Distributed Temperature Sensing: A Transformative Technology in Water Resources

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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 styler_at_unr.ed
u http//wolfweb.unr.edu/homepage/tylers/index.htm
l/
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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
  0.01-0.5oC.
• Spatial location of temperature is resolved
  identically to Time Domain Reflectometry

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Optical Fiber Basic Construction
Total Internal Reflection
Lower Refractive Index
Higher Refractive Index
Ta
Core
Cladding
Ta Acceptance Angle
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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.

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Distributed Temperature Sensing
Rayleigh Scattering
Stokes
Anti-Stokes
shifts with temperature
Brillouin
Raman (Anti-Stokes) in amplitude
Raman (Stokes)
Brillouin in frequency
Amplitude/ Intensity
Frequency
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• 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.
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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.

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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)

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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

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Mammoth Mountain Ski Area (Sierra Nevada)
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Typical DTS Signals
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Bare Ground vs. Buried Cable
Note Scale Difference
Below Snow
Diurnal variations clearly define bare and snow
covered areas
From Tyler et al., 2008
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Lake Tahoe, CA Test Site
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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.

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Weather Conditions June 6

• The previous day was very cold and windy
• Strong westerly's

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Weather Conditions June 7

• Warm, calm day
• Smooth water

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Complete Vertical Profile Single Ended
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Detailed View of the Thermocline at 40 meters
From Tyler et al., 2008
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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, t, to
  estimate the soil thermal diffusivity every 1
  meter along the cable.
• t (x, y, t) f(thermal diffusivity, depth, x, y)
• t (t) f(thermal diffusivity) f(?)
• 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.

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Installing fiber optic cable
1000m of armored cable installed at 15cm
depth Dragged and seeded
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Temperature vs. Time
DRY SOIL
KT 30 cm2/hr
? 7
30 25 20 15 10
35 30 25 20 15
Soil Temperature (ºC)
Air Temperature (ºC)
soil temperature
air temperature
7/26 7/27 7/28
Time
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Soil Moisture Thermal Diffusivity
25 20 15 10 5
100 80 60 40 20
irrigation event
irrigation event
drying
Soil Moisture () - symbols
Thermal Diffusivity (cm2/hr) - lines
drying
DRY SOIL
7/26 7/27 7/28 7/29
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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.

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Cave Air Temperatures
Cave Entrance
Wet Area
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VERTICAL THERMAL PROFILES IN A TALL (gt30 m ) ROOM
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Stream/Meadow MonitoringSequoia National Park
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Stream Temperature Profile
Meadow
Deep Pools and Stream
Ice Bath
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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.