Steven E. Crocker

Geoacoustic Inversion of Ship Radiated Noise in Shallow Water Using Data From a Single Hydrophone

The Centre for Maritime Research and Experimentation conducted a geoacoustic inverse experiment in the Mediterranean Sea in the summer of 2012. Among the objectives was to employ an autonomous underwater vehicle to collect acoustic data to invert for properties of the seafloor. Inversion results for the compression wave speed in the bottom and the source spectrum of the R/V Alliance during a close approach to the bottom moored vehicle are presented. The estimated wave speed was 1529 m/s (σ=10). The source spectrum of the Alliance was estimated across more than six octaves of frequency.

Seabed characterization using ambient noise and compact arrays on an autonomous underwater vehicle.

Estimating the seabed geoacoustic properties at various fidelity levels has been a research topic for several decades. The majority of the applied seabed characterization techniques often require significant involvement of surface vessels, complex experimental setup, and human interaction. Technical advances in underwater autonomy and the development of energy efficient electronics provide new opportunities to optimize underwater environmental surveys in particular of the seabed. In 2012, the CMRE conducted the GLASS’12 experiment in the Mediterranean Sea with the objective to investigate the feasibility of utilizing a hybrid autonomous underwater vehicle equipped with a compact nose array for long-duration seabed characterization over large areas. The vehicle has the capability of operating in traditional propulsion and glider mode, and the nose-mounted array consists of a 5-element vertical and 4-element tetrahedral array. The sound sources used as information carrier were ambient noise, e.g., sea surfa...

Small boat localization using adaptive three-dimensional beamforming on a tetrahedral and vertical line array

Passive acoustic detection and localization of small surface craft has a number of practical applications, such as monitoring and protecting sensitive marine habitats. Moored passive equipment can be cumbersome to deploy and communicate with, so AUV-mounted devices are being investigated as an alternative. The GLASS’12 experiment was designed to assess the feasibility of using a hybrid autonomous underwater vehicle outfitted with a compact volumetric nose array as a data collection platform. The array consisted of five vertical elements and 4 in a tetrahedral arrangement, and the hybrid underwater vehicle had the capability operating in either glider or propeller-driven modes. The rigid design of the array minimized element location mismatch and enabled the use of aggressive adaptive beamforming in 3-D. This facilitated isolation of broadband multipath arrivals originating from the motor of a small rubber boat. Cross-correlation of beams enabled the time-lag between the arrivals to be measured, which, in ...

Nonlinear Inversion of Acoustic Scalar and Vector Field Transfer Functions

A study to investigate the use of the acoustic vector field, separately or in combination with the scalar field, to invert for geoacoustic properties of the seafloor was conducted. The analysis was performed in the context of the 2004 Sediment Acoustics Experiment (SAX04) conducted in the Northern Gulf of Mexico (GOM) where a small number of acoustic vector sensors were deployed in close proximity to the seafloor. The acoustic vector sensors were located both above and beneath the seafloor interface where they measured the acoustic pressure and the acoustic particle acceleration. A variety of acoustic waveforms were transmitted into the seafloor at normal incidence. Motion data provided by the buried vector sensors were affected by a suspension response that was sensitive to the mass properties of the sensor, the sediment density, and shear wave speed. The suspension response for the buried vector sensors included a resonance within the analysis band of 0.4-2.0 kHz. The response was sufficiently sensitive to the local geoacoustic properties, that it was integrated into the inverse methods developed for this study. Inversions of real and synthetic data sets showed that information about sediment shear wave speed was carried by the suspension response of the buried sensors, as opposed to being contained inherently within the vector acoustic field.

Environmental assessment of offshore wind power generation near Rhode Island: Acoustic and electromagnetic effects on marine animals.

An offshore wind farm is planned for Rhode Island coastal waters. The developer has proposed to deploy wind turbines in two stages: 5 turbines in shallow waters 5 km south of Block Island and 100 turbines in deeper waters 30 km to the east. As part of the planning of the proposed offshore wind power generation project under the Rhode Island Special Area Management Plan, ambient acoustic and electromagnetic measurements were made in the area. Two passive acoustic listener (PAL) systems were deployed within 4 km of Block Island from October 6 to November 11, 2008. Data from the PALs were used to compute the ocean acoustic noise budget and other statistics by source. Transmission loss measurements were also made to support the noise budget calculation. Measurements of airborne noise from a 1.5‐MW land‐based wind turbine already in operation in Rhode Island were made. To support the electromagnetic effect study, an underwater magnetometer was towed at the two proposed sites and over an operational underwater ...

On the use of acoustic particle motion in geoacoustic inversion.

Geoacoustic inversion estimates sediment properties based on one or more parameters of an observed acoustic field via inverse mathematical methods. The observed acoustic parameter is usually derived from the acoustic pressure measured at one or more locations. Recent advances in acoustic sensor technology have enabled the simultaneous measurement of the acoustic pressure and particle motion in three dimensions. The additional information provided by such acoustic vector sensors can be used to improve the performance of existing and novel geoacoustic inversion techniques. Current research seeks to use the additional information that is provided by the acoustic vector sensor to pose new inverse problems for the estimation of seabed sediment properties. In particular, data collected during the Sediment Acoustics Experiment 2004 (SAX04) included acoustic pressure and particle acceleration from a small number of acoustic vector sensors arranged in a vertical line, spanning the water‐sediment interface. A varie...

Geoacoustic inversion using specific acoustic impedance.

An inversion method for the estimation of geoacoustic properties of the ocean bottom using data from a small number of acoustic vector sensors was developed. The experimental data used for the inversions were short duration, gated continuous wave transmissions acquired on four acoustic vector sensors that spanned the water‐sediment interface during the Sediment Acoustics Experiment 2004 (SAX04) conducted in the Gulf of Mexico. The acoustic signals were analyzed for their specific acoustic impedance over a bandwidth ranging from 600 to 2400 Hz. The specific acoustic impedance was obtained from simultaneous measurements of the acoustic pressure and particle acceleration for arrivals at normal incidence. A differential evolution algorithm was used for the inversion of the depth dependent sediment properties. A wave number integration routine was used to compute the specific acoustic impedance corresponding to the estimated sediment parameters. Experiment details, forward modeling, inversion methods, and resu...

Effects of Pile Driving on the Residency and Movement of Tagged Reef Fish

The potential effects of pile driving on fish populations and commercial fisheries have received significant attention given the prevalence of pile driving occurring in coastal habitats throughout the world. Behavioral impacts of sound generated from these activities on fish typically have a greater area of influence than physical injury, and may therefore adversely affect a greater portion of the local population. This study used acoustic telemetry to assess the movement, residency, and survival of 15 sheepshead (Archosargus probatocephalus) and 10 grey snapper (Lutjanus griseus) in Port Canaveral, Florida, USA, in response to 35 days of pile driving at a wharf complex. No obvious signs of mortality or injury to tagged fish were evident from the data. Received sound pressure levels from pile strikes on the interior of the wharf, where reef fish primarily occur, were on average 152–157 dB re 1 μPa (peak). No significant decrease in sheepshead daytime residency was observed during pile driving within the central portion of the wharf and area of highest sound exposure, and no major indicators of displacement from the exposure wharf with the onset of pile driving were observed. There was evidence of potential displacement from the exposure wharf that coincided with the start of pile driving observed for 2 out of 4 grey snapper, along with a decrease in daytime residency for a subset of this species with high site fidelity prior to the event. Results indicate that snapper may be more likely to depart an area of pile driving disturbance more readily than sheepshead, but were less at risk for behavioral impact given the lower site fidelity of this species.

Broadband normal mode energy metrics in the presence of internal waves

During the Shallow Water 2006 experiment a J-15 acoustic source deployed from the Research Vessel Sharp transmitted broadband (100–500 Hz) chirp signals 15 km away from a vertical line array. The array was intentionally positioned near the shelf-break front and in an area where internal waves are known to occur. Normal mode decomposition helps provide clues regarding the physics behind signal fluctuations due to internal waves, but often analyses are accomplished at single frequencies. A broadband modal beamformer approach is offered that extracts separate modal arrivals. A method for a mode-dependent matched filter is suggested which helps extract the separate arrivals in a low-signal environment. These methods can be used to compute mode-independent energy statistics which help explain signal fluctuations as an internal wave traverses the source-receiver path. [Work sponsored by the Office of Naval Research.]

Propagation of impulsive broadband signals in a coastal ocean setting during the 1996 Shelfbreak Primer experiment

The 1996 Shelfbreak PRIMER experiment provided an opportunity to study the impulse response of a coastal ocean waveguide using broadband acoustic signals from explosive sources. Acoustic transmission paths in the vicinity of the shelfbreak included various orientations ranging from up‐slope to along‐slope, including interactions with a shelfbreak front. Acoustic receptions were acquired with a 16‐element vertical line array (VLA). In addition, thermistor string data acquired at the receiving array suggest that a group of high‐amplitude, internal waves traversed the test site during this portion of the experiment. Modal decomposition of broadband signal arrivals was performed to investigate variation of the impulse response during source deployments. Variations in the received spectra of individual normal modes were compared with numerical predictions for impulsive broadband signals propagating in range‐dependent environments constructed using oceanographic data available for the test site. The potential f...