Land mine detection measurements using acoustic-to-seismic coupling Ning Xiang * and James M. Sabatier National Center for Physical Acoustics, University of Mississippi, University, MS 38677 ABSTRACT 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 (50-300 Hz) normal component of the soil particle velocity over buried targets. Several different, naturally-occurring ground types were studied in these measurements, 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 (10 Hz) geophones. Higher frequency (1-10 kHz) measurements were made using accelerometers. Three-dimensional 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. Keywords: acoustic-to-seismic coupling, mine detection 1. INTRODUCTION When an acoustic wave strikes the ground surface, energy is coupled into motion of the fluid/solid matrix comprising the ground. The in-phase and out-of-phase motion of the fluid relative to the solid results in two compressional waves that propagate in the ground. This phenomena is well-documented and is termed acoustic-to-seismic coupling in the relevant literature [1]-[6] . In the ground, the pore fluid 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 downwards in the ground [2]-[5] . When an object is buried lower than a few centimeters below the ground surface, it results in distinct changes in the acoustic- to-seismic coupled motion. These changes can be sensed both on the ground surface and below the surface. For purpose of detection of buried objects, it is most practical to sense the acoustic-to-seismic coupled motion on the ground surface. Since early 1980s both theoretical and experimental study of acoustic-to-seismic coupling has been conducted by the University of Mississippi. Different kinds of sensors, including geophones and accelerometers, were used in the early measurements. Taking advantage of a non-contact, remote measurement technique, more recent measurements have been conducted using a Laser Doppler-Vibrometer (LDV). The present paper reviews the early measurements of acoustic-to-seismic coupling and discusses recent measurement results using a scanning LDV. 2. GEOPHONE MEASUREMNTS In early 1980s the acoustic-to-seismic coupled surface vibration was measured using large geophones. A geophone is a sensor of velocity; its output voltage is proportional to the vibration velocity of an object surface under test. A L-4 geophone (manufactured by Mark Products Inc is a cylinder with a diameter of 7.6 cm and a height of 13 cm) was used in the first stage of the mine detection research program using acoustic-to-seismic coupling. This kind of geophone shows high sensitivity and covers a frequency range between 50 Hz and 300 Hz. Flat metal plates or simulated anti-tank mines were buried a few centimeters below the surface of different soil types. These soil types included roadway, gravel road, river bottom sand, and * Correspondence: Email: nxiang@olemiss.edu Ph. (662)-915-5989, Fax: (662)-915-7494. In Detection and Remediation Technologies for Mines and Minelike Targets V, Abinash, C. Dubey, James F. Harvey, J. Thomas Broach, Regina E. Dugan, Editors, Proceedings of SPIE Vol. 4038 (2000)  0277-786X/00/$15.00 645