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Distributed Temperature Sensing: A Transformative Technology in Water Resources Scott W. Tyler University of Nevada, Reno Dept. of Geologic Sciences and Engineering [email protected] http://wolfweb.unr.edu/homepage/tylers/index.html/ What is Distributed Temperature Sensing (DTS) The measurement of temperature (and) using only the properties of a fiber-optic cable. The fiber-optic cable serves as the thermometer, with a laser serving as the illumination source. Measurements of temperature every 1-2 meters for as long as 30 km can be resolved, every 1-60 minutes, with temperature resolution of 0.010.5oC. Spatial location of temperature is resolved identically to Time Domain Reflectometry Optical Fiber – Basic Construction Total Internal Reflection Lower Refractive Index Higher Refractive Index Θa Core Cladding Θa = Acceptance Angle Raman Scattering for Temperature Thermal energy drives oscillations within the lattice of the doped amorphous glass making up the fiber. When excited by photons (from the laser illumination), the interactions between the photons and the electrons of the solid occurs, and results in light being scattered (re-emitted) and shifted to higher and lower frequencies The scattered light is shifted in frequency equivalent to the resonant frequency of the oscillating lattice ( a constant for any particular molecular structure) Higher intensity of thermal oscillation produces higher intensities of the scattered light. Distributed Temperature Sensing Rayleigh Scattering Amplitude/ Intensity Stokes Anti-Stokes shifts with temperature Raman (Stokes) Brillouin Brillouin in frequency Frequency Raman (Anti-Stokes) in amplitude •Currently used in fire monitoring, oil pipeline monitoring, high tension electrical transmission cables, down hole monitoring of oil production, dam seepage. Detector serves as both OTDR (for distance) and intensity (for Stokes and anti-Stokes) Figure courtesy of AP Sensing. Advantages of DTS The cable serves as the measuring device Fiber optic cable is relatively inexpensive ($0.50$10/meter) and robust and have small thermal inertia. Once installed, continuous measurements do NOT disturb the fluid column (wells) or soils. Very high resolution and long cables can provide high density coverage of a landscape, lake, or groundwater reservoir. Installations can be temporary or permanent. Example Applications Snow dynamics (Dozier, McNamara, Burak, Selker) Measuring mixing in the thermocline of Lake Tahoe (Selker, Schladow Torgersen and Hausner Towards developing integrated soil moisture at large spatial scales (Selker, Miller, Hatch) Cave air circulation (Wilson, Barber and Jorgensen) Stream/Groundwater Exchanges (Conklin, Bales, Hopmans) Challenges of Snow Installations Cold Temperatures; Freeze/Thaw common Rodents/Burrowing animals Lack of access throughout winter Significant strains possible due to creep, consolidation, metamorphosis and avalanche Small thermal gradients need to be resolved Solar heating on fiber, particularly in late stages of melt when snow is dominated by ice may affect observed temperatures Mammoth Mountain Ski Area (Sierra Nevada) Typical DTS Signals Stokes/Anti Stokes Cable loss Bare Ground vs. Buried Cable Note Scale Difference Below Snow Diurnal variations clearly define bare and snow covered areas From Tyler et al., 2008 Lake Tahoe, CA Test Site Cable Deployment Cables were deployed from the UC Davis research vessel John LeConte Cable was lowered to the bottom of the lake, then pulled up 20 m Total depth was approximately 411 m. Weather Conditions: June 6 The previous day was very cold and windy Strong westerly's Weather Conditions: June 7 Warm, calm day Smooth water Complete Vertical Profile: Single Ended Detailed View of the Thermocline at ~40 meters Note the wavy pattern of the warm water interface! Causing mixing of nutrients to the bottom waters Measurement of Soil Moisture during Irrigated Agriculture We can measure soil moisture only in the very uppermost portions of the soil with radar, but few methods are available to measure spatially distributed soil moisture IN the root zone! Here, we use a passive approach, relying upon solar heating and time lag at 15 cm, τ, to estimate the soil thermal diffusivity every 1 meter along the cable. τ (x, y, t) = f(thermal diffusivity, depth, x, y) τ (t) = f(thermal diffusivity) ~ f(θ) Active methods, in which a heater cable provides the input have also been developed at OSU and LBL and are analogous to heat dissipation sensors. Installing fiber optic cable • 1000m of armored cable installed at 15cm depth • Dragged and seeded Temperature vs. Time = 7% KT ~ 30 ∆t cm2/hr 35 25 30 20 25 15 20 soil temperature 10 air temperature 7/26 7/27 Time 7/28 15 Soil Temperature (ºC) Air Temperature (ºC) 30 DRY SOIL Soil Moisture & Thermal Diffusivity irrigation event 100 20 15 80 irrigation event 60 drying drying 10 40 DRY SOIL 5 7/26 7/27 20 7/28 7/29 Thermal Diffusivity (cm2/hr) - lines Soil Moisture (%) - symbols 25 Measuring Air Flow in Carlsbad Caverns Nat. Park Air circulation in CCNP an important aspect of cave biology and cave management Air circulation and thermal convection is believed to control many cave feature formation processes. Air circulation may be an analog to fluid convection during cave formation. Hot, saline fluids believed to be dominant cave forming mechanism. Cave Air Temperatures Cave Entrance Wet Area VERTICAL THERMAL PROFILES IN A TALL (>30 m ) ROOM STRATIFIED UPPER ROOM WELL MIXED LOWER ROOM Stream/Meadow Monitoring Sequoia National Park Stream Temperature Profile Deep Pools and Stream Ice Bath Meadow Conclusions and Vision DTS can provide fundamental insights into exchange processes and thermal stratification (Tahoe gravity waves, cave circulation, diurnal variations in stream “dead-zone” volumes). Data “granularity” allows us to probe small scale processes, while at the same time measuring across broad spatial scales (snow monitoring, soil moisture measurement) CUAHSI/NSF-sponsored workshops in 2007 and 2008 have trained ~70 professionals and students, and also shaped our views on technology transfer. Another planned for July 2009 in Denmark. Other applications on-going Borehole logging and fracture flow, ASR Monitoring prescribed fire soil temperatures Lake/atmosphere exchange and evaporation from lakes Vertical snow temperature monitoring Stream/fish habitat recovery, both for cold water species (salmon) and thermophiles (Devils Hole pupfish) Monitoring solar inputs to aquatic systems.