Vladimir N. Fokin

Effect of ground variability on acoustic-to-seismic transfer function and false alarms in landmine detection

The spatial variability of ground properties leads to fluctuations in the acoustic-to-seismic transfer function (A/S TF), the ratio of the normal particle velocity on the ground to sound pressure. In some cases, these fluctuations may lead to false landmine detection alarms. This work shows that small variations in the ground properties may cause strong variations in the A/S TF. Experimental measurements of the A/S TF performed at a US Army eastern temperate site are presented and a correlation between high magnitudes of the A/S TF (false alarms) and moisture content on the surface is shown. A simple model of the ground explaining this correlation is suggested. This model was used to describe spatial distribution of high magnitudes in the TF and natural spatial variability of the TF. Results of calculations were compared with experimental data. Two frequency modulation scales in the A/S TF are observed at positions on the ground where land mines are not located. It was hypothesized that these are due to i...

Determination of effective properties and effective thickness of resonant acoustic metamaterial

Multiple scattering in periodic structures with strong modulation of elastic constants leads to phononic band structures. According to the Bragg’s theory, the spatial modulation of the periodic structure must be of the same order as the wavelength of considered elastic waves. A resonant acoustic metamaterial’s respective wavelength is about two orders of magnitude larger than the lattice constant. Each unit cell consists of a locally resonant structural unit in contrast to simple acoustic scatterers in phononic crystals. In this report, we will address the design of locally resonant sonic materials. These materials may offer dynamic effective negative material properties around resonance frequency. Locally resonant sonic crystals, such as an array of Helmholtz resonators and cylinders or spheres coated with acoustically soft material, were analyzed in the literature. However, existing literature lacks a systematic study of the dependence of effective properties for a finite slab of these metamaterials bot...

Influence of wheeled vehicular traffic on the acoustic-to-seismic transfer function

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.

Wavelet analysis for landmine detection false alarm discrimination

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.

Determination of soil background parameters via acoustic-seismic transfer function

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.

Effect of ground variability on clutter and false alarm in landmine detection

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.

Effects of elasticity and porosity in modeling of the acoustic‐to‐seismic transfer function

Modeling sound interaction with the ground is important both for remote sensing techniques and the elimination of false alarms in the landmine detection application. Elastic and porous‐elastic models of the ground are most frequently used for these studies. Though both of these models are well known, few comparisons have been made between the acoustic‐to‐seismic transfer functions (TF) calculated from these two models for the frequency range 100–1000 Hz, which is typical for geoacoustic applications. In this work, the matrix technique was exploited to solve boundary equations for porous‐elastic layers. This technique allows one to obtain the TF and refraction indexes for arbitrary porous‐elastic stratifications of ground layers. The results of test computations of original codes are presented. Effects connected with the slow wave in the porous‐elastic model are analyzed. Effective parameters of the visco‐elastic model are analyzed to give the best fit to frequency dependence of the TF calculated in the fr...

False alarms associated with the acoustic‐to‐seismic detection of buried land mines

Important problems in landmine detection include false alarms and clutter [high values of the acoustic‐to‐seismic transfer function (TF) in some frequency bands] that mimic the physics of a buried landmine. Many of these high values of the TF are due to the natural variability of the ground. In this paper both the space‐frequency variability of the TF(f,x,y) and the connection with ground variability are discussed. The viscoelastic, layered model of the ground qualitatively explains high values of the TF both in certain spatial regions of the scan site and for some frequencies. This model and the measured spatial dependence of the TF(f,x,y) were used to model the space‐frequency distribution of the acoustic‐to‐seismic transfer function. Comparison between the calculated and measured transfer functions in the space‐frequency volume show that this model can satisfactorily explain variability of the acoustic‐to‐seismic transfer function. To investigate the spatial variability of ground parameters, the measur...

Modal analysis of broadband acoustic signals propagating in the top layer of the ground

Experimental measurements of the frequency dependence of sound propagation losses in the top layers of the ground were performed in the frequency range between 80 and 420 Hz. These measurements revealed that the distribution of energy on the frequency‐range plane has a regular structure. The features of this structure were modeled in the modal approach for a gradient model of the ground. It was shown that average sound speed in the ground and the approximate depth of the layers may be estimated through analysis of the experimentally measured frequency‐range distribution of energy. The technique explores the waveguide properties of the top layer of the ground. According to the mode theory, two asymptotes coming from the point (0,0) should exist on the wave number‐frequency plane. From the slope of these asymptotes, the minimum and maximal sound speeds in the waveguide were found to be 100 and 530 m/s. The approximate depth of the layers was estimated. Comparisons of obtained sound speeds and depth of layer...

Geoacoustic inversion via acoustic‐seismic transfer function

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 ground properties information using the outdoor acoustic–seismic transfer function (admittance). The resonance method developed in A. Nagle, H. Uberall, and K‐B. Yoo [Inverse Prob. 1, 99–110 (1985)] is extended and used for a preliminary estimation of a set of parameters for a three‐layered ground model. The least squares method is used afterward 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 sound source on the acoustic–seismic transfer function is also discussed. [Work supported by ONR.]