Michael S. McMillan

Inversion of airborne geophysics over the DO-27/DO-18 kimberlites — Part 2: Electromagnetics

AbstractWe focus on the task of finding a 3D conductivity structure for the DO-18 and DO-27 kimberlites, historically known as the Tli Kwi Cho (TKC) kimberlite complex in the Northwest Territories, Canada. Two airborne electromagnetic (EM) surveys are analyzed: a frequency-domain DIGHEM and a time-domain VTEM survey. Airborne time-domain data at TKC are particularly challenging because of the negative values that exist even at the earliest time channels. Heretofore, such data have not been inverted in three dimensions. In our analysis, we start by inverting frequency-domain data and positive VTEM data with a laterally constrained 1D inversion. This is important for assessing the noise levels associated with the data and for estimating the general conductivity structure. The analysis is then extended to a 3D inversion with our most recent optimized and parallelized inversion codes. We first address the issue about whether the conductivity anomaly is due to a shallow flat-lying conductor (associated with th...

3D IP Inversion of Airborne EM data at Tli Kwi Cho

In this study, we revisit three airborne EM surveys over Tli Kwi Cho (TKC). These consist of a frequency domain DIGHEM data set, and two time domain surveys, VTEM and AeroTEM. Negative transients have been recorded in both of the time domain surveys and we interpret these as arising from chargeable bodies. The kimberlite pipes are referred to as DO-27 and DO-18. We look in more detail at the transient data and apply the ATEM-IP inversion procedure to recover a 3D pseudo-chargeability distribution. Important components of the analysis involve estimating a background conductivity for the region. For DO-27 we have used a 3D parametric inversion to recover the conductivity from TEM data. The IP signal for the inversion is obtained by subtracting the time domain responses estimated by EM inversion from the observed background signal. This process also removes EM coupling noise that might be contaminating the data. The resultant IP data are inverted with a linear inverse approach using the sensitivity from the background conductivity. This yields a 3D model of pseudo-chargeability.

3D Cooperative Parametric Inversion of Frequency and Time-domain Airborne Electromagnetic Data

We developed a 3D frequency and time-domain parametric level-set AEM inversion code for multiple bodies with independent resistivities and applied a cooperative approach for spatially overlapping data sets. We tested the method on both synthetic and field data, and the synthetic results showed that the cooperative parametric approach recovered the most accurate inversion model. We then applied the method to overlapping AEM data sets at the Committee Bay greenstone belt in Nunavut, Canada to image linear thin dipping conductors where the dip information is crucial for the exploration program.

3D Multiple Body Parametric Inversion of Time-domain Airborne EM Data

We developed a 3D parametric inversion for time-domain airborne EM data using a skewed ellipsoid representation for multiple conductive or resistive anomalies. The approach aims to simplify the task of imaging thin, potentially highly conductive, anomalies with 3D EM inversion. The algorithm finds the optimal location, shape, size and resistivity of the anomalies in a homogeneous or heterogeneous background by employing a Gauss-Newton style optimization. Our parametric method is tested on a synthetic and field data set. The synthetic model is composed of two narrow dipping conductive anomalies in a resistive background along with a vertical narrow conductor. The survey layout and resistivity structure is based off field data from a greenstone setting. The parametric inversion accurately recovers the spatial extent and dips of the three synthetic anomalies, although the depth extent of the anomalies is exaggerated. In the greenstone field example, the inversion defines the spatial location, extent and dips of three conductive anomalies to provide a new conductivity interpretation of an area where little information is known regarding the true nature of the conductors.

Reply to “Cooperative constrained inversion of multiple electromagnetic data sets” (Michael S. McMillan and Douglas W. Oldenburg, 2014, Geophysics, 79, no. 4, B173–B185)

We thank Andre Revil for his interest in our paper (McMillan and Oldenburg, 2014) and for raising some concerns, and we would like to add a few comments in reply. Mr. Revil mentions in his discussion two assumptions that have been made in our paper. The first is that the electric conductivity of rocks containing metallic ores, such as sulfides, is frequency dependent. The second assumption states that we expect to see conductive anomalies in the presence of sulfide and gold mineralization, which goes against theoretical and laboratory studies outlined in Revil (2014). These assumptions are closely connected, and their relevance in interpretation depends upon the total chargeability of the material, the range of electromagnetic (EM) frequencies used in the analysis, and the conductivity values of the various rock units. The frequency dependence of sulfide-bearing rocks is a well-researched phenomenon, and for completeness it could have been addressed in our original paper (e.g., Pelton et al., 1978; Mahan, 1986). The goal of our paper was to present a practical strategy for cooperatively/jointly inverting data from multiple EM surveys to delineate conductive and resistive structures that have important consequences for a field exploration program. Although frequency dependence of conductivity is potentially a factor, when compared with the cumulative effect of other assumptions that are necessary to invert field data (e.g., precise knowledge of survey parameters, uncertainties assigned to data, and stopping criteria for the inversion), we believe that formally including this complicating factor would not have substantially altered the outcome of the paper. It also would not have aided the original goal of presenting …