Low Level Helicopter Magnetic Survey For Uxo Detection

A helicopter-mounted magnetometer survey was used to locate unexploded ordnance (UXO) at the Massachusetts Military Reservation (MMR). The system, called the HM3, was designed and built by High Sense Geophysics of Toronto, Canada, now Fugro Airborne Surveys. The system has threecesium vapor magnetometers housed in three booms attached to the base of the helicopter. Two of the booms extend 6 meters from either side of the helicopter, and the other boom extends 6 meters from the front of the helicopter. Positioning is accomplished using real time Differential Global Positioning System (DGPS). In addition to the cesium vapor magnetometers, a fluxgate magnetometer installed on the aircraft is used to facilitate magnetic compensation due to aircraft movement. The objective of the survey was to locate significant areas containing UXO as well as relatively clear areas. The survey covered about 2200 acres at the MMR, where many years of artillery practice has resulted in munitions scattered across the area, some of which may be unexploded. The area is covered with scrub oak bushes, sometimes quite dense, with heights of about 2 to 3 meters. Also present are areas of pine trees about 10 meters in height. The thick vegetation, along with safety considerations, makes an airborne survey the most realistic option for locating areas of UXO debris. The helicopter flew 3 to 4 meters above the vegetation, giving a flight height of 7 to 15 meters depending on the vegetation height. Many magnetic anomalies were observed, including large anomalies from tanks and other metal objects deliberately positioned as targets. Also observed were linear anomalous patterns with amplitudes of one or two nano Teslas (nT) extending in a northerly direction. These anomalies are thought to result from magnetite minerals in the soil, with the distribution of magnetite being controlled by glaciation and subsequent weathering. In addition, data were recorded at various heights over typical targets buried in the ground. This was done to allow the airborne system to be calibrated. This calibration data shows that magnetic anomaly amplitudes of 2 to 4 nT could be resolved at a flight height of 7 meters, decreasing to background noise above about 15 meters.