Aeromagnetic gradiometry in 1995

Abstract

This paper discusses aeromagnetic gradiometry techniques for fixed-wing survey aircraft and presents results. Some design criteria for airborne gradiometers are briefly noted and specifications for gradiometer surveys are discussed. The following horizontal gradiometry applications are outlined and are illustrated with examples: - Levelling of total field surveys without the use of tie lines. - The use of measured lateral gradient in the total field gridding process to achieve higher resolution of anomalies or alternatively, to enable increased line spacing without loss of resolution. - The identification of 2-D linear structures and their strike angles, on the basis of a single flight line and from these identifications, the formation of accurate estimates of vertical gradient leading in turn to accurate depth estimates using Werner deconvolution. This technique can be useful in the redesign or reorientation of a survey with very little flown data. - The use of directly measured longitudinal gradient to estimate diurnal variations of the total field during the time interval in which a survey line is flown. The first three of these horizontal gradiometry applications are shown to be effective, while the use of directly measured longitudinal gradient to track diurnals has not been successful and the reasons are illustrated. For vertical gradiometry, differences between directly measured vertical gradient and vertical gradient calculated from gridded total field, are discussed with examples. Based on data available, it is concluded that directly measured vertical gradient does not provide better resolution and may, in fact, introduce artefacts. I conclude that a lateral gradiometer offers advantages that are complementary to a total field survey, while a vertical gradiometer, which is more difficult to implement, does not.