John S. Stroud

Optical detection of transient bubble oscillations associated with the underwater noise of rain

A drop of water falling on a water surface entrains a bubble for certain drop diameter–impact velocity combinations. Volume oscillations of the bubble contribute to the underwater noise of rain. A method was developed for optical detection of these volume oscillations in a laboratory environment. The entrained bubble is in the path of a laser beam when the bubble is created. The beam is then directed to a photodetector and the transient signal resulting from oscillations of the optical cross section of the bubble is recorded. This optically obtained record is compared to an acoustic record of each event and the frequencies are in agreement. The general magnitude of the initial radial oscillation is estimated and compared with relevant theoretical results by Oguz and Prosperetti [J. Fluid Mech. 228, 417–442 (1991)].

High‐frequency forward scattering from Gaussian spectrum, pressure release, corrugated surfaces. I. Catastrophe theory modeling

Exact integral equations for the acoustical pressure scattered from a rough pressure release surface can be written down and solved numerically. However, analytical head way into the problem, gained at the expense of various approximations, can lend physical insight not easily obtained numerically. This, in part, motivated the work to be discussed. In particular, a high‐frequency approximation will be presented for forward scattering from Gaussian spectrum, pressure release, corrugated surfaces. The analysis is most directly applicable to forward scattering from an ocean surface dominated by swell. The presentation uses ideas and results from catastrophe theory [M. V. Berry, ‘‘Waves and Thom’s Theorem,’’ Advan. Phys. 25, 1–26 (1976); P. L. Marston, Physical Acoustics (Academic, New York, 1992), Vol. 21, pp. 1–234] to include diffraction. Catastrophe theory allows one to write down the scattered pressure in terms of a finite set of diffraction catastrophes and stationary phase contributions. The catastroph...

Small scale test bed for studying multiaspect and multistatic sonar systems.

A nominally 1:50 scale acoustic test bed is operational at the Naval Surface Warfare Center‐Panama City Division (NSWC‐PCD) to image free‐field, bottom, and buried targets using multiaspect, bistatic, backscatter, forward scatter, and synthetic aperture sonar (SAS) techniques. The test bed is designed to test and study novel geometries and techniques faster and cheaper than can be done in the field. Using precise translational positioning systems mitigates any issues associated with positioning of multiple sonar platforms in the field. The use of two transmit and receive transducer platforms is superior to a single platform, which is restricted to purely backscattered imaging, for producing SAS images. With two platforms, one can generate four images, two using backscattered return data and two using data sent from one platform to the other. Thus, the system is inherently both dual aspect and bistatic. The setup, including the scaled sediment, acoustic sources and receivers, and computer controlled transl...

AUV‐based synthetic aperture sonar: Initial experiences and insights

The ability to do synthetic aperture sonar (SAS) imaging from autonomous underwater vehicles (AUVs) has only recently been achieved. The combination of the two technologies is a milestone in the field of underwater sensing as the combination of high‐resolution SAS imaging with AUVs will provide military, research, and commercial users with systems of unprecedented performance and capabilities. The U.S. Navy took delivery of the first AUV‐based SAS in early 2003. A description of the system will be presented along with the methodologies employed. The emphasis will be on image quality and repeatability, as well as the differences associated with operating an AUV‐based SAS as opposed to a towed SAS. Additional topics will include general comments and recommendations for better AUV/SAS integration such as mission planning and vehicle control strategies intended to maximize the chances of high‐quality imagery, SAS motion measurement requirements coupled with on‐board vehicle navigation, and the potential of using the SAS data to augment the vehicle motion sensors. Lastly, a brief overview of forthcoming Navy SAS systems will be included.

Channel characterization for communications in very shallow water

Broadband acoustic transmissions (7 kHz to 17 kHz) taken from July 4, 2000 to July 8, 2000, in the shallow water near Scripps Pier at La Jolla, California, are used to extract the time‐varying channel parameters of coherence time and multipath time delay spread, as functions of frequency and the environmental variables of wave height and tidal fluctuations. Tidal fluctuations, which have a significant effect on water depth at the receiver and transmitter, are shown to strongly correlate to variations in the multipath delay spread. Variations in the coherence time for the channel, as measured by the drop off in correlation between initial and successive impulse responses, are shown to be inversely related to variations in the measured wave height. The coherence time of the channel was found to decrease with increasing center frequency. A scatter plot of the receiver signal‐to‐noise ratio as a function of wave height and water depth indicates when the shallow water environment allows a viable communications...

Transient bubble oscillations associated with the underwater noise of rain detected optically and some properties of light scattered by bubbles

It is known that a drop of water falling on a water surface will entrain a bubble for a range of drop diameters and velocities. The transient volume oscillations of the bubble can be important to the generation of underwater noise. In the present work the volume pulsations are optically detected and the general magnitude of the initial radial oscillation is estimated. The detection method is based on the dynamic extinction of light by the oscillating bubble and on certain optical scattering properties. General progress concerning the scattering of light by bubbles is reviewed in an Appendix.

Exploiting sediment acoustics properties for subcritical buried target detection using a synthetic aperture sonar aboard the AUV Reliant

The Coastal Systems Station High Frequency/ Low Frequency Synthetic Aperture Sonar (HF/LFSAS) was recently integrated into a Bluefin Robotics, Inc. Odyssey III Family autonomous underwater vehicle (AUV). This vehicle, designated Reliant, has been used in several at‐sea experiments to determine the feasibility of using bottom roughness to enhance detection of buried targets at subcritical grazing angles. Previously, the HF/LFSAS was deployed on a towed 21‐in. vehicle and made some remarkable buried target detections at subcritical angles. However, due to the complexities associated with using a towed platform, only a limited amount of data was obtained during these exercises. This sparse data set made it difficult to quantify the enhancing effects of bottom roughness on buried target detection. An extensive run matrix for buried target detection was completed this year using the Reliant AUV. The primary variables in the experiments were ripple orientation, standoff range, and grazing angle. Results from th...

Using a dual‐band synthetic aperture sonar for imaging various seafloor compositions

The Small Synthetic Aperture Minehunter (SSAM) is a synthetic aperture sonar mounted aboard a 12.75‐in. REMUS600 unmanned underwater vehicle transmitting both low (10–50 kHz) and high (105–135 kHz) frequency bands. The sonar and the vehicle are highly integrated, which allows precise pulse timing, attitude correction, and object geolocation. Operating simultaneously in both bands produces two independent images of each scene. The upper band is capable of 2.5‐ by 2.5‐cm resolution that provides high‐fidelity images of proud targets. The lower band, processed to 7.5‐ by 7.5‐cm resolution, adds clutter reduction as well as capabilities against buried targets. The dual bands of this sonar have proved to be a robust solution for minehunting in the littorals. In the past 18 months, SSAM has been deployed nine times. In these tests, the system has operated against bottom types including mud, sand, rippled sand, layered mud and sand, posidonia, and coral. Imagery from sea tests showing all of these bottom types w...

Analysis of acoustic telemetry signals in the surf

The 2000 Surf Zone Acoustic Telemetry Experiment (SZATE) was conducted off of the Scripps Institute of Oceanography pier in La Jolla, CA. In this experiment, measurements were conducted in a very shallow‐water/surf zone (VSW/SZ) region to investigate acoustic propagation and sound channel stability in the coastal area. These measurements utilized two receivers and a broadband source projecting shoreward toward the receivers. The data to be presented were acquired by a computer‐controlled system designed and operated by Grant Deane of Scripps Institution of Oceanography. This system provided for 12 18‐min data acquisition periods per day. The probative transmissions to be discussed consisted of a variety of linear frequency‐modulated (LFM) and binary phase shift keyed (BPSK), as well as 4‐ary frequency shift keyed (FSK) signals. Each of the types of signals was used during each data acquisition period, providing a record of propagation throughout the tidal cycle. Results of this acoustic measurement to be ...

Acoustic propagation measurements in the surf

As part of the Near Shore Acoustic Network Experiment (NSANE), measurements were conducted in a very shallow water (VSW)/surf zone (SZ) region to investigate acoustic propagation and coherence in coastal areas. These measurements utilized an acoustical array of 16 hydrophones (supplied by Datasonics, Inc.) deployed over many tens of meters, and a broadband source. The data to be presented were acquired from hydrophones on two vertical arrays, separated by 240 m, which comprised half of the total array. The broadband projector was operated in the 3.5–28 kHz range using a variety of pulse types. The projector was aimed seaward, as it was located closer to shore than any of the receivers. Data were acquired in conjunction with environmental data by other NSANE team members. The data were analyzed using coherence techniques. Results of this acoustic measurement will be reported. [Work supported by ONR Code 321OE.]