Michal Kolaj

A multiple transmitter and receiver electromagnetic system for improved target detection

We have developed an alternative strategy for the inevitable deeper inductive electromagnetic (EM) exploration, which will be required as shallow deposits are exhausted. Rather than using very large magnetic moment ground loops, measurement stations are repeated using many smaller sized loops with smaller moments. The multiple transmitter data are then weighted and summed into a single high signal-to-noise ratio (S/N) composite transmitter. The composite transmitter can be thought of as a postprocessing method that uses the collected multitransmitter data to construct/simulate a transmitter, which maximizes the coupling to a particular target. The appropriate transmitter weights to use will depend on the target location and geometry, and, as such, different weighting schemes allow for the construction of different composite transmitters, each of which will maximally highlight different targets. We have assumed no prior knowledge of the location and orientation of the exploration targets, and we constructed composite transmitters for each possible location of a discretized subsurface and 324 possible target orientations (dipole embedded within a fully resistive medium). A modified difference of squares and a dipole look-up table was used to assess the fit between each composite transmitter and the suggested target location and orientation. Synthetic studies using conductive plate target(s) embedded within a fully resistive medium found that the target locations and orientations could be accurately determined and that the S/N of the composite transmitter was significantly higher than that of standard fixed-loop ground and airborne surveys. In a ground time-domain EM field test, 23 transmitter positions were used, and a shallow target (conductive dike) could be identified using the developed methodology. The composite transmitter data we produced was considerably easier to interpret and had a larger amplitude than that of any one single transmitter.

Mapping laterally varying conductance using EM gradients over dry tailings ponds

Summary Mine waste or the saprolite associated with nickel laterites have a conductivity thickness (conductance) that varies laterally. In order for EM methods to be used to easily map lateral changes in conductance, a simple inversion needs to be developed. Through forward modeling we show that, assuming a uniform conductance and inverting for an apparent conductance is sensitive enough to identify lateral conductance changes. However, one of the required terms in the inversion is the vertical derivative of the secondary vertical magnetic field (dHz/dz). The accurate measurement of spatial electromagnetic derivatives requires a good signal-to-noise ratio. Field studies performed over a relatively resistive dry tailings pond showed that the gradient noise is sufficiently lower than the expected signal, suggesting that dHz/dz should be measurable.