USGS Groundwater Information: Branch of Geophysics
F.D. Day-Lewis and J.W. Lane, Jr.
New geophysical methods are required for monitoring hydrologic processes at the catchment and larger scales, and for quantifying fluxes between ground water and surface water. Fiber-optic temperature monitoring (FOTM) is an emerging technology for characterizing estuary-aquifer and stream-aquifer interaction. Although routinely used for monitoring temperature and (or) strain in petroleum wells, FOTM applications in hydrology are rare. Measurements involve sending a laser along a fiber-optic cable and analyzing either Raman or Brillouin backscatter. Analysis of Raman scatter for variation in optical power yields estimates of temperature, whereas analysis of Brillouin scatter for variation in optical frequency yields estimates of both temperature and strain. Commercially available FOTM technology can achieve spatial resolution of less than 1 meter and thermal resolution of less than 1 degree Celsius, at sub-minute measurement intervals. Measurements are taken along a reinforced fiber-optic cable, with cable lengths up to 30 km. Spatial, thermal, and temporal resolution are mutually dependent, and depend on cable length; hence there is a tradeoff between, for example, the scale of a study and the precision of temperature measurements.
In spring of 2006, the U.S. Geological Survey, Office of Ground Water, Branch of Geophysics evaluated a FOTM system. We present results of several FOTM pilot studies, based on Raman scattering, at the 100-meter to kilometer scales. Study goals include mapping submarine ground-water discharge and identification of gaining stream reaches. For each project, additional hydrologic, chemical, or geophysical data are used to help confirm interpretations based on the fiber-optic temperature monitoring results.
Final copy as submitted to Society of Exploration Geophysics Hydrogeophysics Workshop for publication as: Day-Lewis, F.D., and Lane, J.W., Jr., 2006, Watershed-scale temperature monitoring of hydrologic processes [abs.]: Hydrogeophysics Workshop, Vancouver, British Columbia, July 31-August 2, 2006, Proceedings, Society of Exploration Geophysics.