Sensitivity cross-sections in airborne electromagnetic methods using discrete conductors
Richard S. Smith 1 3 Roman Wasylechko 21 Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6.
2 Abitibi Geophysics Inc., 5482 West River Drive, Ottawa, Ontario, Canada, K4M 1G8.
3 Corresponding author. Email: RSSmith@laurentian.ca
Exploration Geophysics 43(2) 95-103 https://doi.org/10.1071/EG11048
Submitted: 9 November 2011 Accepted: 6 March 2012 Published: 13 April 2012
Abstract
A versatile discrete conductor model is used to generate the maximum signal-to-noise ratio along an airborne electromagnetic (AEM) profile. By varying the position of the conductor below and to the side of the airborne traverse, a sensitivity cross-section can be generated that shows the volume of material that influences the AEM response. This type of section accounts for both the coupling of the transmitter with the model and the coupling of the induced current flow with the receiver. Some previous definitions of ‘volumes of influence’ sometimes called ‘footprints’ do not take into account the coupling of the primary field to the target and the secondary field to the receiver. The versatile discrete conductor model can also account for target strike (variable orientation of the current flow) by considering only specific components or orientations of the primary field at the conductor. For a vertical dipole transmitter, the vertical or z-component receiver is generally better for detecting targets at greater depth and the lateral detection range is maximum for the transverse or y component. The in-line or x component is best for sensing conductors where the currents are constrained to flow in a vertical plane perpendicular to the flight direction of the AEM system. The sensitivity cross-sections can also be used for survey design: for example, in order to ensure effective exploration down to 200 m the HeliGEOTEM system must fly with a flight line spacing of 210 m, whereas the more powerful MEGATEM system can achieve equivalent depth penetration with a 300 m line spacing. The discrete conductor model could also be used to estimate the ‘volume of influence’ in ‘moving footprint’ 3D inversion schemes.
Key words: depth, footprint, line spacing, volume, width.
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