Acoustic detection of the landmines, which is based on the analysis of both spatial and frequency dependencies of the acoustic-to-seismic transfer function (A/S TF), exploits the difference between the mine impedance and the impedance of the surrounding ground. However, some deeply-buried mines and some types of the mines are hard to detect due to the natural variability of the ground. This work addresses the problem of false alarms and clutter (high values of the A/S TF in some frequency bands) that mimic the physics of a buried landmine. A time-scale, linear method (wavelet analysis) was utilized for improving the probability of landmine detection. Wavelet analysis of the measured signals resulted in typically stable characteristics for the undisturbed ground, the disturbed ground, and the ground with a mine. These characteristics may be used for the discrimination of false alarms and as an additional criterion to find mines that are hard to locate by traditional methods. The advantages of the suggested technique are illustrated using the experimental data.
Understanding the variability of the grounds acoustic properties will lead to a reduction in false alarms associated with acoustic landmine detection. Experimental measurements of the acoustic-to-seismic transfer functions performed at a US Army eastern temperate site reveal frequency modulation scales in the acoustic-to-seismic transfer function. These modulations have different spatial dependencies along and across the mine lanes. It was hypothesized that these are due to spatial dependencies of the acoustic parameters in the ground layers. It also was speculated that downward gradients in these parameters are due to additional soil strain produced by the wheels of vehicles repeatedly moving down the lane. The measured transfer functions for a few sites were analyzed. It is shown that an elastic layered model of the ground with downward gradients of sound speed in the ground layers successfully models the features observed in the experimental data. Direct time-of-flight measurements of sound speeds in and out of the wheeled tracks confirm the results obtained from the acoustic-to-seismic transfer function analysis.
Inversion methods for estimation of geoacoustic model parameters often use the scattered field data for obtaining the properties of viscoelastic layered media. This work presents a method to retrieve soil background parameters using the outdoor acoustic-seismic transfer function (admittance). Clutter in landmine detection is related to with spatial variations of soil parameters, so knowledge of soil parameters and their spatial variability are very important for landmine detection. The resonance method is extended and used for preliminary estimation of a set of parameters for a three-layered ground model. The least squares method is later used to choose the model with the best fit to experimental data. Results of the reconstruction show good agreement with the experimental data. A description of the resonant technique and the experimental setup are presented. The effect of a finite size of the sound sources often used in acoustic landmine detection on the acoustic-seismic transfer function is also discussed.
Experimental measurements have shown that the use of a multi-layered elastic media is necessary for transfer function numerical modeling. The present work deals with the effect of variability of ground properties (compression and shear wave speeds, density, attenuation and thickness of the layers) on the acoustic-seismic transfer function (admittance) and on clutter in landmine detection. Analysis is performed on the planes of parameters of the ground in a wide frequency range for all angles of incidence. Matrix approach is used to increase the accuracy of computations. It is revealed that the acoustic-seismic transfer function is sensitive to ground properties and that small variations in the shear speed may cause strong variation in the acoustic-seismic transfer function. Results of outdoor measurements of the acoustic-seismic transfer function are presented and a correlation between high magnitudes of the acoustic-seismic transfer function in certain frequency ranges (false alarms) and moisture content on the surface is revealed. A simple model explaining the correlation between moisture content in the upper layer, acoustic-seismic transfer function and ground properties is suggested.