Survey design to maximize the volume of exploration of the InfiniTEM system when looking for discrete targets Jacques K. Desmarais ⁎, Richard S. Smith Earth Sciences, Laurentian University, 935 Ramsey Lake Rd., Sudbury, Ontario P3E 2C6, Canada abstract article info Article history: Received 5 August 2014 Received in revised form 11 November 2014 Accepted 6 February 2015 Available online 11 February 2015 Keywords: Survey design Volume of Influence Impulse response Time-domain electromagnetic Dipolar field Duty cycle Base frequency Transmitter Receiver Traverse Signal-to-noise ratio InfiniTEM Conductor 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 loca- tion 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. 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 dis- tance 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 param- eters 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 0.25), and measurements should be made in the ∂B/∂t domain. Our study confirms that the InfiniTEM system can detect a 100 m radius sphere at up to 925 m 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. © 2015 Elsevier B.V. All rights reserved. 1. Introduction TDEM systems work on the principles of electromagnetic induction. A time-varying electric current is passed through a transmitter loop pro- ducing a time-varying magnetic field in space. According to Faraday's law, this magnetic field induces electric currents in nearby conductors (Nabighian and Macnae, 1991). The induced currents produce a second- ary magnetic field which is sensed by a receiver coil. There are a number of applications of TDEM methods including environmental protection and hydrology, in which the mobility of subsurface fluids is monitored. Such surveys allow for the detection of permafrost and groundwater, the delineation of pollution plumes and saline intrusions in aquifers, the depiction of leakage from tailings dams, and the characterization of acid mine drainage (Keller, 1997; Sheard et al., 2005; Brakni, 2011). Mapping applications have become especially important in shallow environmental surveys (Sheard et al., 2005). The TDEM method also has important applications in the discov- ery and delineation of ore bodies and the characterization of unexposed conductive objects in civil engineering studies (Brakni, 2011). The method is also commonly used as a supplement to the magnetic meth- od in geological mapping, where it provides depth information. The focus of our interest is mineral exploration, where TDEM methods have had extensive application (Nabighian and Macnae, 1991). One weakness of the TDEM method is a result of the large horizontal loop that is normally used, which does not produce strong horizontal primary magnetic fields. In Archean terranes, diapirical plutonism has rendered volcanic strata vertical (Van Kranendonk et al., 2004), so syngenetic ore bodies, which are generally stratabound are often vertically dipping. Faraday's law predicts that horizontal pri- mary magnetic fields will couple strongly with vertically dipping bodies. Thus, horizontal primary magnetic fields are required when exploring for vertically dipping Archean syngenetic mineralization. This type of mineralization includes volcanogenic Zn–Cu, Zn–Cu–Au deposits, mag- matic Ni–Cu–PGE deposits, and syngenetic Au deposits (Thurston et al., 2008). As these mineralizing styles account for a large portion of the mineral resources within the Canadian Shield, the need for a TDEM Journal of Applied Geophysics 115 (2015) 11–23 ⁎ Corresponding author. E-mail addresses: jk_desmarais@laurentian.ca (J.K. Desmarais), rssmith@laurentian.ca (R.S. Smith). http://dx.doi.org/10.1016/j.jappgeo.2015.02.012 0926-9851/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo