b'Data trends Data trendsmaths (Figures 1 and 2). It is the quotient of the First Vertical Derivative divided by the Total Horizontal Derivative and scaled betweenby an inverse tan function.211VD Tilt=tanTHD This measure has the property of being positive over a source and negative elsewhere (Miller and Singh, 1994).Cells equal to zero (0) theoretically mark the boundary of magnetic bodies and the highest values reflect the peak amplitudes of anomalies. To display the boundaries and trends the colour stretch Tim Keepingof the Tilt Derivative raster in Figure 1 has Associate Editor for geophysical been modified along the following lines:data management and analysis technical-standards@aseg.org.au Black is used for values 5% either side of 0 for anomaly boundariesThe top 10% are coloured white for directional trend inside anomaliesFive minute domaining The rest are set to no data/null for Domaining is the method of tracing thetransparency Figure 2.Tilt Derivative with grey scale colour apparent boundaries of magnetic bodiesIts not polished, but quickly outlinesstretch.and their direction/trend. Practitionerswhere you can draw major domain trace along the centre of anomalies inboundaries. The strongest edges are 1VD images to accentuate the trend, anddefinite, while sparse dots reflect theyou still prefer boundary contours then it use one of the edge detecting productsambiguity in the data. It implies but doesmight be much quicker to delete rather for boundaries (Pilkington and Tschirhart,not impose structure. You can play withthan create. Image processing programs 2017). The result is a layer of lines wherethe display ranges to suit your needs. Ifwould allow quick erasing to eliminate the direction trends within boundariesdots that might confuse automated are a proxy for tectonic activity,contouring.classifying them into magnetic domains. As an overlay it may help discern spatialThe survey data used to create Figures 1 relationships between magnetic domainsand 2 was flown in 2015 by GPX for GA and other data types. and GSSA over the Coompana area of South Australia (Kita, 2015). The survey Automated contouring/tracing functionswas flown in an EW direction with a are common in image processingflight line spacing of 400 m with some software but suffer from noise, and200 m infill.even the best data will not produce a convenient series of equivalent grid values for seamless contouring. ContoursReferencescan halt in a series of disjointed lines andKita, J., 2015. Survey Operations and lines can double back on themselvesLogistics Report, GPX Surveys, while buffers can allow jumping thathttps://catalog.sarig.sa.gov.au/swallows neighbouring anomalies intogeonetwork/srv/eng/catalog.search#/one. Manual tracing is normal. metadata/4b3e12db-638d-4af6-880c-However, if the outlines are not an6d754650bd8c.intended final product then how aboutMiller, H. G., and V. Singh, 1994. Potential near enough being good enough forfield tilta new concept for location the human eye? The following exampleof potential field sources. Journal of cheats by not drawing lines but usingApplied Geophysics, 32 (23), 213-217, pointillismthe art of using dots to builddoi: 10.1016/0926-9851(94)90022-1.up an image. Pilkington, M., and V. Tschirhart, 2017. Practical considerations in the use of The First Vertical and Total HorizontalFigure 1.TMI RTP overlain with Tilt Derivative.edge detectors for geologic mapping Derivatives create a Tilt DerivativeBlack indicates Tilt values 5% of 0 and whiteusing magnetic data. Geophysics, (Miller and Singh, 1994), which can behighlights the highest 10% of Tilt values. Tilt Derivative82, J1J8, doi: 10.1190/GEO2016-calculated in any program with rastercells outside those two ranges are transparent. 0364.1.53 PREVIEW DECEMBER 2021'