Publication Type:Journal Article
Source:Journal of Applied GeophysicsJournal of Applied Geophysics, Volume 115, p.11-23 (2015)
Keywords:base frequency, conductor, dipolar field, duty cycle, footprints, impulse response, infinitem, receiver, signal-to-noise ratio, survey design, time-domain electromagnetic, transmitter, traverse, volume of influence
A discrete conductor model was used to estimate the volume of influence of a dual transmitter loop ground time-domain electromagnetic system (the InfiniTEM system). A sphere model in locally uniform field was used to calculate the signal from a subsurface target where the currents are constrained to flow vertically. The noise was determined from two field surveys. The signal-to-noise ratio was determined at each subsurface target location and each receiver location. The sensitivity of the InfiniTEM system at each target location was defined as the maximum of the absolute value of the signal-to-noise ratio for the ensemble of receiver positions in the survey. The volume of influence is defined as the volume where all targets have a sensitivity greater than one.<br/>The manner in which volume of influence varies can be used to determine the optimal design parameters of an InfiniTEM survey. Our analysis reveals that the InfiniTEM system should be operated with a loop separation distance of 1.5 times the loop width (where width and separation are measured parallel to the traverse lines); and that there should be 4 traverse lines between the loops, corresponding to a traverse line spacing of 250 m for a loop width of 1000 m. For the purposes of delineating highly conductive targets, the optimal waveform parameters are a high duty cycle (in our case 0.75), a low base frequency (in our case 10 Hz), and measurements should be made in the B field domain. For the purposes of finding less conductive targets, the base frequency should be high (in our case 30 Hz), the duty cycle should be low (in our case 025), and measurements should be made in the partial derivative B/partial derivative t domain. Our study confirms that the InfiniTEM system can detect a 100 m radius sphere at up to 925 to depth. We have determined that electromagnetic systems are most sensitive to bodies striking perpendicular to the traverse line. As well, we have confirmed that the InfiniTEM system is most effective at detecting vertical targets. (C) 2015 Elsevier B.V. All rights reserved.
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