Accepted Manuscript An improved distributed sensing method for monitoring soil moisture profile using heated carbon fibers Ding-Feng Cao, Bin Shi, Guang-Qing Wei, Shen-En Chen, Hong-Hu Zhu PII: DOI: Reference:
S0263-2241(18)30237-9 https://doi.org/10.1016/j.measurement.2018.03.052 MEASUR 5371
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
8 February 2017 15 January 2018 20 March 2018
Please cite this article as: D-F. Cao, B. Shi, G-Q. Wei, S-E. Chen, H-H. Zhu, An improved distributed sensing method for monitoring soil moisture profile using heated carbon fibers, Measurement (2018), doi: https://doi.org/ 10.1016/j.measurement.2018.03.052
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An improved distributed sensing method for monitoring soil moisture profile using heated carbon fibers Ding-Feng Cao1,2, Bin Shi1*, Guang-Qing Wei3, Shen-En Chen4, Hong-Hu Zhu1* 1 School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China 2 Department of Civil and Environmental Engineering Department, Engineering College, University of Wisconsin-Madison, Madison, WI,53706, USA 3 Suzhou NanZee Sensing Technology Co. Ltd, Suzhou 215123, China 4 Department of Civil and Environmental Engineering, University of North Carolina at Charlotte, NC 28223, USA
Corresponding author 1: Bin Shi, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China Tel :86-025-89680317, Fax: 86-025-83686016, Email:
[email protected]
Corresponding author 2: Hong-Hu Zhu, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China Tel :86-025-89681137, Email:
[email protected]
1
Abstract Soil moisture variation with respect to depth directly affects the engineering properties of soil and the health state of plants. At the present, there are few techniques that can satisfactorily quantify the vertical moisture profile within the soil medium. In this paper, a fiber optic sensor-based distributed temperature sensing (DTS) technique is introduced for soil moisture profile mapping. In this technique, a carbon fiber heated sensing-tube (CFHST) is integrated into conventional fiber optic sensing cable to improve the sensitivity, accuracy and spatial resolution of the measurement of soil moisture profile. The CFHST consists of three parts: the inner tubing, the carbon fiber heated cable (CFHC) tightly wrapped on inner tubing, and the interface screws installed on both ends of the inner tubing. The length of a unit CFHST is adjustable according to the actual demand of a specified application. The power supply model and installation method in field are introduced. Laboratory tests were conducted to establish the relationship between soil moisture and thermal response of CFHST. A foundation pit dewatering test was also carried out to validate the field performance of this monitoring technique. The test results show that the borehole-embedded sensor monitored and recorded the continuous change of soil moisture profile accurately (RMSE = 0.046 m3/m3). This technique can effectively capture the distribution profile of soil moisture along the depth direction, which provides a new approach to investigate the physical and hydrological properties of soils. Keywords: soil moisture profile, distributed temperature sensing (DTS), carbon fiber, fiber optic sensing
2
1 Introduction The variability of soil moisture with depth significantly affects the engineering properties of soil and the health condition of plants. Hence, it is necessary to capture accurate soil moisture distribution in the vertical direction [1-4]. There are several in-situ soil moisture measuring techniques, including time domain refrectometry (TDR), frequency domain reflectometry (FDR), capacitance and microstrip transmission, etc. [1, 5-8]. Most of these approaches are of point measurements, which cannot meet the need of mapping out the soil moisture profile through depth [5]. Some remote techniques such as infrared thermography, ultrasound, electrical resistivity tomography and ground penetrating radar, can obtain indirect soil moisture distributions on the ground surface and over an area. But they are costly, and the measurements are easily disturbed by interfering factors and with low temporal-spatial resolution [9]. Furthermore, these methods can neither obtain moisture distribution in deep soils (depth>1 m) nor can they achieve the goal of long-term monitoring [10-13]. The active heated fiber optic (AHFO) measurement method is based on the distributed temperature sensing (DTS) technology. The AHFO method is well established for estimating the distribution of soil moisture by correlating to the soil thermal properties [14-25]. Although extensive laboratory and field experiments have been conducted to prove its feasibility to obtain distributed soil moisture data, this technique has not been applied to measure soil moisture profile in a borehole. Furthermore, the low spatial resolution of the conventional DTS technology makes it difficult to locate the soil moisture distribution of slip surfaces, weak intercalated layers and flow layers in the soil medium. These weaknesses significantly limit the wide application of AHFO technique in soil moisture monitoring. 3
In recent years, some pioneers have tried to solve the above-mentioned spatial resolution problem by helically wrapping fiber optic cables around a PVC tube, and this method has been used to sense glacier interface temperature, ground water temperature, shallow thermohaline environment and aquifer seepage rates [26-29]. The measuring principles of soil moisture profiles are similar to those of seepage rates, but there are still several major obstacles, which can be divided into two groups. The first group is how to reduce the measurement error, such as selecting a suitable borehole diameter, and optimizing the grouting and calibration techniques. When quantifying seepage velocity by using the AHFO technique in a sealed borehole, water flow will pass through the grouting materials and take away heat from the sensing cables. However, when inferring the surrounding soil moisture, the grout has a large influence on the measurement results, so it is necessary to find a solution to reduce the disturbance [30]. The second group is how to install the sensors inside a borehole, i.e., choice of power supply model, installation procedure and connection means between two adjacent sensors. To fill the gap, in this paper, the AHFO technique is modified by using a carbon fiber heated sensing-tube (CFHST), with a goal to perform continuous and reliable measurement of the distribution profile of soil moisture. The designing of CFHST for in-situ soil moisture monitoring is introduced in details, together with the operation procedures. Both laboratory and field tests have been performed and shown that the modified AHFO technique can accurately monitor the time-varying soil moisture distribution through depth.
2 Principle of CFHST 2.1 The basic principle The principle of DTS soil moisture profile measurement technique is based on assumption of 4
possible correlation between the thermal responses of the temperature characteristic value (Tt) and to the soil moisture [21]. The core element of the CFHST is a sensing tube with a coupled fiber optic sensor and a heating cable (copper wire). CFHST is designed to be installed into soil via a borehole. When a measurement starts, the CFHST is heated under a constant current, and its temperature is recorded by a DTS interrogator during heating. By using the recorded temperature along the length of the CFHST, Tt is calculated and then the soil moisture profile (θ) is determined from Tt. The general schematic of the CFHST monitoring system is shown in Fig. 1. The CFHST technology quantifies temperature through measuring the thermal sensitivity of the relative intensity of the backscattered Raman Stokes and anti-Stokes lights in the optical fiber [31, 32]. The properties of the DTS demodulator (NZS-DTS-M06, Suzhou NanZee 2015) used in our tests are shown in Table 1. 2.2 Soil moisture calculation The most widely used model in soil moisture monitoring by AHFO method is the line heat source. Its effectiveness and reliability are based the following assumptions [33]: (1) Temperature dependence of thermal physical property of heat source can be neglected. (2) Heat capacity of the heat source can be neglected. (3) Length of the line heat source is assumed to be infinite and the soil medium is assumed to extend to infinity in the radical direction. (4) Heat capacity of the line source can be neglected. For the CFHST used in the lab and field tests, the fiber length is much longer than the radius and can be regarded as infinite. The thermal conductivity of the inner air tube is far less than that of the surrounding soil, thus, the heat conduction in the longitudinal direction through CFHST can 5
be neglect. The heating power of every unit of CFHST along the length direction is identical, so the CFHST can be treated as a line heat source. Assuming the soil is homogeneous and isotropic, for a line heat source such as CFHST buried in a boundless soil medium, the temperature ( T ) measured by the DTS technology will satisfy the following relationship [16]: (1) where m-3)，
is the heat power per unit time per unit length (J m-1s-1), is the distance from the line source (m),
is the density of soil (kg
is the heating time (s), and
is the thermal
diffusivity (m2 s-1) which can be calculated as: (2) and
can be approximated as [34]: (3)
where
is the thermal conductivity (W m-1 ºC-1) and
is Euler’s constant (=0.5772). Equation
(2) can be combined into equation (3) and results in: (4) The relationship between soil thermal conductivity ( ) and moisture (θ) has been studied and several empirical or semi-empirical mathematical models have been proposed including an exponential function model [35]; a power function model [36]; a logarithmic function model [37] and, finally, a linear function model [21, 23]. Although the exponential, power and logarithmic functional models have the advantage of having high precision in curve fitting, they also share several disadvantages including the difficulty in obtaining accurate parameters applicable to these models, and the accumulating errors in determining soil moistures. The linear model is albeit simple, is more practical. But it will lose its efficacy when the water state changes. 6
To improve the modeling of the relationship between the soil moisture and the temperature of CFHST, a piecewise-function model is used: The soil moisture-temperature relation is described either as a linear function or a quadratic function. When the soil moisture is lower than a threshold level, a quadratic function is used, and when it is higher than the threshold, a linear function is employed: z 2 z2 z3 0 0 T 1 0 sat k b0
where
(5)
is the threshold moisture which is a function of the soil type and can be determined
experimentally.
represents the transformation of the current water state from absorbed water
to gravity water. sat is the saturated soil water content which should be measured in the laboratory. Once sat is established, it is suggested that the minimum curvature point in the ∆T ~ θ curve is defined as the threshold moisture ( ). In equation (5), z1 , z2 , z3 , k and b0 are contents that are dependent on the soil structure and the soil types. If one correlates soil moisture to the CFHST temperature for only a single temporal data point, the resulted correlation would have high randomness and error. A better approach is to infer the soil moisture from the temperature characteristic value ( Tt ) (or using alternative Tcum [16]) determined from a defined characteristic time interval of [ ta , tb ]. Tt is defined as the arithmetic mean variable value of the CFHST temperature rise for the temperature range [ ta , tb ][21]. The measured soil thermal conductivity is influenced by the heating duration. The heat transfer during the initial few heating minutes is dominated by transient effects which are mainly caused by the heat source thermal properties [35]. Therefore, it is recommended to disregard the reading during the initial few minutes. The number of minutes disregarded is dependent on when the temperature reaches a steady state [36]. For the soils and heating power introduced in this paper, 7
the steady state is reached at the 15th minute after heating, so the interval [15min, 20min] is selected to calculate Tt .
3 CFHST design for soil moisture profile monitoring Conventional DTS soil moisture measurements using optical fiber sensors rely on stainless steel heated cables, which are relatively stiff and are susceptible to corrosion; thus are not suitable for long-term monitoring in some corrosive grounds. The stainless steel heated cable only has a resistance of 0.011 ohms/m, so a low and safe voltage can generate sufficient heat. This makes it very suitable for long distance monitoring. However, for some sites, low resistance and voltage are a weakness because a large electric current is needed to supply heating power enough when resistance is low. In order to provide a sufficiently large electric current, a cumbersome and expensive transformer or voltage regulator has to be used, which increases economic and labor costs. Furthermore, for some places, the supply voltage is not always constant but can fluctuate. If a circuit has a low resistance and large current, a slight voltage change can cause a large change in total heating power. Therefore, using cables with a large resistance is necessary in some circumstances. In this paper, the carbon fiber heated cable (CFHC) is introduced to offset the weakness of traditional metal heated cable in some grounds. As seen in Fig. 2, the CFHC is composed of a multimode optical fiber, a carbon-fiber and a coat. The optical fiber has its own core, cladding, coating and fiber-optic jacket. The optical fiber is then encased by 24,000 carbon-fibers. To accommodate CFHC, a carbon fiber heated sensing-tube (CFHST) is designed to house both optical fiber and the carbon fiber. (Fig. 3) CFHST consists of three parts: the inner tubing, the CFHC tightly wrapped on inner tubing 8
and the interface screws installed on the ends of the inner tubing. A Polyvinyl Chlorid (PVC) tube with a diameter of 5 cm is used as the inner tubing. The length of a unit CFHST is dependent on the actual demands in the specific application and can be designed. In general, it is recommended to choose a 4 m length for ease of processing and transportation. Every two adjacent CFHST are connected via a screw and nut assembly as shown in Fig. 3(c). The screws are fixed on the PVC inner tubing using epoxy glue. The CFHC is wrapped onto the inner PVC tube tightly and forms the CFHST system. The resulting temperature effect during heating has an effective radius that is much larger than that of a straight CFHC element buried in the soil, thus, the CFHST improves the measurement accuracy drastically. Furthermore, this spirally crafted system increases the spatial resolution of the DTS. The enhancement can be expressed as:
S D M
(7)
where is the enhancement ratio of CFHST over the DTS system with straight element, M is the spatial resolution of CFHST, is the diameter of CFHC (4 mm in this study), D is the diameter of CFHST (5 cm in this study), and S is the spatial resolution of DTS (1 m for the interrogator used). According to the results calculated by equation (7), when M reaches into 2.5 cm will be close to a value of 40. Because the electrical resistance of the CFHST is very large, it cannot satisfy the heating requirement through a single power supply. Therefore, a new power supply method is required, which is shown in Fig. 4. In using the segmented power supply method, the CFHC coiled on the CFHST is divided into several sections with a consistent spacing of L. Each section is connected with a copper wire 9
through an annular binding post. One end of the copper wire is connected to the annular binding post, and the other end is connected to a power supply. The heating power of each section of the carbon-fiber between two adjacent annular binding posts should be identical. The CFHST – based DTS system is calibrated in the laboratory and is tested in the field. The following section describes the laboratory and field experiments.
4 Laboratory calibrations 4.1 Relationship establishment between and Tt To establish the relationship between and Tt for the CFHST and determine its monitoring accuracy and range, a test configuration is set up in lab, as shown in Fig. 5. The test soils are collected from a metro foundation pit located in Changzhou, Jiangsu Province, China. The soil-specific (fine sand and clay) calibration equations relating Tt to were obtained from the laboratory tests: The gravimetric clay samples in the field had an average dry bulk density ( d ) of 1.59g/cm3 with a standard deviation (σd) of 0.05 g/cm3. For the fine sand sample, the d was 1.36g/cm3 and σd was 0.04 g/cm3. In order to reduce the errors caused by density variation, the soil sample collected from the field were calibrated in the lab to have the same d as that of the in-situ soil. Fig. 6 shows the grain size curves of the test soil samples. Prior to the calibration tests, all the soil samples were dried in an oven for 48 hours at the temperature of 105 °C to make sure that these soils are almost dry ( approximates zero). If the relative soil weight reduction is less than 0.3 % during the drying process, the soil was considered to be dry enough. The dried clay samples were smashed by a pulverizer and passed through a sieve with 2–mm–diameter holes. The clay sample was divided into 25 groups, and the sand was 10
divided into 30 groups. Each group was injected into different volumetric water. The volume of added water was calculated according to the dry density tests and gauged by a graduated cylinder. In the process of soil samples preparation, the water was added into clay by spraying and stirring by a glass rod. All the preparative soil samples were kept for at least 50 hours to ensure moisture distribution uniformly. For sand samples with a moisture content of less than 0.2 m3/m3, the sample preparation process was the same as that for clay. If it exceeded 0.2 m3/m3, the soil samples with a moisture content of 0.2 m3/m3 was added into the calibration chamber, and the extra water was sprinkled on the surface of the soil samples. Then, the calibration chamber was kept standing for 2 hours to enable the sprinkled water to infiltrate into the soil uniformly. Here, the calibration chamber was a cylindrical metal drum of 90 cm height and 50 cm diameter. The CFHST was installed in the center. The metal drum was used to repack soil samples at different moisture contents, such that by combining the temperature recorded by DTS and the elapsed time for different moisture contents, the Tt can be determined. Because the first centimeters of soil next to the CFHST has a large influential on the measurement, equipment similar to field ones were used—a borehole was set in the drum. The CFHST was firstly fixed in the center of the metal drum, and a PVC tube with an inner diameter of 10 cm was wrapped around the CFHST. The test soil is then put inside the outer space of the PVC tube. Backfill sand is then filled in the space between CFHST and PVC tube. Finally, the PVC tube is drawn out ensuring the backfilled sand and surrounding test soil remained in contact. When a measurement was finished, the triplicate volumetric samples were collected to measure the actual moisture ( ) by oven drying method. After all the measurements were completed, the relationships between Tt and were fitted using the least square method, as shown in Fig. 7. In order to observe the 11
boundary temperature effect of metal drum, 10 Platinum Resistance Thermometers (PRT) with an accuracy of ± 0.2°C were installed into soils at different radial position in the drum. After the calibration was completed, a verification test was carried out. As shown in Fig. 5(b), four soil layers which have a sequence of fine sand, clay, fine sand and gravel filter layer, were filled in the metal drum. The CFHST was installed into the soils by a borehole which was backfilled with coarse sand. The soils were then submerged into water. After all the soils were saturated, the valve was opened and the soils started to dewater. The soil moisture at different dewatering time was measured by the CFHST and drying method to evaluate the errors of the CFHST. The calibration result for fine sand is expressed as: 2251.23 2 318.56 22.61 Tt 7.53 12.06
which has a coefficient of determination (
0 0.07 0.07 0.46
(10)
) of 0.981 and a maximum absolute error of
0.05m3/m3. The monitoring range of CFHS when using this relation is about 0~ 0.23m3/m3. The calibration result for clay is expressed as: 338.86 2 135.92 24.98 Tt 12.64 15.67
which has a coefficient of determination (
0 0.23 cr 0.67
(11)
) of 0.957 and a maximum absolute error of
0.09m3/m3. The monitoring range using this relation is 0~ 0.67m3/m3. It has to emphasize that Equations (10) and (11) are just applicable in the given conditions, i.e., soil types, heating power, backfilled material, diameter of borehole, etc. If any factors changes, new calibration work should be done, that is because any of these factors changes will cause the calibration results to lose efficacy. It can be inferred from equation (4) that heating power directly 12
determined the relationship between thermal response and soil moisture: the larger heating power, the higher temperature will be recorded by DTS. In general, it is suggested using a standard system with a constant heating power per meter of cable. Backfilled material and borehole diameter affect measurement results via the thermal resistance of grout. When monitoring soil moisture around borehole, the smaller the grout thermal resistance is, the better system performance. Among common soils, gravel or coarse have lower thermal resistance, which also provides advantages in terms of construction time and cost savings [40]. Abu-Hamdeh and Reeder [41] discovered that soil density, salt concentration and organic matter have obvious influence on the calibrated Tt ~
relations. Benítez-Buelga et al. [42] sand has the largest thermal
conductivity, follows by loam and clay, so the Tt ~ relationship for these soils should be established separately. They also advised to use numerical methods during the calibration process to reduce endless soil sampling. 4.2 Error analysis Soil moisture data at three different draining moments were presented in Fig. 8. It can be seen from Fig. 8 that the variation of C obtained by CFHST agrees reasonably well with that of D measured by drying method. Both of them reflect the distribution of soil moisture after heating at 30 minutes, 60 minutes and 120 minutes, respectively. Fig. 9 shows the errors of C compared to D . The data points of C and D are scattered close to the 1:1 line to indicate
low errors. As can be seen from Fig. 9, the measured soil moisture between oven drying method and CHFST is in good agreement with high correlation in the sand (R2 = 0.946) and clay (R2 = 0.904) experiments, which suggests the effectiveness of CHFST is similar to the traditional drying 13
method. However, it should be pointed out that this effectiveness is built on accurate calibration. The relationship in equation (11) is only suitable for the soils at the test site, but when the geological conditions changes, its coefficients may vary dramatically. Therefore, for different sites, separate calibration tests should be done to determine the site-specific coefficients. Here, one important thing is how to calibrate for these soils from different sites: performing in in-situ site or bring samples to laboratory. Both the calibrations of Sayde [20] and this study were performed in laboratory, however, during the process of soil sampling and transportation, soil density and structure will inevitably change a lot. Hence, the most effective calibration should be implemented in in-situ sites. However, at the present, there are still many challenges for in-situ calibrations. For examples, it is very difficult to obtain the real moisture in deep soils in real-time. Therefore, further work is necessary to improve calibration techniques in the future. 4.3 Temperature disturbance radius analysis When the CFHST is heated, it will disturb the initial temperature field of the surrounding soils. However, this disturbance has a finite range. In order to eliminate the effect of metal boundary and ensure all the heat released by CFHST can be absorbed by the test soil, the radius of metal drum must be designed to be larger than the temperature disturbance distance. Fig. 10 shows the recorded soil temperature rise by the PRTs in the heating process. Fig. 10 indicates that for both dry and saturated soil, the temperature disturbance range is 7cm (after heating 20 minutes), which is far less than the radius of the metal drum (12.5cm). Hence, the dimension of the metal drum can satisfy the requirements of the laboratory test.
5 Field test study 5.1 Experimental Design 14
The soil at the study site is the same with that of the collected soil samples used in the laboratory tests. The soil in the foundation pit includes alternating clay – fine sand – clay layers. The CFHST was deployed to monitor the soil moisture profile during dewatering before the excavation activities, as seen in Fig. 11. The CFHST used in the field has a length of 20m and composed of five units. In the process of borehole drilling, the core was collected and the in-situ soil moistures were quickly measured by drying method. 5.2 CFHST installation and data collection The CFHST has a diameter of 7 cm after the heating shrinkable tube is installed. Fig. 12 shows the installation processes of CFHST: Fig. 12(a) and (b) show the CFHSTs and the borer for site installation. The borehole with the installed CFHST has a depth of 20 m and a diameter of 10 cm. The CFHST is installed section by section as shown in Fig. 12 (c) and (d). After the CFHST installment is completed, the borehole is backfilled with silty-fine sand which is the same with the backfilled sand used in laboratory tests (Fig. 12(e)). The CFHST was then connected to a DTS interrogator installed in a field observation station. The electric circuits were connected to a controller which provided the power supply and at the same time, controlled the heating time. The controller consisted of an AC current source, a switch, a digital timer with a precision of ±0.1% and a transformer. In the process of measurement, it is important to make sure all the CFHSTs were heated under the same power level.
6 Results and discussions 6.1 Soil moisture variation during dewatering During the foundation pit dewatering, the soil moisture profiles were obtained through combining the temperature ~ time curve measured by DTS in the field and the laboratory 15
calibration results, which is shown in Fig.13. It can be seen from Fig. 13 that the water level continuously decreased throughout the draining process despite the weather conditions: It fell from initial 9.81 m depth to final 14.08 m depth. The moisture of surface soil within 4 m depth was affected by the weather: On the first dewatering day when the weather was sunny, the surface soil moisture gradually reduced; it was rainy on the second day, so the surface soil moisture was found to rapidly increase; from the third day to the fifth day, the surface soil moisture was kept at a very low level (
