Aeromagnetic gradiometers ? a perspective

Abstract

Measurements of horizontal or vertical gradients of the magnetic field intensity can add a new dimension to high resolution aeromagnetic surveys of shallow basement areas. Although a number of vertical, horizontal and triaxial aeromagnetic gradiometers are in operation throughout the world, very little has been documented on the relative merits of horizontal and vertical gradiometer measurements or of measured versus calculated gradients. Our results demonstrate that there are differences in spatial resolution and bandwidth between measured and calculated gradients and that horizontal and vertical gradients provide complementary information. Finally our results indicate that better geo-referencing of horizontal gradiometer data is needed before we can derive reliable vertical gradient data from the measured longitudinal and transverse gradients Development of short-baseline aeromagnetic gradiometers began in 1975 with construction of the Geological Survey of Canada vertical gradiometer. The success of current short-baseline gradiometers depends on the use of high sensitivity optical pumping magnetometers, usually caesium vapour sensors and effective compensation for aircraft manoeuvre noise. Practical advantages of measured gradients include elimination of the diurnal problem and improved spatial resolution of small shallow sources. Transient signals have the same effect on all the magnetometer sensors so the effects are cancelled out in the gradients and diurnal-free total magnetic intensity can then be reconstructed by integrating the gradient data. The gradient data effectively remove the main field of the earth and enhance smaller scale, shallow anomalies while attenuating longer wavelength, deeper seated anomalies. In addition, measurement of the transverse horizontal gradient provides extra information between flight lines, leading to a reduction in flight line dependency of magnetic anomalies and a reduction in aliasing effects. The vertical gradient can also be derived from the longitudinal and transverse horizontal gradients with less line dependence and probably lower noise levels than the measured vertical gradient. Data from a conventional Canadian vertical gradiometer system, an Australian horizontal gradiometer system and a Southern African triaxial gradiometer system are evaluated. The measured vertical gradient data appear to provide better resolution of shallow sources than the vertical gradient calculated from the total field. The horizontal gradient data allow us to derive the vertical gradient from the horizontal gradients and provide much improved enhanced total magnetic intensity grids. The choice of a vertical or horizontal gradiometer system depends on survey objectives and both systems have their advantages. The vertical gradient appears to be marginally easier to measure and simpler to interpret. A triaxial gradiometer system provides the advantages of both at the cost of an additional sensor and retractable boom installation, compared to a horizontal gradiometer. Unfortunately, in Australia, CAA certification of a retractable tail boom may prove difficult. The viability of deriving the vertical gradient profiles from measured horizontal gradients needs more research, but if this can be done routinely, we think this would tilt the balance towards the horizontal gradiometer system. The improvement in total magnetic intensity grids using the transverse gradient information can be quite dramatic in many cases and there is always some improvement in the data. The combination of transverse gradient enhanced total magnetic intensity data and high spatial resolution 'texture' filters provides excellent definition of lower amplitude anomalies. In contrast, the calculated vertical magnetic gradient is often very similar to the measured data and the benefits of the measured vertical gradient may be small in many cases. Gradiometer performance deteriorates rapidly as the source becomes deeper and existing gradiometers are probably close to the noise level at 500m depth. Rugged topography also causes fixed wing gradiometer performance to deteriorate.