Advanced discrimination capabilities address one of the Department of Defense’s (DoD) most pressing environmental problems; the efficient and reliable identification of unexploded ordnance (UXO) without the need to excavate large numbers of non-ordnance items. In recent times, efforts to develop discrimination techniques have focused on electromagnetic induction (EMI) data and not on magnetics. However, very promising results showing low false alarm rates between one and five non-UXO for each excavated UXO have been achieved with magnetic discrimination at sites in Montana. The key to extending these results to other DoD sites lies in improving the fundamental understanding of magnetic remanence, developing a working model for predicting the phenomenology of shock-induced changes to magnetic remanence associated with ordnance impact, and understanding the extent and limitations of this method for practical application.
The objective of this project was to improve UXO discrimination performance and reliability by obtaining a better understanding of magnetic remanence in UXO and shrapnel.
The project team developed a mobile device, the Magnetic Remanence Interrogation Platform (MRIP) for measuring the induced and remanent moments of steel samples. The MRIP comprises 6 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° so that the previous up direction points east.
The MRIP platform was deployed to two field sites (Limestone Hills, MT and Camp Sibert, AL), as well as to the Aberdeen Test Center (ATC), MD where measurements were made on 81-mm mortars before and after firing. At Limestone Hills, 149 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 Camp Sibert, 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.
At ATC, the MRIP device was used to collect measurements of the magnetic remanence of sixty-five 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 charge increments (0, 1 and 2 charge increments). The mortars that initially had low remanent magnetization acquired a magnetization in the direction of the inducing field, with the amount of shock magnetization decreasing with increasing impact velocity. The mortars with medium and high initial magnetization all lost some of their magnetic remanence with the amount increasing with increasing impact velocity (from approximately 50% at 0 charge increment to approximately 70% at two charge increments). Even at the highest impact velocity, shock demagnetization of initially highly magnetized mortars was insufficient to guarantee effective discrimination using apparent remanence.
The conclusion of this project is that discrimination using apparent remanence is not reliable enough to guarantee the excavation of all detected UXO. In the live-site measurements at Camp Sibert and the controlled firing tests at ATC, several ordnance items possessed sufficient magnetic remanence to confound the apparent remanence metric ranking scheme. The measurements made at Limestone Hills suggest that apparent remanence might be more reliable for direct fired artillery projectiles that presumably suffer a larger shock on impact. However, even at Limestone Hills, there was a least one outlier with relatively high magnetic remanence.
The researchers determined that one can never guarantee sufficient demagnetization occurs so that apparent remanence discrimination will be reliable. There is too much potential variability in both the initial magnetic remanence and shock regime experienced by a projectile. For example, consider the case where a direct fired round skips and tumbles several times before coming to rest and never suffers a large shock or where an indirect fired round impacts an easily penetrated soil instead of a nearby rock outcrop.