3D inversion of total magnetic intensity data for time-domain EM at the Lalor massive sulphide deposit
Dikun Yang 1 2 Douglas W. Oldenburg 11 Geophysical Inversion Facility, Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2020-2207 Main Mall, Vancouver, British Columbia, Canada V6T 1Z4.
2 Corresponding author. Email: yangdikun@gmail.com
Exploration Geophysics 48(2) 110-123 https://doi.org/10.1071/EG15070
Submitted: 24 July 2015 Accepted: 5 December 2015 Published: 13 January 2016
Abstract
The Lalor deposit is a massive sulphide that is characterised as a stack of conductive ore lenses buried more than 600 m deep. We invert helicopter sub-audio magnetics (HeliSAM) data from Lalor collected using a ground large transmitter loop and an airborne total magnetic intensity (TMI) magnetometer measuring the time-domain electromagnetic (EM) responses. The TMI data are modelled as a projection of the anomalous field onto the earth’s magnetic field. Inversion of these data is considered a significant case study because of two challenges. First, the early-time data are contaminated by the infrastructure on the surface. Second, inverting the data with a uniform half-space as the initial model results in a mathematically acceptable, but non-geologic, model. We create workflows to overcome these difficulties. For the contaminated data, we use a locally refined mesh and a constrained inversion to recover highly conductive cells near the surface that effectively represent the infrastructure. This allows us to safely extract geologic information from the early time data. The non-uniqueness in the inversion is reduced by warm-starting the voxel 3D inversion with a more reasonable initial guess, for example, a block model from geometric inversions. Those procedures greatly improve the inversion image from the surface to the bottom of the target at Lalor, and they can easily be incorporated into the industrial production workflows.
Key words: 3D inversion, Lalor, mineral exploration, time-domain electromagnetic, total magnetic intensity, volcanogenic massive sulphide.
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