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Evidence from overprinting of magnetizations of Late Permian and Mesozoic rocks and from the rank of Permian coals and Mesozoic phytoclasts (coal particles) suggests that surface rocks in the Sydney Basin, eastern Australia, have been raised to temperatures of the order of 200°C or higher. As vitrinite reflectance, an index of coal rank or coalification, is postulated to vary predictably with temperature and time, estimates of the paleotemperatures in the Sydney Basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin. These estimates give maximum paleotemperatures of present day surface rocks in the range 60–249°C, depending on factors such as location in the basin, the thickness of the sediment eroded, and the maximum paleogeothermal gradient. Higher coal rank and, consequently, larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the Tasman Sea on the basin's eastern margin. A theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperatures in the Sydney Basin. This estimate places the maximum paleotemperature in the range 250–300°C along the coastal region. Both coalification and thermal activation of magnetization models provide strong evidence of elevated paleo‐temperatures, which in places exceed 200°C, and the loss of sediment thicknesses in excess of 1 km due to erosion.

Airborne magnetic surveys have improved dramatically over the past three decades with advances in both data acquisition and image processing techniques. Magnetic surveys form an integral part of exploration programs and are now routinely undertaken before geological mapping programs. These advances have been made despite treating the magnetic field as a scalar, wherein various processing procedures that assume a potential field are compromised. If the vector information could be retrieved, either by direct measurement or by mathematical manipulation, magnetic surveys could be improved even further. For instance, the total magnetic intensity (TMI) could be corrected so it represents a true potential field. Vector surveys, where the direct measurement of vector components has been attempted, have met with mixed success. The accuracy of direct measurement of the field vector is largely governed by orientation errors, which for airborne platforms are so large that the theoretical derivation of the components from the TMI is actually preferable. For this reason, and others listed below, it is desirable to measure the field gradient(s) rather than the field vector. We discussed the calculation of vector components and magnetic moments from the TMI in previous articles (Schmidt and Clark, 1997; Schmidt and Clark, 1998) which compare theoretical derivations with laboratory measurements and demonstrate the validity of the approach. Phillips (2005) has since taken these ideas further by using a moving window to generalize the technique to tackle larger areas, and also to search for sources with specified directions of magnetization. This article is largely drawn from, and updates, an earlier contribution of ours (Schmidt and Clark, 2000). Gradient measurements are relatively insensitive to orientation. This is because gradients arise largely from anomalous sources, and the background gradient is low. This contrasts with the field vector, which is dominated by the background field, i.e., arising from the earth's core. Gradient measurements are therefore most appropriate for airborne applications. Another advantage is they obviate the need for base stations and corrections for diurnal variations. They also greatly reduce the need for regional corrections, which are required by TMI surveys because of deeper crustal fields that are normally not of exploration interest, or the normal (quasi-) latitudinal intensity variation of the global field. Gradient measurements also provide valuable additional information, compared to conventional total field measurements, when the field is undersampled. Undersampling is common perpendicular to flight lines in airborne surveys, is usual in ground surveys, and always applies in downhole surveys. Conditions under which calculation is preferable to measurement of vectors and gradient tensors have yet to be characterized by modeling and case studies. Synergistic interpretation of calculated vectors and measured gradients may allow significantly more information to be extracted from airborne su...

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