b'Data trends Data trendsGeoscience offers to de-survey your drill holes and make cross sections (https://www.spatialintegration.com/). This is a useful function.AGT is designed for archaeologists collecting electrical and magnetic gradiometry data in xyz text format, but I did not have a magnetic gradiometry dataset to hand to test the plugin (https://plugins.qgis.org/plugins/AGT/). Coincidently, the lack of gradiometry data in Australia was noted at our last Technical Standards Committee meeting.As an alternative, I tried to find a functionFigure 1.Normalised TMI generated using Tim Keepingto create a magnetic grid, and I stumbledthe Slope function with a histogram equalised Associate Editor for geophysical across the Slope function help page andgreyscale stretch for range 0 (dark)1 (light).data management and analysisand saw this equationtechnical-standards@aseg.org.audz 2 dz 2Slope = Atan dx + dyPlaying with open source software which reminded me of this equationQGIS is the hot, free GIS kid in town and is proving very popular withdz 2 dz 2explorers, so I thought Id try it out.THD = dx + dyQGIS is not a polished proprietary GIS (like some I could mention)So, there is a (almost) Total Horizontal but exploration is not a shape-fileDerivative (THD) function in this open orientated profession, and opensource program.source software may offer better file interoperability.As a quick comparison I ran the QGIS uses the GDAL (Geospatial Datasame magnetic data through both Abstraction Library) raster and vectorfunctions and normalised the results forFigure 2.Normalised Total Horizontal Gradient standard, an open standard dedicated tocomparative purposes. The result of theof TMI with histogram equalised greyscale stretch open file formats, which is right up theSlope function (Figure 1) produces a closefor range 0 (dark)1 (light).ASEG Technical Standards Committeeslikeness to the result of the THD (Figure2). alley. The Raster - Translate menuThe small difference (grey) is likely to be turned my ers file (our beloved archivethe tangent scaling (Figure 3).format) into an industry favourite - grd. Ignoring the tangent scaling of the Slope function, the common Pythagorean But, it was not all a love-in for QGIS, asresult contrasts Slopes comparison of making 3 band radiometric files provedimmediate neighbours and the THD baffling. The process was annoying andleast squares fitting of a third order the results were inconsistent. At thesurface to a 5x5 window (Lyatsky etal., most fundamental level I was unable to1992). While its an unusual method, verify which input raster was assigned toIm sure that there is worse magnetic which band. Needless to say, I could notdomain mapping out there!expect Gary to make a state radiometric grid with those results, and it was easier to code my own procedure in Python. ReferenceThere are two plugins available thatLyatsky, H.V, Thurston, J.B., Brown, R.J., might be useful for geophysicists andLyatsky, V.B., 1992. Hydrocarbon-explorersGeoscience for drill holeExploration Application of PotentialFigure 3.Difference between normalised THD visualisation and the ArchaeologistsField Horizontal-Gradient Vector Maps,and Slope files with standard deviation greyscale Geophysical Toolbox (AGT).CSEG Recorder, pp 1014. stretch of range -0.16 (dark) to 0.2 (light).AUGUST 2021 PREVIEW 40'