How Warm is the Water? / Scientists Hope Fiber-Optic Technology Can Assist in Measuring Temperature
Posted on: Saturday, 16 September 2006, 03:00 CDT
By A.J. HOSTETLER
Fiber-optic cable can provide commercial TV, Internet access, phone service and now, temperature readings along the Shenandoah River.
The U.S. Geological Survey is conducting a six-month pilot project to test fiber-optic temperature-measuring technology to explore waterways across the country from Cape Cod to the Shenandoah to Wyoming and possibly, the chilly depths of Lake Tahoe.
The fiber-optic system, designed by German-based LIOS Technology, typically is used in fire-detection systems or in power-line-load monitors. The USGS leased the equipment to see what it can do for hydrological research. Each location gets the equipment for two weeks.
"The real benefit is . . . in two minutes I can measure the temperature of a mile of river," said David Nelms, a groundwater specialist with the USGS Virginia Water Science Center in Richmond. He was one of about a dozen federal and state employees involved in the Shenandoah project last month.
Using conventional methods, researchers who either paddle out in a boat or wade into waterways with temperature probes can take about 50 readings in a day or two, Nelms said.
The fiber-optic system allowed the USGS-led team to take about 1 million readings from the upper Shenandoah in Clarke County in about three days.
Nelms and his colleagues hope the fiber-optic cable, combined with an infrared laser, will tell them more about where groundwater enters the Shenandoah River.
Groundwater is the precipitation that seeps into the soil until it reaches saturated rock.
It moves slowly downward underground and may eventually enter rivers, streams, lakes and oceans.
"The ground is your storage vessel," said Don Hayes, a surface- water specialist with the USGS office in Richmond.
Previous research has shown that the Shenandoah's primary contributor is groundwater, not surface water. This finding holds implications for the movement of water contaminants in the Shenandoah Valley, according to Nelms.
Identifying where groundwater enters the Shenandoah would help uncover more about its relationship to the region's unusual geology. The same rocks - siliciclastic rock such as shale and sandstone or carbonates such as limestone - exist east and west of the river, but the two sides of the valley have different structures and deformations, Nelms said.
The Shenandoah branch of the pilot project involved laying by hand about 3,280 feet of fiber-optic cable down the center of the river in Clarke County.
"We spent three days walking up and down the river like a sidewalk," Nelms said.
To measure temperature differences, the lightweight cable, thinner than a pencil, must rest on the river bottom.
The USGS employees cleaned out a Winchester hardware store of 500 washers and returned to the river, where they attached the weights to the cable with rubber bands. The washers provided enough weight to keep the cable on the riverbed while the rubber bands allowed it to flex in the water's current.
"Sometimes you don't need expensive high tech," Nelms said.
The river depth ranges from about 1 foot to 9 feet in the Lockes Landing area. At one point, Hayes bobbed under water to arrange the cable along a hole in the riverbed.
"When we were in the river, there's people tubing, canoeing and fishing," Nelms said. "You had these 'river socials,' where they're telling you where the springs are, saying, 'There's cold pockets out here,' a lot of helpful information" that might shed light on the river's groundwater sources.
Once the researchers positioned the cable and returned to their laptop computers at their truck parked on the bank, they beamed an infrared laser inside the cable to measure temperature every meter, or a little more than a yard.
The cable contains quartz fibers. When the laser, traveling about two-thirds the speed of light, strikes fibers that are hotter or cooler, the light when it returns scatters into a pattern displayed as peaks on a computer graph. The peaks represent possible temperature changes.
A sudden temperature difference could reveal a location where colder groundwater enters the river. Or it might indicate a hole in the river bottom where, when the sun goes down, the temperature approximates other readings.
Readings were taken during the day as well as at night to filter out the effects of the sun warming the river.
A spring feeding into the Shenandoah's shallow water provided some of the coldest readings. The spring was near a spit of land by a take-out site the researchers used.
"We would come down and take out here, and everybody would say, 'Boy, that's cold,' " Nelms said.
Generally, however, the river temperatures were fairly constant, shifting with sunlight. On a sunny day, temperatures along the riverbed ranged in the low 80s.
The delicate cable stretched from Clarke County's Watermelon Park to Lockes Landing. It took just under two minutes for the laser to make a round trip of the cable.
The group conducted nearly 600 round trips, or traces. The experiment was abruptly halted on its third day when the fragile cable snapped, possibly from a stick pushed against it in the current or as the result of inquisitive resident beavers.
"We had two prime candidates," Nelms said of the beavers.
A new cable, one wrapped in a more protective coat, is on order for the next location, according to Hayes.
Aside from the critters' curiosity, early August was a good time for the experiment. Not only was the river water pleasantly warm for laying cable, groundwater is coolest in the summer. The temperature of any groundwater could be more easily detected from the river's ambient temperature, Nelms said.
The summer's dry conditions also aided the project by keeping the water shallower and clearer than if it had been rainy, he said.
While valuable, the data gathered with the fiber-optic equipment needs to be considered in context before interpretation, said Hayes. The cable used in the Shenandoah was placed down the center of the river. That meant temperature differences near the riverbanks, where groundwater might be entering, were ignored. For the data to be truly meaningful, more readings taken over a longer time period and over a wider area would be critical, he said.
Once the USGS gains experience with the equipment, it could "give us another set of clues" to help learn more about the source of the Shenandoah's waters and how to maintain and regulate the supply and quality, Hayes said.
The USGS financed the pilot project for about $100,000, including equipment and personnel time. When the pilot experiments are finished, the agency will review the findings to determine whether to incorporate the equipment in the agency's work, said Bill Cunningham of the USGS groundwater division.
Fiber optics and hydrology
The U.S. Geological Survey is testing fiber-optic temperature- measuring equipment to see how it can be used in hydrological research.
The Shenandoah River was one of the test sites. Temperature changes along the river might enhance the agency's understanding of the movement of groundwater in the Shenandoah Valley.
To learn more about groundwater, go to: http://ga.water.usgs.gov/ edu/earthgw.html
Contact staff writer A.J. Hostetler at ahostetler@timesdispatch.com or (804) 649-6355.
ILLUSTRATION: PHOTO, MAP
(c) 2006 Richmond Times - Dispatch. Provided by ProQuest Information and Learning. All rights Reserved.
Source: Richmond Times - Dispatch
User Comments (0)
RSS Feeds