On the value of including x-component data in 1D modeling of electromagnetic data from helicopterborne time domain systems in horizontally layered environments Casper Kirkegaard a, ⁎, Nikolaj Foged a , Esben Auken a , Anders Vest Christiansen a, b , Kurt Sørensen a a Department of Geoscience, Aarhus University, Hoegh-Guldbergsgade 2, DK-8000 Aarhus C, Denmark b Geological Survey of Denmark and Greenland, Lyseng Allé 1, DK-8270 Hoejbjerg, Denmark abstract article info Article history: Received 2 December 2011 Accepted 4 June 2012 Available online 23 June 2012 Keywords: TEM X component 1D layered modelling AEM Airborne geophysics SkyTEM Helicopter borne time domain EM systems historically measure only the Z-component of the secondary field, whereas fixed wing systems often measure all field components. For the latter systems the X-component is often used to map discrete conductors, whereas it finds little use in the mapping of layered settings. Measur- ing the horizontal X-component with an offset loop helicopter system probes the earth with a complemen- tary sensitivity function that is very different from that of the Z-component, and could potentially be used for improving resolution of layered structures in one dimensional modeling. This area is largely unexplored in terms of quantitative results in the literature, since measuring and inverting X-component data from a he- licopter system is not straightforward: The signal strength is low, the noise level is high, the signal is very sensitive to the instrument pitch and the sensitivity function also has a complex lateral behavior. The basis of our study is a state of the art inversion scheme, using a local 1D forward model description, in combination with experiences gathered from extending the SkyTEM system to measure the X component. By means of a 1D sensitivity analysis we motivate that in principle resolution of layered structures can be im- proved by including an X-component signal in a 1D inversion, given the prerequisite that a low-pass filter of suitably low cut-off frequency can be employed. In presenting our practical experiences with modifying the SkyTEM system we discuss why this prerequisite unfortunately can be very difficult to fulfill in practice. Having discussed instrumental limitations we show what can be obtained in practice using actual field data. Here, we demonstrate how the issue of high sensitivity towards instrument pitch can be overcome by including the pitch angle as an inversion parameter and how joint inversion of the Z- and X-components produces virtually the same model result as for the Z-component alone. We conclude that adding helicopter system X-component to a 1D inversion can be used to facilitate higher confidence in the layered result, as the require- ments for fitting the data into a 1D model envelope becomes more stringent and the model result thus less prone to misinterpretation. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Airborne time domain electromagnetic systems can be divided into two categories; helicopter and fixed wing systems. Helicopter systems such as SkyTEM (Sørensen and Auken, 2004), VTEM (Witherly et al., 2004), AeroTEM (Balch et al., 2003) and HoistEM (Boyd, 2004) carry the instrument as a sling load beneath the helicop- ter. These systems have relatively fixed transmitter/receiver geome- tries with loops that remain close to horizontal during operation. This type of controlled geometry operating almost parallel to the ground is very well suited for measuring the component of the sec- ondary field perpendicular to the ground, i.e. the Z-component, which is most often the component of interest. The characteristics of the field component along the flight direction, i.e. the X-component, are very different from those of Z since the signal is weaker, decays faster with time, has a higher noise level, and is more sensitive to minor changes in geometry. Fixed wing systems such as GeoTEM (Annan, 1991), Spectrem (Leggatt et al., 2000) and TEMPEST (Lane et al., 2000) have a trans- mitter mounted around the aircraft itself and carry a set of receiver coils being towed in a “bird.” This implies that the relative transmitter/ receiver geometry is constantly varying and the receiver coils are often exposed to significant movement (pitch, roll and yaw). For such systems, the field components can be of almost equal magni- tude and hence all 3 field components are typically measured in modern instruments. Historically, fixed wing systems have used the X-component to qualitatively locate discrete conductors, since this component couples strongly with vertical conductors as dis- cussed by Smith and Keating (1996). These authors further describe how the Z-component is more appropriate in the case of layered tar- get structures, but examples of X-component data used for mapping of layered environments also exist, e.g. Huang and Palacky (1991) and Palacky and West (1973). Regardless of target structure, the Journal of Applied Geophysics 84 (2012) 61–69 ⁎ Corresponding author. E-mail address: casper.kirkegaard@geo.au.dk (C. Kirkegaard). 0926-9851/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2012.06.006 Contents lists available at SciVerse ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo