b'Environmental geophysics m of a resistivity sounding (only 28 ofon two lines of data collected duringthe differences. More interestingly, the 314 readings qualified). These invertedthis field stint. Figures 1 and 2 compareworld as described by the resistivity is resistivity sounding became the forwardvarious representations of the CMDfar more interesting than the relatively model for simulated CMD data. In thisdata collected over the two lines anduninteresting versions portrayed by step I forward modelled CMD Explorercompares them with the resistivitythe CMD. In general, the CMD and the data using Aarhus universitys Aarhusinvsurvey data that were collected on theresistivity are finding similar resistive 1D modelling program (Aarhusinv is thesame line. features toward the ends of the lines, engine that runs Aarhus GeoSoftwaresalthough the resolution of the shallowest Workbench). I then calculated the ratioAt first I started writing in some detail toresistive units in the CMD appears less between the observed GCM data anddescribe the results and compare themthan what is resolved with the resistivity the calculated (forward modelled) databut, as always, that is largely unnecessary(assuming that the resistivity inversion is at each location. The average ratios andas the pictures speak for themselves.the truth).their associated standard deviations, forInstead, I will note a few things that I each transmitter-receiver separation,think are interesting (and may still beAgain, more interestingly, the CMD are shown in Table 1. Note that whilevery obvious). First off, observe that Iis not resolving any features that are I ran the forward models on both thehave added a red line to each of the CMDmore conductive than about 1 ohm-m, in-phase and out-of-phase data, I onlyinversions to show where the inversionwhile it appears that the resistivity is used the out-of-phase results, as that hashas estimated the depth-of-investigationmaking claims in that ultra-conductive a relatively direct link to conductivity/ (Christiansen and Auken, 2012). Somespace. While we are no longer limited resistivity (McNeill 1980). These ratiosof what is interesting in the resistivityby the LIN approximation with this were then used as the correctionsection is often below that depth inGCM, I wonder if we are hitting some factors to calibrate the CMD readingsthe CMD, and may explain some ofsort of a low-limit related to LIN. Or is it something that I have done, in my rush to get this article to press (late again Table 1.Ratios of observed data to modelled data for each CMD transmitter -Mike)? Obviously, a) I need to check receiver separation.my work, and b) I need to experiment with the forward modelling to see Input data/modelled data Standard deviation what it tells me. I am hoping to do both faster than the five years it took me In-phase Out-of-phase In-phase Out-of-phaseto get here. And I would love to hear Shortest separation 0.853 0.401 0.566 0.135 advice and input into this problem. Mid separation 0.569 0.517 0.342 0.137 Do any readers have another set of Longest separation 0.583 0.845 0.300 0.142 data, in a different (maybe not quite so conductive) setting that I can play with? No time guarantees.Stay healthy!ReferencesChristiansen, A. V., and E. Auken. 2012. A global measure for depth of investigation. Geophysics 77 no. 4: WB171WB177.Davis A. C., Ley-Cooper A. Y. and Kirkegaard C., 2010. SkyTEM system calibration: Two systems, one dataset, ASEG Extended Abstracts, 2010.Foged, N., et al. 2013. Test-site calibration and validation of airborne and ground-based TEM systems. Geophysics 78 no. 2: E95E106.McNeill J. D., 1980 Technical Note TN-6, Figure 2.Comparison plot showing various representations of GCM data. a) inverted section usingElectromagnetic terrain conductivity uncalibrated data; b) inverted section using calibrated data; c) ERT data used as calibrate known. Redmeasurement at low induction lines in b) and c) indicated approximate depth of investigation for these GCM inversions. Black line in d)numbers. pp. 17. Mississauga, Ontario, delineates depth extent shown in b) and c). Canada: Geonics LimitedJuNE 2020 PREVIEW 28'