Evaluating The Effectiveness of Varying Transmitterwaveforms for Uxo Detection In Magnetic Soil Environments

Magnetic soils are a major source of false positives when searching for landmines or unexploded ordnance (UXO) with electromagnetic induction sensors. In adverse areas up to 30% of identified electromagnetic (EM) anomalies are attributed to geology. The EM phenomena that give rise to the response of magnetically viscous soil and metal are fundamentally different. The viscosity effects of magnetic soil can be accurately modelled by assuming a ferrite relaxation with a log-uniform distribution of time constants. The EM response of a metallic target is due to eddy currents induced in the target and is a function of the target’s size, shape, conductivity and magnetic susceptibility. In this presentation, we compare the range and variation of time decays of metallic targets (UXO and scrap metal) with the time domain electromagnetic response of magnetically viscous ground. Electromagnetic sensor measurements and modelling are used to assess the feasibility of multiple pulse widths to differentiate the ground response from metallic targets. We present ground and metal responses measured by Geonics EM61-MK2 sensors with differing transmitter waveform characteristics. Introduction Magnetic soils are a major source of false positives when searching for landmines or UXO with electromagnetic induction sensors. In regions with high concentrations of viscous remnant magnetic (VRM) soils, the large background signal can produce electromagnetic anomalies of the same spatial wavelength and magnitude as buried metallic targets. In adverse areas up to 30% of identified electromagnetic (EM) anomalies are attributed to geology (Cargile et al., 2004). Several techniques have been developed to distinguish between anomalies originating from UXO and geology. These techniques generally include a combination of (1) spatial filtering of data and (2) comparing the EM response of the data to a soil model. Spatial filtering is based on the assumption that the spatial wavelength of anomalies from magnetic geology are larger than the anomalies of metal targets. This process is only partially effective in areas of highly magnetic soil since variations in sensor height and orientation, as well as small variations in the surface topography can produce anomalies similar to those from UXO (Walker et al., 2005). Due to the difficulties in spatially distinguishing between soil and metal anomalies, it is important to compare the TEM decay, or equivalently the frequency response, to the soil response. Viscous remnant magnetic soils produce a well known 1/t step-off time domain electromagnetic voltage response. Clearly, there is a danger of misclassification when the response of the soil closely matches the response of a target. Ware (2003), in addition to spatial filtering, compared how soundings measured on Kaho’olawe by the Geonics EM63 deviated from a magnetic soil model. A direct comparison is feasible with the Geonics EM63 due to the number of time gates (26) and the range of times at which the EM63 records the decay (gates centered from 180 microseconds to 25 milliseconds). Bosnar (2001) compared the expected soil model using 4 time channel Geonics EM61-MK2 TEM data. Candy (1996) suggested a technique which compares the measured response to a soil model when the transmitter illuminates the ground with pulses of different lengths. The success of Candy’s procedure relies on relative differences between excitations from short and long pulse lengths for metal and VRM soils. This technique has been implemented in the Minelabs F1A4 sensor. In this paper, we study the effectiveness of varying the transmitter waveform characteristics applied to an EM61-MK2 sensor. This study is motivated by several practical and theoretical reasons. Although the Minelabs F1A4 sensor is effective in detecting small targets in a VRM soil setting, it has difficulties with the larger deep targets. In addition, it is difficult to perform advanced processing, in particular inversion, on Minelabs F1A4 data due to proprietary on-board data processing. Decay curve comparisons using a Geonics EM63 sensor may be possible in principle, but is difficult to use in many realistic field settings due to its large weight on a wheeled cart platform. The EM61-MK2 is the most common EM tool used in UXO remediation. It has a greater depth of investigation than the Minelabs F1A4, and is a much lighter, field ready alternative to the EM63. Although the EM61-MK2 only has four channels, the addition of multiple pulse widths provides additional data that will enhance the differences between a metal target decay to a soil decay. In this paper we study soil and metal TEM responses and show how these responses differ as a function of the transmitter waveform. We investigate how size variations in the buried metal targets affect the effectiveness of the differential illumination EM technique. The TEM Response of VRM Soils and Metallic Objects The TEM method of detection consists of an EM sensor illuminating the subsurface by time varying magnetic field. A secondary field is produced by decaying eddy currents induced in conductive bodies, or by magnetic domains rotating to align themselves with the primary field. This secondary field is then sensed by the receiver. The background geologic response measured by EM sensors in highly magnetic regions is primarily due to viscous remnant magnetization, or magnetic after-effect, that is characteristic of ferri-magnetic minerals such as magnetite and maghaemite. Viscous remnant magnetization refers to the finite time required for the magnetic grain’s magnetization vector to rotate from its minimum energy orientation prior to the application of field, to its new orientation. Neel (1949) developed the theory for non-interacting single domain grains. The time for a rotation of the magnetization vector is characterized by the Neel relaxation time τ . Let us consider a primary field H that illuminates the soil, then is turned off at t = 0. The magnetization of a collection of grains with time constants distributed log-uniformally between τ1 and τ2 is M (t) ≈ Hχo log (τ2/τ1) (−γ − log t− log τ2) (1) where χo is the dc susceptibility, H is the magnitude of the primary field before step-off, and γ ≈ 0.577 is the Euler constant. In the derivation of (1) it is assumed τ1 ? τ2 and the time of interest t lies within the range 1/τ2 ? t ? 1/τ1. Most TEM sensors measure the time derivative of the secondary field, and are therefore sensitive to the time derivative of the magnetization ∂M ∂t ∝ −Hχo log (τ2/τ1) 1 t (2) For the derivation of equations (1) and (2) we direct the reader to Billings et al. (2003). The step-off 1/t response predicted by equation 2 has been observed in mineral exploration (Buselli, 1982), archaeological prospecting (Colani and Aitken, 1966), and UXO detection (Ware, 2003). To model a spatial distribution of VRM material, a complex, frequency dependent susceptibility based on the assumption of a log-uniform distribution of time constants is used (Pasion et al., 2002a). The theory of the TEM response of metal targets is well understood. Similar to the response of VRM soils, the response of a compact metal target can be expressed as a function of time constants. Kaufman (1994) derived a general form for the field caused by currents induced in a confined conductor. By assuming quasi-static fields, the secondary field produced by currents in a confined conductor can be written as