Yellowknife Geoscience Forum

At a regional scale and for the purpose of delivering base geological cartography, federal and provincial government agencies plan and commission aeromagnetic surveys over blocks that are of higher priority at the time of planning. Airborne magnetic surveys are a part of the reconnaissance work for projects and cultivate a patchwork of different sized surveys.  The difference in sizes between surveys creates a patchy effect in the combined data.  This results in a block-style distribution of surveys that were flown with different acquisition parameters such as platform, flying height, line spacing and line orientation.

The advantage of compiling these individual grids into a single grid is to provide a consistent portrayal of the geophysical response of the imaged rock. This facilitates interpretation of the data for the end users, who can now look into a single file instead of having to browse, clip and display multiple blocks.

The varying sensor height of the multiple surveys will be a challenge for any compilation effort, since surveys flown higher will register a weaker signal than those at lower elevations.  One approach to account for this is to level the high-resolution surveys up to the flying height of more regional surveys (upward continuation). The second possibility is to compromise at an intermediate flying height between the regional and detailed surveys, and level all the data to that level. A third possibility is to bring all the surveys to the lowest possible height.  Downward continuation methods have the potential to introduce noise that obscures geological signal while upward continuation methods have the potential to lose signal but have greater computation capacity.  For all these approaches, there are a variety of computational procedures that could be used.

This project will use a draping downward continuation on profile data to create a compilation of aeromagnetic data available for the NWT Cordillera. There were 18 government surveys flown between 1961 and 2018 by both the GSC and NTGS, with flying heights ranging from 110 to 2000 m and line spacings between 400 and 1207 m; and 2 industry surveys flown between 2006 and 2011, with line spacings between 100 and 300 m and 100 m flying height with variable line orientation. Using the radar altimeter data to calculate the downward continuation distance, profile data from all the surveys were draped line by line to the lowest possible elevation by using a Taylor expansion. The height-corrected profile data were gridded and microlevelled and resultant grids were compiled.  The final levelled grid will be published in the NWT Open File series.

A further goal is to refine the structural map of the area. The current geological maps for the region are a mixture of 1970’s reconnaissance work that has been digitized and reinterpreted, 1:250,000-scale maps based on 1980’s mapping, and 1:100,000 scale maps of selected areas from the 2000’s. Interpreted faults and folds from the aeromagnetic data will be validated and refined using the available bedrock geology along with publicly available radiometric data.