Distributed Temperature Sensing using Fiber Optics

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Distributed Temperature Sensing using Fiber Optics
Scott W. Tyler University of Nevada, Reno Dept.
of Geologic Sciences and Engineering tylers_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.

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How Does it Work?

• Rayleigh, Raman and Brillouin scattering all
  occur as light is passed through a fiber optic
  cable.
• Raman scattering is produced by inelastic
  collisions of photons with atoms or molecules
  within the fiber optic cable. If a photon loses
  energy to the fiber, the scattered wavelength is
  longer (Stokes Signal) if a scattered photon
  gains energy from interactions, its energy is
  larger and therefore its wavelength is shorter
  (anti-Stokes Signal).

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From Agilent, Inc.

• The Raman wavelengths are predictable and
  symmetric.
• The anti-Stokes (energy gaining) is strongly
  temperature dependant, but the Stokes is
  relatively independent of the temperature of the
  colliding molecule.
• The temperature of the scatterer is calculated
  from the ratio of the anti-Stokes/Stokes
  Intensity.

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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.
• Cable lengths up to 30 km
• Resolution of Temperature every 1-2 meters!
• Temperature accuracy up to 0.01 oC

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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 (more on that
  later!) 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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Instrument Response and Costs

• Tool costs range from 30K-60K
• Cable 1K-10K/km
• Several manufacturers interested in environmental
  applications.

30K Instrument, 2 m resolution
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Traditional Applications

• Geothermal Well Logging
• Circulation in boreholes, fracture mapping,
  continuous borehole flowmeters
• Limnology and fluid column monitoring

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Sakaguchi and Matsushima (2000) detected
fractures in geothermal wells during Injection,
and showed growth of steam front.
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Circulation in Mine Shafts(Selker, Stekjal,
Zeman, Tyler and Lockington)

• Circulation in flooded Czech mines
• Thermal and salinity stratification
• Double diffusive steps clearly present
• DTS produced a very high resolution data without
  disturbance to the fluid column

Steps in both T and S occur over lt1 m
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The Revolutionary or High Spatial Resolution
Applications

• Groundwater inflows to streams and estuaries.
  (see Selker et al, WRR and Day-Lewis et al., EOS)
• High spatial and temporal resolution snow/ground
  interface temperatures, ground freezing and snow
  melt.
• Very high spatial resolution of lake
  stratification and mixing.
• Fish habitat monitoring and thermal regime
  restoration efficacy.

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Mapping GW Inflows in Streams

• DTS allows continuous measure of stream
  temperatures to identify groundwater inflows
• Diehl Temp changes allow calculation of gw flux.

From Selker et al., 2006
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Role of Tidal Forcing on Ground Water

• 500 m cable installed in SF Bay to monitor
  estuary temps.
• Cost 900 for 500 m armored cable
• 500 spatial data points every 5 minutes with no
  data loggers!

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Installing fiber optic along the channel
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Measuring Spatial Structure of Snowmelt

• Snow pack temperature experiment currently in
  place at Mammoth, CA.
• 500 m DTS cable laid to measure ground/snow
  interface temperature.
• High winds relocated the cable several times
  before the snow fell.

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Issues and the Future

• Instrument costs and power requirements are
  coming down.
• Expensive cables are needed for high pressure
  installations.
• Surface placement can put the cable at risk from
  breakage and rodents, and reuse can be
  challenging.
• Cables can be repaired AND cable manufacturers
  are working with Agilent, Lios and Sensornet for
  improved cables for environmental.
• The future looks bright. UNR will be campaigning
  a tool in April-May 2007.

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Recent Hydrology-Related Publications

• Selker, J. S., L. Thévenaz, H. Huwald, A. Mallet,
  W. Luxemburg, N. van de Giesen, M. Stejskal, J.
  Zeman, M. Westhoff, M. B. Parlange (2006),
  Distributed fiber-optic temperature sensing for
  hydrologic systems, Water Resour. Res., 42,
  W12202, doi10.1029/2006WR005326.
• Selker, J.S., N. van de Geisen, M. Poolman, W.
  Luxemburg and M. Parlange, In Press, Fiber Optics
  Open Window on Stream Dynamics, Geophysical
  Research Letters.
• AGU NS24A   MCS220   Tuesday  1600h
• NS24A-02 INVITED   Monitoring Submarine
  Ground-Water Discharge Using a Distributed
  Temperature Sensor, Waquoit Bay, Massachusetts
  Day-Lewis et al.

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Sensornet DTS Halo Unit, Preliminary