The quantitative advantages of using B-field sensors in time-domain EM measurement for mineral exploration and unexploded ordnance search

Abstract

The use of simple models for decay of conductive targets under conductive overburden and for the decay of magnetically permeable conductive steel objects allows quantitative consideration of the advantages of the use of magnetic-field detectors in time-domain electromagnetic (TEM) measurements, or more generally, the advantage of step response over impulse response TEM systems. We identified eight advantages of the step response versus impulse-response systems. The first two advantages relate to the inductive limit (early time) decay behavior, in which a target response amplitude is largely dependent on geometrical rather than conductivity parameters. Five further advantages occur when measuring response of a target in a conductive host or under conductive overburden; the maximum target-tooverburden response occurs 25%-30% earlier in time, the earliest target detection time occurs a factor 2-4 earlier, and the amplitude advantage of target-to-overburden response is a factor in the range of 1-10 for the step versus impulse-response systems, respectively. These advantages agree quantitatively with field observations on a chalcopyrite orebody under conductive cover. We used a model response for a conductive permeable sphere to derive mathematically consistent approximations for the power-law and exponential decay behaviors for step and impulse responses of metal objects, from which the onset of late-time exponential decay of EM responses of unexploded ordnance occurs about a factor of two earlier in time for the step response. This earlier-time transition together with the higher signal-to-noise ratio available from the step-response measurement makes measurement of the fundamental timeconstant of unexploded ordnance (UXO) possible for medium and large UXO where the time constant is in the range of tens of milliseconds. This time-constant thus becomes accessible as an additional parameter for UXO characterization and discrimination.

On‐site calibration of air‐coil sensor for transient electromagnetic exploration

“…). With the use of TEM shallow detection (Asten and Andrew ; Swidinsky, Hölz and Jegen ; Abu Rajab and Elnaqa ; Frenkel and Yakovlev ; Kukita and Mizunaga ), it is inevitable to eliminate the transition process.…”

“…1, these signals tend to coincide at the later stage of the response, which deems the shallow detection result unreliable (Spies 1989;Xu et al 2010). With the use of TEM shallow detection (Asten and Andrew 2012;Swidinsky, Hölz and Jegen 2012;Abu Rajab and Elnaqa 2013;Frenkel and Yakovlev 2013;Kukita and Mizunaga 2013), it is inevitable to eliminate the transition process.…”

An Improved Finite‐difference Time‐domain Solution for Three‐dimensional Transient Electromagnetic Modeling in Source‐free Media

“…In recent years, the transient electromagnetic method has been widely used in the exploration of mineral deposits, oil, groundwater, and geothermal sources (Ji et al, ). It is also a prior choice for subsurface metal objects detection, such as abandoned wells, pipelines, unexploded ordnance and seafloor mineral deposits (Asten & Duncan, ; Kukita & Mizunaga, ; Swidinsky et al, ). High‐accuracy transient electromagnetic modeling is important in providing a theoretical basis for subsequent data inversion and prospective instrument design.…”

Two-Dimensional Magnetotelluric Modelling of Ore Deposits: Improvements in Model Constraints by Inclusion of Borehole Measurements

“…In particular, the need to refill liquid nitrogen or helium regularly makes SQUIDs unattractive and expensive to operate in MT measurements that last from days to weeks or months, especially when located in remote localities. Nevertheless, owing to the shorter duration of measurements, SQUID magnetometers have been in use for surface or airborne timedomain CSEM measurements for 30 years (Kalberkamp et al 1997;Chwala et al 1999;Bick et al 1999;Panaitov et al 2002;Lee et al 2002;Vallee et al 2011;Asten and Duncan 2012;Smith 2014). For borehole AMT and BHEM measurements, SQUID sensors have the advantage that they are only 2 cm in side length.…”