Magnetic data over the continental margin of eastern Canada: preparation of a data base and construction of a 1:5 million magnetic anomaly map

Abstract

This report describes the implementation of a data base containing marine and aeromagnetic data over eastern Canada and adjacent offshore areas. The primary marine data set consisted of about 2.9 million observations collected over nearly one million line kilometres on 91 cruises during the period 1963-84. A small additional quantity of data was obtained from the National Geophysical Data Center (NGOC ) at Boulder, Colorado. The aeromagnetic data set consisted of: a ) : numerous surveys eastern Canada since Division of the GSC, Geochemistry Division; carried out over land portions of the late 40's by the Geophysics formerly the Resource Geophysics and b ) : three recently acquired data sets over Georges Bank, the Laurentian Channel, and the Northeast Newfoundland Shelf; c ) : additional data sets obtained from the US Naval Research Laboratory and the US Geological Survey. The aeromagnetic data were obtained in gridded form or in flight line format, after prior checking elsewhere for internal consistency and accuracy. A large part of the data base implementation consisted of similar checks on the marine data set, and these are described in this report. The collection of the marine magnetic data spanned a 21-year period, and temporal variations of the earth's magnetic field are therefore an important factor. The International Geomagnetic Reference Fields (IGRF) were applied to all the data in order to compensate for these variations, and to reduce the observed total field values to anomalies. The reference fields are defined at 5 year intervals, and values in the intervening years are obtained by linear interpolation. In order to determine the performance of the reference fields in eliminating time variations, we grouped the calculated anomalies by 5-year periods , and gridded each group separately. We found that anomalies in overlapping areas did not have the same values in the different periods. Since anomalies are related to geological features that remain stable over the time periods involved, this suggested that there is still some time dependency left in the reference fields. To gain some insight into the nature of the residual time dependency in the reference fields, we calculated the errors at nearly 60,000 track crossover points in the marine data , and undertook an analysis of their statistical properties. After comparing histograms of the crossover errors for different time periods and latitude bounds, we concluded that the pre-1975 reference fields are too low in the southern part of the study area (i.e. between 35N and 50N), resulting in derived anomalies that are too high. We used a shadowgram technique to identify bad points in the marine data, and eliminated some 3% of the data set on that basis. After this process, an analysis of the crossover errors in the remaining data gave a mean value of -0.9 nT and a standard deviation of 69.9 nT for 53361 crossovers, excluding errors greater than twice the standard deviation of the mean value. Having cleaned up the marine data set to a practicable level, we then proceeded to combine it with the different aeromagnetic sets. Level differences occurred at several locations between the two types of data, and it was decided to make all adjustments with respect to the marine data . At all locations save one, we applied constant level corrections, and eliminated small differences at the seams with a cosine taper. The only exception was Georges Bank, where we used an inclined plane for the level correction. The resulting data set, at a geographical grid of 0.05 degrees latitude and longitude, was then used to produce a map of the magnetic anomalies over the continental margin of eastern Canada, bounded by latitudes 40N and 84N, to accompany the volume prepared by the AGC staff as their contribution to the commemorative series for the Decade of North American Geology (DNAG} . The map was produced in a Lambert projection at a scale of 1:5 million.