Systems (EARS) at the site closest to the BP oil spill, specifically for Rizzo, bottlenose, Clymene, and Pantropical-Spotted dolphins. Clustering and trained classifier techniques were used to isolate dolphin species. A population model developed by LADC-GEMM based on acoustic data collected from the EARS buoys is used to estimate the strength of delphinid recovery in the northern Gulf of Mexico after the BP oil spill. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Future deployment corrections for passive acoustic localization and tracking using real data for the Northern Gulf of Mexico

The Littoral Acoustic Demonstration Center–Gulf Ecological Monitoring and Modeling (LADC-GEMM) project collected underwater acoustic data in the northern Gulf of Mexico during the summer of 2015 using Environmental Acoustic Recording Systems (EARS) buoys. A localization and tracking method developed for the EARS hydrophones uses Monte-Carlo based simulations based on these data but was only tested using synthetic whale clicks at random times along a sinusoidal path. The location of the synthetic source was tracked. Real data obtained in 2015 could not be used by the method developed due to excessive clock drift between moorings with no way to correct the phase differences. It will be shown, using the geometry of the site and acoustic theory, how a simple, low powered, periodic acoustic ping can aid in correcting the phase difference of the clocks after significant time has passed. [This research was made possible by a grant from The Gulf of Mexico Research Initiative. Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org.]

Long-term spatially distributed observations of deep diving marine mammals in the Northern Gulf of Mexico using passive acoustic monitoring

This paper will present the results of processing long-term passive acoustic monitoring (PAM) data collected July through October 2015 in the Northern Gulf of Mexico in the vicinity of the Deep Water Horizon oil spill site to aid in understanding factors driving the distribution of sperm and beaked whales in the Gulf of Mexico. The Littoral Acoustic Demonstration Center -Gulf Ecological Monitoring and Modeling Consortium (LADC-GEMM) deployed five bottom-anchored acoustic moorings (LADC EARS buoys) at 10, 25, and 50 nmi distance from the 2010 oil spill location. Autonomous surface vehicles and a glider were simultaneously operated in the area for the additional collection of PAM data. The daily and monthly activity of three species of beaked whales exhibits spatial and seasonal variability, which appear to be correlated with levels of anthropogenic noise at the monitoring sites. Acoustic detection data are used to estimate abundances at three sites and compare them to the estimates obtained from baseline data collected before (2007) and right after (2010) the oil spill. Long-term abundance trends for both beaked and sperm whales are discussed. [Research supported by GOMRI.]

Classification of beaked whale and dolphin clicks measured by environmental acoustic recording system buoys in the northern Gulf of Mexico

The Littoral Acoustic Demonstration Center (LADC) has used its Environmental Acoustic Recording System (EARS) buoys to record sperm and beaked whales and dolphins, with frequencies up to 96 kHz, in the northern Gulf of Mexico in 2007 and 2010. The 2007 experiment was the first to record beaked whales in the Gulf. It has been found that there is considerable overlap in the band of beaked whale signals from 20 to 60 kHz with deep-water dolphin clicks, so traditional energy-band detectors have a high occurrence of false positives. Although acoustic measurements in this frequency range validated by visual observations have been limited, for the Gulf of Mexico species, progress is being made in automatically delineating clicks that belong to beaked whale species observed in the Gulf and those originating from dolphins. Spectrograms of the classified clicks are shown and compared to known spectrograms for beaked whale and dolphin species. Many of the spectrograms show an upsweep in the observed spectrum but others do not. Improved classifiers can provide higher accuracy estimates of the regional abundance trends and effects of environmental changes on both beaked whale and dolphin groups. [Research supported by BP/GOMRI, SPAWAR, ONR, NSF, and Greenpeace.]

Mixed-methods analytic approach for determining potential impacts of vessel noise on sperm whale click behavior.

The Gulf of Mexico is a center of marine activities from seismic exploration to shipping, drilling, platform installation, lightering, and construction, among others. This analysis explored whether sperm whales respond to the passage of vessels using changes in total number of clicks during vessel passages as a proxy for potential variation in behavior. The data for this analysis were collected in 2001 as part of a larger Littoral Acoustic Demonstration Center project using the Environmental Acoustics Recording System buoys. These buoys were bottom moored, autonomous, and self-recording systems consisting of an omni-directional hydrophone and instrument package. Data from 36 days of continuous acoustic monitoring were recorded at a sampling rate of 11.725 kHz, and produced reliable recordings from 5 Hz to ∼5.8 kHz. Multiple preparatory steps were executed including calibration of an automatic click detector. Results indicate a significant decrease (32%) in the number of clicks detected as a ship approached an area. There were also significantly fewer clicks detected after the vessel passed than before (23%).

Prospects for short-term and long-term passive acoustic monitoring of environmental change impact on marine mammals

The Littoral Acoustic Demonstration Center, LADC, a consortium of scientists from four Gulf state universities and the U.S. Navy, was begun in 2001 to study underwater noise and acoustic propagation, and the impact of human activities in the ocean on marine mammals, with emphasis on the Gulf of Mexico (GoM) region. LADC has a library of broadband passive acoustic data, collected by autonomous bottom-moored buoys, which sampled the GOM region ambient noise state, seismic airgun array emissions, and/or marine mammal activities six times during the last decade. LADC acoustic data represent an opportunity to study the short- and long-term effects of environmental changes on the marine mammal population. Environmental factors include baseline anthropogenic noise levels, passages of tropical storms, seismic exploration surveys in the area, and the 2010 Deepwater Horizon oil spill accident. The talk summarizes recent findings on the relationship between regional population dynamics of sperm and beaked whales and abrupt environmental changes with emphasis on the recent GoM oil spill. Statistically significant results of the study suggest a need for establishing consistent acoustic monitoring protocols in the oceanic areas of current or potential industrial activities. [Past data acquisitions were supported by ONR, SPAWAR, JIP, NSF, and Greenpeace.]

Identification of individual beaked whales in the northern Gulf of Mexico

Recent Littoral Acoustic Demonstration Center (LADC) multi-mooring Environmental Acoustic Recording System (EARS) data from the northern Gulf of Mexico are analyzed to deduce identifications of individual beaked whales. Procedures are built on previously applied self-organizing map techniques for clustering sperm whale clicks and beaked whale clicks from workshop data. Associating the clicks from beaked whales is difficult because, compared to sperm whales, beaked whales have lower source level, a narrower beam, and a faster rate of turning. Recordings of individual clicks can be clustered according to their time domain signal, frequency spectrum, or wavelet spectrum. For example, clicks clustered according to the magnitude of their frequency components show similarities for all the clicks in a class (representing an individual), but significant differences from class to class, suggesting that this approach has promise for identifying individuals. Recent work by Baggenstoss (J. Acoust. Soc. Am. 130, 102–112 (2011); J. Acoust. Soc. Am. 133, 4065–4076 (2013)), who has used cross correlations of clicks to assist in associating beaked whale clicks into trains, reinforces the idea that single click properties can be associated with individuals. [Research supported by SPAWAR and ONR.]

Three-dimensional primary acoustic field characterization for a seismic airgun array

The Littoral Acoustic Demonstration Center conducted the Source Characterization Study in 2007 (SCS07) to measure the 3-D acoustic field of a seismic airgun array in the Gulf of Mexico. Three moorings with sensitive and desensitized hydrophones at different depths were deployed as well as hydrophones suspended from a ship, while a seismic source vessel shot specified lines. Hydrophone positions were measured. Peak pressures, RMS sound pressure levels (SPL), sound exposure levels, total shot energy spectra, one-third octave band analyses, and source directivity studies are used to characterize the field. Summary results are first calculated for each hydrophone. These are then combined to give isopleths for azimuthal cuts at 0, 45, 90 degrees, etc., for the spatial domain measures. Plots for each solid angle bin give these and frequency measures analyzed versus range. Zero-to-peak pressures directly under the array go from 210 dB for depths less than 200 m down to 195 dB at 1200 m. At 2000 m horizontal range the pressures go from 160 dB near the surface to 175 to 180 dB at 1200 m. RMS SPL is about 5 dB smaller. [Research supported by the Joint Industry Programme through the International Association of Oil and Gas Producers.]

Comparison of the first-year response of beaked and sperm whale populations to the Northern Gulf oil spill based on passive acoustic monitoring

This paper continues a discussion on using passive acoustic methods to study the environmental impact of the recent oil spill in the Northern Gulf of Mexico on resident populations of marine mammals. The Littoral Acoustic Demonstration Center, possessing several broadband acoustic datasets collected near the spill site before and after the event, is in a unique position for monitoring long-term environmental impacts in the vicinity of the incident The pre-spill recordings provide a baseline which, when combined with post-spill measurements, give important indicators of changes in the local populations. Ackleh et al., J. Acoust. Soc. Am. 131, 2306-2314, provide a comparison of 2007 and 2010 measurements showing a decrease in acoustic activity and abundance of sperm whales at the 9-mile distant site, whereas acoustic activity and abundance at the 25-mile distantsite has clearly increased. This may indicate that some sperm whales have relocated farther away from the spill subject to food source availability. This paper reports on applying developed population estimation techniques to monitor beaked whale response that appears to be different from that of sperm whales. Follow-up experiments will be critical for understanding the long-term impact on different species. [Research supported by SPAWAR, NSF, and Greenpeace.]

The measured 3-D primary acoustic field of a seismic airgun array

The Littoral Acoustic Demonstration Center has conducted an experiment to measure the 3-D acoustic field of a seismic airgun array in the Gulf of Mexico. A seismic source vessel shot specified lines to give solid angle and range information. Hydrophone positions were measured by an ultra-short baseline (USBL) acoustic system while the source ship was turning between lines. An acoustic Doppler current profiler measured currents so the positions could be modeled between USBL measurements. The position locations were refined by using information from the acoustic arrival times on the hydrophones. Peak pressures, sound exposure levels, total shot energy spectra, one-third octave band analyses, and source directivity studies are used to characterize the field. One third octave band analysis shows received levels up to 180 dB re 1 µP for emission angles from 0 degrees (vertically down) up to 45 degrees for horizontal ranges up to 200 m at endfire, between 10 Hz and 200 Hz. The levels decrease with increasing frequency above 200 Hz, with increasing horizontal ranges, and for emission angles above 45 degrees. The levels are lower at broadside than at endfire. [Research supported by the Joint Industry Programme through the International Association of Oil and Gas Producers.]