Practicalities, pitfalls and new developments in airborne magnetic gradiometry

Scott Hogg, whose company Scott Hogg & Associates, based in Toronto, Canada offers services to the exploration and airborne geophysical industry, reviews the latest refinements in the revitalised use of airborne magnetic gradiometers for the mining industry. The oil and gas exploration industry spurred the development of the first airborne magnetic gradiometers. The motivation was to use Euler’s equation with measured vertical gradient to calculate depth to magnetic source. Aeroservice introduced both a helicopter system and a fixed wing towed bird system in the 1960s. Interest in the Euler theory for magnetic analysis was rekindled in the mining industry 20 years later and continues to be the foundation of many new interpretation methods. In Canada, the GSC developed a vertical magnetic gradiometer in the 1970s. A national mapping programme began in the early 1980s and in response Canadian airborne survey contractors created a variety of fixed-wing and helicopter systems. The mining sector’s interest in airborne gradient measurement was based primarily on the increased spatial resolution and detail: small anomalies on the flanks of large features could be clearly resolved. Calculated vertical gradient maps, produced by simple filtering of total field, were found to provide almost the same benefit as measured gradient, at less cost, and the commercial interest in measured vertical gradient faded. Geometrics introduced a horizontal gradiometer in 1983. This development was of particular significance since, at the same time, they incorporated a technique developed by Nabighian and Hansen to derive a pseudo total field residual from measured horizontal gradients. The demise of the Geometric survey division took horizontal gradients off the horizon for a while. A decade later, this same concept was used by Nelson of the NRC, and more recently implemented in a variety of forms by De Beers and others. Geodass in Botswana, now Fugro, introduced the first 3- axis gradiometer in the early 1990s. In conjunction with the gradient measurements it provided a variety of compilation and mapping services that made use of the information. In Canada, Terraquest was the first to provide horizontal gradient measurement and Goldak the first to provide a full 3-axis fixed-wing gradiometer. At present, almost all of the airborne contractors offer horizontal gradient systems, and several can now provide full 3-axis configurations for simultaneous vertical and horizontal gradient measurement. Considerable interest in magnetic gradient measurement has arisen over the past few years. Some of the benefits of gradient are well founded and some are overstated and many are poorly understood. Magnetic gradient measurements can be used to advantage in interpretation. Vertical gradient maps, analytic signal maps, and a host of Euler based methods all use gradient information. The gradient information for these purposes may be calculated or measured. This review addresses methods that rely specifically on measured, not calculated, gradient. At present there are three such primary applications. The first is the potential to avoid diurnal interference, the second is to correct total field for variations in aircraft altitude, and the third is to make significant improvements in the accuracy and resolution of magnetic maps.