One-dimensional spectral analysis of aeromagnetic data from Temiscouata area and the western sector of the Gulf of St. Lawrence

Abstract

1-D spectral analysis is useful to determine the mean depths to buried magnetic Precambrian basement rocks. The average thickness of the sedimentary cover which may contain favourable petroleum reservoirs overlying the basement of the Northern Appalachians of Quebec was determined by this method. Additional information related to structural geology of the basement and the sedimentary cover can also be obtained with this technique. The center of the Temiscouata area is located at longitude 69° W and latitude 47° 45′ N and the one of the Gulf at 67° 45′ W and 49° 05′ N. The aeromagnetic data were digitized from the 1 mile = 1 inch total field contour maps published by EMR. In the digitization process, the magnetic intensities were read on contour lines only. Interpolation at fixed intervals were calculated with the help of a computer program subroutine. The fixed intervals were chosen at 0.25, 0.50 on shore and 1.00 mile offshore in the Gulf of St. Lawrence. The magnetic gradient was removed using the IGRF formula or numerical filters. The least square method was avoided because of the biasing it involves. Additional smoothing was occasionally done. After these different steps were executed, the transformation to obtain the Fourier co-efficients was carried out using an algorithm for computing the Mixed Radix Fast Fourier Transform. Mean depths are then calculated from the energy spectra; this constitutes a statistical approach. The mean depths obtained from the 1-D energy spectrum are comparable within 10–15% with the ones obtained from the 2-D energy spectrum. 1-D spectral analysis of AM data offers some advantages on the 2-D counterpart. Among these, we can mention the eventuality of obtaining mean depths to the basement from the analysis of only a few AM flight lines in a new area and of detailing more accurately structural features of local nature in the basement. Such structural features may be controlling agents in eventual reservoirs within the sedimentary rocks.