Resolving Power of Some Geophysical Methods as Applied to Detecting Buried Unexploded Ordnance

Abstract

Various geophysical sensors are employed for the purpose of detecting and clearing unexploded ordnance (UXO) items buried in the battlefield including inland, beachhead, surf‐zone, and the ocean. Similar sensors are also used for environmental investigation of ordnance and explosive waste (OEW) sites including bombing ranges, target practice areas, and explosive manufacturing facilities, as well as nuclear, biological, and chemical (NBC) waste sites. One of the most important issues is the concept of “standoff” detection so that the survey crew and their equipment are reasonably safe from accidentally setting off buried explosives or contacting hazardous materials. Despite the long history of geophysical sensors used at UXO sites, there exists a knowledge gap between geophysicists and the end‐users in understanding the applicability and limitations of the geophysical tools. I attempt in this article a heuristic review of classical geophysical methods from a perspective of their usefulness for standoff, non‐intrusive target detection. The most routinely used UXO sensors are magnetometers and metal detectors. A magnetometer is a passive sensor that measures the geomagnetic distortion caused by a nearby ferrous target. A metal detector actively broadcasts an electromagnetic field and receives the secondary field induced by conductive targets. Barring unforeseen developments, I advocate in this article that magnetic and electromagnetic methods are, and will be, perhaps the only acceptable and effective tools in the near future for detecting buried UXOs. A discussion follows on the resolving power of the magnetic and electromagnetic methods in terms of UXO sizes and standoff distances. This quantitative discussion provides a theoretical basis for determining whether a given survey platform and a specified modus operandi would detect and resolve a specific, buried UXO at a stipulated standoff distance. The relationship between the standoff distance and the required resolution also governs whether survey platforms may be ground‐based or airborne.