Field Measurements of Induced and Remanent Moments of Unexploded Ordnance and Shrapnel

Magnetometry is a widely used technique for clearance of areas contaminated with unexploded ordnance. The discrimination of hazardous ordnance from nonhazardous items is possible using apparent remanence if ordnance is shock demagnetized on impact with the ground. We developed a mobile device, the Magnetic Remanence Interrogation Platform (MRIP), for measuring the induced and remanent moments of steel samples. The MRIP comprises six three-component fluxgate magnetometers symmetrically distributed around a rotating sample holder. Samples are placed on the holder and are slowly spun through two complete rotations. The measurement is repeated after the sample is physically rotated by 90^deg so that the previous up direction points east. The MRIP platform was deployed to two field sites. At the first site, 76- and 90-mm projectiles were measured. Most had relatively low remanent magnetization relative to the induced, suggesting that shock demagnetization occurs. Variations in the remanent moments of different specimens of 90-mm projectiles were attributed to variations in the type of steel used. There was a strong correlation of the direction of remanent and inducing field during burial, indicating evidence of shock magnetization and/or magnetic viscosity. At the second site, intact and partial 4.2-in mortars as well as shrapnel, base plates, and cultural debris were measured. Most of the base plates and intact mortars had low remanent magnetization, whereas the shrapnel and partial mortars tended to have higher remanent magnetization. The results indicate that there is an inherent risk in using apparent remanence for discrimination as not all ordnance is demagnetized on impact.