Geology topics
Publications and source records attributed to Chloe Danielle Gustafson.
High-resolution airborne radiometric surveys are covering more ground than ever to provide insights into unseen geology, mineral resource potential, and possible health hazards.
Airborne radiometric surveys use passive geophysical techniques to characterize geochemical variations at or near earth’s surface. These methods have been used for a variety of mapping applications, including mineral resource evaluation. However, detailed characterization of smaller geologic targets, including mine waste features, requires flying at lower altitudes and with tighter line spacing than is feasible with traditional aircraft. Here, a small uncrewed aircraft system (sUAS) equipped with a radiometric sensor was used to acquire high-resolution gamma-spectrometry over small mine waste features and a low-grade stockpile in southwestern New Mexico. The sUAS radiometric system mapped local variability within each survey area and revealed ~2–10 m wide zones where radioelements K, Th, and U may be elevated 2–10× the surrounding material. Additionally, the sUAS radiometric data revealed radioelement variability across survey sites, which correlated reasonably well with variability seen in geochemical samples at each survey site, even though samples collected from individual sites showed high local variability. The sUAS data characterized local heterogeneity within mine waste and other small geologic targets at scales of a few meters to tens of meters, which is not possible with traditional crewed aircraft, and with continuity of coverage that is not possible with ground surveys, thus filling a key gap in geophysical survey spatial resolution.
We investigate the utility of audio-magnetotelluric (AMT) data for detecting high electrical conductivity zones indicative of fresh and saline groundwater lenses at depths of < ∼1000 m at seven survey sites in the southwestern corner of Bradford County, PA. Dimensionality analysis indicates the AMT data can be interpreted using two-dimensional (2-D) models. 2-D electrical resistivity models obtained from inverting the AMT data reveal conductive zones at depths of <30 m at all sites, between depths of 50–150 m at six sites, and between depths of 400–600 m at one site. The models show slightly lower resistivities (∼15–30 Ω m) in the deeper zones compared to the shallower zones (>30 Ω m). Consistent with well log data from two boreholes in the study area and salinity estimates obtained from the imaged resistivities, we interpret the <30 m deep conductive zone as the freshwater aquifer, and the two deeper conductive zones as salty/briny groundwater lenses. Our salinity estimates for the deeper zones vary from 1 to 380 (unitless Practical Salinity Scale), which agrees well with previous estimates of Appalachian brine salinity of 10–343. We attribute the high-salinity groundwater lenses to Appalachian Basin brine migrating upwards from deeper in the basin via bedrock fracture and fault systems. Our findings indicate AMT surveying can be useful for imaging fresh and saline aquifers at shallow depths (< ∼1000 m), at least within the northeastern section of the Appalachian Basin in Pennsylvania.
The Basin and Range province of North America hosts substantial lacustrine evaporite mineral systems prospective for lithium, a critical mineral currently listed for mineral resource assessment by the U.S. Geological Survey. Airborne electromagnetic (AEM) surveys are being conducted to support these assessments by identifying shallow clays and brines, as well as through improving the shallow subsurface geologic framework of the regional fluid flow system. In 2022-2023, three focus areas with proven lithium resources or considered highly prospective for lithium are being surveyed. Results from this effort can help to improve our understanding of the geologic conditions and geophysical signatures associated with known resource regions and benefit future lithium resource assessments by identifying regions with similar geophysical and geologic characteristics.