On the imaging of radio-frequency electromagnetic data forcross-borehole mineral exploration

Abstract

Summary Radio‐frequency (typically from 0.1 to 20 MHz) electromagneti c methods are powerful tools for locating conductive mineralization in ore exploration and mine development. Yet data interpretation is complicated by the non‐linear relationship between the observed electric and magnetic fields and the electrical parameters of the Earth. The principal means of quantifying inversion capabilities is to compute synthetic data sets using accurate numerical models and to perform the inversion under controlled conditions. Our specific interest is in locating 3‐D bodies that are highly conductive relative to the host rock. An excellent approximation for this class of targets, at least at radio frequencies, is to assume that the bodies are infinitely conductive. The numerical advantage of this assumption is that inhomogeneities can be represented simply as internal boundaries where the total electric and magnetic fields are identically equal to zero. Ensuring numerical stability thus does not require excessive discretization in conductive regions since the maximum grid cell size is determined only by the electrical parameters of the host material. We use a finite‐difference time‐domain approach to compute the total electric and magnetic fields everywhere within the background medium and validate the code by comparisons with two analytical solutions. One common means of interpreting radio‐frequency electromagnetic data is to assume linearity between the model parameters and physical response and to apply tomographic image reconstruction methods. While relatively simple and inexpensive, the limitations and applicability of tomographic imaging methods to non‐linear electromagnetic data acquired in complicated, 3‐D mineral exploration environments are not well understood. Our initial study involves applying the simultaneous iterative reconstruction technique to recover images of the electrical properties of a conductive inclusion. Several examples show that the structural geometry of bodies between boreholes can be reliably imaged using both frequency‐doma in and time‐domain data. Phase data seem more amenable to recovering geometry information from tomographic reconstruction methods than amplitude data. However, attenuation data provide better constraints on the electrical properties of the geological media and thus form an essential complement to primarily geometrical information obtained from phase tomography. Non‐linear inversion methods will probably be required to incorporate the amplitude data for accurate reconstructions of the subsurface.

Modelling and straight‐ray tomographic imaging studies of cross‐hole radio‐frequency electromagnetic data for mineral exploration

“…The electromagnetic waves in the radio-frequency range (typically 0.1-10 MHz) provide a good balance between imaging resolution and the signal-to-noise ratio level (Wilkinson 2005). A few RIM systems based on EM fields radiated at discrete frequencies have been developed, which can measure the amplitude and phase data simultaneously (Yu et al 1998;Karimi-Sharif 2013;Korpisalo and Heikkinen 2014). A few RIM systems based on EM fields radiated at discrete frequencies have been developed, which can measure the amplitude and phase data simultaneously (Yu et al 1998;Karimi-Sharif 2013;Korpisalo and Heikkinen 2014).…”

“…Since then, the straight-ray imaging method has become the primary RIM data interpretation method used in the mining exploration community (Steven et al 2000;McDowell, Stewart and Monteiro 2007;Korpisalo 2016). Of these methods, the SIRT algorithm is that primarily used in the mining exploration community (Jackson and Tweeton 1994;Yu et al 1998). Of these methods, the SIRT algorithm is that primarily used in the mining exploration community (Jackson and Tweeton 1994;Yu et al 1998).…”

“…This exploration method has been applied to coal-seam hazard detection (Hill 1984), ore body delineation (Thomson and Hinde 1993;Mutton 2000;Stevens, Watts and Redko 2000), and site selection for underground disposal of nuclear waste (Korpisalo and Heikkinen 2014). A few RIM systems based on EM fields radiated at discrete frequencies have been developed, which can measure the amplitude and phase data simultaneously (Yu et al 1998;Karimi-Sharif 2013;Korpisalo and Heikkinen 2014). The information provided by cross-borehole imaging becomes more critical when the mining activities go deeper, as drilling becomes more expensive and people need to extract more information from the boreholes to understand the geology between holes.…”

“…Straight-ray imaging algorithms used to solve the inverse problem include the back-projection technique, the algebraic reconstruction technique, the simultaneous iterative reconstruction technique (SIRT), and the conjugate gradient least squares method (Radcliff and Balanis 1979;Hansen and Saxild-Hansen 2012). Of these methods, the SIRT algorithm is that primarily used in the mining exploration community (Jackson and Tweeton 1994;Yu et al 1998). A critical question usually asked after a survey is whether the reconstructed image is reliable.…”

Characterising borehole radio antenna performance using finite-difference time-domain modelling

“…The pattern of the antenna is usually taken into account when inverting radio tomography data by assuming the pattern for a perfect dipole. Here, a Finite-Difference Time-Domain (FDTD) numerical model (Yu et al, 1998;Vogt, 2000) is applied to the electrically thin wire dipole antenna, typically used for RT, leading to rules of thumb for the selection and operation of RT antennas.…”