R. Lee Culver

Application of the coherent-to-incoherent intensity ratio to estimation of ocean surface roughness from high-frequency, shallow-water propagation measurements

For acoustic propagation through a shallow ocean channel or waveguide, the coherence between different transmissions is controlled primarily by the roughness of the ocean surface and to a lesser degree by fluctuations in the volume. In this study, the coherent-to-incoherent intensity ratio (CTIR) is defined as a way to quantify the coherence between multipath transmissions and ocean surface rms wave height and wind speed. A theory that connects the CTIR and the coherent surface reflection coefficient is developed using both Kirchhoff and small-slope approximations as rough surface scattering models. The CTIRs have been evaluated over a period of several days using broad-band experimental results from shallow-water deployment of source and receiver arrays that span most of the water column. Estimates of wind speed and rms wave height obtained using these CTIR calculations are compared with environmental measurements to demonstrate the validity of the theory.

Scintillation index of high frequency acoustic signals forward scattered by the ocean surface

Ocean measurements of the scintillation index (SI) of surface forward-scattered signals made in August 2002 are presented and compared with a model developed by Yang and McDaniel [Waves in Random Media 1, 419–439 (1991)]. The acoustic measurements employed continuous wave (CW) pulses and linear frequency modulated (LFM) sweeps with center frequencies of 20 and 40 kHz. Simultaneously, measurements of wind speed, directional surface wave height spectrum, and ocean sound speed profile were made. The sea state was between 0 and 1 during the four days of the experiment, in part because the location is very much in the lee of San Clemente Island. The measured values of SI are found to agree with Yang and McDaniel model predictions, except for measurements with the largest signal bandwidth and/or the narrowest beamwidths, which violate model assumptions of continuous signals and omnidirectional projectors and hydrophones. In addition, the data show that SI decreases with increasing signal bandwidth (or decreasing temporal extent). An extension to the Yang and McDaniel model is developed that accounts for a reduction in signal temporal extent or ocean surface ensonification. The extended model is in qualitative agreement with the measurements, in that SI is predicted to decrease with increasing signal bandwidth.

Sonar signal processing using probabilistic signal and ocean environmental models

Acoustic signals propagating through the ocean are refracted, scattered, and attenuated by the ocean volume and boundaries. Many aspects of how the ocean affects acoustic propagation are understood, such that the characteristics of a received signal can often be predicted with some degree of certainty. However, acoustic ocean parameters vary with time and location in a manner that is not, and cannot be, precisely known; some uncertainty will always remain. For this reason, the characteristics of the received signal can never be precisely predicted and must be described in probabilistic terms. A signal processing structure recently developed relies on knowledge of the ocean environment to predict the statistical characteristics of the received signal, and incorporates this description into the processor in order to detect and classify targets. Acoustic measurements at 250 Hz from the 1996 Strait of Gibraltar Acoustic Monitoring Experiment are used to illustrate how the processor utilizes environmental data to classify source depth and to underscore the importance of environmental model fidelity and completeness.

Waveguide invariant analysis for modeling time-frequency striations in a range-dependent environment

The waveguide invariant is a useful parameter for understanding the behavior of interference patterns (e.g., striations in time-frequency plots) resulting from broadband acoustic sources in shallow water waveguides. It is possible to model these striations for range-dependent environments using conventional parabolic equation methods; although this approach can be computationally intensive as a full field must be created for each frequency and azimuthally dependent geometry. This letter discusses the formulation and use of a range-dependent waveguide invariant distribution that can be used to describe spectral striation patterns using a fraction of the computing power required by parabolic equation methods.

Single-Hydrophone Model-Based Passive Sonar Source Depth Classification

This paper introduces a source depth classification technique applicable to passive low-frequency narrowband sonar signals received by a single hydrophone in shallow water. The classifier is based upon Rice probability density functions with model-derived parameters for the received amplitude of a tonal signal that has been modulated by propagation through the ocean. Differences in modal excitation due to source depth result in different probability density functions of received amplitude that allow for source depth classification. The performance of the technique is demonstrated using data from the SWellEx-96 experiment.

Processing methods for coprime arrays in complex shallow water environments

Utilizing the concept of the coarray, coprime arrays can be used to generate fully populated cross-correlation matrices with a greatly reduced number of sensors by imaging sensors to fill in gaps in the physical array. Developed under free space far-field assumptions, such image sensors may not give accurate results in complicated propagation environments, such as shallow water. Taking shallow water acoustic models under consideration, it will be shown that image sensors can still be used, but to a more limited extent based on spatial variability. Performance of a coprime array with limited image sensors and full image sensors will be compared with that of a fully populated array. [This research was supported by the Applied Research Laboratory, at the Pennsylvania State University through the Eric Walker Graduate Assistantship Program.]

An information theoretic performance bound for passive sonar localization of a moving source

Several passive sonar signal processing methods have previously been developed for determining the location of a source radiating tonal acoustic energy while moving through a shallow water environment. These localization algorithms rely on the complex interference pattern resulting from multipath acoustic propagation. By treating passive sonar localization as a communications problem, an information theoretic upper bound on performance can be derived. The bound is based on acoustic propagation, and depends on radial distance the source travels through the waveguide, signal to noise ratio, frequency of the radiated acoustic tone, and minimum sound speed of the problem, and resolution of the localization. An example using parameters from the SWellEx-96 experiment is shown.

The effect of medium variability on acoustic variability: Internal waves and thermohaline intrusions (spice)

In July 2002, an experiment was conducted off the coast of southern California in which the two‐dimensional temperature and salinity field were measured directly using a towed conductivity, temperature, and depth chain. The spatial scales are approximately 3500 m long by 80 m deep, with a horizontal resolution of 1–2 m. The resulting sound‐speed field has been decomposed into fields of internal waves and density‐compensated, thermohaline intrusions. The medium variability in these two fields covers many spatial scales. High‐frequency acoustic propagation (1–5 kHz) through the derived fields, as well as through the original field, was computed using the PE‐based model RAM and the ray‐trace model RAY. Using the metric of scintillation index, the contributions of internal waves and thermohaline intrusions to the acoustic variability are examined, and their relative contributions to the total acoustic variability quantified.

Tracking aircraft flybys using a microphone array

Ferguson and Quinn (JASA 96(2), 1994) develop equations to describe the time-frequency characteristics of an acoustic signal radiated by an aircraft and received by a stationary microphone. These equations enable construction of curves in time-frequency space whose shape depends upon several parameters, namely, aircraft speed and height above the microphone. We have utilized Ferguson and Quinn’s equations to track an aircraft fly by under very low signal-to-noise conditions using six microphones which are spaced too far apart to form beams. We make use of signal coherence across the array, and we develop a time-frequency “matched filter” with replicas that are based upon aircraft speed, height above ground, and closest point of approach distance.

Frequency-dependent beam pattern of acoustic arrays for unmanned aerial vehicle tracking

There are practical constraints on the geometry and construction of acoustic arrays which are designed to detect small unmanned aerial vehicles (UAVs). The constraints depend to a large degree on the environment in which the array must operate. For example, an acoustic array which provides UAV detection to protect a public stadium is likely to differ considerably from one which soldiers can carry into the field and deploy to protect their position. The former can use almost unlimited power, be very large and be composed of many elements, while the latter will probably run on batteries or solar and must be man-portable. These different designs provide different performance as measured by detection range, array gain, and angular resolution, for example. This talk presents the performance of several common array configuration over the band of frequencies in which passive AUV detection at tactically useful ranges is possible.