Summary

The Pentamag is the next generation of magnetic field sensors designed by Mobile Geophysical Technologies of Germany specifically to be carried by a custom-designed octocopter in a survey pattern close enough to the ground to detect and accurately locate small metal objects, e.g. unexploded ordnances (UXO). This system carries an array of five fluxgate magnetic sensors in a linear array. An earlier system, with a 2- fluxgate magnetometer array, has already been shown as a tool that can be useful to detect and accurately locate UXO. The Pentamag system is specifically designed with RTK location accuracy and for operation close to the ground to sense small metal objects. The magnetometer system is integrated into a specifically designed octocopter that can operate in survey mode for up to 25 minutes. Standard operational procedure is to specify a series of GPS locations to represent a regular survey, including both location and elevation above grade, based on a lidar system to guide elevation control.

Testing of the performance of this system was carried out on a military test facility in Northern Germany, on which more than 50 metal objects (amunition) were buried at known locations and depths. The size of the area is about 1 hectare. The time needed to screen this area was about 20min. The field test demonstrated that the system prototype is fully functional in an operational environment. As result the multicopter borne Pentamag system is capable for detecting and locating objects of sizes of at least 3.7cm.

Introduction

and efficiently cover large tracts of land and wide areas for the purpose of screening and identifying areas that potentially contain unexploded ordnance (UXO). Large areas across many countries are potentially contaminated with UXO, with some ranges encompassing tens to hundreds of thousands of hectares. Ultimately, safe clearance of contaminated areas requires a standoff or an unmanned, remote detection capability due to the inherently unsafe nature of UXO. Technologies are needed which will allow for cost effective wide area scanning with near 100% coverage and near 100% detection of subsurface ordnances. But current sensor systems have very limited capabilities to discriminate clutter from UXO. As a result, nearly every anomaly must be excavated to determine if it is, in fact, an UXO. Current technologies, either walking on ground or manned helicopter, can achieve these requirements. Two manned helicopter based systems have been developed over the past decades in the USA, the ORAGS (Oak Ridge Airborne Geophysical System) system developed by Oakridge National Lab (Doll et al. 2001; Gamey et al. 2003) and the Multisensor Towed Array Detection System (MTADS) system developed by the Naval Research Laboratory (Nelson et al. 2004). Both systems are similar in design, consisting of a boom equipped with a number of cesium vapor magnetometers mounted in the front of a manned helicopter. But these systems are not cost effective, dangerous to operate, and the associated economics make it impossible for potential clients to apply universally. In order to be effective for small UXO detection, the sensing altitude for magnetic site investigations needs to be on the order of 1 – 2 meters above ground level (AGL). An unmanned aerial vehicle (UAV) magnetometer platform is an obvious alternative to ground magnetic measurements or manned helicopters. The motivation behind such a system is that it is safer for the operators, cheaper in initial and operational costs, and more effective in terms of site characterization. Key features of our proposed system are:

1. The availability of a customized octocopter platform with autonomous flight capability, payload capacity and endurance
2. assembling of five fluxgate magnetometers on this octocopter instead of only one or two sensors, which considerably enhances the resolution and detectability of small objects
3. the low power consumption of the five sensors and ancillary systems, which is negligible compared to the power ratings of the octocopter
4. a positioning system which encompasses RTK accuracy for accurate localization of objects
5. a laser altimeter that controls the flight altitude enabling flight operations with a ground clearance of about 1m. This ability is a key feature for maintaining low flight altitudes above ground.

The ideal multicopter magnetometer system should allow for the effective and accurate detection of metal objects of concern. Flight operations should be done by the acquisition of regularly spaced values of the magnetic field as closely as possible to the ground. Here we demonstrate that the Pentamag owns these capabilities. We have invested much time and effort to:

1. improve the in-flight sensor resolution, sensor temperature stability and the reduction of sensor weight
2. design and build an octocopter optimized for the deployment of five magnetometers and maximizing the endurance for this payload
3. integrate a laser altimeter to maintain a constant altitude of the UAV and sensor package above the ground level
4. ensure that the total weight of the system does not exceed 10 kg. In this configuration, this system is very flexible and can be used worldwide. This UAV category requires less effort to successfully apply for a flight permission and to receive export permissions for most countries.
5. The data acquisition system shall allow highly advanced positioning and synchronization of the position and magnetic data. The sample rate shall be at least 100Hz.

The detection and delineation of UXO places many highly demanding requirements on the measuring system:

The ideal system needs to demonstrate near 100% detection over all terrains with very limited false alarms. This is not a technically feasible goal even for ground based systems. Therefore, a series of thresholds and requirements will be established to demonstrate a leap ahead in the capabilities and performance over current capabilities.