Experimental measurements of shock induced changes to the magnetization of unexploded ordnance

•We measured the magnetic remanence of ordnance items before firing and after impact.•Shock demagnetization on impact erased part of the existing remanent magnetization.•Ordnance acquired a shock induced magnetization on impact.•Discrimination of unexploded ordnance using magnetometry is unreliable.

Millions of acres of land around the world are potentially contaminated by unexploded ordnance (UXO). Magnetometry is a technique widely used to both detect and characterize buried UXO. It has been hypothesized that ordnance suffer a large shock on firing and impact that erases any preexisting remanent magnetization. If such demagnetization occurs, an apparent remanence metric has been shown to be effective at distinguishing hazardous ordnance from non-hazardous metallic debris. To test the shock demagnetization hypothesis, an experiment was conducted at a firing range to measure the magnetic remanence of sixty-five inert 81 mm mortars before firing and after impact. As delivered, 64 of the 65 rounds had very low remanent magnetization and a magnetizer had to be used to impose various amounts of remanence on the mortars. Three different categories of initial remanent magnetization were created (low, medium and high remanence) and these were fired at three different initial velocities. The mortars that initially had low remanent magnetization acquired a magnetization in the direction of the Earth's inducing field after impact, with the amount of re-magnetization decreasing with an increasing impact velocity. This effect is known as shock magnetization. The mortars with medium and high initial magnetization all lost some of their magnetic remanence, with the amount of demagnetization increasing with an increasing impact velocity. However, even at the highest impact velocity, shock demagnetization of initially highly magnetized mortars was insufficient to guarantee effective discrimination using apparent remanence.