b'Mark Lackies best of Exploration GeophysicsFeaturewith geologically and petrophysically unrealistic sophisticatedIt should be understood quite clearly that for MAGMOOS I, computer models. There is much to be said for a simple analyticII, III the models give correct results only for vertical resultant approach with remanence and other factors considered. Thesemagnetizations. For inclination (of resultant magnetizations): programs endeavour to provide such an approach. 90 |I R | 60, the results may be acceptable but only approximate owing to the presence of poles on the sides of the bodies.The total magnetic intensity (B T ) and vertical magnetic intensity anomaly (B z ) anomalies are computed. A considerable amountThe exposition for each model includes: formulae, program of B zdata still exists even though total field data acquisitioninstruction, key diagram, worked numerical example, and a is now virtually the norm. A diagram depicting the naturelisting of the program.of measured (B m ), theoretical total (B T ), and vertical (B z ) anomalies has been included as these concepts appear to beThe instrument requirements are: HP 41C calculator, card very poorly understood. reader, and thermal printer. If there is no printer, users can modify programs up to the LBL E step by inserting SF21 For units, the authors have a perverse preference for the cgsat the beginning of the program and substituting AVIEW emu system as it is based on magnetostatic concepts ideallyeverywhere for PRA. When division by zero occurs the program suited to the problems arising in magnetic interpretation. If SIsubstitutes a very small number, but depth h cannot be set units are used, factors of 4 crop up in the equations and it isequal to zero metres at any body vertex- instead a small finite best to avoid this complication. However, all calculations can benumber is required. The plotting routine may be useful in done in SI providing the correct conversion factors are used. Insome circumstances, but it is more of an ornament with limited the magnetic formulae distances can be in any units providedapplication. When using the plotting subroutine |x| cannot they are consistent e.g. all in metres. The flux densities (magneticexceed 999. This problem can be avoided simply by changing field strengths) B and F must be in the same units: gammasthe distance units (e.g. using kilometres instead of metres). (10 5 gauss, cgs) or nanoteslas (SI). The gamma and nanoteslaThe HP 41C is not meant to give detailed plots which may be are numerically equivalent. The magnetic volume susceptibilityrigorously interpreted; the plots are simple visual aids. The k is a dimensionless ratio. It is the magnetic moment generatedresolution is limited because the plot field consists of either 119 per unit applied field divided by the volume. The emuor 126 columns, therefore the anomaly value is rounded to one susceptibility must be inserted into the equations presentedof 119 or 126 values (inclusive) between nominated min. and herein. Accordingly SI susceptibilities must be divided by 4. Selfmax. values. If B MIN , B MAXare too large an error in the printout demagnetisation factors N are also dimensionless, but the emuarises caused by the finite size of alpha register. If x values are demagnetizing factor is 4 times the SI value. Then 0 N emu 4too large, then plots appear on two lines causing an apparent and 0 N SI I. Pole strengths are calculated in pole units whichorigin shift- too large varies for different models as different arc hybrid units involving gammas and metrestheir SI andplot fields are used.emu relationship is not important because they are simply part of an internal computation. The magnitude or intensity ofSome of the programs will require extra memory (modules) for natural remanent magnetisation is expressed in gammas (), sothe basic HP 41C calculator.as to be consistent with the induced magnetisation. NRMFor further reading and background theory it may be intensities are usually quoted in the literature in microgaussworthwhile to consult:(emu) or milliamp/metre (SI) which are numerically equivalent. One gamma is equal to 10 microgauss. Southern hemisphere(i)the classic paper (the basis for a lot of this work) by D. H. magnetic field inclinations are negative (upward pointing). Hall, Directions of polarization determined from magnetic As an example of the units consider MAGMOO I withanomalies, J. Geophys. Res. 64, 1945-1959, 1959;a monopole 20 m radius depth 100 m and resultant(ii)The ASEG Short Course Notes on Pole and Dipole Models magnetisation vertically up: in Magnetic Exploration by D. W. Emerson and D. A. Clark, unpub. 1982, 1983;(iii) the book: Magnetic Models in Geophysical Exploration by D. A. Clark and D. W. Emerson (in prep.);(iv) The Applied Magnetic Interpretation Symposium Proceedings ed. D. W. Emerson, Bull. Aust. Soc. Explor. Geophys. 10, 1-139, 1979;(v)the paper: by D. A. Clark, Comments on magnetic petrophysics, Bull. Aust. Soc. Explor. Geophys., 14, 49-62, 1983.It is a pleasure to acknowledge the indispensable assistance of: Mr Len Hay (Sydney University) in drafting the MAGMOD example Figures and for providing the cover design which accurately portrays the feeling of many an interpreter; Mrs D. Garbler (Sydney University) for cheerfully typing many drafts; Mrs Pat Godden (CSIRO), who drafted several Figures; and Miss Diana Bridgewater (CSIRO), who transferred pages of The body oriented co-ordinate system used is the Cartesianhandwritten formulae into camera-ready copy.right-hand convention with the z axis positive downwards. Principal profile x axis analysis is presented. A principal profile isFinally, a reminder to users that they are on their own with these a traverse over the centre of a 3D body or a traverse normal toprograms. Any user accepts and uses any, some, or all of these the strike of a 2D body. programs at the users own risk and responsibility entirely.DECEMBER 2020 PREVIEW 60'