James M. Sabatier

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

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.

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

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.

Acoustically induced seismic waves

When an airborne acoustic wave is incident at the ground surface, energy is coupled into the ground as seismic motion. In a previous publication [Sabatier et al., J. Acoust. Soc. Am. 78, 1345–1352 (1986)] the ground surface was modeled as an air‐filled poroelastic layer overlying a semi‐infinite, nonporous elastic substrate. In this work, the model is extended to include calculations of the normal seismic transfer function (ratio of the normal soil particle velocity at a depth d to the acoustic pressure at the surface). Measurements of the seismic transfer function for three sites are considered and compared to the predicted values. Generally good agreement between theory and experiment is achieved by best fits assuming the soil or seismic attenuation. This is accomplished by specifying the ratio of the imaginary to real part of the measured seismic p‐ and s‐wave speeds. The seismic transfer functions quite typically exhibit minima and maxima which are associated with the seismic layering of the ground su...

The interaction of airborne sound with the porous ground: The theoretical formulation

The surface of the ground is modeled as that of an air‐filled poroelastic soil layer of known thickness overlying a semi‐infinite nonporous elastic substrate. Using a modified form of the Biot–Stoll differential equations for wave propagation in fluid‐saturated porous media, propagation constants for the two possible dilatational waves and the shear wave in the poroelastic layer are determined. The dilatational waves are identified as a fast wave, moving predominately in the solid frame, and a slow wave, moving predominately in the pore air. The elastic moduli in the substrate are assumed to be those of the solid grains of which the poroelastic soil layer is composed. Intergranular friction in the soil and substrate is assumed to be negligible. Boundary conditions at the air–soil interface and at the porous soil–substrate interface are applied to determine, numerically, the displacement amplitudes of the allowed wave motions. From the incident and reflected amplitudes at the air–soil interface, the normal...

Vibration and sound signatures of human footsteps in buildings

The acoustic signature of a footstep is one of several signatures that can be exploited for human recognition. Early research showed the maximum value for the force of multiple footsteps to be in the frequency band of 1-4 Hz. This paper reports on the broadband frequency-dependent vibrations and sound pressure responses of human footsteps in buildings. Past studies have shown that the low-frequency band (below 500 Hz) is well known in the literature, and generated by the force normal to the ground/floor. The seismic particle velocity response to footsteps was shown to be site specific and the characteristic frequency band was 20-90 Hz. In this paper, the high-frequency band (above 500 Hz) is investigated. The high-frequency band of the vibration and sound of a human footstep is shown to be generated by the tangential force to the floor and the floor reaction, or friction force. The vibration signals, as a function of floor coverings and walking style, were studied in a broadband frequency range. Different walking styles result in different vibration signatures in the low-frequency range. However, for the walking styles tested, the magnitudes in the high-frequency range are comparable and independent of walking style.

Laser-doppler based acoustic-to-seismic detection of buried mines

Airborne acoustic waves coupled into the surface of the ground excite Biot Type I and II compressional and shear waves. This coupling of airborne sound into the ground is termed acoustic-to-seismic coupling. If a land mine or other inhomogeneity is presented below the surface, the ground surface vibrational velocity or S/A ratio will increase due to reflection and scattering of the Type II compressional wave. The dispersion characteristics of this wave in solids determines the mine detection limits. The S/A ratio is read with a laser doppler vibrometer (LDV). The loud speaker and LDV were mounted onto a large forklift at Fort AP Hill. This system was used to scan patches of ground at the Fort AP Hill calibration mine lanes. An investigation on the variability of surface velocity over different background types and mine types is described. The results of these initial field exercises are described.

A Review of Human Signatures in Urban Environments Using Seismic and Acoustic Methods

Techniques for sensing footstep vibrational frequencies, typically below 100 Hz, by seismic sensors are well- developed human detection methods. Walking styles (standard, soft, or stealthy) and the background noise floor limit the detection range of footsteps. Walking style changes the dynamic footstep force on the ground and the influences the footstep detection range. The seismic background noise floor is much higher in urban areas and in buildings than in rural areas, dramatically influencing detection range. Alternatively, high- frequency passive and active ultrasonic methods for human detection are being developed. High-frequency sound produced by friction forces between a foot and the ground/floor allow passive footstep detection in urban areas and in buildings. The active method utilizes continuous-wave Doppler ultrasound. Simultaneously collecting Doppler motion and footstep ultrasonic signals reveals correlated timing features between the footstep friction and the Doppler shift from the human motion. Discrimination between human and other moving targets is accomplished by analyzing the envelopes of footstep signatures.

Choosing Biot parameters for modeling water-saturated sand

The work of Chotiros [J. Acoust. Soc. Am. 97, 199–214 (1995)] has stimulated renewed interest in detecting the Biot type II (slow) P wave for the purpose of ocean bottom characterization. Chotiros postulates that total internal reflection of the type I (fast) P wave can occur at a water–sediment interface and that the observed refracted energy below the critical angle is associated with the type II (slow) P wave. The choice of parameters used in the Biot theory to model these observations is discussed. Two major differences between the parameter set proposed by Chotiros and previous parameter sets for sandy sediments exist. First, the value for the unjacketed bulk modulus, Kr, is about five times less than the bulk modulus for quartz crystals, which is commonly used. This value is obtained by equating it to a value of effective grain modulus measured by Molis and Chotiros [J. Acoust. Soc. Am. 91, 2483(A) (1992)]. Second, the value for the frame bulk modulus, Kb, obtained by fitting to measured values of t...

Probe microphone instrumentation for determining soil physical properties: testing in model porous materials

Abstract An acoustic technique for evaluating soil physical properties is described and tested in model porous materials. A probe microphone is used to measure the acoustic signal attenuation and phase speed. Emphasis is on the probe construction, insertion in soils and reduction of acoustic data to predict tortuosity and an effective air-flow resistivity of the soil. Well-known capillary-tube models of porous materials which incorporate these two porous material properties are used to invert the acoustic data. The probe microphone, associated hardware, data acquisition and analyses are implemented on a personal computer. The system can provide real time prediction of the soil properties in the field. Measurements in three materials: glass beads, washed sand and loess soil are analyzed and discussed. The instrumentation works well in the glass beads and washed sand tested; however, in the loess soil tested some restrictions occur.

Range limitation for seismic footstep detection

Seismic methods for footstep detection exploit low frequency vibration waves, typically below 100 Hz. There are two limiting factors for detection of human footsteps at these frequencies: walking styles and the background noise floor. The walking style changes the dynamic footstep force on the ground and, therefore, limits the maximum distance at which walkers may be detected. For seismic frequencies, the background vibration noise floor is higher in urban areas than in quiet areas. This article presents and discusses test results of human footstep measurements as a function of distance using the seismic method in quiet and urban areas.