Benefits of using multi-component transmitter–receiver systems for determining geometrical parameters of a dipole conductor from single-line anomalies
Jacques K. Desmarais 1 3 Richard S. Smith 21 Earth Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, Canada, S7N 5E2.
2 Department of Earth Sciences, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6.
3 Corresponding author. Email: jkd788@mail.usask.ca
Exploration Geophysics 47(1) 1-12 https://doi.org/10.1071/EG14076
Submitted: 24 July 2014 Accepted: 17 February 2015 Published: 25 March 2015
Abstract
We show the advantages of using multi-component transmitter–receiver systems for determining the geometry of a compact planar target whose electromagnetic response can be approximated by a dipole. Our approach is based on a modified version of an algorithm that we previously published using a single-component (vertical) transmitter.
Tests on synthetic models reveal that single transmitter systems are unable to resolve the orientation of a dipole conductor that approaches axial symmetry with respect to the traverse line. This occurs as a result of lack of a noticeable y-component anomaly, where the y-component is oriented transverse to the flight-line direction. For a plate-like conductor, axial symmetry equates to being at a small offset and having a strike parallel or perpendicular to the traverse line. Here, the term ‘offset’ is used to denote the lateral distance from the centre of the conductor to the flight line. The ambiguities can be resolved through measuring specific components of a multi-component transmitter–receiver system; namely, Rxz and Rzz with one of Rxy, Ryy, Rzy, Ryx and Ryz, where the first letter denotes the orientation of the transmitter and the second letter denotes the orientation of the receiver. However, for the case of a MEGATEM system geometry, measuring Rzx, Rzz and Ryx is most suitable for determining the geometry of conductors striking nearly perpendicular or parallel, and at small offset to the traverse line. The minimum system capable of determining the correct geometrical parameters of a dipole conductor for the small-offset symmetric case would therefore consist of a two-component (y- and z-directed) transmitter, as well as a two component (x- and z-directed) receiver.
Tests on line 15701 of the MEGATEM survey in Chibougamau, Quebec, confirm the inability of single-transmitter systems to determine geometrical parameters of a dipole conductor for the case where y-component data is unavailable.
Key words: airborne electromagnetic, conductor, dipole, multi-component, receiver, transmitter.
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