Predictive Analysis of Acoustic Demining Efforts, Project 14-R9658

Principal Investigators
Jerome A. Helffrich
Walter M. Gray
James D. Walker

Inclusive Dates:  10/01/06 – Current

Background - There is a need for detectors that can sense non-ferrous land mines and improvised explosive devices (IEDs) that may be buried in roadways. One proposed method of finding these usually plastic devices is to project seismic waves from rollers that are pushed along the roadway and scan the undulations of the road ahead using a laser beam. This approach relies upon some assumed properties of the soil underlying the roadway to facilitate quick detection of the response of the mine. Previous researchers in this arena have been unable to do more than come up with a phenomenological explanation of how it works, based on two adjustable parameters. This is insufficient for field use; what is needed is a more detailed model of the interaction of seismic waves and buried objects, and a way to condense this model into a few simple heuristics that can be put into hardware.

Approach - The technical approach to the problem is to conduct both experimental and numerical investigations of a model system consisting of a fiberglass plate buried in a soil-filled box. The numerical analysis is also informed by research on models of granular media in compression and expansion. The project is attempting to numerically model the soil-mine system using LS-DYNA, a Finite Element Analysis (FEA) code, so that different soil models can be compared with measured experimental data on the resonances of a buried plate.

Accomplishments -

• Three test cells were built and instrumented with strain gages and accelerometers; these are used for experiments and long-term monitoring of soil consolidation effects.
• The plate response was measured at realistic vibration levels and was found to be highly nonlinear.
• Nonlinearity was characterized by measuring the resonance frequency shift with drive amplitude and by measuring the harmonic spectrum of the response to sine wave excitation. It is consistent with the Preisach-Meyergoyz theory of hysteretic behavior in solids, indicating that soil has something in common with limestone and concrete.
• Numerical modeling has produced resonance frequencies in agreement with measurements on the buried plate in sand.
• A new soil model involving nonlinear stress-strain relations is being developed for use in this work and by others in the field.

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