Jennifer Giard

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 ...

A measurement system for shear speed using interface wave dispersion

Recent research has highlighted the effect of shear on compressional wave attenuation and its frequency dependence and modal arrival times near the Airy Phase. Shear properties of sediments are directly related to the strength of the sediments in geotechnical applications. One of the most promising approaches to estimate shear speed is to invert the shear speed profile using the dispersion of seismo-acoustic interface (Scholte) waves that travel along the sediment-water boundary. A shear measurement system is being developed at the University of Rhode Island based on this concept and data collected during a sea-test is presented along with preliminary results.

Monitoring the acoustic effects of pile driving for the first offshore wind farm in the United States

The Block Island Wind Farm, the first offshore wind farm in the United States, consists of five turbines in water depths of approximately 30 m. The turbines have a jacket type substructure with piles driven to the bottom to pin the structure to the seabed. A number of acoustic sensors were deployed to monitor the acoustic properties of the pile driving activity. The acoustic sensor systems consisted of an eight element towed hydrophone array, two fixed moorings with four hydrophones each, and a fixed sensor package for measuring particle velocity. The towed array was towed from 1 to 8 km from the pile driving location. The fixed moorings were deployed at 7.5 and 15 km from the pile location. The particle velocity sensor package was deployed at 500 m from the pile driving location. This sensor package consisted of a three-axis geophone on the seabed and a tetrahedral array of four low sensitivity hydrophones at 1 m from the bottom. Data collected on these sensor systems will be presented. Acoustic levels a...

Inversion of shear wave speed in coastal sediments using interface waves

Shear speeds in semi-consolidated and consolidated shallow water sediments can significantly impact compressional wave attenuation and arrival times of acoustic normal modes. In addition shear properties of sediments are directly related to the strength of the sediments in geotechnical applications. All of these factors emphasize the importance of estimating shear speeds in shallow water sediments. One of the most promising approaches to estimate shear speed is to invert the shear speed profile using the dispersion of interface waves (Scholte waves). Interface wave data from a small scale experiment conducted in very shallow water in coastal Rhode Island will be presented. The University of Rhode Island’s shear measurement system consisting of vertical axis and 3-axis geophones were used to collect data in 3 m of water. Interface waves were excited by dropping a weight from a research vessel. Modeling of interface waves will be carried out using Finite Element Method (FEM) and a dynamic stiffness matrix m...

Explosive offshore structure removal noise measurements

The underwater structures that support wind turbines and oil drilling rigs must eventually be removed. Explosive severing is a commonly used removal method in which charges are inserted into a pile and placed below the seafloor to sever the pile. The open-water source level of an explosive charge is readily determined from its composition and weight (Urick 1986). However, the sediment and pile absorb much of the explosion’s energy. A recent study (BSEE project M13PX00068) measured explosive removals in the Gulf of Mexico. Peak pressure, impulse and energy flux density metrics were measured with a 12-element, two-dimensional array spanning 90 ft vertically and at distances out to 200 ft. Peak amplitudes, compared with theoretical open-water predictions, were reduced from 76% (75 lb charges) to 54% (200 lb charges). Measured results were also compared to predictions from the UnderwaterCalculator (Dzwilewski and Fenton 2003) that included pile and sediment attenuation effects. It accurately predicted the pro...

Visualization of spatially explicit acoustic layers in an underwater soundscape

Soundscapes emphasize the way in which the acoustic environment is perceived by an individual or species. Anthropogenic sounds have the potential to cause behavioral responses or even injury in marine mammals. Exposure criteria include sound pressure level (SPL) and sound exposure level (SEL) thresholds that, when exceeded, have the potential to cause auditory injury to marine mammals. While there is a general understanding of the hearing capabilities in representative marine species, the behavioral responses of these animals are not well understood. These responses are thought to be strongly affected by the context of exposure (animal activity at time of exposure, habituation/sensitization to the sound, etc.) in addition to acoustic metrics, such as received level (RL). To investigate the complexity of sound exposure, a user-friendly, interactive tool has been developed that utilizes a holistic and multi-dimensional approach to the exposure scenario in order to address the spatial relationships among noi...

Exposures v. Individuals: Effects of varying movement patterns and animal behavior on long-term animat model exposure predictions

Predictions of animal exposure to anthropogenic acoustic sources have become increasingly sophisticated through simulating animal behavior. A long-standing issue has been the question of how many exposures occur during an activity and how those exposures are distributed over individual animals. A sensitivity study evaluated the effects of simulation duration, source movement, animal movement and group size. Two airgun array survey patterns (2D and 3D) were modeled, each with a one-month duration. During each simulation, animal movement was modeled for low-frequency, shallow and deep diving mid-frequency, and high-frequency cetaceans. The unweighted 160 dB RMS exposure threshold for behavior was used to evaluate the effect of different modeling parameters. Results found that simulating animals in groups does not alter the predicted level of exposure, but it does increase its variance. Examining the full 30 day exposure records found that deeper divers had a greater number of exposures. The more wide-rangin...

Acoustics program at the University of Rhode Island

The undergraduate and graduate program in Ocean Engineering at the University of Rhode Island is one of the oldest such programs in the United States. This program offers Bachelors, Masters (thesis and non-thesis options), and Ph.D. degrees in Ocean Engineering. The Ocean Engineering program has a strong acoustic component both at the undergraduate and graduate level. At the graduate level, students can specialize in several areas including geoacoustic inversion, propagation modeling, marine mammal acoustics, ocean acoustic instrumentation, transducers, etc. Current acoustics related research activities of various groups will be presented. Information regarding the requirements of entry into the program will be provided. Many graduates from the program hold faculty positions at a number of universities in the United States and abroad. In addition, graduates from the ocean acoustics program at URI are key staff at many companies and organizations. The opportunities and challenges facing the program will be...

Shear wave inversion in a shallow coastal environment

Estimation of the shear properties of seafloor sediments in littoral waters is important in modeling the acoustic propagation and predicting the strength of sediments for geotechnical applications. One of the promising approaches to estimate shear speed is by using the dispersion of seismo-acoustic interface (Scholte) waves that travel along the water-sediment boundary. The propagation speed of the Scholte waves is closely related to the shear wave speed over a depth of 1–2 wavelengths into the seabed. A geophone system for the measurement of these interface waves, along with an inversion scheme that inverts the Scholte wave dispersion data for sediment shear speed profiles have been developed. The components of this inversion scheme are a genetic algorithm and a forward model which is based on dynamic stiffness matrix approach. The effects of the assumptions of the forward model on the inversion, particularly the shear wave depth profile, will be explored using a finite element model. The results obtained from a field test conducted in very shallow waters in Davisville, RI, will be presented. These results are compared to historic estimates of shear speed and recently acquired vibracore data. [Work sponsored by ONR, Ocean Acoustics.]