| Mechanical resonances in landmines are often exploited by acoustic landmine detection schemes. This work, therefore, focuses on a single plastic, blast-hardened, anti-tank landmine, the Italian VS 1.6. Modal analysis experiments show seven modes below 1.6 kHz in the pressure plate of the VS 1.6. Also observed is a large shift of the first symmetric mode such that it occurs at a frequency higher than that of the first asymmetric mode.{09}With the exception of the first symmetric mode, the landmine's modes are adequately modeled by an elastically supported, elastic plate of uniform thickness. The shift of the first symmetric mode is shown to be caused by a large volume below the pressure plate. Simulated burial experiments in water exhibit downward shifts in modal frequency with depth that saturate at shallow depth. Similar experiments in sand show a re-ordering of modes upon flush burial of the landmine. Said phenomenon is qualitatively due to shear contact between the edge of the pressure plate and the sand. Burial simulation in sand on an acoustic simulant created to mimic the in-sand flush-buried modes of the VS 1.6 shows saturation of the shift in the first symmetric mode at a shallow depth of 4 cm. |
