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New technology tackles depth to bedrock mapping

April 29, 2014 | 0 comments


When accuracy of mapping for soil depth to bedrock is the goal, Wisconsin's diverse geology means that the same method will not work at every site.

This has spurred the development of new technology, attendees learned at a series of four regional workshops sponsored by the Wisconsin Discovery Farms program and the United States Geological and Natural History Survey.

One of those sessions was held in Calumet County because it has some of that geology, it undertook a special project to map its shallow bedrock sites and a landowner offered a site for developing and testing some new technology. Knowing the soil depth to bedrock is important because it provides a basis for identifying the landscape surface practices that would protect or threaten groundwater quality.

Presenter Steve Mauel, who is a mapping specialist with USGS, noted that both paper and digital sources have been used to identify the soil depth above bedrock. In addition to the Calumet County project, similar ventures have been carried in Chippewa County and in parts of several southwest Wisconsin counties that are in the state's driftless area.

Well construction data

In Calumet County, logs on construction of wells obtained from the departments of Natural Resources and Health identified about 2,800 locations, Mauel said. Most of those wells served barns, houses or ponds.

The well data is important because it suggests the depth to bedrock at each site, he said. Multiple sources of data are needed for accurate mapping, but in some cases, surface rocks and rock outcropping make the task very easy.

In Chippewa County, the geology is very different, resulting in what Mauel described as "a strange mix" when the mapping was completed. He noted that the area does not have a mature drainage pattern, has some exposed sandstone and has disagreements on depth to bedrock in three sets of maps that have been created during the past 25 years.

Driftless features

Across the 770 square miles mapped in the driftless terrain of the Kickapoo River Valley in Vernon, Crawford, Richland and Monroe counties, there are many sites with 30 to 60 inches of soil above bedrock, Mauel noted. What makes this area different is the much longer period since it was touched by a glacier.

At least 70 percent of Wisconsin's counties have somewhat of a database on the soil depth to bedrock. All of them could obtain the information that USGS has, he added.

Mapping challenges

A method of mapping that is used in many areas, but not Calumet or Manitowoc counties, is light imaging detection and ranging, Mauel said. It is an intense technology that can identify sinkholes and related drainage areas, but it does not recognize the effect that roadbanks and culverts have on surface water flow. "That's a tough challenge," he said.

The amount of carbonate in much of Wisconsin's soil limits the use of ground-penetrating radar to identify subsurface profiles and structure, according to Wisconsin Discovery Farms co-director Eric Cooley. He credited Gold Star Farms (David Geiser and Deb Reinhart) with cooperating on the testing of new technology at the Niagara Escarpment outcropping on the farm in the town of Charlestown in Calumet County.

Imaging technology

This new technology for depicting an image, titled electrical sensitivity imaging, is no more technical than the real time kinetic base sites that provide guidance for global positioning system units, according to David Hart, who has been with USGS for just over 10 years. He said this imaging has been showing a strong correlation to groundwater problems.

An electrical resistivity imaging system costs less than $15,000, is fast, reliable and non-invasive, requires little training, goes to a depth of about 50 feet and can be transported by two people. The technique works because there is a difference in how the earthen materials conduct electricity, he explained.

This electrical approach is most effective when there is a significant difference in those materials and when clay and water are present, but it is not a precise or reliable indicator of the depth to groundwater, Hart said. The resistivity technique is also not a good choice with dolomite or sandstone rock.

Hart showed layered, color-differentiated maps created from voltage measurements that are obtained with electrical-resistivity imaging. Various placements of cables, a control box and a battery, ideally by a crew of three workers, will capture a lateral view of changes in subsurface materials.

A software program matches the resistivity images with data from other sources in order to determine at what point sediment changes to bedrock. He said this technology has been used near the Mink River in Door County, at the Rountree clay and karst geological formation in southwest Wisconsin and in Outagamie and Calumet counties.

Resistivity advantages

What can be learned with the resistivity technique is a time-saving alternative to hiring a backhoe digger for about $600 a day, Hart said. A seismic identifying unit would cost a minimum of $60,000, ground-penetrating radar is effective on sandy soils but not heavy ones and costs more than $40,000 per unit and gravity measurements and equipment are time-consuming and expensive.

Passive seismic, which is the recording of wavelength frequencies, can be conducted in about 20 minutes per site with equipment costing about $7,000. He added that vibration technology is being researched.

Hart indicated that he and colleagues are willing to carry out small-scale projects with scientific value that can be completed in a day or two. This offer does not apply to any commercial ventures, he pointed out.

Cooley noted that one or more of the techniques available for determining the soil depth to bedrock can be used for the planning and construction of manure storage lagoons. Hart suggested cooperative ventures on the purchase and use of equipment.

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