b'Magnetics in the mountainsFeatureThe modelled response over the mountains and peaks doescorrelation between the observed, modelled and transformed not look like the observed response, which is both encouragingdata is impressive, even down to the small anomalies either and not unexpected, given that they are sandstone. The gorges,side of the river valley. There are also correlations away from the however, show on both the observed magnetics and theriver, which while not obvious in the images shown in Figure 10, modelled or transformed topography, so one would have to bewould have a big impact on any interpretation of these data.careful in ascribing the magnetic patterns they create to changes in geology. This is illustrated in Figure 11 which shows an eastThere may be a simple relationship between the value of west profile along flight line 11381 over the Hellyer River. Becausepseudomag transformed topography and modelled response of the coarse line spacing used for the survey, the observed datathat will be dependent on the constant used in the FFT to shown in Figure 11 is the line data, not the gridded magneticcompute the pseudomag transform, the susceptibility and data, while the other profiles have been extracted from grids. field strength used in the forward model, and perhaps the field direction used in both. However, Ill leave that for others to The forward modelled response was multiplied by 4.09 tosolve. For any given area, the transformed data could be scaled scale it to a similar range as the observed data, implying abased on some smaller blocks of forward modelled data or susceptibility of 0.04 SI. This compares with the measuredeven on the observed data itself although that would not give range of susceptibilities for the Tertiary basalt here of 0.0078you confidence that the susceptibility required to generate the to 0.0215 SI (Duffett pers comm). However, the samples alsoanomaly was realistic for the environment being studied. While showed a relatively high Knigsberger ratio of up to 2 in athis transform does not replace more rigorous modelling, it direction that was dominantly normal. A susceptibility ofdoes give the interpreter a very quick way of knowing whether 0.04 SI is therefore not out of the question, although it seemsor not topography is likely to be impacting on their data, and for more likely that the magnetic response here is a combinationvery large areas may be the only practical way to evaluate the of changes in rock susceptibility and topography, which istopographic response.probably a more general case.Hopefully this simple transform will improve your next magnetic The upward continued, pseudomag transformed topographyinterpretation in areas of strong relief.was re-scaled to the same range as the observed data using a multiplier of 0.2. The modelled data sets contain a higher frequency content than the observed data, indicating that IReferenceshave modelled the drape height too low or perhaps too exactly for this survey. In this case, the difference between flying heightBlakely, R, J. 1995 Potential Theory in Gravity and Magnetic and model height might be explained by weathering, whichApplications. Cambridge university Press.will have the dual effect of increasing the apparent flyingLi, Y. and Oldenburg, D.W. 1988. 3-D inversion of gravity data. height and low pass filtering the topography. Nevertheless, theGeophysics, 63 (1), pp 109119.Appendix 1SuBROuTINE INVF01!!!! PERFORM THE SPECIFIED OPERATIONS! - code originally written for a grid stored in row, column order. Converted to Surfer 7 column, row I/O but! variable names not changed! - note that X and Y are reversed relative to those given in Blakely - because of the way the fft is stored!!REAL, PARAMETER :: PI = 3.14159265REAL, PARAMETER :: TWOPI = 2.0 * PIREAL, PARAMETER :: DEGRA = PI/180.0REAL :: DELWX,DELWYREAL :: XLFAC,YLFACREAL :: AI,ADREAL :: SINI,COSI,SIND,COSDREAL :: WY,WX,WYSQ,WYSD,WXSQ,WXCD,YLANC,YARG,YHANN,WNuMREAL :: XLANC,XARG,RLANC,XHANN,RHANNREAL :: RPOLER,RPOLEIREAL, PARAMETER :: CMTOG=53.0E-4 ! THIS IS A BIT ARBITRARY - EQuATION IS ! (G*dDEN)/(2PI*dMAG)! WHERE G IS BIG g, dDEN= DENSITY CONTRAST, dMAG=MAG CONTRAST IN nT! TO CONVERT PSEuDO SuSCEPTIBILITY INVERSION TO PSEuDO DENSITY! STARTING WITH A BOuGuER GRAVITY GRID IN um/s\x1e2, PSEuDOMAG AND INVERT uSING A FIELD ! STRENGTH OF 60000 nT! THEN MuLTIPLY PSEuDO SuSCEPTIBILITY BY 188 TO GET DENSITY CONTRAST, ADD TO! BOuGuER DENSITY TO GET PSEuDO DENSITY. FLIP THE PSuEDOMAG GRID AND INVERT TO GET41 PREVIEW JuNE 2020'