With development initially driven by the military, drones have expanded into a plethora of new sectors and are now common in mapping geology and targets of interest. Also known as UAVs (Unmanned Aerial Vehicles), they are changing how we investigate the surface and subsurface of the Earth. Their advantages are obvious, they can easily and cheaply go where humans can’t, they don’t put people at risk, they are lightweight and powerful and, with advances in battery technology, range and flight times are on the rise. Couple that with small lightweight UAV mounted magnetometers such as GEM Systems DRONEMag™ and Geometrics MagArrow™ and the ability to add other sensors from visible light cameras to gamma ray, infrared or LiDAR and you have a winning solution in mapping the subsurface.
To help make effective use of this new surveying technique, dynamic 3D geoscience platform, Oasis montaj is offering even more drone capabilities. Its UAV Geophysics extension provides a complete workflow for planning and processing drone magnetic surveys for extracting information for accurate decision-making.
Ron Bell, Senior Geophysicist and self-proclaimed geoDRONEologist at Denver-based International Geophysical Services sees UAV geophysical surveys as a “game changer” for environmental and engineering subsurface characterization and detailed subsurface geologic mapping. “Typically, it takes less time to execute a drone magnetic survey than a comparable ground one. In rugged topography, there is also a clear reduction in the risk of injury to field staff. UAVs also traverse over the area of interest with the sensor at a consistent height above the ground along remarkably straight lines, unlike human beings.”
Oasis montaj’s latest release, 9.7, now offers even more capabilities when working with drones, optimizing and simplifying the UAV Geophysics extension workflow for this type of surveying and offering solutions to common issues. The latest release supports the import of previously designed and collected drone surveys and the export of surveys in the commonly used KML file format. It also supports defining the location of more than one observer for Line of Sight compliance and the application of several tools for multiple sorties at once.
Ron comments, “I use Oasis montaj’s survey design tool to help set the parameters of the survey and estimate how much time is required. I then use this information to cost out the survey and prepare the project proposal. There is also a tool for selecting the optimum location for the pilot to occupy for Line of Sight compliance.” Regulations in some countries, most notably the United States, require that the UAV pilot in command maintains visual line of sight with the UAV while occupying airspace. “The net result of these innovations is that the size of the survey areas will continue to grow larger and so will the volume of data. The bottom line is that there will be a net gain gleaned in the reduction in the time required to trim data volume.”
UAV Magnetic Survey Data Processing and Visualization
Bell also uses Oasis montaj’s UAV Geophysics extension for processing and visualizing his drone magnetic data. “I begin processing while in the field, first by creating flight path and data verification maps for each sortie and in between each sortie. Then at the end of the survey day, once I have the diurnal and heading error corrections calculated, I continue the processing resulting in a database of the corrected and tie line leveled magnetic data. I also use the MAGMAP Geophysical Filtering extension to process the data into a set of map products that I use for interpretation. I even use Oasis montaj to create my own trademarked presentation to quickly show areas of high induced magnetization.”
The UAV Geophysics extension allows the user to subdivide the survey area into adjacent sorties based on the drone-specific time. This helps determine survey cost much more reliably. Planned survey paths can then be exported to many standard formats such as KML for direct upload to the drone.
The high sample rates of UAV geophysical sensors result in large volumes of high-resolution data. “Software such as Oasis montaj, with its roots in traditional airborne geophysics, are designed to handle large magnetic data sets made up of a lot of flight lines and tie lines,” says Bell. “It makes good sense to use Oasis montaj and, in particular, the UAV Geophysics extension to process the drone magnetic data to increase the workflow efficiency and, best of all, save time.”
Mapping the Crestone Crater
In 2017, Bell and his team were invited by UAS Colorado, a UAV industry Colorado based trade association, to conduct a drone magnetic survey within the boundaries of Colorado’s Great Sand Dunes National Park. The drone magnetic survey was accomplished over a 500 m by 500 m flight block. Upon analyzing the data, Bell concluded even though the survey adequately defined the magnetic response of the crater, a survey over a larger area was required to better understand the basement geology, this second survey was completed in 2018 and was conducted over a square mile and in 1.5 days. The data volume consisted of 25 sorties acquired using four (4) flight operation site locations (see Figure 1).
The data was processed and visualized in Oasis montaj and the colour contours of the total magnetic intensity (TMI) were superimposed on a 2D plan view Google Earth base image for the project area (see Figure 2). The TMI color contour image was then exported as a KML file for import into Google Earth (see Figure 3).
“In September of 2019, we returned once again, this time equipped with Oasis montaj’s latest software tools and the experience that comes from having flown over 500 sorties. In two days, my team and I were able to collect drone magnetic data that effectively repeated the 2017 and 2018 surveys plus we expanded the overall coverage to 2.7 square miles.” (see Figure 4). A detailed reporting of drone magnetic surveys over the Crestone Crater will be presented at SAGEEP 2020 in a special session titled: The Enigmatic Crestone Crater – Part II.