Download Fiber Optic Temperature in Hydrology

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)
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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
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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
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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
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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
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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
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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
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The previous day was very cold and windy
Strong westerly's
Weather Conditions: June 7
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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
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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
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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
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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.