Murray S. Korman

Linear and nonlinear acoustic velocity profiles over buried land mines

Acousto-to-seismic coupling has proven to be an extremely accurate technology for locating buried landmines. Most of the research to date has focused on linear acoustic techniques in which sound couples into the ground, interacts with the buried mine, and causes increased vibration of the ground above the mine. However, Donskoy has suggested that nonlinear acoustic techniques may be applicable to acoustic mine detection. This technique has recently been used with success in field tests at the University of Mississippi and US Army mine lanes. In the nonlinear acoustic technique, airborne sound is produced at two primary frequencies which couple in to the ground and a superimposed compressional wave interacts with the mine and the soil. Because the mine is compliant, contact between the soil and the mine is maintained during the compression phase of the wave, but they are separate during the tensile phase. This creates a bimodular oscillator that is inherently non-linear. This effect has been demonstrated on inert landmines at the University of Mississippi and at US Army test lanes. Results of these tests indicate that nonlinear measurements over buried landmines have more sensitivity than linear measurements. Non-compliant objects such as concrete disks do not exhibit nonlinear phenomena but can be located using linear techniques.

Nonlinear acoustic experiments involving landmine detection: connections with mesoscopic elasticity and slow dynamics in geomaterials

The vibration interaction between the top-plate of buried VS 1.6 and VS 2.2 plastic, anti-tank landmines and the soil above it appears to exhibit similar characteristics to the nonlinear mesoscopic/nanoscale effects that are observed in geomaterials like rocks or granular materials. In nonlinear detection schemes, airborne sound at two primary frequencies f1 and f2 (chosen several Hz apart on either side of resonance) undergo acoustic-to-seismic coupling. Interactions with the compliant mine and soil generate combination frequencies that, through scattering, can effect the vibration velocity at the surface. Profiles at f1, f2, f1-(f2-f1) and f2+(f2-f1) exhibit a single peak while profiles at 2f1-(f2-f1), f1+f2 and 2f2+(f2-f1) are attributed to higher order mode shapes. Near resonance the bending (softening) of a family of increasing amplitude tuning curves (involving the surface vibration over the landmine), exhibits a linear relationship between the peak particle velocity and corresponding frequency. Subsequent decreasing amplitude tuning curves exhibit hysteresis effects. New tuning curve results for buried M 14 and VS 50 plastic anti-personal landmines along with experiments with a buried “plastic drum head” mine simulant behave similarly. Slow dynamics explains the amplitude difference in tuning curves for first sweeping upward and then downward through resonance, provided the soil modulus drops after periods of high strain.

Nonlinear tuning curve vibration response of a buried landmine

Measurements of the acoustic impedance of a VS 2.2 anti-tank plastic landmine reveal significant resonances in the frequency range between 80 and 650 Hz. The top surface resonances are due to its complicated mechanical structure vibrating in air. The lowest mode of the landmine results in a high Q simple harmonic oscillator resonance of the top surface, which behaves like a rigid mass. At higher frequencies the top surface behaves like thin circular plat acoustic modes. When these landmines are buried in soils, the modes are mass loaded. Resonances from measurements of the normal component of the acoustically induced soil surface particle velocity (due to sufficient acoustic-to-seismic coupling) are used for detection schemes. Since the interface between the top plate and the soil responds to pressure fluctuations nonlinearly, characteristics of landmines, the soil, and the interface are rich in nonlinear physics and allow for new methods of buried landmine detection not previously exploited. Here, the structure of a family of resonant tuning curves for relatively low amplitude, but nonlinear drive levels, reveals the “nonclassical” nonlinear resonant behavior of the soil-landmine oscillator.

Nonlinear acoustic techniques for landmine detection: Experiments and theory, Part II

A nonlinear acoustic technique for detecting buried landmines has been suggested by Donskoy [SPIE Proc. 3392, 211 (1998); 3710, 239 (1999)]. Airborne sound at two primary frequencies f1 and f2 undergo acoustic‐to‐seismic coupling and a superimposed ‘‘slow’’ compressional wave interacts with the compliant mine and soil. The nonlinear mechanism involves a simple model of the top surface of the mine–soil planar surface separating two elastic surfaces. During the compression phase of the wave, the surfaces stay together and then separate under the tensile phase due to a relatively high compliance of the mine. This ‘‘bouncing‘‘ soil–mine interface is thought to be a bimodular oscillator that is inherently nonlinear. Geophone measurements scanning the soil’s surface (at the difference frequency) profile the mine, but off the mine some nonlinearity exits. Amplitude‐dependent frequency response curves for a harmonically driven mass‐soil oscillator are used to find the nonlinearity of the soil acting as a ‘‘soft’’...

Design and development of PC‐IMAT: Teaching strategies for acoustical oceanography

The PC‐IMAT (Personalized Curriculum for Interactive Multisensor Analysis Training) project is proving to be a flexible and effectively evolving computer‐based training/educational platform needed to help tackle ASW and other tasks which require extensive analysis, classification, and interpretational skills. Midshipman taking SP411 (Underwater Acoustics and Sonar) are currently using PC‐IMAT [J. Acoust. Soc. Am. 101, 3096A (1997)] to help investigate effective instructional strategies which convey understanding of a complex multivariate domain (like ray trace or propagation loss models). Classroom demonstration lectures and out‐of‐class projects allow students to successfully interact with experimental apparatus and make actual measurements (e.g., Fourier analysis, detection theory, beam pattern functions, sound speed versus temperature, computer ray tracing, reflection and transmission at an interface, and target strength versus angle of a scale model sub). However, a link between textbook theories, the...

Nonlinear acoustic landmine detection: Comparison of ‘‘off target’’ soil background and ‘‘on target’’ soil‐mine nonlinear effects

When airborne sound at two primary tones, f1, f2 (closely spaced near a resonance) excites the soil surface over a buried landmine, soil wave motion interacts with the landmine generating a scattered surface profile which can be measured over the ‘‘target.’’ Profiles at f1, f2, and f1−(f2−f1), f2+(f2−f1), 2f1−(f2−f1), f1+f2 and 2f2+(f2−f1) (among others) are measured for a VS 1.6 plastic, inert, anti‐tank landmine, buried at 3.6 cm in sifted loess soil. It is observed that the ‘‘on target’’ to ‘‘off target’’ contrast ratio for the sum frequency component can be ∼20 dB higher than for either primary. The vibration interaction between the top‐plate interface of a buried plastic landmine and the soil above it appears to exhibit many characteristics of the mesoscopic/nanoscale nonlinear effects that are observed in geomaterials like sandstone. Near resonance, the bending (softening) of a family of increasing amplitude tuning curves, involving the vibration over the landmine, exhibits a linear relationship bet...

Nonlinear acoustic experiments for landmine detection: the significance of the top-plate normal modes

In nonlinear acoustic detection experiments involving a buried inert VS 2.2 anti-tank landmine, airborne sound at two closely spaced primary frequencies f1 and f2 couple into the ground and interact nonlinearly with the soil-top pressure plate interface. Scattering generates soil vibration at the surface at the combination frequencies | m f1 +- n f2 | , where m and n are integers. The normal component of the particle velocity at the soil surface has been measured with a laser Doppler velocimeter (LDV) and with a geophone by Sabatier et. al. [SPIE Proceedings Vol. 4742, (695-700), 2002; Vol. 5089, (476-486), 2003] at the gravel lane test site. Spatial profiles of the particle velocity measured for both primary components and for various combination frequencies indicate that the modal structure of the mine is playing an important role. Here, an experimental modal analysis is performed on a VS 1.6 inert anti-tank mine that is resting on sand but is not buried. Five top-plate mode shapes are described. The mine is then buried in dry finely sifted natural loess soil and excited at f1 = 120 Hz and f2 = 130 Hz. Spatial profiles at the primary components and the nonlinearly generated f1 - (f2 - f1) component are characterized by a single peak. For the 2f1+f2 and 2f2 + f1 components, the doubly peaked profiles can be attributed to the familiar mode shape of a timpani drum (that is shifted lower in frequency due to soil mass loading). Other nonlinear profiles appear to be due to a mixture of modes. This material is based upon work supported by the U. S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate under Contract DAAB15-02-C-0024.

Nonlinear scattering of crossed ultrasonic beams in the presence of turbulence: Experiments performed with pulses

The nonlinear scattering of two finite‐amplitude mutually perpendicular crossed beams—interacting in the presence of turbulence—generates a sum frequency component that radiates outside the interaction region. Experiments are reported where two primary pulsed (f1=2.35 MHz and f2=1.65 MHz) focused beams are generated by 2.54‐cm‐diam concave spherical transducer units (T1 and T2) of focal length 14 cm. The 4‐MHz receiving unit (R) is a 2.54‐cm‐diam circular plane array. The turbulence is generated by a D=0.635‐cm‐diam submerged circular water jet (nozzle exit velocity 7 m/s) whose orifice is located at 56D from the interaction region. All transducer beam axes and jet axis form a common plane. A scattering region is formed at the intersection of the focal points of the primary beams. While T1 and T2 rotate on radius arms—always keeping the beams perpendicular—R is fixed. Symmetry suggests a scattering angle θ*, where θ*=0 defines forward scattering. Ensemble averaged rms pressure spectra (near the sum freque...

Turbulent energy spectrum predictions by nonlinear acoustic scattering.

An underwater nonlinear experiment is performed involving the propagation of finite‐amplitude sound waves through a turbulent flow. The experimental geometry involves the interaction of two mutually perpendicular crossed ultrasonic beams that are generated by individual focused transducer units (with frequencies of 2.0 and 2.1 MHz, respectively). The alignment positions the center of the interaction at the overlapping focal regions. A mechanical apparatus allows the crossed beams to rotate in a horizontal plane containing the submerged circular water jet. The beam axes always remain perpendicular. In the presence of turbulence, a radiated nonlinear sum frequency component (f+=4.1 MHz) is scattered and detected by a receiving transducer unit located outside the interaction region. In the absence of turbulence, there is virtually no nonlinear scattering. Measurements of the broadened intensity spectrum versus angle contain statistical information about the turbulent velocity components. Further analysis sho...

The use of microcomputers in an undergraduate acoustics laboratory

Microcomputers are used in the laboratory as a tool to collect, store, analyze, and graph data from acoustics experiments. Emphasis is placed on teaching the physics involved from individual experiments and not on particular features of the computer. These microstations allow students to record data from the experimental apparatus using the computer. Simple menued software instructions are available to organize the collection of data for printout or for X‐Y pairs to be plotted and labeled. Students can also scale their data and store or plot these variations. This laboratory facility speeds up the tedious and routine data taking chores of the experiment and frees up time for discussion of the physics. In many cases time allows the student a chance to try a few things on his own. Simple acoustics experiments will be shown involving transient vibration and Fourier analysis using our mini‐computer station. This station has a 6502 microprocessor, a two‐channel analog to digital plug‐in board, a disk drive, mo...