Doru Velea

An effective fluid model for landmine detection using acoustic to seismic coupling

A model is developed to describe the response of the ground to airborne sound in the presence of a buried landmine. The model describes both the near-field acoustic wave phenomena as well as the mechanical dynamics of the mine itself. Since buried landmines are typically close to the surface of the soil, the induced ground vibration is the result of near-field scattering so that classical asymptotic scattering theory cannot be used. Instead, the near-field problem is solved in a right circular waveguide with rigid walls. The waveguide contains air in the upper half, soil in the lower half, and a buried mine placed concentrically on the waveguide’s axis. The advantage of a waveguide model over infinite space models is that the resulting computer models are much more straightforward to code and run much more quickly. As the radius of the waveguide increases, the results of the waveguide model converge to those of the infinite space model. For low frequencies (a few hundred Hz) this convergence is quite rapid: it is found that a waveguide radius of 10 times the mine radius is more than sufficient. The results obtained are found to qualitatively explain the phenomena observed in the field.

Modal expansions for infrasound propagation and their implications for ground-to-ground propagation

The use of expansions in vertical eigenmodes for long range infrasound propagation modeling in the effective sound speed approximation is investigated. The question of convergence of such expansions is related to the maximum elevation angles that are required. Including atmospheric attenuation leads to a non-self-adjoint vertical eigenvalue problem. The use of leading order perturbation theory for the modal attenuation is compared to the results of numerical solutions to the non-self-adjoint eigenvalue problem and conditions under which the perturbative result is expected to be valid are obtained. Modal expansions are obtained in the frequency domain; broadband signals must be modeled through Fourier reconstruction. Such broadband signal reconstruction is investigated and the relation between bandwidth, wavetrain duration, and frequency sampling is discussed.

A wide-angle high Mach number modal expansion for infrasound propagation

The use of modal expansions to solve the problem of atmospheric infrasound propagation is revisited. A different form of the associated modal equation is introduced, valid for wide-angle propagation in atmospheres with high Mach number flow. The modal equation can be formulated as a quadratic eigenvalue problem for which there are simple and efficient numerical implementations. A perturbation expansion for the treatment of attenuation, valid for stratified media with background flow, is derived as well. Comparisons are carried out between the proposed algorithm and a modal algorithm assuming an effective sound speed, including a real data case study. The comparisons show that the effective sound speed approximation overestimates the effect of horizontal wind on sound propagation, leading to errors in traveltime, propagation path, trace velocity, and absorption. The error is found to be dependent on propagation angle and Mach number.

A fast effective fluid model for the scattering of normally incident sound off of a buried landmine

Landmines buried in the ground can be found acoustically by insonifying the ground and detecting a contrast between the vibratory motion of the ground surface directly above the mine and away from the mine. A model to predict the near‐field scattering of low‐frequency (∼100 Hz) acoustic waves off of a landmine is presented. The mine is assumed to be a rigid cylinder with a compliant top. The ground (soil) is modeled as an effective fluid. Because of the short range of the scattered field at low frequencies it has been possible to replace a full space model with a waveguide model that is straightforward to implement numerically. The predictions of the model will be discussed. [Work supported by the U. S. Army Communications‐Electronics Command Night Vision and Electronic Sensors Directorate and the U.S. Army Research Office.]

PERMANENT REMOVAL OF THE WALL POROSITY IN MONOLITHIC CATALYST SUPPORT CERAMICS

A technique is described that allows for the permanent removal of the wall porosity from monolithic catalyst support ceramic samples in order to have a better characterized porous medium. Specific acoustic impedance measurements for a rigid‐backed ceramic sample have confirmed the effects of wall porosity on sound propagation in square‐tube, capillary‐type porous ceramics previously observed by Arnott and co‐workers [J. Acoust. Soc. Am. 90, 3299–3306 (1991)].

NCPAprop—A software package for infrasound propagation modeling

Developed by the infrasound group at the National Center of Physical Acoustics, University of Mississippi, and a few collaborators, NCPAprop is an open source software package aiming at providing a comprehensive set of tested and validated numerical models for simulating the long range propagation of infrasonic signals through the earths atmosphere. The algorithms implemented in NCPAprop are designed for frequencies large enough that the effects of buoyancy can be neglected and small enough that propagation to ranges of hundreds to thousands of kilometers is possible without significant signal attenuation. Nominally, NCPAprop can, without modification, be used to efficiently model narrowband propagation from 0.1 to 10 Hz and broadband propagation from 0.05 Hz to 2 or 3 Hz. NCPAprop provides both geometrical acoustic and full wave models which will be presented. The geometrical acoustics part consists of 2-d and 3-d ray tracing programs as well as a non-linear ray theory model. The full wave models consist of a suite of normal mode models of increasing complexity and a Parabolic Equation (PE) model.Developed by the infrasound group at the National Center of Physical Acoustics, University of Mississippi, and a few collaborators, NCPAprop is an open source software package aiming at providing a comprehensive set of tested and validated numerical models for simulating the long range propagation of infrasonic signals through the earths atmosphere. The algorithms implemented in NCPAprop are designed for frequencies large enough that the effects of buoyancy can be neglected and small enough that propagation to ranges of hundreds to thousands of kilometers is possible without significant signal attenuation. Nominally, NCPAprop can, without modification, be used to efficiently model narrowband propagation from 0.1 to 10 Hz and broadband propagation from 0.05 Hz to 2 or 3 Hz. NCPAprop provides both geometrical acoustic and full wave models which will be presented. The geometrical acoustics part consists of 2-d and 3-d ray tracing programs as well as a non-linear ray theory model. The full wave models consis...

Comparision of measured versus predicted buried mine resonant behavior

The resonant behavior of landmines has been exploited by an acoustic detection technique to find buried mines. The resonance of the buried mine is induced by broadcasting an acoustic wave, which couples into the ground. The resonating mine causes the soil above it to vibrate and this vibration is measured with either a laser Doppler vibrometer (LDV) or a geophone. A set of resonance frequencies, which can be attributed to the design, material, and dimensions of the mine, is exhibited when the mine, sitting on a rigid surface above the ground, is excited by an acoustic wave. These resonance frequencies shift when the mine is buried. Acoustic models have been developed to predict these burial effects on mine resonant frequency behavior. This paper will discuss measurements made of several mines of the same type buried at various depths and will compare these measurements to predictions made by a lumped element model.

Resonant frequency response and surface particle velocity profiles for buried land mines: Theory and experiment

Acoustic‐to‐seismic coupling is currently used to detect buried landmines by measuring a contrast in the particle velocity of the air–soil interface directly above the mine (on‐target) and away from the mine (off‐target). Field measurements reveal a resonance phenomenon of the mine–soil system at frequencies around 100 Hz. The resonance frequency and the spatial profile of the on/off‐target velocity ratio depend on the type of the mine, depth, and soil characteristics. Experimental results for antitank mines will be presented. These results will be compared with predictions by a recently developed model for the scattering of normally incident sound off of a landmine. The model assumes that the mine has a compliant top and the soil is an effective fluid. [Work supported by the U.S. Army Communications‐Electronics Command Night Vision and Electronic Sensors Directorate and the U. S. Army Research Office.]

Acoustic scattering by buried objects in a rigid porous material

Land mines buried a few inches below the surface of the ground can be found by acoustic excitation of the porous ground surface and measuring the particle velocity at the surface. There are various theoretical models describing the ground: from a rigid porous frame model to a compete layered poroelastic description. The goal of this paper is to use the approach of Berry et al. to calculate the acoustic field at points on the ground surface in the vicinity of an object buried in a rigid, porous soil. The excitation is point sound source placed in the air above the ground, which is modeled a rigid, porous frame. A boundary element method is used for numerical integration to calculate the scattered acoustic field due to the presence of the object. This study represents the first step towards developing a complete model of acoustic scattering from near-surface objects embedded in a layered poroelastic material. The predicted disturbance associated with the buried object is much smaller than observed in field measurements.

Acoustic scattering by a buried object with a compliant top in a rigid porous material

Recently, a method for the detection of landmines buried a few inches below the surface of the ground has been developed at the University of Mississippi. This method utilizes acoustic‐to‐seismic coupling and measures the particle velocity of the insonified ground surface on‐ and off‐target with a Laser Doppler Vibrometer. As part of the on‐going theoretical effort to explain the measurements, this paper is concerned with acoustical scattering from an object buried in a rigid porous half‐space. The object is assumed to be a rigid right circular cylinder with the exception that its top can move in the vertical direction in a pistonlike fashion with a mechanical resonance at around 100 Hz. A boundary element method has been employed to predict the on‐target/off‐target particle velocity ratio. Comparisons with observations are made. [Work supported by the U.S. Army Communications‐Electronics Command Night Vision and Electronic Sensors Directorate.]