Patrick Edson

Nonlinear wave propagation from a discrete annular array: Experiments

To date, efforts to model nonlinear wave propagation from a bounded beam source have chiefly been concerned with unfocused and weakly focused circular piston sources [Lee and Hamilton, J. Acoust. Soc. Am. 97, 906–917 (1995)]. When the peak pressure does not exceed approximately 150 MPa, the Khokhlov–Zabolotskaya–Kuznetsov (KZK) nonlinear parabolic wave equation yields numerical solutions in agreement with experiments. However, little research has been reported on nonlinear wave propagation from an array of discrete sources. Theoretical and experimental results for an eight‐element annular array will be presented. The source signal is typically a ten‐cycle sine wave tone burst with a center frequency of 500 kHz. The outer diameter of the array is 0.14 m, and the surface areas of the elements are equal. While theoretical predictions will be shown, the experimental aspects of this research will be emphasized. [Work supported by ONR.] by Kargl

Nonlinear wave propagation from a discrete annular array: Theory

Theoretical investigations of finite amplitude wave propagation from bounded beam sources has been considered by Lee and Hamilton [J. Acoust. Soc. Am. 97, 906–917 (1995)] as well as others. These researchers have restricted their analyses to a source geometry of either an unfocused circular piston or a weakly, spherically focused circular piston. When the peak pressure does not exceed approximately 150 MPa, the Khokhlov–Zabolotskaya–Kuznetsov (KZK) nonlinear parabolic wave equation yields numerical solutions in agreement with experiments. However, little research has been reported on finite amplitude wave propagation from an array of discrete sources. Time domain simulations, based on the KZK equation, will be discussed for an eight element discrete annular array immersed in fresh and sea water. The source signal is typically a short sine wave tone burst with a center frequency of 500 kHz. The transitions from linear acoustics to nonlinear acoustics will be discussed, and comparisons between a spherically focused circular source and a properly phased array will be presented. While some data from recent experiments may be shown, the scope of this talk will be restricted to the theoretical treatment and the numerical simulations for the discrete annular array. [Work supported by ONR.] by Edson

Noise measurements of a prototype tidal energy turbine.

Characterizing the acoustic footprint of power‐generating tidal turbines is necessary to assess possible effects of the radiated noise on local marine life. However, as is the case for a wide‐range of offshore installations, cohesive and accurate measurement of both the radiated noise of the turbine and the ambient noise conditions is a difficult task. In this case, the usual difficulties with propagation and contaminating noise sources are exacerbated by flow induced noise in the measurement system. Here we present results from noise measurements made of the Ocean Renewable Power Company prototype TidGenTM power generation turbine unit deployed in Cobscook Bay near Lubec and Eastport, Maine. These measurements were made using a drifting measurement system in currents upwards of 6 kn.

On the role of acoustic cavitation in enhancing hyperthermia from high‐intensity focused ultrasound

There are several physical mechanisms that lead to localized heating of tissue and tissue‐like media with high‐intensity focused ultrasound (HIFU). Experimental results obtained in vivo [Hynynen, Ultrasound Med. Biol. 17, 157 (1991)] and in vitro [Edson et al., J. Acoust. Soc. Am. 104, 1844 (1998)] clearly indicate the existence of an insonation pressure threshold above which cavitation activity can profoundly enhance heating rates at MHz insonation frequencies. The dominant mechanisms through which bubbles facilitate heating have yet to be determined. Candidate mechanisms include viscous dissipation in the flow near the bubble surface, absorption of radiated acoustic waves, attenuation of multiply scattered waves, heat transfer across the bubble wall, etc. Results of numerical simulations designed to investigate the relative impact of these mechanisms on heat deposition from single bubbles and bubbly assemblages are reported. The critical roles of equilibrium bubble size and rectified diffusion are addre...

Active Acoustic Monitoring of Aquatic Life

Active acoustic monitoring (AAM) can be used to study the behavioral response of marine life and to mitigate harm during high-danger anthropogenic activities. This has been done in fish studies for many decades, and there are now case studies in which AAM has been used for marine mammal monitoring as well. This includes monitoring where the ranges, AAM frequency of operation, and species are such that the AAM operation is completely outside the hearing range of the animals. However, it also includes AAM operations within the hearing range of marine life, although this does not necessarily that imply AAM is not a suitable tool. It is just not always possible to have a sufficient detection and tracking range and operate at a frequency outside the marine life hearing range. Likely, the best and most important application of AAM is when the anthropogenic activity to be conducted is temporary and presents a clear danger to aquatic life.

Practical applications of track segment association algorithms to an active sonar network for underwater port surveillance

Acoustic detection and tracking of small (swimmer-size) targets while minimizing the incidents of false alerts can be challenging in a shallow, multipath, high-clutter harbor environment. One common problem involves track intermittency where periods or locations of high clutter or multipath interference inhibit detection and cause tracks to fail and restart. This paper describes the results of applying track segment association and related algorithms from airborne and ground radar applications [e.g., Zhang and Bar-Shalom, IEEE Trans. Aerosp. Electron. Syst. 47(3), 1899–1914 (2011)] to an active sonar network for underwater port surveillance [Edelson et al., J.Acoust. Soc. Am. 129(4), 2598 (2011), Stein et al., J. Acoust. Soc. Am. 121(5), 3084 (2007)] using real-world swimmer and synthetic target data.

Assessing shear property variability in shallow water sediments using a wide aperture geophone array

The significance of seabed shear properties to low-frequency propagation in shallow water is well established, and since the early 1980s the measurement of the properties of the seismic interface, or Scholte waves, has been recognized as the most direct tool fordetermining the shear properties. In Sep. 2011, an experiment was carried out in Singapore harbor aimed at investigating the excitation of Scholte waves by objects dropped from the surface and impacting the seabed. For that purpose, a 100 m aperture array of 10 ocean bottom seismometers (OBS) with logarithmic spacing was deployed in approximately 16 m of water, in an area with close to range-independent seabed stratification. Spherical and cylindrical objects were equipped with 6 degree-of-freedom motion packages which allowed for accurate measurement of the impact forcing. The objects were dropped onto the seabed at various bearings and distances relative to the array, providing a unique, rich data set which provides the opportunity of estimating...

Two‐stage diver tracking in a high‐clutter harbor environment.

One approach to detecting swimmers with active sonar is to deploy a larger number of relatively simple nodes and network them together. Instead of using a complex multi‐element phased array with electronic beamforming, this system uses air‐backed parabolic reflectors, each with an omni‐directional transducer. To avoid performance degrading acoustic interactions, the available operating frequency band is managed at the channel level. The physical configuration of this sonar system presents challenges for tracking through and across beams, and between nodes. Swimmers can exhibit low target strength and the acoustic clutter fields can be dense and highly dynamic. To detect and track swimmers in this environment, we employ a windowed Hough‐transform (HT) tracker at the beam level. The HT has received wide use for track initiation. However, because of the processing gain required to continually track a weak target in such a significant clutter field, the HT is used in this case to maintain as well as to initia...

Marine life detection and tracking using a swimmer detection sonar network.

There is a growing need for real‐time monitoring of marine life and floating debris during a wide variety of commercial operations. These include seismic exploration for oil and gas, explosive removal of offshore structures, pile driving for the installation of marine structures including offshore wind farms, and the operation of tidal turbines and wave power generation devices. Active acoustics is likely the best method for monitoring where there is a high‐danger region with a limited range around the activity. The SSI Swimmer Detection Sonar Network was originally designed as a human swimmer and diver detection and tracking system. However, extensive trials have demonstrated that the system is also capable of tracking marine life ranging from a large fish or marine mammals to schools of smaller fish. Analysis of the detection characteristics and movement behavior of marine life is being conducted as a means of tracking and classification at ranges out to roughly 500 m. Real‐time identification and track...

A distributed micro sonar network for marine mammal monitoring.

The design of a distributed micro sonar network (DMSN) is presented. The DMSN is a miniaturized, low‐power, and high‐frequency version of the swimmer detection sonar network used to detect and track marine mammals at ranges out to 500 m. The intent is to use it for monitoring during potentially harmful military and commercial activities such as high‐powered Navy sonar, oil and gas exploration, explosive removal of offshore structures, seismic exploration for research and oil and gas, pile driving to install maritime structures including offshore wind farms, and marine hydrokinetic energy devices such as free turbines. The system will consist of multiple single‐beam sonars, each transmitting and receiving a unique signal in a narrow beam. The hardware and software will initially be ported to a Xilinx FPGA, with future versions done within a custom made ASIC. Initial hardware and software research and development will be described, as well as the work done to create computationally efficient signal processi...