Volume 1 2011

Water Resources Research

Cable Delivers More Than Entertainment

By Moira Harrington

A team of University of Wisconsin Water Resources Institute researchers conducting field work on groundwater-test and municipal wells around the state are proof there is a connection between tuning in the HBO vampire series “True Blood” and turning on a kitchen faucet.

Fiber optic cable can not only deliver entertainment, it can also be used in empirical pursuits. In this case, it is crucial to a study on a new and potentially economical method for profiling groundwater movement. Jean Bahr, a UW-Madison geoscience professor, and David Hart of the Wisconsin Geological and Natural History Survey (not the same person as the ASC’s David Hart) are leading the year-long analysis. Bahr said urbanization is, in part, driving the need for this research. As development pushes outward, municipal water systems are called upon to tap different and multiple aquifers to meet demand. It is important to know the effects of water withdrawals from layers of bedrock, including shale and sandstone, particularly if they affect critical discharge zones such as springs.

“One of the big challenges in hydrogeology is the geology – how the different rocks conduct water. Water can move much faster in some layers than in others. It depends on how the rocks were deposited. What’s challenging about that is this is all happening underground and we can’t see it,” said Andrew Leaf, a former UW-Madison graduate student on the project. “Temperature is one way to measure the movement of water because water carries heat with it as it moves.”

Bahr explained, “Older water, coming through longer flow paths and that’s gone deeper, is generally warmer than newer, near-surface recharge water.”

To see how the water flows, Bahr and her team can force hot water down a well. The hot water becomes a kind of liquid sherpa, providing insight about the terrain. But the question remains: Beneath turf and topsoil, where is the sherpa and what path is it blazing? That’s where a method called distributed temperature sensing (DTS) comes in.

DTS provides nearly continuous measurement of temperature in time and space along fiber optic cable. A sensor box pulses a laser light the entire length of sinuous cable feeding temperature readings to an above-ground computer – yielding a profile of geologic framework. It provides “a level of detail we haven’t been able to get before,” Leaf said.

Using fiber optic cable for non-entertainment/non-communication purposes isn’t new. Cable has monitored dams’ structural integrity and buildings’ fire risk for the last 25 years, for example. Yet this study is innovative. “We’re pioneering its application down-hole, in a more hydrogeologic setting,” Leaf said. “That’s an unusual use for this technology.”

“We’ll be seeing DTS used more and more in the future,” predicted researcher David Hart. Plus, the cost is economical when stacked against more common mapping tools.

In this study, the cable cost approximately $3,000, and another university loaned the sensor box. The boxes can be expensive, perhaps $20,000 apiece, Hart said. However, he said lower costs are within reach due to an economy of scale as DTS is increasingly adopted, particularly when compared with the electromagnetic flow meter method and its $70,000 price tag.









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