b'Environmental geophysics Environmental geophysicsinterpret data collected using Ground(uncalibrated) GCM inverted results Conductivity Meters (GCM). Specifically,were just a bit off, while the far older I am most interested in instruments likeTEM data appeared to do better. Ever the GF Instruments CMD or the DualEMsince then, I have been hoping to have 421 that collect data using a combinationan opportunity to see if I could come of frequencies and/or transmitter- up with some calibrates for the GCM. receiver separations so that data areAnd finally, I have started making some collected at a number of depths. progress in this space - after almost five years.As most of you will know, it irritates me that a) these data are still presentedThe methodology that I am about to using the LIN approximation (see, fordescribe is inspired by a number of example, my diatribe on this in Previewarticles (e.g. Davis et al, 2010 and Foged Mike Hatch191 - December 2017), and b) thatet al, 2013) along with conversations Associate Editor for the data are usually presented usingwith the various authors and others on a Environmental geophysicsExcel (dont get me wrong, Excel issimple (too simple?) way to calibrate this michael.hatch@adelaide.edu.au fantastic), with each conductivity datatype of data.set presented on a graph with labels like Shallowest Data, Medium DepthIn this exercise, a few of us from the and Deepest Data (Figure 1a) and thenuniversity of Adelaide collected a line some estimate of depth given basedof shallow resistivity over a project area Improving interpretationin the Riverland of South Australia. The on the instruments manualcome on of ground conductivityfolks, we can do better than that. Thesedata were collected using a ZZResistivity data can be inverted and the results64-channel system, using 3 m station data are improved when this is done. Thespacings. These data were inverted using problem is that data collected using aZZResistivitys ZZResinv64 2.5D program. Welcome readers to this issuesGCM usually need to be calibrated. ThisThe next day we collected GCM data over column on geophysics applied to thebecame very obvious to me a numberthe same line using Flinders universitys environment. I wasnt going to writeof years ago when a colleague testedGF Instruments CMD Explorer (many about COVID-19, so I wont, other thana GCM against some just-drilled (andthanks to Flinders university for use of to comment that I dont get out much,logged) shallow bores in an area wherethe instrument).and therefore this months column iswe had some shallow time-domainThe inverted resistivity results were then even more in my space than usual. Inelectromagnetics (TEM) data that hadtreated as individual soundings at each this column I am going to continue inbeen collected a number of yearsstation on the line. I ran a search to find my quest to improve our (my?) ability tobefore the drilling. Interestingly, thethe GCM readings that were within 0.5 Figure 1.Comparison plot showing various representations of GCM data. a) uses Excel to plot data for each transmitter - receiver separation; b) inverted section using uncalibrated data; c) inverted section using calibrated data; d) ERT data used as calibrate known. Red lines in b) and c) indicated approximate depth of investigation for these GCM inversions. Black line in d) delineates depth extent shown in b) and c).27 PREVIEW JuNE 2020'