Steven R. Baker

Seismic sonar sources for buried mine detection

Prior research on seismo‐acoustic sonar for detection of buried targets [J. Acoust. Soc. Am. 103, 2333–2343 (1998)] has continued with examination of various means for exciting interface waves (Rayleigh or Scholte) used to reflect from targets. Several seismic sources were examined for sand beach applications, including vibrating shakers, shaker devices configured to preferentially excite interface waves, linear force actuators, and arrays of shaker sources to create directional interface wave beams. Burial of some plate‐like or rod‐like portion of the vibrating devices was found to ensure good coupling to the beach. The preferential interface excitation device employed two degrees of freedom to mimic the two components of elliptically polarized interface waves, and was successfully demonstrated. However, it was found that at long ranges, the medium itself created two component interface waves from vibrating source radiations operating with one degree of freedom in the vertical plane. Linear force actuato...

Interface wave source concepts for a seismic sonar concept

A seismic sonar concept is being developed which employs guided interface (Rayleigh or Scholte) waves to detect objects buried in sea floor and beach sediments. These waves have the desirable characteristics for this application that (1) they are localized near the interface and (2) the propagation from a compact source is cylindrical. As a result, the wave intensity on targets of interest is enhanced and the reverberation suppressed, compared to a bulk wave. To selectively excite Rayleigh or Scholte waves, a harmonic source employed at the interface must excite elliptical particle motion in the vertical and propagation, or radial, directions. Several such source concepts will be described. Results to date of field tests employing these sources in narrow‐band pulse tests will be described. [Work supported by ONR Code 321OA.]

A primary method for the complex calibration of a hydrophone from 1 Hz to 2 kHz

Primary calibrations of hydrophones at frequencies less than about 1 kHz are typically performed in a coupler reciprocity chamber (“coupler“); a closed test chamber where time harmonic oscillations in pressure can be achieved and the reciprocity conditions required for a primary calibration can be realized. The closed and controlled environment in the coupler allows for the performance of primary calibrations over the temperature and hydrostatic pressure range found in the ocean. The coupler reciprocity system employed by the United States, in service since the 1960s, provides only the magnitude of the pressure sensitivity and not the phase. Recent work has demonstrated a method for the primary calibration of both the magnitude and phase of the complex sensitivity for a hydrophone at frequencies ranging from 1 Hz to 2 kHz. The combined expanded uncertainties of the magnitude and phase of the complex sensitivity at 1 Hz were 0.1 dB re 1V/μPa and ±1°, respectively.

Optimizing seismo‐acoustic sonar source array configuration for maximum radiated Rayleigh wave energy

A three‐dimensional (3‐D) continuum mechanics approach to the development of a time‐dependent finite element model for optimizing the position and excitation of a seismo‐acoustic sonar source array to detect the presence of buried landmines will be presented. Various source configurations will demonstrate the use of constructive and destructive interference, which maximizes the radiated energy of unidirectional Rayleigh waves while suppressing the radiation of body waves. Radiation characteristics are analyzed in a linear, horizontally stratified (isotropic and homogeneous within each layer) half‐space with a discrete number of transient seismic sources. Results for Rayleigh wave strengths are presented in both a homogeneous half‐space and a layered medium.

Transmitting beam patterns of an echolocating bottlenose dolphin

Measurements on a free‐swimming subject, echlocating under a simple go paradigm, were conducted at SSC San Diego, with a linear array of seven simultaneously and individually processed wideband hydrophones (response to 400 kHz), arraigned either vertically or horizontally, and centered on a small underwater video camera, with the video output synchronized with the recording of acoustic data. The measurement apparatus itself served as the test target. Lowering it into the water provided the cue for the blindfolded subject to locate its position in the test pen, swim to the apparatus, and touch it with the tip of its rostrum; whereupon the trainer provided a bridge signal, indicating reward due. During this process, acoustic beampattern measurements were made on echolocation clicks, as a function of frequency, which support previous, non‐simultaneous, statistical beampattern measurements. Early in the experiment, the subject emitted echolocation clicks that peaked at 78 kHz, with −3 dB beamwidths of 8 to 10...

Time‐dependent finite‐element model for optimizing source array element position and excitation for a seismic sonar for buried mine detection

A three‐dimensional (3‐D) continuum mechanics approach to the development of a time‐dependent finite‐element model for optimizing the position and excitation of source array elements for use in a seismic sonar to detect buried landmines is presented. Mathematical formulation of the problem consists of the coupling of a system of linear, second order, partial differential equations and related boundary conditions into one single wave equation, from which a composite elastic finite element is derived. The hp‐adaptive finite‐element kernel, ProPHLEX [Altair Engineering, Inc., McKinney, TX], is used to perform the numerical computations. The radiation characteristics of a discrete number of transient seismic sources are analyzed in a linear, isotropic, homogeneous half‐space. Results for radial and vertical radiation fields, and for radiated Rayleigh wave strength will be presented for various source configurations. Particular attention will be paid to those configurations which maximize the radiation of unidirectional Rayleigh waves, while suppressing the radiation of unwanted body waves.

High frequency components in cetacean echlocation signals

The rich literature of high‐resolution biosonar capability is sometimes baffling as to attainable sonar resolution. This has led us to measurements in San Diego Bay, on captive research dolphins, utilizing an extremely wide band piezo‐composite hydrophone, with a frequency response extending to 2 MHz. The results indicate that cetacean echolocation signals contain frequency components, above ambient noise, that can extend to the neighborhood of 500 kHz. The study was conducted on two bottlenosed dolphins and one beluga whale. Attempts were made to determine if these animals were actually using these high frequency components in echolcation, but they were not completely successful. However, measurements with rho‐c matched, acoustic, low pass filter panels, placed between the blindfolded subjects (on a bite bar) and their test targets, showed that as the detection tasks became more difficult, the animals increased the intensity of the high frequency content of their transmissions (to ∼500 kHz) and their pulse repetition frequencies also increased. [Work supported by the U.S. Navy ONR.]

Demonstration of an end‐fire array Rayleigh wave source for a seismo‐acoustic sonar

A linear array of four vertical‐motion sources was deployed on the sand in the near‐surf zone of Del Monte Beach, Monterey, CA. The sources were spaced 25 cm apart (approximately one‐quarter wavelength at 100 Hz, the nominal operating frequency) and were driven with a transient signal in a sequential fashion so as to preferentially radiate Rayleigh waves in one end‐fire direction. Beam patterns were measured at a radius of 3.5 m. Measurements were made with the array directing the radiation toward, away from, and parallel to the surf line. In general, results were in fair agreement with simple (nondispersive) theory, except for the depth of the nulls. A measured front‐to‐back radiation suppression of approximately 15 dB was routinely achievable.

Detection of buried mines with seismic sonar

Prior research on seismo‐acoustic sonar for detection of buried targets [J. Acoust. Soc. Am. 103, 2333–2343 (1998)] has continued with examination of the target strengths of buried test targets as well as targets of interest, and has also examined detection and confirmatory classification of these, all using arrays of seismic sources and receivers as well as signal processing techniques to enhance target recognition. The target strengths of two test targets (one a steel gas bottle, the other an aluminum powder keg), buried in a sand beach, were examined as a function of internal mass load, to evaluate theory developed for seismic sonar target strength [J. Acoust. Soc. Am. 103, 2344–2353 (1998)]. The detection of buried naval and military targets of interest was achieved with an array of 7 shaker sources and 5, three‐axis seismometers, at a range of 5 m. Vector polarization filtering was the main signal processing technique for detection. It capitalizes on the fact that the vertical and horizontal componen...

Relationship between the diffraction constants, the reciprocity parameter, and the mutual radiation impedance of a pair of compact radiators: An improved Pritchard approximation

By the principle of reciprocity, the mutual radiation impedance matrix of an acoustical array in a stationary fluid must be symmetric. However, use of the classical ‘‘Pritchard approximation’’ to compute array mutual (mechanical) radiation impedance (Zmut=j Re[Zself]e−jkd/kd;  ejωt assumed) [R. L. Pritchard, J. Acoust. Soc. Am. 32, 730–737 (1960)] leads to a matrix which is not symmetric for nonidentical radiators. Also, it is restricted to low frequencies (ka≪1). By a simple application of the principle of reciprocity, it is shown that the mutual mechanical radiation impedance of a pair of reasonably compact radiators in an otherwise unbounded stationary fluid is expressible in terms of their (complex) diffraction constants D1 and D2 and the (complex) spherical wave reciprocity parameter Js as Zmut=S1S2D1D2/Js, where S is the radiator surface area, Jsf=2d/jρf, d is the separation between radiator acoustic centers, ρ is the fluid density, and f is the frequency. (For mutual acoustical radiation impedance,...