Roger W. Meredith

Measurements of high-frequency shallow-water acoustic phase fluctuations

A shallow-water high-frequency (HF) acoustic propagation experiment was conducted just off shore in Panama City, FL. Several broad-band high-resolution sources and receivers were mounted on stable platforms and deployed in water depths of 8-10 m. Signals covering the frequency range from 20 to 200 kHz were transmitted from the sources to two spatially separated receivers. The data were analyzed to provide estimates of the signal phase variances as a function of frequency and source-to-receiver range. These phase variabilities are correlated with small-scale water column thermal variabilities and ocean swell conditions.

Panama City 2003 Broadband Shallow‐water Acoustic Coherence Experiments

In June 2003 a series of acoustic propagation experiments were conducted off the coast of Panama City, Florida. The experiments were designed to measure and provide an understand of signal phase and amplitude fluctuations, and signal spatial and temporal coherence over several large horizontal and vertical arrays. The propagation measurements were conducted in a water depth of 8.8m and at ranges of 70 m and 150 m. The acoustic measurements cover frequencies from 1 to 140 kHz. The propagation measurements were supported by data obtained by wave rider buoys, CTD’s, thermister chains and current meters. Bottom penetration data was also obtained using a buried hydrophone array. The experiments will be outlined and the data sets described.

Coherence estimation for high-frequency narrowband cw pulsed signals in shallow water

Signal coherence is an important environmental predictor for sonar performance and is often difficult to estimate. This paper examines different methods for estimating signal coherence from data not amenable to coherence estimation. Methods are compared using narrowband, high-frequency, short pulse length continuous wave signals (cw) that typify those used by mine-hunting sonars in complex shallow-water environments in which the spectral frequency resolution is poor using classical overlapping segments and Fourier techniques. Alternate approaches for the spectral and coherence estimation were the autoregressive parametric based approach, harmonic wavelet approach, and concatenation. These methods provide a statistic similar to coherence that is shown to be useful for specialized data. Low model orders in the parametric method yield results that are difficult to interpret. The wavelet approach is more suited for signals whose frequency spans several wavelet levels. Concatenation via narrowband spectral ave...

Broadband Temporal Coherence Results From the June 2003 Panama City Coherence Experiments

During the month of June, 2003, the Naval Research Laboratory conducted a series of coherence experiments in shallow water (approximately 9 meters) off Panama City Beach, Florida. Examined here are preliminary mid frequency (1 – 10 kHz) results of analyzed temporal coherence data. For this experiment, a G34 omnidirectional source, mounted approximately 2.7 meters from the bottom, ensonified a vertical and horizontal array of hydrophones mounted on a submerged tower 70 and 150 meters down range in the along shore direction. Results will be shown for both macro (5 minutes), and micro events (<20 seconds).

Panama City 2003 Acoustic Coherence Experiments: Environmental Characterization

During June 2003, the Naval Research Laboratory conducted a series of acoustic propagation experiments to measure both high (20 to 150 kHz) and low (1 to 10 kHz) frequency spatial and temporal coherence in very shallow water. Environmental data collected to support the acoustic measurements included water column current, bottom current, sea‐surface wave height, tide height, CTD water column profiles and mid‐water time series, two‐dimensional micro‐scale seawater temperature, and weather parameters. Wave periods varied from 3 to 7 seconds and wind speeds ranged from 4 to 35 knots throughout the experiment. Temperature and salinity profiles characterized periods when the water column was isovelocity and periods when the water column was stratified with a strong depth dependence of temperature and salinity. Current magnitudes were always less than 25 cm/s. Experimental geometry and methods of environmental data collection are briefly described and environmental conditions and their impact on the propagation ...

A High‐Speed, Multi‐Channel Data Acquisition System

The Naval Research Lab is currently conducting research programs in MCM detections and classifications using both low and high frequency acoustics. These include target detections, target imaging, proud and buried target detections and classifications using structural clues. To determine the limitations that a fluctuating environment places on these target detection methods, a data acquisition system was developed. The data acquisition system consists of multi‐channel, high‐speed A/D’s with remote, variable gain control, and FPGA technology. Each A/D is synchronously sampled at a rate of 1 MHz and using time ‐division multiplexing techniques, is sent down an optical fiber at 1.3 Gbps. The sampled data is then separated back to its original channel and recovered back to an analog signal along with the original clock. Precision filters and high speed transient recorders utilizing fast CAMAC crate controllers are then employed to sample, simultaneously, all data channels with sample rates up to 3Msps. Acoust...

Broadband Horizontal and Vertical Spatial Coherence Measurements

Initial results of broadband (1 to 10 kHz) spatial coherence measurements taken during the June 2003 shallow‐water (8 m) propagation experiments will be presented. The results will show spatial coherence estimates over a 12 m long horizontal array and over a 6 m vertical array. The data was taken over a range of sea states and at ranges of approximately 70 and 150 m.

Probability distribution functions for shallow water acoustic bottom scattering at high frequency

When designing signal processors to discriminate against underwater sound background noise and scattering, it is important to know the statistics of the unwanted signal disturbances. A study was undertaken for the Naval Research Laboratory, Stennis Space Center, Mississippi to review the literature reporting on a number of bottom scattering experiments that were conducted during the past 15–20 years. The scope of the investigation is limited to high frequencies (10 to 200 kHz) and shallow water of depths less than 100 m. Many of these tests were performed by the Naval Research Laboratory, Stennis Space Center; the Applied Research Laboratories, University of Texas at Austin; and the Applied Physics Laboratory, University of Washington. Other experiments reviewed were performed by Thomson‐Sintra ASM, France and by the University of Wisconsin. Each experiment will be briefly described, and the statistical results discussed and summarized for various parameters, such as frequency, beamwidth, pulse length, gr...

High‐frequency under‐ice reflection losses and spatial coherence

Propagation of high‐frequency acoustic signals under a multiyear pack ice results in a scattered acoustic field of fluctuating amplitude and coherence. High‐frequency scattering from under‐ice surfaces is complicated by out‐of‐plane and multiple scattering because acoustic wavelengths may be many orders of magnitude smaller than ice relief. In addition, the transition layer between water and ice, ice inhomogeneities, and ice anisotropy become important scattering sources. Measurements are presented of horizontal spatial coherence at 24 and 42 kHz from linear FM slide pulses of 800‐Hz bandwidth and 100‐ms duration. Measurements were made with a linear array of 16 hydrophones distributed over a 16‐m aperture and deployed 61 m below the ice surface. The data presented here were taken at a nominal range of 1000 m and results from array bearings at endfire, broadside, and 45° are compared. Results are presented for a fixed source at depths of 30, 61, and 91 m. Deconvolution of the source waveform is used to se...

Impact of organic matter on the physical and dynamic properties of marine mud

The physical and dynamic properties of marine mud are a function of the interaction of clay fabric, organic matter (OM), and seawater physico-chemistry, and, when present, free gas. OM is the focus of this presentation and is a determinant of several properties in mud deposits including (1) free water available versus total water content in clay fabric pore space as a result of (2) OM seawater hydration, (3) reduction of permeability by volumetric contribution of hydrated OM in pore space, (4) OM density reduction by seawater hydration versus dry OM density, (5) dynamic behavior by physico-chemical attachment of OM to clay particles in potential energy fields created by the clay fabric signatures (edge-to-face, edge-to-edge, and offset face-to-face) in seawater, (6) high percentages of OM (TOC >2%) result in a high degree of compressibility at considerably lower stress compared to mud deposits with <2%TOC. Measurement and characterization of OM, previously often neglected, is expected to provide new signi...