Technical Papers - Airborne Magnetics

Mapping Remanent Magnetisation Using the Local Phase

Jeffrey B. Thurston, formerly Fugro Airborne Surveys, Ottawa, Ontario K1G 3P5, Canada; presently University of Western Ontario, Richard Ivey School of Business, London, Ontario, Canada.

Abstract

For uniformly magnetised 2-D contacts, thin sheets, horizontal cylinders, and dykes, the apparent inclination of the magnetisation vector (i.e., the inclination projected into a plane perpendicular to the strike of a 2-D anomaly) can be computed from the difference in the anomaly shapes of the total field and the vertical field. The calculation can be performed using gridded data, resulting in a grid containing estimates of apparent inclination. These estimates take into account varying source-strike direction. This quantity is important for computing source dip wherever the data are likely to be affected by the presence of remanent magnetisation. As well, it is useful for input to forward models computed along profiles perpendicular to strike. Further, without knowledge of source dip, the apparent inclination can be used to map the presence of remanent magnetisation. This is an aid to geologic mapping, allowing isolated paleomagnetic measurements to be extrapolated over a broad area. Knowing the extent of remanent magnetisation can also help indicate how well the reduction to the pole process will work.

The method requires both total-field and vertical-field component data. Most modern airborne surveys are conducted with optically pumped magnetometers that record only the total field. To apply the method in these cases requires estimation of the vertical component from the total field using a 2-D Fourier operator, which in turn requires the assumption that the earth's field direction is constant over the study region and the induced field is small compared to the ambient field. This restricts application to areas of several hundred square kilometers. 

Apparent inclination is estimated from synthetic data computed over a generalised cross-section of the center of the Manicouagan impact structure (Canada) and to measured aeromagnetic data over this structure. The results of the synthetic test indicate that for the northern and southern edges of this source, the method can distinguish the present-day and paleomagnetic fields with the same spatial resolution as the analytic signal has for identifying magnetisation boundaries. Application to the airborne data indicates that where the source strikes at high angles to the declination of the present-day field, the estimated apparent inclination is influenced by both the remanent and inducing fields. For other strikes the field is almost entirely remanent or no distinction between the present-day and paleomagnetic fields can be made. As well, the source dip, estimated from the local phase and the estimated apparent inclination, is consistent with previously published results.

GEOPHYSICS, VOL. 66, NO. 4 (JULY-AUGUST 2001); P. 1082-1089, 6 FIGS., 1 TABLE. 

©2001 Society of Exploration Geophysicists. All rights reserved.

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