b'Data trends Data trendsinstead of a measurement with a unit.of core favours core from magnetic Hence you will see NOUNIT listed foranomalies.specific gravity. Magnetics has moreHow this affects your software is glamour in physics so it gets the SI,debatable. It may treat the numbers the official dimensionless unit of theas relative and generate equally valid Standard Internationale, but you mightratios in the solution. It may assume find old logs with cgs (centimetre an implied order of magnitude and gramseconds system). The notableaccidentally treat uninteresting differences between the two systems: sediments as signs of haematite.cgs uses the centimetre as the standard length instead of the metre Some more tips under general topics:SI uses different dimensions for the two magnetic fields susceptibility isGauging the reliability of mag sus:comparing, and you need to multiplyMeasurements over 1 SI in Figure 1 by 4for the SI values were taken from a magnetite core that SG often gives the equivalent densityresembled iron bars more than rockTim Keepingto gcm-3, the cgs unit, and you might20 is the theoretical maximumAssociate Editor for geophysicalneed to multiply by 1000 for the SI unitNote that zero is very common for datamanagement and analysis of kgm-3 metal free mineralstechnical-standards@aseg.org.auNegatives are possibleHowever, it is the numbers that areDo not take readings while the core is most confusing because of the ordersitting in a metal trayof magnitude of numbers displayed by If the logs name the device, you can try different devices. Different conventions A quick mineralin displaying numbers with scientificto gauge the implied order of magnitude:petrophysics primer notation have (sort of) taken holdTerraplus KT (5, 9, 10) meters are x 10-3. between handheld (10-3) and downholeThe display is floating point, where the (10-5). Enough for the National Datadecimal moves while maintaining the When explorers download mineralSubmission guidelines to expect eitherorder of magnitude on the displaypetrophysics data from the Geologicalof those formats. But this is not a hardFugro GMS-2 is opposite and changes Survey of South Australia I am invariablynorm since the handheld GMS-2 andorders of magnitude of the display asked what are these numbers? Anewer downhole logs claim the moreso it can be a judgement call to work fair point since magnetic susceptibilityaccurate 10-6. For those who arebackwards with old logsmeasurements (mag sus from here on)interested, MRIs use 10-7.can be as confusing as an electricity bill,When in doubt, use the lithology logs:and specific gravity has its own oddities.For simplicity, it was decided bySandstone/sediments 0.000010.0001This quick primer might help. the GSSA that all mineral values beHaematite 0.00010.01changed to their plain old valuesHaematite/Magnetite mix 0.0010.01A simple explanation for mag susdevoid of scientific notation. ThatMagnetite dominate 0.0110measurements is that unpairedmeant moving the decimal point by electrons emit magnetic fields, anddividing by the number of zeros (i.e. x(Now you can see why downhole loggers in some molecules those electrons10-3 divides by 1000). As a rough guide,prefer using scientific notation)can be moved by outside magneticyour resulting values should be withinQuick points on specific gravity:fields. Susceptibility will be positivethe range of the logarithmic graph inAccuracy is limited to tenths of a gram if the internal magnetic field alignsFigure 1 (0.00001 to 10 SI). Note thisat bestwith the outside field. Why doesgraph does not reflect a general rockDensity should increase with significant this matter? It indicates how muchpopulation since magnetic samplingincreases in susceptibilityof the geomagnetic field in a TMI map is amplified by this physical phenomenon.Measurement of Specific Gravity (SG) relies on the Archimedes Principle; the ratio between weighing a rock in air versus weighing a rock in water. This reduces to the ratio of rock density to water density, and since water has a density of 1 (one), this leaves only the density of the rock.Mag sus and SG measurements are both dimensionless quantities where the units cancel out. Divide a metreFigure 1.A graph of open file magnetic susceptibility values taken from SA Geodata. Population was by a metre and you have a ratiodecimated to enable graphing in Excel. Logging favoured magnetic anomalies.49 PREVIEW DECEMBER 2019'