Report from IUGG Assembly: IAGA session on lithospheric mapping from local anomaly charts

Abstract

The objective of this session was to review recent advances in utilizing magnetic survey data to delineate the geological formations that comprise the earth's crust, and to provide information on their genesis.P. J. Hood commenced the session with a review of the national aeromagnetic survey, which has been carried out by the Geological Survey of Canada, since 1947, as an aid to both geological mapping and mineral exploration programs in Canada. Since that time some 8 million line kilometers of aeromagnetic survey data have been obtained and have resulted in the production of, 8100 aeromagnetic maps at various scales. The specifications for aeromagnetic surveys in Canada have been continually improved upon over the years, mainly as a result of developmental work undertaken by the Geological Survey of Canada. This developmental work has also resulted in the fabrication of an inboard aeromagnetic gradiometer survey system, which measures the vertical gradient of the earth's total field in addition to being a high‐resolution total field instrument. The main advantages of the aeromagnetic gradiometer as a geological mapping tool, compared to the single‐sensor (total‐field) technique, are: (a) superior resolution of anomalies produced by closely spaced geological formations; (b) anomalies produced by nearsurface features are emphasized with respect to those resulting from more deeply buried rock formations; (c) regional gradient of the earth's magnetic field and diurnal variation are automatically removed; (d) direct delineation of vertical contacts by the zero‐gradient contour value, i.e., vertical contact mapper. In 1967 the first, colored Magnetic Anomaly Map (MAM) of Canada was produced, at a scale of 1:5,000,000, in which the dominating effects of the main earth's field were removed. Two more editions of the map have been produced in the subsequent 10‐year period. The main uses of such a magnetic anomaly map are threefold: first, as an index map it presents an overview of the aeromagnetic survey coverage of Canada; second, the map presents the major patterns produced by the continental rocks of the Canadian landmass and may be utilized both as an aid in the delineation of structural provinces and in the interpretation of regional features in the basement rocks; third, the map stimulates comparisons of magnetic features with other geological and geophysical compilations on a regional scale. More recently, commencement of colored 1:1,000,000 MAM's has also begun, and these are clearly superior to the 1:5,000,000 MAM in showing the finer details of the regional anomalies, since short wavelength features, especially dykes, tend to be eliminated at the smaller scale.