Magnetic depths to basalts -- extension of Spector Grant method

Abstract

Previously essentially manual, the Spector-Grant method requires inspection of the power spectrum to assign a single slope characterising the depth to a large flat body. In effect, there is only one subjectively chosen point on the spectrum to apply the SG formula. Using animations, the (oral) presentation demonstrates a fresh derivation of the formula for slopes extended to the entire length of the power spectrum, cutting off where the noise level rises. By repeatedly taking grid samples 20 km wide, a depth spectrum can be drawn repeatedly across a landscape. The presentation thus demonstrates tracking the Antrim basalts across several hundred kilometres. 60,000 profiles for the entire Northern Territory have already been drawn up algorithmically, currently visible on the NTGS' Image Web Server, http://geoscience.nt.gov.au/giws/. Here, the base of the basalts can be tracked and a first estimate of their depths obtained. A degree of manual intervention reappears when fining up the depth estimates, because the different survey line spacings require separate calibrations, which change the apparent depths by 20%. Whereas the original Spector Grant paper modelled a few prisms, the fresh derivation is NOVEL in that the power of modern computers allows the modelling of a thousand randomly-placed dipoles at each of 500 depths. The new method can be applied to flat magnetic bodies GLOBALLY. The power of the technique has made UNDERSTANDABLE by demonstrating magnetic profiles of depths derived every 5 km across several hundred kilometres of the Northern Territory basalts. By the time of the conference, the paper of the presentation will have been submitted to Exploration Geophysics for PEER REVIEW as part of my Ph.D.