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UNL hydrogeologist Steve Sibrary shows a resistivity tool, which is lowered into test holes to measure electrical resistance of underground geological layers.
Mining industry could use aerial survey techniques now used for groundwater, UNL hydrogeologist says
Posted October 25, 2012
By David Ostdiek, Communications specialist
Panhandle Research and Extension Center
A technology used increasingly to search for groundwater in Nebraska is airborne electromagnetic survey (AEM), where an electromagnet sensor is suspended beneath a helicopter that flies parallel, systematic routes over a designated area.
The same technique is useful in locating uranium-bearing formations, according to a water scientist with the University of Nebraska-Lincoln.
Steve Sibray, a hydrogeologist with UNL’s Conservation and Survey Division based at Scottsbluff, said data from recent AEM surveys in the Nebraska Panhandle indicated some “anomalies” below the base of the groundwater aquifer, at the base of the brule formation.
The data was generated at the limits of the resolution used for groundwater flights, which were calibrated for the shallower geologic formations containing most of the groundwater important for human consumption, irrigation and other uses. Mining companies could fly over areas of interest, with the equipment calibrated for deeper formations, he said.
Uranium is found in western Nebraska in a deeper formation known as the basal White River group sands, and some of the shallower portions of that formation were indicated in the groundwater airborne survey data.
“I think it would be helpful for companies exploring for uranium to help delineate the uranium-bearing sands,” Sibray said of AEM surveying.
Sibray is based in the part of Nebraska where uranium is mined. Nebraska’s only uranium mine, the Crow Butte Facility, is located about 4 miles southeast of Crawford. According to a Nuclear Regulatory Commission fact sheet, Crow Butte uses the in situ process to extract uranium from basal Chadron sandstone 400 to 800 feet below the land surface. Injection wells place an oxidizing solution into the formations, and the solution flows to recovery wells. Uranium-rich solution is drawn from the wells for processing into yellowcake.
Sibray said Nebraska is the No. 2 state in uranium production, behind Wyoming.
Much of Sibray’s work for Conservation and Survey has focused on groundwater in the Panhandle. But he also has been sharing information with people who drill into the earth to extract petroleum and uranium.
He spoke about the use of AEM surveys recently at an American Association of Petroleum Geologists conference. In 2011 Sibray gave a talk to the uranium industry in Casper about the same topic. Sibray also was a coauthor on a USGS report on the use of AEM in western Nebraska. It is online at http://pubs.usgs.gov/sir/2011/5219/.
Prior to the 2011 talk, he wrote an article for a regional journal on geology about uranium deposits in northwest Nebraska. In the article he proposed the idea that the source of the uranium was paleosols (ancient soils) towards the base of the White River group.
Sibray said his theory is based on similarities between uranium deposits in the Nebraska Panhandle and those in Wyoming.
Although AEM surveys have been used recently in Nebraska to help locate groundwater, Sibray said it is not novel to employ the technique in other settings. In fact, the first practical use of AEM was looking for metal deposits. Electromagnetic surveying was developed around the time of World War II in mine detectors. Used in that setting, it was earthbound, not airborne; it was similar to metal detector technology.
Among those who developed the technology were oilfield technologists. Later, the Canadian Province of Ontario publicly funded airborne surveys to help identify ore deposits.
Sibray predicts that someday the entire state of Nebraska will be mapped for groundwater purposes using AEM. It has been used quite extensively in the central and southern Panhandle in addition to eastern Nebraska. These surveys have been a combined effort of several natural resources districts, the U.S. Geological Survey, and UNL’s Sibray.
Sibray’s role in the project is to partner with the USGS to help the federal agency interpret local geology. Conservation and Survey has drilled a lot of test holes over the years in the North Platte and South Platte natural resources districts, covering the southern half of the Panhandle.
Sibray makes use of down-hole probes in test holes to measure some of the same electrical properties of the rocks that is measured in airborne surveys. USGS coordinated their airborne measurements with Conservation and Survey’s data gathered from down-hole probes.
“If it’s a good match, we feel that their methods are providing what they need,” he said.
Sibrary explained that electrical properties can indicate where groundwater will be found because freshwater sands do not conduct electricity as well as the clay-bearing formations that hinder underground water movement. Thus, aquifers show up as areas with higher resistitvity.