b'A Sliding Doors momentFeatureWe all know that FIR calculated gradients are not capable ofbelow the surface, associated with the black shales. In the estimating past a \x1drst derivative without noise starting toversion shown in the top image the predicted magnetic \x1deld swamp the coherent signal. is not clipped by topography, allowing the full expression of north/south poles to show. The folded quartzite formation In contrast to this current situation, Cauchy methods completelyhas no magnetic response - as expected. As there can be Cu re-write the rule book (Figure 2). in the ma\x1dc porphyry, with a magnetic response, some of the indicated sub-vertical veins deserve another look.Downwards continuationContinuing work has shown that several of exploration One immediate consequence of this development has provengeophysics standard methodologies need to be reconsidered to be direct prediction of the potential \x1deld below the groundand re-imagined. Astoundingly, gravity practice may well end surface. Derivatives that remain coherent to above order 10 areup being the biggest bene\x1dciary and subject to more change, fed into a Pad series expansion. as the notion of terrain correction may become a thing of the Flat-land thinking can be relegated to the trashcan because 3Dpast. The magnetic \x1deld also perfectly re\x1cects any remanence in representations of the measured magnetic and gravity \x1deld,the rocks.extending hundreds of metres below and above the groundFor me personally, this story is reminiscent of Moses going to surface, are now routinely possible to estimate. 3D processing andthe top of Mt Sinai to see the promised land, without having the visualisation for all potential \x1deld survey datasets can be madetime left in his life to watch the full implications being realised. available to the interpreting geologist. This is not an inversion butI thank all my colleagues who have contributed their time and a simple continuation, made possible by the Cauchy-stabilisedpassion to this on-going journey on the road less travelled.high-order derivatives; there is no modelling and no arti\x1dcial means to alleviate the ambiguity in the algorithm (Figure 3). So back to the title:The black shale Coocerina Formation wraps the KnapdaleLets shut the Hamming door and step through the Cauchy door Quartzite. This syncline has two most distinctive magneticand, in the words of W.S. Gilbert, lets polish up the handle of dipole responses (as seen in Figure 3, middle to right) justthe big brass door.Figure 2.GSSA Gawler Craton Regional Mapping, Total Magnetic Intensity, Variable Reduced to Pole. The image on the left-hand side is \x1drst vertical derivative. The image on the right-hand side is third vertical derivative, using Cauchy. These images are downloadable from the SARIG website.Figure 3.Two versions of a depth section of TMI data, continued from survey line 16101, Dugald River VTEM survey, showing a range of in-section dipoles, with a variety of polarities, using uncompensated TMI data. The version shown in the top image is smoother, and not clipped to topography. The version shown in the bottom image is at sampling resolution, is clipped to topography, shows the depth scale to 500 m, and better indicates the recent near surface cover depths.35 PREVIEW JUNE 2023'