When an acoustic wave strikes the ground surface, energy is coupled into the motion of the fluid/solid frame comprising the ground. This phenomenon is termed acoustic-to-seismic (A/S) coupling In the ground, the Biot Type Il or Biot slow waves travel with a speed well below the speed of sound in air. The porous nature of the ground causes the entering acoustic wave to bend toward the normal and the acoustic wave propagates downward into the ground. When an object is buried a few cm below the ground surface, it distinctly changes the A/S coupled motion. These changes can be sensed by measuring vibrational particle velocity on the ground surface. Taking advantage of a noncontact remote measurement technique, the A/S coupling measurements for antitank landmine detection are conducted using a laser Doppler-vibrometer (LDV). Recent field measurements in both calibration and blind mine lanes and the resulting data analysts, which demonstrate the effectiveness of this technique, are described in this paper.

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An investigation of acoustic-to-seismic coupling to detect buried antitank landmines

Review of ground effects on outdoor sound propagation from continuous broadband sources

An acoustic-to-seismic system to detect buried antipersonnel mines exploits airborne acoustic waves penetrating the surface of the ground. Acoustic waves radiating from a sound source above the ground excite Biot type I and II compressional waves in the porous soil. The type I wave and type II waves refract toward the normal and cause air and soil particle motion. If a landmine is buried below the surface of the insonified area, these waves are scattered or reflected by the target, resulting in distinct changes to the acoustically coupled ground motion. A scanning laser Doppler vibrometer measures the motion of the ground surface. In the past, this technique has been employed with remarkable success in locating antitank mines during blind field tests [Sabatier and Xiang, IEEE Trans. Geosci. Remote Sens. 39, 1146-1154 (2001)]. The humanitarian demining mission requires an ability to locate antipersonnel mines, requiring a surmounting of additional challenges due to a plethora of shapes and smaller sizes. This paper describes an experimental study on the methods used to locate antipersonnel landmines in recent field measurements.

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An experimental study on antipersonnel landmine detection using acoustic-to-seismic coupling.

[1] During the early stages of the 2004–2008 Mount St. Helens eruption, the source process that produced a sustained sequence of repetitive long-period (LP) seismic events also produced impulsive broadband infrasonic signals in the atmosphere. To assess whether the signals could be generated simply by seismic-acoustic coupling from the shallow LP events, we perform finite difference simulation of the seismo-acoustic wavefield using a single numerical scheme for the elastic ground and atmosphere. The effects of topography, velocity structure, wind, and source configuration are considered. The simulations show that a shallow source buried in a homogeneous elastic solid produces a complex wave train in the atmosphere consisting of P/SV and Rayleigh wave energy converted locally along the propagation path, and acoustic energy originating from the source epicenter. Although the horizontal acoustic velocity of the latter is consistent with our data, the modeled amplitude ratios of pressure to vertical seismic velocity are too low in comparison with observations, and the characteristic differences in seismic and acoustic waveforms and spectra cannot be reproduced from a common point source. The observations therefore require a more complex source process in which the infrasonic signals are a record of only the broadband pressure excitation mechanism of the seismic LP events. The observations and numerical results can be explained by a model involving the repeated rapid pressure loss from a hydrothermal crack by venting into a shallow layer of loosely consolidated, highly permeable material. Heating by magmatic activity causes pressure to rise, periodically reaching the pressure threshold for rupture of the “valve” sealing the crack. Sudden opening of the valve generates the broadband infrasonic signal and simultaneously triggers the collapse of the crack, initiating resonance of the remaining fluid. Subtle waveform and amplitude variability of the infrasonic signals as recorded at an array 13.4 km to the NW of the volcano are attributed primarily to atmospheric boundary layer propagation effects, superimposed upon amplitude changes at the source.

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The source of infrasound associated with long‐period events at Mount St. Helens

During the early 1980s, the phenomenon of acoustic-to- seismic coupling was used to detect buried objects or mines. In these early measurements, large 2 Hz geophones measured the low frequency normal component of the soil particle velocity over buried targets. Several different, naturally- occurring ground types were studied in these measurement, including grass-covered ground; bare, sandy soil surfaces; and 'dirt' roads. Since the large geophone averages the particle velocity over the area of the sensor case, acoustic-to-seismic transfer function measurements were made with new, smaller-sized geophones. Higher frequency measurements were made using accelerometers. 3D maps of the surface particle velocity were made using measured seismic/acoustic transfer function data. Recognizing the need for a non-contact sensor and the need to investigate the geophone/soil coupling effect in the acoustic-to-seismic transfer function, additional measurements were made using a laser Doppler vibrometer (LDV). This paper explains the acoustic-to-seismic coupling mine detection measurement technique using both geophones and an LDV. The early measurements of the acoustic-to-seismic coupling transfer function for mine-like targets are discussed as well as some recent measurements using a LDV.

Land mine detection measurements using acoustic-to-seismic coupling

Simultaneous measurements have been made of sound pressure above the ground and seismic velocity below the ground surface resulting from a source suspended in the air a variable distance from the surface. The ratio of seismic velocity to acoustic sound pressure has been determined; there are peaks in the ratio in the vicinity of 45 and 90 Hz depending on the height of the speaker. The source–receiver distance was 10, 30, and 60 m; the source height was varied between 1 and 10 m. The frequency of maximum acoustic coupling was found to agree well with theory; the first and third shear modes appear to be excited. Results for vertical, horizontal, and radial motion indicate the coupled seismic signal is greatest for vertical, next greatest for radial, and least for transverse, though the difference between radial and vertical displacement velocities was not great and depended on the speaker altitude.

Coupling of airborne sound into the earth: Frequency dependence

: Acoustic-to-seismic coupling is investigated using measurements of acoustic and seismic signals generated from acoustic sources. Source types used for the measurements were: pure tones, band limited pink and white noise, and broad band noise from an acoustic impulse (gun shot). The measurements showed that the frequency dependence of acoutic-to-seismic coupling was independent of source type. However, the efficiency of coupling favored certain frequency bands for a fixed source height. Dependence of the coupling on source height was also investigated. Measurements showed that the favored frequency bands were lowered as the source height was increased for frequencies below 250 Hz. Two techniques were evaluated for separating the acoustic-to-seismic coupled signature from the measured seismic signature in order to support development of methods for suppressing acoustic-to-seismic coupled background noise. Alternatively, such separation could help improve target classification. The two techniques (one using velocity measurements, the other using coherent measurements) were shown to be potentially useful for separating the acoustic-to-seismic coupled part of the seismic signature from the measured seismic signature. A theoretical approach with a simplified model of plane waves, a smooth surface, and no soil layering was not adequate to explain the measured dependence of acoustic-to-seismic coupling on frequency and source height. (Author)

Mark D. Flohr

Papers

Coupling of airborne sound into the earth: Frequency dependence

Acoustic-to-seismic coupling : properties and applications to seismic sensors