Breaks in lithology: Interpretation problems when handling 2D structures with a 1D approximation

Abstract

Most airborne electromagnetic ͑AEM͒ data are processed using successive 1D approximations to produce stitched conductivity-depth sections. Because the current induced in the near surface by an AEM system preferentially circulates at some radial distance from a horizontal loop transmitter ͑sometimes called the footprint͒, the section plotted directly below a concentric transmitter-receiver system actually arises from currents induced in the vicinity rather than directly underneath. Detection of paleochannels as conduits for groundwater flow is a common geophysical exploration goal, where locally 2D approximations may be valid for an extinct riverbed or filled valley. Separate from effects of salinity, these paleochannels may be conductive if clay filled or resistive if sand filled and incised into a clay host. Because of the wide system footprint, using stitched 1D approximations or inversions may lead to misleading conductivity-depth images or sections. Near abrupt edges of an extensive conductive layer, the lateral falloff in AEM amplitudes tends to produce a drooping tail in a conductivity section, sometimes coupled with a local peak where the AEM system is maximally coupled to currents constrained to flow near the conductor edge. Once the width of a conductive ribbon model is less than the system footprint, small amplitudes result, and the source is imaged too deeply in the stitched 1D section. On the other hand, a narrow resistive gap in a conductive layer is incorrectly imaged as a drooping region within the layered conductor; below, the image falsely contains a blocklike poor conductor extending to depth. Additionally, edgeeffect responses often are imaged as deep conductors with an inverted horseshoe shape. Incorporating lateral constraints in 1D AEM inversion ͑LCI͒ software, designed to improve resolution of continuous layers, more accurately recovers the depth to extensive conductors. The LCI, however, as with any AEM modeling methodology based on 1D forward responses, has limitations in detecting and imaging in the presence of strong 3D lateral discontinuities of dimensions smaller than the annulus of resolution. The isotropic, horizontally slowly varying layered-earth assumption devalues and limits AEM's 3D detection capabilities. The need for smart, fast algorithms that account for 3D varying electrical properties remains.

Accurate quasi 3D versus practical full 3D inversion of AEM data – the Bookpurnong case study

“…The debate regarding the need for full 3D inversion in semilayered environments is intensifying, and the issue is the subject of ongoing research (e.g., Ley-Cooper et al, 2010). Along these lines, the May 2010 issue of Preview (Issue 146) featured a paper by Wilson et al entitled 'Practical 3D inversion of entire airborne electromagnetic surveys'.…”

On the value of including x-component data in 1D modeling of electromagnetic data from helicopterborne time domain systems in horizontally layered environments

“…In this sense one can argue that including the Xcomponent in a joint 1D inversion facilitates an equally good model result, but further introduces a more sensitive intrinsic measure of how well the 1D assumption is fulfilled. Ley-Cooper et al (2010) further shows how 1D modeling of Z-component data acquired in areas of 2D/3D conductivity variation can lead to serious errors when interpreting the model result, which can obviously be of major concern in many cases of decision making. As such, there are many scenarios where more stringent requirements towards fulfilling the 1D assumption can be useful and the added measure of model trustworthiness facilitated by adding the X-component can be highly valuable.…”

“…In the fixed wing literature X-component data also finds little use for detailed mapping of layered structures, but Ley-Cooper et al (2010) show its importance for mapping sharp discontinuous boundaries with a TEMPEST instrument and Lane et al (2004) show how X-component data is in fact necessary in order to include pitch as a model parameter in 1D inversion of TEMPEST data. Taking the existing literature into account we find it very relevant to quantitatively investigate the potential use of helicopterborne system X-component data for mapping of structures that are normally regarded 1D.…”

Review on airborne electromagnetic inverse theory and applications

“…Airborne EM inverse theory/application W21 The LCI algorithm uses the interval thicknesses and resistivity from adjacent stations and profiles in the lateral constraints as well as a priori information that can be added at any point of the profile Auken et al, 2008;Ley-Cooper et al, 2010). The lateral constraints are included in a roughness matrix that is solved simultaneously with the Jacobian matrix in an iterative eigenparameter inversion and can be applied to the resistivities and the thicknesses, separately and together (Vallée and Smith, 2009b).…”