While many efforts involving optical instrumentation for the monitoring of natural resources have focused on terrestrial and atmospheric studies, a growing number of hydrologic scientists are beginning to experiment with these same technologies to increase their understanding of water bodies (oceans, lakes, rivers, streams, glaciers, and groundwater) and how they are being affected by global warming and pollution.
Over the past decade, remote-sensing techniques such as airborne laser scanning, lidar, and forward-looking-infrared systems have enabled researchers to achieve sub-square-meter resolution of hydrologic processes. Most of these systems provide expensive single-point-in-time data, however, which limits the utility of the information for monitoring these processes. The ability to continuously in time collect multipoint data from temperature at various water depths and locations in rivers, lakes, streams, and glaciers can help hydrologists better assess changes in the water and ice over time and determine what is causing those changes.
Fiber-optic sensing
With this in mind, distributed temperature sensing (DTS) along optical fibers is beginning to find favor among hydrologists. According to John Selker, professor of biological and ecological engineering at Oregon State University (Corvallis, OR) and a member of the board of directors of the Consortium of Universities for the Advancement of Hydrologic Science, DTS offers a number of advantages to the study of water resources.
“One of the key things in hydrology is that things happen in very short scales but often over many meters or kilometers,” Selker said. “One of our big questions is looking at streams and rivers and determining where and how much water joins the river as it grows from its souce (see figure). We know that the aquifers release groundwater to streams at a very stable cool temperatures compared to surface water which fluctuate over the course of the day/night cycle. The stream temperature changes are damped where groundwater enters, showing where the water enters the stream. By looking at the time-changing temperature immediately above and below groundwater inflows we can calculate both the temperature and the amount of water and determine where it comes in. The DTS’ ability to measure temperatures to 0.01°C precision in each meter of up to more than 10 km of cable opens a completely new window on river dynamics.” Such information would be useful not just for understanding how rivers and streams work but in improving, for example, the management of these resources for irrigation and near power plants.
While hydrologists have been aware of this process for 20 years, Selker says they had no way to quantify it. But DTS seemed promising. “I thought, ‘Here is a technology that really spans these issues,’” said Selker. For six months starting in April of 2006, based at the EPFL in Lausanne, Switzerland, he participated in a number of projects in Europe that involved experimenting with Raman-backscatter DTS along fiber-optic cables in glaciers, lakes, rivers, streams, abandoned mines, and power plants. The scientists at EPFL had pioneered these techniques. In Lake Geneva, Switzerland, in the late 1991s, Selker and colleagues used Brillouin scattering from an experimental DTS system attached to a spare telecommunications cable to determine season temperature profiles on the lake bed.1 The data were taken at 1319 nm, achieving spatial resolution of 3 m and a maximum range of 10 km with 20-minute precision of 0.25°C. According to Selker, current commercial Brillouin instruments operate at 1550 nm and have a spatial resolution of 1 m, range of 30 km, and one-minute precision of 0.1°C.
Using the same Raman technology in a different configuration, Selker and his team also studied water 1400 m deep in a mineshaft in the Czech Republic and air-snow-interface temperature profiles in a glacier in Switzerland. In the latter experiment, which was designed to better understand the vulnerability of snow to global climate change by understanding the dynamics of energy transfer within a snow pack, the researchers needed exceptionally fine vertical resolution. They achieved this by helically wrapping 500 m of plastic-encased multimode cable about a thin-walled 0.075-meter-diameter PVC pipe, compressing the entire 500 m fiber into a 2 m measurement section. Each meter of fiber corresponded to 0.004 m of vertical distance. The entire probe was wrapped in two layers of aluminized Mylar to limit radiant-energy adsorption. With a two-minute integration time, the DTS achieved a 0.05°C standard deviation in temperature. The DTS-based system was collocated 1 m from a thermocouple-based system with discrete measurements at 0.025 m intervals near the air-glacier interface. According to Selker, the two systems agreed well at common depths, although the DTS provided both greater vertical and temperature resolution.
REFERENCE
1. J. Selker et al., Water Resources, in press.