Assessment of an active electromagnetic sensor for hunting buried naval mines

Electromagnetic (EM) sensors potentially provide a valuable tool for buried naval mine classification in fusion with acoustic sensors because their sensor modalities are very distinct from imaging sonars. Passive magnetics provide moderate ranges against ferrous mines and shorter ranges against low-ferrous mines. While active EM sensors, limited by practical constraints on size and power, would provide only shorter-range sensing against sea mines, their ranges against low-ferrous mines are projected to approach those obtained by passive magnetic sensors. In addition, active EM sensors can detect and characterize nonferrous metallic objects, and in some circumstances even nonmetallic objects. Given new tactics in which short-range sensors are incorporated into reacquisition minehunting systems for short-range classification, active EM sensors using multifrequency sources for target illumination can provide valuable multispectral discrimination for the classification of buried mines. The US Navy is investigating use of active broadband EM sensors as a candidate technology for its Buried Minehunting (BMH) sensor suite. Geophex, Ltd. has demonstrated that broadband EM sensors can detect metallic ordnance buried in the ground and under the sea bottom at short ranges. Geophex also demonstrated that electromagnetic induction spectroscopy (EMIS) techniques could be used to measure the spectral "fingerprint" of a target, which is a function of the shape and metallic composition of the target. In this paper, we report results of initial investigations to measure the performance of Geophex active EM sensors moving past ferrous and nonferrous targets in seawater under controlled conditions. Results would be used to provide guidance for design of an active EM sensor as part of a multisensor suite in a BMH reacquisition and identification system.