Peter J. Stein

The effects of high-intensity, low-frequency active sonar on rainbow trout

This study investigated the effects on rainbow trout (Oncorhynchus mykiss) of exposure to high-intensity, low-frequency sonar using an element of the standard Surveillance Towed Array Sensor System Low Frequency Active (LFA) sonar source array. Effects of the LFA sonar on hearing were tested using auditory brainstem responses. Effects were also examined on inner ear morphology using scanning electron microscopy and on nonauditory tissues using general pathology and histopathology. Animals were exposed to a maximum received rms sound pressure level of 193 dB re 1 microPa(2) for 324 or 648 s, an exposure that is far in excess of any exposure a fish would normally encounter in the wild. The most significant effect was a 20-dB auditory threshold shift at 400 Hz. However, the results varied with different groups of trout, suggesting developmental and/or genetic impacts on how sound exposure affects hearing. There was no fish mortality during or after exposure. Sensory tissue of the inner ears did not show morphological damage even several days post-sound exposure. Similarly, gross- and histopathology observations demonstrated no effects on nonauditory tissues.

Interpretation of a few ice event transients

This article discusses some of the observables important to the study of sound radiated into the water by ice fracturing events. Such studies are important in order to understand the mechanisms of Arctic Ocean ambient noise generation. Theoretical work for a homogeneous ice plate indicates that there are three important paths of elastic wave propagation over which energy travels away from an ice event source. These are the flexural wave in the ice, the longitudinal wave in the ice, and the acoustic wave in the water. Each has a distinctive propagation speed, attenuation, and associated pressure field in the water. The experimental work, consisting of the interpretation of four ice events, supports the theoretical description. In each case the event is located and the contributing wave paths identified. In all cases the acoustic wave dominates. The work also demonstrates how event analysis can be used to determine ice properties.

The Kauai Experiment

The Kauai Experiment was conducted from June 24 to July 9, 2003 to provide a comprehensive study of acoustic propagation in the 8–50 kHz band for diverse applications. Particular sub‐projects were incorporated in the overall experiment 1) to study the basic propagation physics of forward‐scattered high‐frequency (HF) signals including time/angle variability, 2) to relate environmental conditions to underwater acoustic modem performance including a variety of modulation schemes such as MFSK, DSSS, QAM, passive‐phase conjugation, 3) to demonstrate HF acoustic tomography using Pacific Missile Range Facility assets and show the value of assimilating tomographic data in an ocean circulation model, and 4) to examine the possibility of improving multibeam accuracy using tomographic data. To achieve these goals, extensive environmental and acoustic measurements were made yielding over 2 terabytes of data showing both the short scale (seconds) and long scale (diurnal) variations. Interestingly, the area turned out...

Gray whale target strength measurements and the analysis of the backscattered response

One of the current Integrated Marine Mammal Monitoring and Protection System (IMPAS) directions is concentrated on the design and development of the active sonar modality representing just one component of the global system. The active sonar was designed, built, and tested during the Marine Mammal Active Sonar Test (MAST 04), producing whale detections and whale tracks. The experiment was conducted in January 2004 off the coast of California. One of the objectives of the current work is to distinguish whale backscattered responses from the ones generated by the environmental clutter in a waveguide. Furthermore, the work aims to identify and analyze the target signature features that are necessary for enhanced active sonar detection and classification of marine mammals. Over the years there have been very few documented attempts to capture and analyze the backscattering response of whales using an active sonar system. Nevertheless, whales, mostly owing to their size, their motion, and the aspect dependence of their backscattered field, possess desirable properties that help distinguish their scattered response from clutter and other environment related false alarms. As an initial step, data collected during the MAST 04 experiment are presented, and gray whale target strength measurements are obtained. Results are compared to the previously published whale target strengths. Additionally, an investigation is conducted in an effort to provide whale feature identification points suitable for automated detection and classification, as means of relating gray whale active acoustic signatures to their inherent characteristics and their motion.

Model-oriented ocean tomography using higher frequency, bottom-mounted hydrophones.

A tomographic scheme is presented that ingests ocean acoustic measurements into an ocean model using data from bottom-mounted hydrophones. The short distances between source-receiver pairs (1-10 km) means arrival times at frequencies of 8-11 kHz are readily detectable and often distinguishable. The influence of ocean surface motion causes considerable variability in acoustic travel times. Techniques are presented for measuring travel times and removing the variability due to surface waves. An assimilation technique is investigated that uses differences in measured and modeled acoustic travel times to impose corrections on the oceanographic model. Equations relating travel time differences to oceanographic variables are derived, and techniques are presented for estimating the acoustic and ocean model error covariance matrices. One test case using a single source-receiver pair shows that the tomographic information can have an impact on constraining the solution of the ocean circulation model but can also introduce biases in the predictions. A second test case utilizes knowledge of a bias in a model-predicted variable to limit grid cells that are impacted by the tomographic data. In this case, using the tomographic data results in significant improvements in the model predictions without introducing any biases.

Observations and modeling of thermally induced stresses in first-year sea ice

During spring 1992, ice property, geophone, meteorological, and stress data were collected on first-year ice southwest of Cornwallis Island within the Canadian archipelago. One of the goals of the study was to specify the average characteristics of the ice, use these characteristics in a model of thermally induced stresses in the ice, and examine the fracturing associated with the occurrence of those stresses. The results of simulations with a thermal stress model indicate that stress variations within the ice can be reasonably approximated by a viscoelastic rheology. The rheology takes into consideration thermally induced strains generated locally as well as strains generated elsewhere and then mechanically transmitted through the ice. The geophone data showed both ice-borne and water-borne propagation paths for individual fracturing events. The data imply a detection radius out to 500–600 m for the ice-borne signatures of fractures. An investigation of a region after fracturing showed that (1) fracturing occurred in an area with a 10- to 15-cm snow cover, (2) the snow cover had been scored down to the surface of the ice, and (3) cracks in the ice were found under each location where the snow had been scored. The cracks were 5–6 m long and at least 15 cm deep. A review of these and other experimental results draw us to the conclusion that the forces required to produce fractures in response to natural forcing is greater for first-year floes than for multiyear floes.

Inversion of pack ice elastic wave data to obtain ice physical properties

Geophones mounted on floating ice sheets can potentially provide data for a robust method to monitor mechanical properties of the ice. The technique relies on the measurement and inversion of low-frequency elastic waves propagating in the ice. To test this hypothesis, several geophone systems were deployed on the Arctic ice as part of the Office of Naval Research Sea Ice Mechanics Initiative (SIMI), including a winter-over system of 20 triaxial geophones at the fall 1993 SIMI camp. An inversion technique is discussed here through references to the literature and by analysis of geophone data, specifically those data collected on “clean” first-year ice off Resolute Bay, Canada, and on highly irregular multiyear ice found at the fall SIMI camp. For undeformed first-year ice, the inversion technique gave an estimate of ice properties that agreed well with known values, although extensive work remains to determine the effects of anisotropy and inhomogeneity. The geophone data for the nonuniform ice at the SIMI camp were very complex and difficult to analyze. The inversion yielded only trends in what can, at best, be termed effective ice thickness and bending rigidity; it is uncertain how these relate to actual ice properties. The SIMI geophone system demonstrated the feasibility of performing autonomous measurements of the characteristics of propagating waves in the Arctic ice; with continued study, it seems evident that such systems can be used in clean first-year ice to monitor ice properties. However, extensive research is required to make this technique useful in multiyear or complex first-year ice.

Pressure gradient sensors for bearing determination in shallow water tracking ranges

Underwater acoustic tracking has traditionally used only the arrival time of tracking pings to localize targets. This implies that the ping transmitted from a target must be received at a minimum of three separate nodes (receiver locations) in order to determine the position. For deep water ranges this was acceptable. In shallow water, where propagation ranges are limited, this requires a large number of nodes. This makes shallow water ranges very costly. An effort is underway to use pressure gradient hydrophones as receivers and measure the bearing of the ping arrival along with arrival time, thereby locating the target using only one tracking node. This allows for increased node spacing and greatly reduced cost. However, the accuracy required for training ranges is on the order of 1 degree. Further, the directional receiver must be housed so as to withstand impacts from fishing operations. Research including design, fabrication, and testing of conventional and unconventional pressure gradient hydrophone...

Under-ice noise resulting from thermally induced fracturing of the arctic ice pack: Theory and a test case application

A theory is presented that relates thermally induced fracturing of pack ice to under-ice noise level variations. It begins with the governing equations for the thermomechanics of pack ice. The thermomechanics relates thermally induced strain rates to the stresses within various vertical layers of the floe. In addition, paradigms are developed which specify the relative quantity of fracturing and stress relief in the floe as the tensile yield strength of the ice is exceeded. The thermomechanics is complemented by an acoustic propagation model that relates the number of fracture events at a given time to the acoustic levels at arbitrary frequency and depth below the ice. The acoustic theory assumes that each fracture acts as a simple monopole source, the fractures are evenly distributed horizontally, and the energy of each fracture propagates through the ice and the water column on the basis of the governing equations for elastic waves in a horizontally stratified medium. The results indicate that noise episodes resulting from fracturing occurring over the top 40 cm of a 160 cm thick floe will propagate over distances of up to 100 km. However, noise episodes associated with fracturing occurring in the lower 100 cm of the floe will only propagate over a range of ∼10 km. The thermomechanics and acoustic propagation theories were used to develop a numerical model for predicting under-ice noise levels for a given thermal forcing of floes within the arctic ice pack. The model was used to simulate stresses in a multiyear floe and under-ice noise levels at 500 Hz at 305 m below the floe. Model-predicted ice stresses and under-ice noise levels compare quite well to observed stresses and noise variations during the fall of 1988 in the eastern Arctic Ocean. The model predicts that most of the thermally induced, under-ice noise at 500 Hz was a result of fracturing occurring between 5 and 30 cm below the ice surface for a 1.6 m thick multiyear floe.

Active acoustic monitoring systems for detecting, localizing, tracking, and classifying marine mammals and fish.

Detection, localization, tracking, and classification (DLTC) of marine mammals and fish is necessary for a wide range of bioacoustic studies. This includes those related to understanding anthropogenic effects and to the development of methods for mitigating harm. Active acoustic monitoring (AAM) is a robust method for monitoring marine life as it can detect and accurately localize a silent target, enabling full DLTC. With the growth of the offshore renewable energy industry and the need to mitigate harm from pile driving, seismic surveys, and military sonar operations, there is strong interest in developing AAM systems and integrating them with current mitigation techniques. There are a host of significant issues including the standard sonar problems of reverberation and propagation in high‐clutter shallow water environments, false alarms, classification, methods of deployment, and cost. Furthermore, AAM systems transmit acoustic energy that has the potential to disturb marine life. Much work lies ahead t...