Written by: Ronald S. Bell, Senior Geophysicist/geoDRONEologist – Collier Geophysics, LLC.
In 2018, four (4) drone enabled magnetic surveys were conducted in the San Luis Valley. The footprint for one survey is in the northeastern reach of the valley just north of the Colorado-New Mexico border. The fourth survey was conducted along a stretch of Monte Vista. Though the purpose for each survey was slightly different, the geological information extracted from the data is being applied to improve and enhance the understanding of the groundwater issues specific to each study area.
The magnetic geophysical method responds to variations in magnetization in the subsurface geologic fabric. Because inverse model solutions of magnetic data are unique, accurate geologic interpretation of magnetic data sets function is available knowledge of the geology and, where feasible, the correlation with results obtained from surface geophysical surveys and well logs. This is best illustrated for the project area near Crestone, where ground transient electromagnetic (TEM) soundings and electrical resistivity tomography (ERT) surveys were conducted over a unique topographic feature unofficially named the “Crestone Crater”.
The drone magnetic data for the one (1) mile section centered approximately on the Crestone Crater strongly suggests the presence of down dropped block of basement rock under a surface layer of aeolian sands overlying an alluvium layer sourced from the Sangre De Cristo Mountains to the east. A range front fault known to be recently active along with the crosscutting faults interpreted from the magnetic survey data as striking westerly towards the center of the valley provides the basis for a new narrative that engages the powerful forces of groundwater to explain the genesis of the slightly elongate crater-like topographic feature.