Airborne Magnetic Survey, Colorado Mineral Belt, Northeast Block, 2024

Magnetic data collected as part of a high-resolution airborne magnetic and radiometric survey over the northeastern portion of the Colorado Mineral Belt in northern Colorado are provided as digital flight-line and grid files. Magnetic surveys measure subtle variations in the Earth’s magnetic field and are sensitive to differences in the magnetic properties of rocks from ground level to many kilometers below. Data for this survey were acquired by NV5 Geospatial, Inc. and sub-contractor's Precision GeoSurveys, Inc. and EDCON-PRJ, Inc under contract with the USGS (U.S. Geological Survey). The survey was flown over 39 flight days in August and September of 2024. Magnetic data were measured from a magnetometer mounted in a stinger extending from the nose of the aircraft. The helicopter pilots followed pre-planned flight paths in a grid-like pattern, with east-west lines spaced 200 meters apart and north-south lines spaced 1,000 meters apart. Lines were flown 100 meters above ground as much as possible to resolve details of the magnetic field. This clearance could be realized in areas of low relief but higher clearances, as much as 200-500 meters, were required over rugged terrain and populated areas for safety reasons. In the northeast portion of the area, the drape surface incorporated a flat surface at 2,000 meters above sea level to enable smooth transitions from close terrain-following over the mountains to FAA mandated safe clearances over the population centers along the base of the foothills. The main objective for the flat surface was to follow regional patterns in the magnetic data east of the mountain front while minimizing noise due to infrastructure on the ground. Areas with restricted airspace, such as Wilderness Areas, were avoided. A total of 35,748 linear kilometers of data were collected along the lines, covering a 5,937 square-kilometer irregular area. EDCON-PRJ performed extensive data processing after completion of flying and delivered the final data and report in March 2025. Files available in this publication include: flight‑line data for the magnetic survey in ASCII .csv format; flight‑line data for the radiometric survey in ASCII .csv and NetCDF formats (the latter accompanied by an NCML metadata file); GeoTIFF images of the residual magnetic anomaly, reduced‑to‑pole magnetic anomaly, first derivative of the reduced‑to‑pole magnetic anomaly, radiometric total count, potassium, thorium, and uranium values, and element ratios (thorium‑to‑potassium, uranium‑to‑thorium, and uranium‑to‑potassium); a ternary plot of radiometric uranium, potassium, and thorium values; grids of all these data; images depicting the reduced‑to‑pole magnetic anomaly and the ternary plot; and a report describing survey parameters, field operations, quality control, and data‑reduction procedures. A zip file containing the contractor's deliverable products is also included. These files are provided as received from the contractor. Binary files in this package are readable using the free software "Geosoft Viewer"" or commercial software "Oasis montaj". References cited: Day, W.D., 2019, The Earth Mapping Resources Initiative (Earth MRI): Mapping the nation's critical mineral resources: U.S. Geological Survey Fact Sheet 2019-3007, https://doi.org/10.3133/fs20193007.

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