Airborne electromagnetics help detail underground valleys, aquifers
By helicoptering an electromagnetic device above southeast Nebraska, a research team from the Nebraska Geological Survey is generating 3-D maps of ancient underground valleys that could help inform water management by predicting how groundwater will flow.
Researchers Jesse Korus, Matt Joeckel and Dana Divine conducted their recent study in southeast Lancaster County, which contains deposits created when glaciers passed through the central lowlands of the United States and Canada more than 640,000 years ago. The team discovered a complex network of overlapping and cross-cutting sedimentary deposits that affect the region’s aquifers and streams.
“Airborne electromagnetic surveys are on the forefront of groundwater research,” said Korus, a groundwater hydrogeologist with the University of Nebraska-Lincoln’s School of Natural Resources. The technology has been used in mining for decades, he said, but “its application to aquifers is a new development, and no one has used it to study buried valleys in (these) deposits. This is the first demonstration of it.
“Fundamentally, we are trying to understand why aquifers are so variable and why they seem to occur randomly below the surface,” Korus said. “With this research, we are beginning to understand that there is indeed an order to the complexity.”
Traditional borehole drilling doesn’t work well for mapping buried valleys because it is too time-consuming and expensive. So the team, which included Nebraska’s Natural Resource Districts and the U.S. Geological Survey, used a helicopter to tow an electromagnetic tool that recorded the electrical properties of the subsurface. Clay, which is electrically conductive, registers as blue, whereas sand and gravel appear red.
This method allowed the researchers to collect extremely detailed information 200 feet beneath the surface and along lines more than 900 feet apart. Limitations in resolution, though, still left a gap in data, requiring the use of borehole and water-well data collected along the same flight line.
“We see as many as six generations of valleys in this area, overlapping in complex ways,” Korus said. “This pattern has relevance in predicting how groundwater flows in the area. Groundwater availability depends on the nature of the sediment fill. Some buried valleys contain fine sediments that act as barriers to groundwater flow. Other valleys act as hydraulic windows, allowing water to flow from the land surface to deep aquifers below. The geology is directly related to the sustainability of the resource.”
Airborne electromagnetic surveys have created a surge in data collection for the region, which could prove useful in making water management decisions, Korus said.
The team has submitted a proposal to expand this work to other areas of Nebraska, hoping to focus on the cost-effectiveness of airborne electromagnetics and how underground maps can be used to plan for water development and management.
The researchers published their findings in the journal Sedimentology.