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.

It has been suggested that the tomographic approach that feeds the 8–11 kHz ocean acoustic measurements into an ocean model using data from bottom-mounted hydrophones, suffers from a significant travel-time variability due to the surface waves. In this work, the influence of the ocean surface motion on the acoustic travel times has been studied using both the experimental data collected at the Pacific missile range facility (PMRF) off Kauai, and an appropriate acoustic model. The ray-based acoustic model developed is an adaptation of the range-dependent version of Bellhop with the modified top surface characterizing ocean surface waves. The model-generated time series suggests a direct relationship between the phase of the top surface wave and the time-of arrival, which is further confirmed by the experimental data. Furthermore, an approach is developed that automatically decouples the effect of the top surface waves from the environment-related phenomena, making it possible to assign the travel time variations produced by the physical phenomena independently of the surface geometry, in turn eliminating the surface related error. [Work supported by the ONR.]

We investigated the effects of exposure to Low Frequency Active (LFA) sonar on rainbow trout (a hearing non-specialist related to several endangered salmonids) and channel catfish (a hearing specialist), using an element of the standard SURTASS LFA source array. We measured hearing sensitivity using auditory brainstem response, effects on inner ear structure using scanning electron microscopy, effects on non-auditory tissues using general pathology and histopathology, and behavioral effects with video monitoring. Exposure to 193 dB re 1 microPa (rms received level) in the LFA frequency band for 324 seconds resulted in a TTS of 20 dB at 400 Hz in rainbow trout, with less TTS at 100 and 200 Hz. TTS in catfish ranged from 6 to 12 dB at frequencies from 200 to 1000 Hz. Both species recovered from hearing loss in several days. Inner ears sensory tissues appeared unaffected by acoustic exposure. Gross pathology indicated no damage to non-auditory tissues, including the swim bladder. Both species showed consistent startle responses at sound onsets and changed their position relative to the sound source during exposures. There was no fish death attributable to sound exposure even up to four days post-exposure. [Work supported by Chief of Naval Operations.]

The Kauai Experiment (22 June–9 July 2003) was designed to study high-frequency (8–50 kHz) acoustics in a shallow-water waveguide. In contrast to much of the previous literature, emphasis was placed on multipath arising from multiple boundary interactions. Various participants were interested in different applications; however, a core theme was the role of the environment on acoustic communications. A great deal of effort was made to characterize the environment including the surface wave spectrum, 2D temperature structure along the propagation path, salinity, currents, and bottom properties. Most of these parameters were measured continuously over the 2 weeks of the experiment, providing information on the diurnal cycles. At the same time, extensive acoustic measurements were made using a variety of vertical line arrays, some of which spanned the entire water column. The acoustic measurements included channel probes to characterize the variation of the impulse response. These probes were interleaved with a variety of modulation schemes for communications including noncoherent methods such as MFSK (multifrequency shift keying), and DPSK (differential phase-shift keying), as well as coherent schemes such as QAM (quadrature amplitude modulation), OFDM (orthogonal frequency division modulation), and PPC (passive-phase conjugation) methods. Thus, the experiment provides a vast amount of information relating environment to acoustic propagation to modem performance. This talk will present an overview of key lessons learned to date.

The Pacific Missile Range Facility, located off the west coast of Kauai, offers a unique opportunity to perform perpetual tomographic imaging of the sound-speed structure in the region. The range covers over 1100 square miles of ocean and features 172 bottom-mounted hydrophones and 15 bottom-mounted projectors. Data collected during an August, 2001 feasibility experiment suggest that a small subset of the total number of acoustic propagation rays connecting sources and receivers can be used to image prominent oceanographic features such as internal tides. However, acoustic travel time fluctuations occurring over durations of only a few seconds are a potentially significant source of error. Uncertainty in bathymetric relief also prohibits effective utilization of ray paths with multiple surface interactions in the inversion. Results of research intended to better manage these and other uncertainties are presented. The basis of this research is the use of repeated measurements, made every few seconds over many hours, along with a sophisticated ocean circulation model of the region and error decomposition techniques to isolate errors in assumed bathymetry and travel time noise. Inversion improvements as well as approaches to assimilating tomographic measurements into the ocean model are also discussed. [Funding provided by CEROS and ONR.]

An ocean acoustic laboratory (OAL) is being implemented at the Pacific Missile Range Facility (PMRF). The range covers 1100 sq nm off the west coast of Kauai, HI, with water depths ranging from very shallow to abyssal plane. There are 172 bottom-mounted hydrophones and 15 bottom-mounted sources permanently cabled to shore. The range is used extensively for training and test and evaluation. The general OAL concept is to improve the range products by providing an accurate depiction of the four-dimensional ocean environment. A high-resolution ocean model has been implemented for the region with 48-h forecasts available on the Scientific Solutions web site (www.scisol.com/hawaii). We are currently implementing a tomographic imaging capability using the bottom-mounted sources and receivers. These sound-speed images will be assimilated into the ocean model to improve accuracy. The long-term goals include providing a real-time picture of this well-described ocean environment over the global network. Applications of the ocean acoustic laboratory include oceanographic research, ocean acoustic research, system evaluation, training, and virtual war fighting. [Work supported by ONR and CEROS.]

The High Frequency Marine Mammal Mitigation (HF/M3) sonar system was specifically designed to meet the marine mammal mitigation plan proposed by the Surveillance Towed Array Sensor System Low Frequency Active (SURTASS LFA) Draft Environmental Impact Statement. The system is integral with the SURTASS LFA, residing at the top of the vertical transmit array, utilizes PC based processing and control, and is comprised primarily of commercial off-the-shelf (COTS) components. The HF/M3 is an active sonar operating in the 30-40 kHz frequency range. The system utilizes four independent transponders mounted on a rotating carousel. Each transponder consists of an omni-directional hydrophone located at the focal point of a 12"/spl times/18" air-backed, parabolic reflector. The reflector provides for an on-axis transmit and receive directivity factor of 20-25 dB to depths of 1000 ft. The system has been tested in numerous field trials, where its ability to detect marine mammals of various size has been qualitatively verified. Quantitative performance estimates, generated using empirical interference and target echo models, suggest that a single, moderately-sized (/spl sim/5 m in length) marine mammal swimming radially toward the system has less than a 1 in 1000 chance of penetrating the LFA's designated 180 dB re /spl mu/ Pa/sup 2/ isoplath.