Search

This report describes a computer model for predicting levels of acoustic scattering at sonar frequencies from an important class of clutter objects, i.e., rocks and seashells lying on the ocean floor. A simplified modeling approach has been adopted that incorporates basic concepts and principles. Rocks are represented by either grid movable spheres or elastic spheres, a technique that has been previously used to model aqueous suspensions of sand over the equivalent frequency range. Seashells are represented by water-filled spherical shells. The model predicts scattering strength values for areas of the ocean floor covered by ensembles of rocks and shells of sizes, numbers, geologic type, and material compositions typically found in littoral and coastal regions. The results of the model show that ensembles of cobbles and pebbles may give rise to scattering strengths of sufficient magnitude to mask sonar targets or to appear as false sonar targets themselves., Prepared in collaboration with Planning Systems Inc., Slidell, LA.

From 30 March to 19 April 1995, the Naval Research Laboratory conducted a series of high frequency acoustic coherence measurements in the shallow-water (6-12 m) coastal environment off Panama City Beach, FL. This report contains descriptions of the equipment used to collect the data, the data quantity and quality, plots and tabulations, and an analysis of the oceanographic environment in which the acoustic data were collected. From 30 March to 22 April, wind speed and direction, air temperature, barometric pressure, and solar irradiation were collected every 15 min. Oceanographic measurements were taken as sea conditions permitted from 5 April to 19 April. A total of 29 CTD profiles and two CTD time series of 2 and 4 days each were collected. Wave spectra and small scale temperature variations were also collected during each acoustic run. Weather and oceanographic conditions were typical of the northeastern Gulf of Mexico during spring. In general, air and sea temperatures rose over the course of the experiment except during the passage of cold fronts. Water column temperatures were close to isothermal and had not developed the typical late spring, mid-depth thermocline. However, changes in the water mass produced by effluence from St. Andrews Bay during falling tides were evident on calm days in the temperature, salinity, and sound speed time series. On at least three occasions, internal wave activity was also associated with these water mass changes.

Results investigating the effects of variable bottom composition on modeled high-frequency backscattered levels are presented for a typical shallow-water, variable bottom (range-dependent) environment. The modeled environment consisted of a single sound speed profile, a flat sea bottom with range-dependent bottom composition, and a smooth, flat sea surface. Coarse-and fine-grained sandy areas were partitioned in range to create range dependence. Bottom backscattering and reflection loss for each partition were obtained from the recent University of Texas high-frequency ocean bottom backscatter model (N. P. Chotiros and F. A. Boyle, J. Acoust. Soc. Am., vol. 96, no. 5, pt. 2, 3264, (1994)). Maximum differences of S dB were discernible between the fine- coarse-fine sand bottom and a range-independent fine-sand bottom for ranges <500 m. The trend and structure of the backscattered levels were nearly identical for both bottom types. In all cases, surface reverberation had a strong impact on the backscattered levels, trends, and structures. (MM), Prepared in collaboration with Planning Systems, Inc., Slidell, LA.

This report examines the horizontal spatial coherence of broadband acoustic energy in the frequency range 100-1200 Hz from explosive sources under the ice pack. The data were recorded on four hydrophone pairs with a nominal sensor separation of 370 m from source ranges from 3 to 25 km. Mean-square coherence is computed separately for the direct and ice-reflected propagation paths, and octave band frequency averaging reduces variability inherent in the coherence estimation associated with explosive acoustic sources. Coherence varies significantly with sensor depth, range, and frequency. Background noise coherence is also examined briefly. Changes in coherence because of under-ice reflection are smallest for the 100-200 Hz octave band, but the change may be as high as 50% for the higher frequency bands.

This article compares both real and complex outputs from sizeable numeric computations using identical code on several computer systems. The digital signal processing technique known as the modified covariance method was used as the computational engine. It is a recursive algorithm for solving the covariance equations of a linear predictor that seeks to predict an input signal by a linear combination of past signal samples. Single precision and double precision results are presented but the study focuses primarily on differences between the VAX Fortran 4.8 and MacFortran/020 compilers. Differences in the first digit for single precision arithmetic were found and double precision differences occurred in the eighth digit. Arithmetic with complex data types was found to be less precise than with real data types. Although differences exist among various computer systems, they all show the same order of magnitude accuracy with respect to CRAY-YMP results. The algorithm used here required a double precision implementation to obtain agreement between different computer systems. Acoustic propagation, sea ice, noise., Pub. in Computers in Physics, v6 n5 p506-512, Sep/Oct 1992.

Oceanographic data acquisition is usually done with instrumentation that has taken many hours to configure, program, and learn to use. The advent of self-recording oceanographic data collection systems based on microprocessors has greatly facilitated the acquisition of oceanographic data from ships of opportunity and in harsh environments. Two notable examples are the Ocean Data Equipment (ODEC) model 302A CSTD unit, which measures ocean conductivity, salinity, and temperature as a function of depth from which acoustic sound speed is calculated, and the InterOcean Model S4 current meter, which measures ocean current speed and direction as a function of depth or time. Networked with Apple Corporation's Macintosh microcomputer, the acquisition, processing, and analysis of CSTD and current data become dependable and virtually automatic. Communication between the Macintosh and the oceanographic probes, as well as data processing, is accomplished with commercially available software. Ease of use is accomplished in both instruments through probe features and the Macintosh user interface. Both systems have a proven at-sea track record and are ideally suited to Arctic applications by virtue of their portability, ability to fit through a small 8-in ice hole, and ruggedness. The Macintosh has been modified to run on dc, and the entire suite of instrumentation can be operated entirely from battery power. The commercially available software described here enables the user to collect, reduce, analyze, and even report results in the field minutes after probe recovery., Pub. in IEEE Jnl. of Oceanic Engineering, v13 n1 p1- 4 Jan 1988. No copies furnished by DTIC/NTIS.

From 27 April through 28 May 1988, personnel from NORDA and other institutions used the USCGC Northwind to establish an ice camp to conduct an environmental acoustics exercise in the Marginal Ice Zone (MIZ) between Greenland and Svalbard in the Fram Strait. This report documents the comprehensive set of environmental data collected in support of the acoustic measurements. These data include expendable bathythermographs, conductivity- temperature-depth profiles, both vertical and time series current meter casts, meteorological and navigational measurements, and satellite imagery. This report also provides a brief synopsis of the sequence of events that transpired during the exercise, a preliminary environmental analysis, and some suggestions for future MIZ experiments.

Estimation of time delays between coherent transient signals is biased by the choice and location of the sampling window. For transients that do not tend to zero within the sampling window (self-windowing), the uniform or boxcar window may not be the best choice due to spectral leakage. This is especially important when processing highly asymmetrical transient signals that are not self windowing as in the case of acoustic ambient noise. When a sample window is used to suppress spectral leakage, a bias in time delay estimation results from a suboptimal window location. Even though the boxcar window produces the least bias in time delay when the transient is self windowing, it is shown that sidelobe interference is significant for highly asymmetric signals. This work critically examines these effects and describes an iterative procedure to determine optimal location of the transient within any sample window., Availability: Naval Ocean Research and Development Activity, Stennis Space Center, MS 39529-5004.

The Broadband-87 acoustics exercise was an at-sea efforts designed to characterize broadband propagation. This experiment was conducted in two areas of the North Atlantic Ocean Site Alfa, 31 N, 76 W, was chosen for its acoustically bottom-limited conditions and characteristically stable oceanographic environment. Site Delta, 28 N, 67 W, represented an area in which the sound channel is unobstructed by the seafloor. This report documents the comprehensive set of environment data taken at Site Alfa in support of the acoustic measurements. The oceanographic environment was, basically very stable throughout the exercise despite its location in a cold-core eddy of the Gulf Stream. Thermometric satellite imagery collected during the exercise quantified the surface temperature within this eddy to be about 1 C cooler than the surrounding waters. The measurements were collected fully within the confines of the eddy, precluding potentially marked variability resulting from acoustic propagation across dissimilar water masses. A 36 hour time series of temperature data collected during the exercise exhibited a maximum sound speed variation of approximately 6 m/sec in the area of the main thermocline below the 18 C water layer. However, a 15 mile displacement over the duration of these measurements precluded an accurate interpretation of this variability. The most profound effect of the eddy on the conduct of the exercise was the extremely strong surface currents experienced. Expendable Current Profile measurements indicate that near surface currents approaching 5.5 kt were present. Plots and tabulations of all data collected during phase Alfa of Broadband-87 are in the appendices.

The performances of the maximum entropy method (MEM) and Wagstaff-Berrou broadhand (WB squared) high-resolution spatial processing techniques are compared with each other and with the conventional fast Fourier transform (FFT) beamformer. The comparisons are based on the analysis of three sets of measurements acquired at sea during two different environmental acoustics field exercises. Chosen for comparison are the effects of signal-to-noise ratio (SNR) on beamwidth and the effect of the number of array elements on the bearing estimation accuracy and beamwidth of the spatial processors. Results from these data indicate that both of the high-resolution methods yield smaller beamwidths than the conventional FFT processor, and that the high-resolution beamwidths show little dependence on SNR for values greater than 8 dB at the hydrophone input. Also, MEM beamwidths are generally slightly smaller than WB squared beamwidths and show less dependence on aperture. Measured MEM beamwidths show the same general trend as theoretical calculations for sensitivity to SNR, and the average arrival angles of all three beamformers show the same general dependence on the aperture. (Author) (Reprints), Presented at the Meeting of the Acoustical Society of America (108th) Held in Minneapolis, MN in Oct 84.

The Ocean Data Model 302A CSTD oceanographic profiling system is self-recording, preprogramming device. The system has been integrated with the Apple Macintosh computer to yield a portable, durable and easy-to-use system for measuring ocean conductivity, temperature, and salinity as a function of depth, and for calculating accurate sound speed values (roughly + or - 0.2 m/sec) for acoustic and oceanographic analysis and data quality assurance. This system enables the user to tabulate and plot the above values in near real time for field analysis. Little operator training time is required to operate this system, which has been used successfully in the northeast Pacific and in the Arctic. CSTD data output from the Model 302A reader/recorder are stored on Macintosh 3.5-inch, hard-shelled floppy discs and processed using Microsoft Corporation's program EXCEL. A description of the system, specifications, and user's guide are included, as well as a listing of the routines and samples of the outputs. Keyword: Oceanographic data; Acoustic data; User manuals; Acoustic measurements; Electrical conductivity; Data processing.