The combined use of magnetic and electromagnetic sensors for detection and characterization of UXO

Data has been collected on a test field of inert UXO with both a Geometrics 822A total field magnetometer and a Geonics EM61 pulsed, electromagnetic induction sensor. This study allows comparison of the two instrument's detection capabilities. While the EM61 as currently configured is not as sensitive as the 822A in terms of depth of detection, it does detect objects of interest down to 10-15 feet, as well as detecting non-ferrous materials. While the location, depth, and size of single UXO targets are readily estimated from magnetic measurements, it is not as clear what can be done with an electromagnetic sensor that will synergistically add to the capability of the magnetic sensor. Based on laboratory measurements, electromagnetic sensors may be capable of size estimation based on their temporal response. For frequency domain instruments such as the EM31, this involves measuring the phase angle response. For time domain instruments such as the EM61, this involves recording the time decaying response. This size estimation is not affected by remnant magnetization and also reflects the size of individual objects in a collection of objects. Because the spatial response of an electromagnetic sensor involves the relative geometry of the transmitter, the object, and the receiver, it is expected to be sensitive to object orientation. Measurements made on large elongated UXO in different orientations confirm this.