Technical Papers - Airborne Magnetics

A Heuristic Method of Removing Micropulsations from Airborne Magnetic Data

M.D. O'Connell, Fugro Airborne Surveys, Ottawa, Ontario, Canada

Introduction Geophysicists have long been able to use detailed maps of the earth's magnetic field to extract geologic information such as the depth to basement in sedimentary basins. Other features that can be identified from aeromagnetic data are the locations of possible salt dome structures, kimberlites, faulting, and zones of mineralisation. 

The magnetic data for large-scale surveys are usually collected by high-sensitivity magnetometers, such as optically pumped cesium or helium vapor magnetometers. The magnetometers are carried by either fixed- or rotary-wing air-craft in a "stinger" or in a "towed bird" configuration. Collecting these data is a very time consuming and expensive task; a single line of data could take from several minutes to hours to collect, and the entire survey could take weeks to several months. During acquisition, the earth's magnetic field is affected by diurnal variations associated with the electrojet and solar wind (Rigoti et al., 2000). In addition to these daily variations, there are also shorter variations caused by solar activity known as micropulsations. At the present time, there is a peak in the sun spot cycle that can cause a large number of these variations in the earth's field. 

Peak amplitudes of some magnetic markers can be very small, for example the amplitude from a salt dome may be only a few tenths of a nanotesla (nT) but will correlate across several lines. The width of the response from a salt dome is 1-2 km (Rowe, 1998). At typical flying speeds, it would take 10-20 s to acquire the magnetic response of a salt dome with modern aircraft. The micropulsations vary in width from 0.3 to 100 s and in amplitude from 1 to 3 nT (sometimes up to a few tens of nT). Standard aeromagnetic leveling will remove micropulsations if the tie-line spacing is significantly less than the width of the micropulsation (i.e., less than 1 km). Otherwise for shorter pulses, there is the possibility that the micropulsation may not be removed and hence completely mask the low-amplitude response of features such as a salt dome. Nor is it cost effective to fly a survey with a line and tie-line spacing of 500-1000 m when the depth to the target may be as great as 3000 m. 

Millegan et al. (1993) concluded that the pulsations from 10 base stations showed small variations in amplitude and were uniform in phase across a 40 x 40 km study area in the Ballarat area of Victoria, Australia. The small differences were attributed to local variations in conductivity. In "The total-field geomagnetic coast effect" (Exploration Geophysics, 2000), Hitchman et al. show how coastal effects will change the character of the micropulsation, causing amplitude and phase changes. 

The Leading Edge, November 2001, Vol. 20, No. 11.  

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