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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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S U M M A R YThe field and frequency dependences of the initial susceptibility of pyrrhotite have been analysed as a function of grain size, motivated by a strong field dependence recently observed for large (mm-sized) pyrrhotite crystals (Worm 1991) and smaller field dependences determined on smaller grain sizes by Clark (1984). In the present study, the frequency ranged from 30 Hz to 27 kHz. At 2 kHz, a field range from 0.05 to 1500 p T was investigated. Separate determinations of in-phase ( k ' ) and quadrature (k") susceptibility components allow for the analysis of eddy current effects.Up to 4 kHz the in-phase susceptibility of a pyrrhotite-ore specimen is practically independent of frequency whereafter it decreases while the quadrature component increases linearly with frequency to a valuc on the order of k' at 20 kHz for large grains. k" is proportional to d2p.af where d is grain diameter, p the intrinsic permeability, u the electrical conductivity and f the frequency. The frequency response of magnetite is essentially flat up to frequencies >20 kHz. Both frequency dependences agree well with calculations based on the theory by Wait (1 951). The conductivity of the pyrrhotite ore has been determined to be u = 1.40 (+0.05) * 1 O S E ' m -' .The susceptibility of pyrrhotite and its field dependence increase strongly with grain size. While the susceptibility of grains smaller than 30 p m is field independent (up to 1.5 mT) it may increase as k cx H025 for mm-sized crystals in fields >10pT. For most samples the Rayleigh law is inadequate to characterize induced magnetizations in weak alternating fields. When susceptibilities are measured for geomagnetic anomaly modelling, laboratory fields should be of similar intensity as the Earth's field and of frequency 5 1 kHz.

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Monoclinic pyrrhotite (Fe7S8 with 4C superstructure) was magnetically separated from a massive pyrrhotitic ore for preparation of nine grain‐size fractions ranging from ∼ 80 µm (large multidomain grains) to < 3 µm (single‐domain grains). Grains smaller than 100 µm were found to be magnetically hard with coercive forces ranging from 135 oe for 83 µ grains to 920 oe for < 3 µm grains. For grain sizes between 83 µm and 7 µm the coercive force is given by Hc ⧜ d−0.79. Variations in hysteresis properties with grain size appear to be gradual, with no evidence of sudden changes associated with domain structure transitions.

Geological Controls on Magnetic PropertiesInterpretation of magnetic surveys in terms of geology is hampered by poor correspondence between broad lithological categories and magnetic properties, and by lack of knowledge of the geological factors that influence the magnetisation of rocks. Magnetic petrology is the integrated application of rock magnetic and conventional petrologic techniques to identify and characterise the magnetic minerals in rocks. This information elucidates the factors that produce, alter and destroy magnetic minerals and thereby influence the bulk magnetic properties of the rocks and their associated magnetic anomalies. Improved understanding of magnetic petrology is therefore essential for maximising the geological information that can be obtained from magnetic anomaly patterns.