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Feasibility of soil moisture monitoring with heated fiber optics


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Accurate methods are needed to measure changing soil water content from meter to kilometer scales. Laboratory results demonstrate the feasibility of the heat pulse method implemented with fiber optic temperature sensing to obtain accurate distributed measurements of soil water content. A fiber optic cable with an electrically conductive armoring was buried in variably saturated sand and heated via electrical resistance to create thermal pulses monitored by observing the distributed Raman backscatter. A new and simple interpretation of heat data that takes advantage of the characteristics of fiber optic temperature measurements is presented. The accuracy of the soil water content measurements varied approximately linearly with water content. At volumetric moisture content of 0.05 m3/m3 the standard deviation of the readings was 0.001 m3/m3, and at 0.41 m3/m3 volumetric moisture content the standard deviation was 0.046 m3/m3. This uncertainty could be further reduced by averaging several heat pulse interrogations and through use of a higher-performance fiber optic sensing system.
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Feasibility of soil moisture monitoring with heated fiber optics
Chadi Sayde,
Christopher Gregory,
Maria GilRodriguez,
Nick Tufillaro,
Scott Tyler,
Nick van de Giesen,
Marshall English,
Richard Cuenca,
and John S. Selker
Received 12 February 2009; revised 18 February 2010; accepted 29 March 2010; published 5 June 2010.
[1] Accurate methods are needed to measure changing soil water content from meter
to kilometer scales. Laboratory results demonstrate the feasibility of the heat pulse
method implemented with fiber optic temperature sensing to obtain accurate distributed
measurements of soil water content. A fiber optic cable with an electrically conductive
armoring was buried in variably saturated sand and heated via electrical resistance to
create thermal pulses monitored by observing the distributed Raman backscatter. A new
and simple interpretation of heat data that takes advantage of the characteristics of fiber
optic temperature measurements is presented. The accuracy of the soil water content
measurements varied approximately linearly with water content. At volumetric moisture
content of 0.05 m
the standard deviation of the readings was 0.001 m
, and at
0.41 m
volumetric moisture content the standard deviation was 0.046 m
. This
uncertainty could be further reduced by averaging several heat pulse interrogations
and through use of a higherperformance fiber optic sensing system.
Citation: Sayde, C., C. Gregory, M. GilRodriguez, N. Tufillaro, S. Tyler, N. van de Giesen, M. English, R. Cuenca, and J. S. Selker
(2010), Feasibility of soil moisture monitoring with heated fiber optics, Water Resour. Res., 46, W06201, doi:10.1029/2009WR007846.
1. Introduction
[2] Soil water accumulation, storage, and depletion play a
central role in the hydrologic cycle and the global water
balance. Though many accurate methods are available for
point measurement of soil water content, there are currently
no precise in situ methods for measurement of soil water
content from meter to kilometer scales. The goal of this
article is to demonstrate the feasibility of the Active Heat
pulse method with Fiber Optic temperature sensing (AHFO)
to obtain precise, distributed measurements of soil water
content across these spatial scales and over a broad range of
soil water contents.
3] The ability of fiber optic Distributed Temperature
Sensing (DTS) systems to retrieve temperature readings
each meter along fiber optic cables in excess of 10,000 m in
length at high temporal frequency has afforded many
important opportunities in environmental monitoring [e.g.,
Selker et al., 2006a, 2006b; Tyler et al., 2008, 2009; Westhoff
et al., 2007; Freifeld et al., 2008]. Recently, SteeleDunne et
al. [2010] demonstrated the feasibility of using the thermal
response to the diurnal temperature cycle of buried fiber
optic cables for distributed measurements of soil thermal
properties and soil moisture content. Unlike the AHFO
method, the SteeleDunne et al. [2010] method does not
require an external source of energy. Nevertheless, its appli-
cation remains challenging under conditions where the
thermal response to the diurnal temperature cycle is not
large enough to allow accurate estimation of soil moisture
content (e.g., under dense vegetative canopy, at depths
beyond the top few centimeters of the soil column, cloudy
days, or other surface energy flux limited systems).
4] The principle of temperature measurement along a
fiber optic cable is based on the thermal sensitivity of the
relative intensities of backscattered Raman Stokes and anti
Stokes photons that arise from collisions with electrons in
the core of the glass fiber [see Tyler et al. , 2009]. A laser
pulse, generated by the DTS unit, traversing a fiber optic
cable will result in Raman backscatter at two frequencies,
referred to as Stokes and antiStokes. The DTS quantifies
the intensity of these backscattered photons and elapsed
time between the pulse and the observed returned light.
The intensity of the Stokes backscatter is largely independent
of temperature, while antiStokes backscatter is strongly
dependent on the temperature at the point where the scat-
tering process occurred. Temperature can be inferred from
the Stokes/antiStokes ratio. The computed temperature is
attributed to the position along the cable from which the
light was reflected, computed from the time of travel for the
light [Grattan and Sun, 2000].
5] Heat pulse methods are well established for the
determination of soil thermal properties, soil water content
and water movement. These methods usually apply a line
source of energy to the soil with the resulting temperature
fluctuation monitored by one or more parallel probes
[Bristow et al., 1994]. The rate of radial transmission of heat
depends on the soil bulk density, mineralogy, particle shape,
and, principally, soil water content [e.g., Shiozawa and
Campbell, 1990]. Geometries where the thermal observa-
tions are colocated with the heated probe are referred to as
Department of Biological and Ecological Engineering, Oregon State
University, Corvallis, Oregon, USA.
Departmen t of Rural Engineering, Technical University of Madrid,
Madrid, Spain.
Department of Geological Sciences and Engineering, University of
Nevada, Reno, Reno, Nevada, USA.
Water Management Civil Engineering and Geosciences, Delft
University of Technology, Delft, Netherlands.
Hydrologic Sciences, Divi sion of Earth Sciences, National Science
Foundation, Arlington, Virginia, USA.
Copyright 2010 by the American Geophysical Union.
WATER RESOURCES RESEARCH, VOL. 46, W06201, doi:10.1029/2009WR007846, 2010
W06201 1 of 8
single probe methods [de Vries and Peck, 1958; Shiozawa
and Campbell, 1990; Bristow et al., 1994]. Heat pulse
methods have also been widely implemented in multiprobe
geometries, with one or more sensing probes in proximity of
the heat source [e.g., Lubimova et al., 1961; Jaeger, 1965;
Larson, 1988; Campbell et al., 1991; Bristow et al., 1993,
1994; Heitman et al., 2003; Ren et al., 2003, 2005].
6] Many analytical and numerical methods have been
developed for the interpretation of heat pulse experiments in
soils. Typically, the solutions assume an infinitely small
radius and infinitely long line source geometry. The thermal
properties of soil are calculated from the heat pulse response
via the solution of the radial heat conduction equation
[Carslaw and Jaeger, 1959]. During heating, a pulse of
duration t
(s) is applied to an infinite line heat source in a
homogeneous isotropic medium which is taken to be at
uniform initial temperature. The solution for the resulting
temperature change following the commencement of heating
that is given by [de Vries and Peck,1958;Shiozawa and
Campbell, 1990; Bristow et al., 1994]
!Tr; tðÞ¼$
f or 0 < t % t
and during cooling
!Tr; tðÞ¼
4" t $ t
$ Ei
f or t > t
where q is the energy input per unit length per unit time
(J m
), r is the density of the medium (kg m
), c is the
specific heat of the medium (J kg
), r is the distance
from the line source (m), " = l/rc is the thermal diffusivity
), l is the thermal conductivity (W m
), and
Ei donates the exponential integral.
7] In this implementation of the line source transient
method, the radius of the heat source is assumed to be
infinitely small. A correction factor can be added to the long
time solution to account for the nonzero radius of the heat
source. The validity of such a correction decreases with an
increase of the probe radius [Blackwell, 1954]. To account
for the finite dimensions of the cable, the cylindrical tran-
sient method can be used as described by Jaeger [1965] for
a perfectly conducting cylinder with constant heat supply
per unit length per unit time (q)
!T ¼
1 $ exp $&u
! uðÞ
; ð3Þ
! uðÞ¼uJ
uðÞ$$ $ hu
þ uY
uðÞ$ $ $ hu
& ¼ "t=a
; ð5Þ
$ ¼ 2!a
#c=S; ð6Þ
h ¼ %=aH; ð7Þ
with a being the heat source radius (m), S the heat capacity
per unit length of the cylinder (J m
), 1/H the thermal
contact resistance per unit area between the perfect con-
ductor and the surrounding material (m
°C W
), and J
and Y
(u) the Bessel functions of u of order n of the first and
second kind (dimensionless).
8] Most of the existing heat pulse method literature
focuses first on calculating l and rc from the thermal
responses of the soil to a heat pulse. From these values, the
soil moisture content is then inferred, since both l and rc of
the soil monotonically increase with increasing water con-
tent. The wellknown advantage of using the dualprobe
method for soil water determination is that both thermal
conductivity and volumetric heat capacity can be accurately
obtained from a single measurement, while the single probe
method is primarily sensitive to the thermal conductivity
[e.g., de Vries, 1952, 1963; Campbell, 1985; Kluitenberg et
al., 1993; Bristow et al., 1994]. The main advantage of
obtaining the volumetric heat capacity of the soil is that it
allows estimation of the change in soil water content without
information on soilspecific thermal prope rties [Bristow et
al., 1993]. Some have tried to directly correlate soil mois-
ture content to the temperature rise during heating [e.g.,
Shaw and Baver, 1940; Youngs, 1956]. A disadvantage of
such methods is that a calibration curve that relates soil
moisture content to temperature change is needed for each
soil type, and for each probe design.
9] Systems using more than two probes provide addi-
tional information (e.g., direction of flux), and are an active
area of investigation [e.g., Bristow et al., 2001; Mori et al.,
2003, 2005; Hopmans et al., 2002; Ren et al., 2000; Green
et al., 2003; Kluitenberg et al., 2007]. Concerns regarding
the accuracy of the different heat pulse methods remain,
related to soil bulk density [Tarara and Ham, 1997], soil
mineralogy [Bristow, 1998], contact resistance between the
probe and the surrounding material [Blackwell, 1954], and
temperature sensitivity [Olmanson and Ochsner, 2006].
10] The use of actively heated fiber optic cable for obser-
vation of subsurface water movement has been demonstrated
[e.g., Perzlmaier et al., 2004, 2006; Aufleger et al., 2005],
though for determination of soil water content it was con-
cluded that (1) the method could only distinguish qualita-
tively between dry, wet and saturated soils [Perzlmaier et al.,
2004, 2006; Weiss, 2003] and (2) small changes in soil
water content could not be detected at levels above 6%
volumetric water content [Weiss, 2003]. Weiss [2003] con-
cluded that only with dramatic improvement of the signal
tonoise ratio of the DTS instrumentation could sufficiently
accurate thermal conductivity be obtained by a DTS heat
pulse method to quantify soil water content above this level.
11] Although we agree that better DTS performance
improves accuracy, here we argue that the DTS method can
quantify moisture content more precisely than suggested
previously by using a different approach to data interpreta-
tion. Both Weiss [2003] and Perzlmaier et al. [2004] used
the long time approximation of either the line source or the
cylindrical source transient methods to calculate the thermal
conductivity of the soil, deriving the thermal conductivity
from the slope and intercept of a line fit to the temperature
response following an extended heat pulse. They then
computed the moisture content using a calibration equation.
Unfortunately, this fitting routine made use of data which
varied little between moisture contents (particularly the fit-
2 of 8
ted slope). Our approach was, in part, motivated by their
data, where it was evident that though the slope of heating
was rather insensitive to water content, the overall magni-
tude of the temperature change was quite sensitive to
moisture content. This is partially due to the impact of the
early time data that is not fully incorporated into the late
time analysis. In addition, there is an intrinsic improvement
in sensitivity found in integral methods compared to deriv-
ative (slope) approaches.
12] Recent work has shown that more robust estimates of
soil thermal properties are obtained using analyses that fit
the entire data set of temperature change with time to a
model [Mortensen et al., 2006]. In this article we do not
attempt to optimize the data interpretation, but rather dem-
onstrate the power of a simple interpretation methodology
that appears to make better use of information contained in
the heat pulse data obtained with a DTS system. Opportu-
nities for optimization of this method are manifold, and will
be the topic of further research.
2. Materials and Methods
[13] We seek a response variable that monotonically
varies with soil water content and is suited to the char-
acteristics of the DTS measurement method. To this end, we
propose quantifying the thermal response of the soil to the
heat pulse in the form of cumulative temperature increase
over a certain period of time
!T dt; ð8Þ
where T
is the cumulative temperature increase (°C.s)
during the total time of integration t
(s), and DT is the DTS
reported temperature change from the prepulse temperature
(°C). T
is a function of the soil thermal properties. Higher
heat capacity and higher thermal conductivity, both of
which monotonically increase with soil water content ( '),
increase the rate at which heat is conducted away from the
probe and reduce the integral for sufficiently long heat
pulses. Thus, there exists a 1 to 1 function relating T
' (under conditions where flow can be taken to be negli-
gible) for a given soil, heating rate, integration time, and
fiber optic cable characteristics.
14] One may ask about the advantage of the integrated
parameter compared to the maximum temperature increase
approach described by Shaw and Baver [1940] and Youngs
[1956]. The variance of the computed parameter is mini-
mized by taking advantage of the fact that the DTS read-
ings are fundamentally based upon cumulative photon
counting. The standard deviation of DTS temperature mea-
surements reduces with the square root of reading time
[Selker et al., 2006a]. This method allows use of relatively
long reading times (photon integration) and low sampling
rates. In fact, the value of T
is largely unaffected by
sampling rate since the DTS will internally compute this
integral as it reports lower time resolution data requiring, for
example, a less expensive DTS recording instrument. It
will be shown later that T
allows for more accurate
estimation of soil water content than DT in our experi-
mental setup.
15] The highspeed DTS unit used in this experiment
(Sensortran DTS 5100 M4) allows high frequency data
collection for comparison of more traditional interpretations
of the integral method. This DTS unit recorded temperature
every 0.5 m along the fiber optic cable, with a spatial res-
olution of 1 m for each measurement. The average reading
frequency was 0.2 Hz.
16] A 0.61 m diameter sand column was supported by
a 1.46 m tall smooth interior, corrugated exterior HDPE
pipe (Figure 1). The bottom of the pipe was sealed with a
rubber membrane, and an outlet was installed 0.05 m above
the membrane seal. A 0.012 m diameter perforated hose
was fitted to the inside of the drainage port and wound in a
spiral laying flat on the bottom of the rubber seal to provide
an easily controlled lower boundary condition. The drain-
age was actively controlled using a peristaltic pump.
17] Within the column, 31.5 m of BruSteel (Brugg
Cable, Brugg, Switzerland) fiber optic cable was distributed
Figure 1. Images showing (a) the sand column and (b) the
fiber optic section (in helical coils) before inserting into the
sand column.
3 of 8
in a helicoidal geometry supported by five vertical 0.006 m
diameter fiberglass rods (Figure 1). The 3.8 × 10
m outer
diameter cable made twentyone 0.48 m diameter helical
coils, spaced 0.06 m vertically, starting 0.05 m from the
bottom and ending at the surface of the sand (1.30 m from
the bottom). The fiber optic cable employed was composed
of two optical fibers encased in a central stainless steel
capillary tube (OD 1.3 × 10
m / ID 1.07 × 10
m) sur-
rounded by stainless steel strands (12 4.2 × 10
m OD
stainless steel wires), all of which were enclosed in a 2 ×
m thick nylon jacket. The metal components were used
as an electrical resistance heater (0.365 W/m).
18] Airdried medium sand (d
= 0.297 mm) was added
in 0.30 m deep lifts with vibration of the entire column using
a rubber mallet to settle the sand between lifts. No further
settling was observed during the remainder of the experi-
ment. The total depth of sand in the column was 1.30 m with
0.12 m of the HDPE pipe extending beyond the top of the
19] Computation of T
requires a precise value of the
temperature before the start of the heat pulse. A 5 min DTS
reading preceding each heat pulse was used as the baseline
temperature. Thereafter, a 44.5 m section of the cable
(including the section in the sand column) was heated by
connecting the stainless steel windings at both ends of the
heated section to a variable voltage AC current source
(Staco® Variable Autotransformer Type 3PN1010). The
drop in voltage along the 12 AWG copper connecting wires
was 0.1% of the total, and thus was assumed to be negli-
gible. A digital timer with a precision of ±0.01%
(THOMAS® TRACEABLE® Countdown Controller
97373E70) controlled the duration of the heat pulse. A wide
range of combinations of power and time were tested,
though in this article we discuss only the results of 2 min
heat pulses at 20 W/m (120.2 VAC) which appeared to
provide an appropriate balance of temperature response and
duration relative to the DTS resolution. The measurements
were repeated three times. T
was calculated using the
data obtained over the entire heating period of 120 s. The
temperature increase observed at the end of the heating
period (DT
) will also be reported to compare its per-
formance in predicting soil water content with that of T
We chose to employ DT
because among all values of
DT for heating and cooling it had the highest signalto
noise ratio. A reference temperature reading was obtained
from a 33 m coil of fiber optic cable kept in an icefilled
water bath (0°C) (Figure 2).
20] DTS readings were taken in dry, saturated and
drained conditions. The drained condition was obtained one
month after establishing the water table at 0.4 m above the
bottom. Following the final DTS measurements in the
drained column, triplicate volumetric samples were obtained
from eight depths between the sand surface and the water
table (spanning 0.9 m) for calibration.
3. Results and Discussions
[21] Volumetric soil moisture content of samples taken
from the drained column varied from 4% to 41% (saturated),
with a sharp transition 0.3 m above the water table, typical
of sands (Figure 3). Repeatable, distinct values of T
were obtained up to saturation (Figure 3). The slope in the
' T
and ' DT
relationships decreased with water
content (Figures 4 and 5), suggesting lower sensitivity at
higher water contents, as found in previous studies [e.g.,
Weiss, 2003].
22] To estimate the error in soil water content (' )
obtained from T
, a function f (') was fitted to the T
versus ' data using least squares regression (Figure 4). For
each value of ', the estimated error ( s
) was calculated as
df 'ðÞ
Figure 2. Temperature readings along the fiber optic cable before (solid line) and at the end (dashed
line) of a 2 min , 20 W/m heat pulse for the drained soil column condition. The before temperature
was obtained by averaging all readings during the 5 min directly proceeding the heat pulse start.
4 of 8
where s
is the standard deviation of T
df ð'Þ
d '
is the local
slope of the T
response evaluated at '. In general, the
standard deviation of DTSmeasured temperature depends
on the distance from the DTS recording unit, increasing with
light loss as it potentially travels kilometers of distance from
the unit [e.g., Tyler et al., 2009]. However, over shorter
cable distances, such as the 50 m span employed here, this
effect is negligible. Therefore, the standard deviation of T
, should only depend on the performance of the DTS
system. In this experiment, s
was computed as the aver-
Figure 3. Measured soil water content (circles) and cumulative t emperatu re increase (triangles) as
function of depth for a 2 min, 20 W/m heat pulse.
Figure 4. Average cumulative temperature increase (T
water content (') for three 2 min, 20 W/m heat pulses and fi tted function. For each soil water content
value, the error bars are obtained from the standard deviation of three repetitions. The R
of the fitted
function is 0.994.
5 of 8
age of all standard deviations of T
observed along the 30
m cable section in the sand column. The same method was
employed to estimate the error in soil water content ob-
tained from DT
. The error analysis shows that s
tained from either T
or DT
increased approximately
linearly with soil water content (Figure 6). As expected, the
error in soil water content obtained from T
was much
smaller than that obtained from DT
(Figure 6). This
error could be further reduced by increasing the signalto
noise ratio, which could be accomplished by averaging
several heat pulse results, using a more precise DTS unit,
increasing the heating intensity, or increasing the duration
of heating.
23] A large heat pulse could cause water to evaporate
and/or diffuse away from the cable [Farouki, 1986]. To
avoid this, and to minimize the energy required to complete
Figure 5. Average temperature increase at 120 s (DT
) as function of soil water content (') for three
obtained from the standard deviation of three repetitions. The R
of the fitted function is 0.987.
Figure 6. Calculated err or (s
) in soil water content derived from T
(solid line) and from DT
(dashed line) as function of soil water content (').
6 of 8
a measurement, it is desirable to reduce both the magnitude
and duration of temperature increase. An important advan-
tage of the integral method is that a relatively good estimate
of soil water content can be obtained with a brief heat pulse.
In this experiment, the maximum cable temperature never
exceeded 17°C over the ambient soil temperature (Figure 5).
The injected energy was less than 2.4 kJ/m, compared to the
11.7 kJ/m for Weiss [2003] and greater than the 72 kJ/m
employed by Perzlmaier et al. [2004]. The much shorter
heating interval employed here (120 s), compared to 626 s
used by Weiss [2003] and 7200 s by Per zlmaie r et al.
[2004], greatly reduces the potential for such disturbance.
That said, Weiss [2003] indicated that his approach did not
give rise to water displacement, and our experiment
showed no change in T
with replication, suggesting
there were no significant distortions due to the heat pulse
measurements. Sequential measurements did not show
persistent cumulative heating in our experiments, but this
would ultimately provide a practical limit to the feasible
sampling frequency using this method. Fortunately, this
cumulative heating can easily be measured with DTS.
24] Currently marketed DTS systems have both a tenfold
higher speed of reading performance and four times better
spatial resolution than that employed here. The magnitude
of the heat pulse required to obtain a particular level of
precision is scaled linearly with reading speed, thus we have
by no means explored the instrumentation limitations on
accuracy or energetic requirements of the DTS approach.
25] While the laboratory results are encouraging, field
measurements of soil water content using the DTSbased
heat pulse method are expected to bring additional sources
of uncertainty. Expected primary sources of error include
poor contact between the probe and soil, and the spatial
variability of soil thermal properties.
26] Finally, in addition to varying with moisture content,
is expected to be a function of the convective flow of
water around the heated cable. An increase in convective
flow will further increase the rate at which heat is dissipated
away from the probe and thereby reduce T
. Thus, this
method has the potential to not only detect soil water content
but also to monitor water fluxes in saturated soils, as dem-
onstrated by Perzlmaier et al. [2004], with long heated
durations. The ability to use shorter pulses based on the
method proposed here allows greater separation between
measurements of moisture content and flux.
4. Conclusion
[27] We have shown that the heat pulse method using
coaxial heating and a DTS system is feasible for determi-
nation of soil water content across a much broader range
of values than previously reported. This result was found
by using a response metric that has not been previously
employed: the time integral of temperature deviation. This
strategy is especially appropriate to the DTS method wherein
precision of temperature reporting is a direct function of the
interval of photon integration. Though we have used high
temporal resolution in the DTS measurements, this method
can provide the same level of precision with less expensive,
slower DTS instruments since the data can be integrated in
time for analysis. Further, using more sensitive DTS sys-
tems, the technique could be more accurate and use shorter,
lower energy heat pulses which may be of importance in
remote application of the method.
28] While this study demonstrates feasibility, additional
work is required to develop optimal heating and interpre-
tation strategies for DTSbased heat pulse methods, building
upon the rich literature related to needle heat pulse systems.
The key finding of this work is to confirm the potential to
employ DTS systems to monitor soil water content at tem-
poral resolutions well under one hour and at high spatial
resolution (1 m). In principle, this DTS method could
monitor soil moisture along cables exceeding 10,000 m in
extent. This would allow for concurrent observation of
thousands of adjacent locations, which will likely provide
new insights into the spatial structure of infiltration and
evaporation. Such measurements could be transformative in
our understanding of soil hydrology in natural and managed
systems at field and watershed scales. Many challenges
remain (e.g., installation in the presence of stones and
roots), calling for significant further effort in developing
this methodology. For example, we presented only results
from a singleprobe DTS approach, though multiple probe
approaches using DTS are expected to be of utility just as
they have been in other soil heat pulse applications.
29] Acknowledgments. The authors gratefully thank Atuc Tuli, Jan
Hopmans, Jim Wagner, Mark Hausner, and Christine Hatch for their very
helpful conversations about this method. We also wish to express our
appreciation to Water Resources Research reviewers, Editor, and Associate
Editor for their valuable suggestions and comments concerning this manu-
script. We acknowledge the NationalScienceFoundation(grantsNSF
EAR0930061, NSFEAR07115494) and the Oregon Experiment Station
for their critical financial support.
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... The FO technology has led to the observation of previously unobserved low frequency signals [7,8] which can provide valuable information related to complex and highly coupled hydrogeological and geochemical processes. Some FO sensors including Fiber Bragg Sensors (FBG) and Distributed Temperature Sensors (DTS) are currently used for the monitoring of fluids flow [9,10], hydrogeological processes [11,12], water content, and energy exchanges along the VZ [13][14][15]. Infrared optical fibers based on chalcogenide glass are used as sensors for the determination of volatile organic pollutants in groundwater. The system works following the fiber evanescent wave spectroscopy (FEWS) principle. ...
... The Raman DTS is based on the measurement of the power amplitude and can be disturbed in certain places of the fiber (curvature, connectors, splices). DTS can also be used to monitor the thermal response of the material to the active heating of the sensing fiber [Active Heated Fiber Optics method (A-DTS)] [14,15]. This method represents an interesting alternative to traditional sensors to monitor the variations of the water content within the VZ. ...
... These studies contributed to the validation of this technology and to the take-off of its use in environmental applications. As discussed above, many researchers have investigated the potential of DTS to measure water content in the soil using A-DTS [14,75]. The objective was to quantify the thermal response of the soil to a heat pulse over time in the form of a cumulative increase in temperature [14]: ...
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The structure and dynamics of the Vadose Zone (VZ) play a major role in the groundwater recharge process and in the transport of contaminants. By monitoring the mass and heat transfer processes within the VZ, it will be possible to predict the contaminants travel time and implement suitable solutions to preserve the groundwater resources. Several environmental monitoring solutions have been developed in recent years to better understand the complex hydrogeological processes that occur along the VZ. The use of Fiber Optic (FO) sensors is a promising technology for environmental monitoring. Compared to conventional sensors, the FO sensors allow measuring and monitoring different parameters, while offering interesting specificities. To improve our knowledge on the reactive processes occurring during mass and heat transfers within the VZ of the Beauce aquifer, the Observatory of transfers in the VZ is being developed near Orléans (France). Three types of distributed FO sensors (DTS, DSS and DAS) have been installed at the O-ZNS experimental site in July 2020. This chapter presents the state of the art on the use of FO sensors for environmental monitoring. The installation of these sensors at the O-ZNS site is then discussed along with the future developments and targeted results.
... In addition, both passive-and active-DTS experiments can be used to estimate soil thermal properties. As soil thermal properties depend on moisture content, monitoring temperature changes through fiber optic DTS not only provide estimates of thermal conductivity (Steele-Dunne et al., 2010;Abesser et al., 2020), and thermal diffusivity but also of soil moisture content (Sayde et al., 2010(Sayde et al., , 2014Ciocca et al., 2012). ...
... This issue can be addressed by applying longer heating and cooling periods. Unfortunately, prolonged-time heat injection can significantly disturb the soil water content field and then reduce the accuracy of the soil water content estimate (Sayde et al., 2010;Wang et al., 2020). ...
Traditional methods to assess soil properties or monitor soil processes typically rely on point data. They often fail to represent the heterogeneity of soil. Airborne remote sensing techniques allowed us to measure proxies for soil properties or variables at regional scale, but often covering only the first few centimeters of soil. Ground-based geophysical methods can fill in the gap between these two scales. They primarily observe variations in the thermal, electrical, magnetic, electromagnetic or seismic properties of the subsurface to map its characteristics or to monitor dynamic changes and can yield 2- or 3-D quantitative information on soil properties or processes. Geophysical methods have been applied in various soil-related research fields: from soil mapping and precision agriculture, to geotechnical engineering and soil remediation and to archeology and forensic investigation. This chapter briefly introduces the ground-based geophysical techniques appropriate for soil applications.
... 水分和渗流都是影响岩土体稳定性的关键因素. 水分主要针对非饱和土而言, 渗流是指流体在岩土 孔隙介质中的流动, 它常常是岩土体失稳致灾的关 键因子. 2010 年美国 Selker et al. [19] 提出了土中水分主 动加热 DTS 光纤法, 但只适用于岩土体温度在 0℃以 上低精度监测; 2018 年施斌等根据不同水分和渗流 的岩土体导热性, 将传感光纤研制成主动线热源, 通 过建立光纤所测温度与渗流导热性之间的关系, 发 明了水分与渗流热脉冲光纤法, 并提出了冻结与非 冻结统一的水分场光纤计算方法, 实现了岩土体水 分场的全天候感知. ...
... The method used this fact to calculate the water content from the temperature rise due to heated optical fiber. More details on the method can be found in [111], [115]. ...
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Seepage is the most important indicator in the safety assessment of embankments which requires continuous monitoring to maintain structural integrity. Punctual monitoring techniques for seepage in embankments can only provide limited information on the process. As the thermal behavior of embankments is affected by seepage, a continuous temperature measurement system can be established in embankments to identify the possible seepage flow. With the advancement of fiber optic measurement technology, measurement equipment is available that allows distributed temperature sensing. Optical fiber Distributed Temperature Sensor (DTS) was developed to measure the ambient temperature along the entire length of the fiber optic cable at an arbitrary time interval. There are two techniques to perform this measurement: passive measurement technique and active measurement technique, which, unlike passive measurement, require an additional power supply. The passive measurement technique measures the natural temperature of the soil and is therefore suitable for long-term temperature monitoring in embankments. In this thesis two objectives were set: first, that it is possible to detect the onset of seepage in embankments with a DTS fiber optic cable measurement system based on the passive measurement technique, and second, that the correct distribution of the DTS fiber optic cable system increases the ability to detect the onset of seepage in embankments at an early stage. The experimental part of the task was carried out in two phases. In the first part, a small-scale experimental model was developed to simulate the seepage phenomenon in embankments. The soil material was replaced in the model by quartz sand with three different granulometric compositions. The hydraulic and thermal properties of the materials were obtained from laboratory tests or the relevant literature. In the second part, a larger-scale experiment was carried out under laboratory conditions, using only one type of silica sand. By simulating the seepage process under laboratory conditions, it has been shown that the DTS optical cable measurement system can detect any small temperature changes in the sand layer as a result of the seepage phenomenon. The system can detect the start of the seepage process in the initial stages, as well as monitor the progress of the seepage process, even in cases where there are small temperature differences between the soil and the seepage water. The numerical analysis was conducted for both the seepage process and the heat transfer progression in soil. A clear relation between the variation in the degree of saturation and temperature measurements was observed visually in experiments and in the numerical simulation. The application of passive DTS for seepage detection in embankments was validated. The efficiency of DTS application in seepage monitoring strongly depends on the optical fiber installation approach, calibration technique, temperature data interpretation, and post-processing. To achieve the accuracy of the measurement capture, a special method of laying the fiber-optic cable with loops was used, based on the configuration of the sensory ring, which allows precise point-to-point measurement capture. Some guidelines for optimization of optical fiber DTS application were provided by a comprehensive review of different techniques to interpret temperature data in terms of seepage information. Additionally, different methods for the calibration of DTS data were reviewed. Various techniques both in calibration and interpretation were classified and their comparison was provided that can be used for a more efficient DTS employment in seepage detection. A method was also suggested for the interpretation of DTS data based on the comparison of numerical results with the obtained temperature measurements in embankments.
Actively heated fiber Bragg grating (AH-FBG) can perform quasi-distributed monitoring of soil water content. However, the analysis method needs to be improved to minimize measuring errors. In this study, the artificial intelligence method is proposed and a model test was used to verify its feasibility and to explore the influence of cover conditions. Three cover layers were considered, including bare soil (S), grass (G), and biochar mixed soil (B). The water content measurements based on maximum temperature increase have higher accuracy for G, followed by S, and the worst is B. Fluctuation in the heat power and the longitudinal heat transfer are the main sources of errors. Artificial neural network (ANN) models can effectively improve monitoring accuracy. Cover conditions have a significant influence on the measurements by affecting initial ground temperatures and water content gradients. For field monitoring, the cover layer should be considered when analyzing AH-FBG measurements.
Instrumentation and measurement technologies are currently playing a key role in the monitoring, assessment, and protection of water resources. The whole water sector involves multiple technological contexts for the monitoring of the resource, given the broad multidisciplinary context, which covers water from its natural domains up to the various man-made infrastructures. Water cycle management refers to a very complex framework, which requires reliable technological responses to the questions raised in meteorology, hydrology, water resources management, hydraulic engineering, and, more in general, environmental management, with the related societal implications. Measurement techniques and sensing methods for observing water systems are rapidly evolving, requiring a continuous update in measurement technologies and methods. It is clear that effective and sustainable planning of the water cycle management requires the design and implementation of a systematic monitoring approach. In particular, instrumentation and measurement technologies have a pervasive presence in all the necessary aspects of the assessment, monitoring, and control of water systems. Thus, the assessment of the water resource and its relationship with the various environmental stressors, including the anthropic pressures on it, requires adequate knowledge, technologies, and infrastructures to deal with the challenges of today. It is also important to underline that this aspect applies to both quantitative and qualitative monitoring activities, being the threats to the quality of the resource also indirectly affecting its availability and quantity. This book provides an updated framework of observational techniques, sensing technologies, and water management and protection data processing. In data analytics, attention is given to the synergy between different sensing systems and between measurements and modeling approaches. The coexistence in this book of measurement techniques, sensing methods, and data science implications for the observation of water systems, emphasize the strong link between measurement aspects and computational and modeling The present volume provides a portrait of current measurement technologies and data analysis approaches for water systems monitoring and management, also offering insights into the enabling technologies that are today fostering the concept of smart water systems. The 23 chapters of this book are organized to give a survey of current technologies and available methods for the assessment and monitoring of water resources in multiple domains. In particular, the selected contributions are intended to cover the following thematic areas: (i) remote sensing methods; (ii) instrumentation for direct water sensing; (iii) water sensor networks and ICT infrastructures; (iv) geophysical techniques; (v) synergy between measurements and modeling. For more details, please visit the Springer website:
Over the last 37 years, particular interest has been directed towards exploring the characteristics of the optical environment for sensing, giving rise to what would now be one of the largest applications of well-known optical fibers, typically employed to transmit data at high rates. Sensing temperature, pressure, liquid level, deformation, and other physical parameters utilizing optical fibers has become a growing branch of research and a business competing with well-established electrical sensors in the industry. Optical fiber sensors have all the inherent characteristics of a fiber optic cable, such as electromagnetic immunity, small size and weight, multiplexing, and so on. These exclusive features have made fiber sensors so versatile as to become a transformative technology by enabling several industrial processes to be carried out with higher reliability. Nowadays, there are several optical sensors, including fiber Bragg grating, interferometric, polarimetric, polymer fiber, distributed, and several others. Specifically, Raman-based distributed temperature sensor (RDTS) is a class of fiber optic sensors broadly employed in temperature measurement of large structures such as oil and gas wells, tunnels, and pipelines. Since 1985, many techniques have been proposed to break through the barriers of exploring Raman scattering as a distributed temperature measurement method. Range, spatial and temperature resolutions have been the most investigated parameters. In this perspective, this paper presents a comprehensive review focused on the progress of the RDTS technology over the past 37 years (1985–2022), covering an analysis of over 500 journal papers. First, a brief introduction to fiber optic sensor technology is presented as a theoretical basis, discussing the emergence of distributed sensors. Subsequently, Raman scattering in optical fibers is introduced, as well as how this nonlinear effect can be used to build temperature sensors. Next, RDTS technology is detailed, followed by a discussion of its applications and evolution over nearly four decades of development. Lastly, future perspectives are addressed in this review for the advancements in distributed temperature sensor technologies.
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A great number of temperature measurements in deep bore-holes are made on the territory of the Soviet Union. The results of these measurements are partly systematized in the work " Problems of Geothermie " t1). However they can be hardly used for the calculations of heat flow because of the absence of the corresponding determinations of the thermal conductivity of rock cores taken directly from these boreholes. In the present paper the temperature gradient and the thermal conductivity of samples of rocks for the same place are determined and by means of these data the value of the thermal flow is estimated. For temperature measurements the electric resistance thermometer was used that is designed by Dergunov I. D. and improved for the conditions of work in deep bore-holes (up to 4-5 km) at rather high pressures and temperatures. The electrical resistance was measured by means of the compensation methods by a special potentiometer and a high sensitive mirror galvanometer. Four bringing wires are used for doing away with the erratic currents and the influence of the wires. As a result of it the accuracy of the temperature measurements is about 0.01°C. Fig. 1 represents the exterior of the apparatus and the thermometer. The thermometer is a hollow cylinder on the surface of which a copper wire is winded bifllarily, whose resistance is measured. When the cylinder is buried into a bore-hole it is washed by a solution from outside and inside. Together with the application of ftorplast isolation this decreases the thermal inertia of the thermometer down to 1.5 sec. For the sake of strength annular justs are made on the cylinder. The thermometer stands the pressure up to 600 atm and the temperature up to 200°C.
One way to detect leakage in a dam is to monitor temperatures in the structure and its foundation. That’s because seepage flows affect the temperature field within the structure. A method for measuring temperature in dams - distributed fiber-optic temperature (DFOT) measurement - has been used for almost a decade.Yet, researchers continue to improve the tool's effectiveness and develop new applications. Temperature measurement is a vital tool for detecting leakage in dams. In particular, the distributed fiber-optic temperature (DFOT) measurement method is a well-established tool, having been in use for nearly ten years. Researchers at several universities continue to adapt this tool for new applications at dams. For example, researchers are studying new applications for the gradient and heat-up methods of temperature monitoring, as well as measuring temperatures in roller-compacted-concrete (RCC) dams. New applications for the method continue to make it a key technology for dam monitoring.
A simple heat-pulse device is described for determining the volumetric heat capacity and hence water content of porous media. The device consists of three needle probes mounted in parallel. The heater probe contains a heating element while the sensor and reference probe contain thermocouples. Measurements made at three depths during drying of a laboratory soil column yielded accurate changes in soil water content provided actual heater-to-sensor probe spacings were used in the calculations. A simple procedure is also described for checking on the probe spacings. -from Authors
Time domain reflectometry (TDR) and the heat pulse method are both used to measure soil water content. Changes in ambient temperature have been shown to affect TDR measurements, but less is known about the behavior of heat pulse sensors in response to changes in temperature. This study directly measured and compared the temperature sensitivity of the TDR and heat pulse methods. Both methods were used to estimate water content in silt loam and sand at two fixed water contents across a wide temperature range. An increase in temperature led to an increase in measured water content in most cases. Across the 40°C temperature range, changes in measured water content were generally 0.04 m 3 m-3 or less for both methods. Weighted linear regression showed that in these soils the heat pulse method exhibited greater temperature sensitivity than the TDR method, although the differences were not statistically significant. A previously proposed correction for the temperature sensitivity of the TDR method produced mixed results. The temperature sensitivity of the heat pulse method was attributed to the changes in the density and specific heat of water and specific heat of soil with respect to temperature. When the changes in these parameters were accounted for, the temperature sensitivity was eliminated in three out of four cases.
This paper presents the results of an experimental study of the temperature dependencies of the heat capacity and the lattice parameters of terbium tetraboride (TbB4) measured in a temperature range of 2–300 K. This study reveals the anomalies of the above characteristics of TbB4 at the temperatures of the corresponding magnetic phase transitions (Neel points) T N1 = 44 K and T N2 = 24 K. The transitions of TbB4 from the tetragonal crystalline structure into the orthorhombic one at temperatures below T N1 were identified. Within our experimental accuracy, no structural transition into the orthorhombic symmetry at temperatures above 80 K was observed. The individual temperature variations of lattice-related and magnetically induced components of the measured properties of TbB4 are extracted using the data for the diamagnetic LuB4. As a result, separate contributions of lattice and magnetic subsystems of TbB4 are determined.
Water, solute, and heat transport processes in soils are mutually interdependent as each includes convective water flow and each transport mechanism is partly controlled by fluid saturation, pore geometry, temperature, and other soil environmental conditions. Therefore, their measurement in approximately identical measurement locations and volume is essential for understanding transport phenomena in soils. We introduce a 2.7-cm-diameter multi-functional heat pulse probe (MFHPP), which consists of a single central heater, four thermistors, and four electrodes (Wenner array) that together are incorporated in six 1.27-mm-o.d. stainless-steel tubes. The bulk soil thermal properties and volumetric water content of Tottori Dune sand were determined from the measurement of the temperature response of all four thermistor sensors after application of an 8-s heat pulse by the heater sensor. Simultaneously with the temperature measurements, the bulk soil electrical conductivity (ECb) was measured using the Wenner array, from which soil solution concentration (ECw) can be obtained after calibration. All measurements were taken during multistep outflow experiments, which also allowed estimation of the soil's hydraulic properties. We demonstrated that the MFHPP can effectively measure volumetric water content, thermal properties, and ECb, and can be used to indirectly estimate soil water fluxes at rates larger than 0.7 m d-1 in the sand.
Interpretation of shallow temperature surveys conducted during exploration for shallow geothermal aquifers can be improved by incorporating thermal diffusivity measurements in the survey. A multineedle thermal probe can be used to obtain these measurements directly, using simple curve-matching analytical procedures. Errors caused by thermal gradients can be minimized by using a pulsed heat source, rather than a continuous heat source. The method has been tested successfully in the laboratory and is easily adaptable to field studies.