Many unexploded ordnance (UXO) sites present unique challenges to deployment and precise positioning of geophysical instrumentation. These difficulties result in increased costs for any given survey deployment as well as diminished precision in the spatial correlation of data collected in subsequent passes or resurveys. Diminished sensor location precision makes it difficult to correlate targets selected from separately collected data sets. As one would expect, this also has a detrimental effect on the ability to distinguish UXO from ordnance scrap when applying physics-based analysis algorithms. Electromagnetic (EM) and magnetometer arrays deployed simultaneously may provide the relative positioning precision required to support these analyses.
The objective of this project was to develop a laboratory prototype frequency-domain electromagnetic induction (FDEM) sensor array suitable for simultaneous operation with an array of cesium vapor total field magnetometers.
Arrays of total magnetic field (TMF) and EM sensors are complementary for detection of UXO. Unlike EM sensors, magnetometers do not respond to non-ferrous metals. EM sensors have a limited depth of investigation, while the depth of investigation of TMF surveys is limited only by the size of the target. It follows that combined surveys could result in increased probabilities of detection as compared to surveys using only one or the other technology.
Researchers initially developed a notional sensor array design based on the GEM-3 sensor active primary field cancellation technology, which creates a "magnetic cavity" for each receive coil in the array. The design has the magnetometers mounted inside the receive coils, within the magnetic cavities. This is important because the primary transmit field would otherwise be strong enough to shift the TMF vector outside the operating envelope of the magnetometers. Because the array configuration does not have the symmetry of a standard GEM- 3, in-phase drift tends to be more severe.
The project team then built and tested a working laboratory prototype FDEM sensor array and performed measurements of a set of standard objects, verifying the viability of a co-planar FDEM sensor array suitable for co-deployment with standard cesium vapor magnetometer sensors. Tests with the prototype array showed drift rates roughly one order of magnitude larger than with a standard GEM-3. However, demedian filtering such as that used for the MR-200033 towed GEM-3 array can correct for the drift.
The most serious issue for the array is structural rigidity. When the array is bent, the bucking coils do not properly cancel the field from the outer transmit loop. Calculations indicate that bucking errors can be caused by bending the array by a few tenths of a degree over a distance of about 30 cm. The array would have to be held stiff to reduce these errors. For towed applications on land, this may be difficult because the array will be subjected to bending stresses as it is towed over any realistic terrain. The situation may not be so serious for marine applications where the array would be buoyed up and not subject to such severe stresses.
As compared to separately deployed EM sensor and magnetometer arrays, a combined array could have the advantage of increased detection efficiency. The relative position accuracies for all sensors on a single pass also are correlated and precise to apply advanced physics-based analysis algorithms, which are required for successful discrimination of UXO from clutter.