ROCK WATER CONTENT MEASUREMENTS BASED ON ELECTRICAL IMPEDANCE SPECTROMETRY METHOD

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3 ROCK WATER CONTENT MEASUREMENTS BASED ON ELECTRICAL IMPEDANCE SPECTROMETRY METHOD Maria Clementina Caputo 1, Francesco De Benedictis 1, Rita Masciale 1 Abstract The characterization and modelling of flow and transport through the vadose zone are receiving increased attention in the last years, particularly as regards the pollutant phenomena. Hence water content estimation and its monitoring, from which strongly depend the hydrological processes that occur within the unsaturated zone, is an important task. A number of methods and devices are available for measuring the water content in soils but, usually, they are not usable with the rocks too and very few are the attempts to verify their applicability. The installation of the probes is the main difficulty in applying the devices for water content estimation in rocks, because the installation procedure has to ensure a good contact between the rock and the sensor in order to reduce the uncertainty due to the gap effects. In this study a non-traditional method, known as electrical impedance spectrometry (EIS), was used. Using the new apparatus Z-meter, developed within the European International Project E!3838 of EUREKA Program, the electrical impedance in complex form and its changes over time were monitored in order to verify the suitability of the device for water content estimation in rocks. Both the real component of impedance (resistance R) and the imaginary component (capacitive reactance X) were independently investigate. One kind of stainless steel probes have been installed into samples of calcarenite, a sedimentary rock coming from a quarry located in Canosa (South of Italy). The relationship between the output device and water content was investigated within a wide frequency range (from 100Hz to 10000Hz), using different water solutions with different electrical conductivity (EC) values. Due to the influence of temperature variation on all indirect moisture measurement methods, the experimentation was performed in a temperature controlled room provided with a ventilation system. Correlating the real component of the impedance with the directly measured water content, determined using a gravimetric method, functions of exponential type, with a good correlation coefficient, were obtained for different water solutions. The independence of the real component of impedance to changing the frequencies is confirmed; any frequency can be used. On the contrary, the sensitivity of resistance to changing the EC value of the solution used for sample saturation is evident. The analysis of reactance data confirms a strong dependence on the applied frequency. The reactance outputs are not continuously recorded for all investigated range of water contents (from saturation to dry) but appear in correspondence of water contents which depend not only on applied frequency but also on the EC of the solution used to saturated the sample. Particularly, 1 Maria Clementina Caputo Water Research Institute (IRSA), National Research Council (CNR), via F. De Blasio, Bari, Italy, Corresponding author: phone , maria.caputo@ba.irsa.cnr.it) 55

4 higher frequency and lower EC of solution seem to assure reactance values in a wider range of water content. This evidence, together with a lower precision of the device on reactance outputs, makes it more difficult to use reactance to obtain any indirect measurement of water content. Though the experimentation is still ongoing, these preliminary results are encouraging for further studies both on different type of rocks, and in order to better investigate the capacitive reactance, that could provide more information about the physical properties of the investigated rock. Key words Rock water content, calibration, electrical impedance spectrometry. 1 Introduction Monitoring the hydrological processes that occur within the vadose zone is important, although it is usually considered difficult and expensive. When the vadose zone consists of rock, the difficulties are greater for several aspects. Many methods and techniques commonly utilized to investigate the unsaturated zone flow process in soils are not easily adaptable to rocks (Bogena et al., 2007; Kizito et al., 2008). Among these, the technologies used to measure the water content in the subsurface, such as TDR (Time Domain Reflectometry) (Jones et al., 2004; Robinson et al., 2003), FDR (Frequency Domain Reflectometry) and multi-sensor capacitance probes (Baumhardt et al., 2000, Seyfried et al., 2004), are hard to use mainly because the probes are delicate which makes difficult their insertion in the rock with good contact. The Z-meter 2 device works with probes that can differ in shape, design, material, and hardness; these features, therefore, could influence the possibility for the probes to be installed into rock samples. The objectives of this study, that is still ongoing, are: (i) to verify the possibility to use the Z-meter 2 to measure the water content in the rocks, (ii) to characterize the effects of electrical conductivity (EC) and frequency on Z-meter 2 measurements under laboratory conditions, and (iii) to develop specific laboratory calibration curves for the investigated lithotype. 2 Materials and Methods 2.1 Z-meter 2 device The Z-meter 2 is a new device, developed within the European International Project E!3838 of EUREKA Program, based on the Electrical Impedance Spectrometry (EIS) method. This method allows the measurement of the changes in electrical impedance caused by changes in saturation, textural characteristics of the medium, and/or the presence of solutes. The electrical impedance, Z, is characterized from two parameters: the resistance, R, and the reactance, X, combined as in the follow equation: Z=R+jX, (2.1) The Z value of the portion of the medium comprised between two electrodes represents the electromagnetic property of the measured profile. The electrical impedance of the unsaturated medium is a complex number that describes the properties of the medium: the resistance (R), that represents the real part of the measured impedance and is strongly influenced by the degree of saturation (water is a high conductivity material), and the reactance (X), that represents the imaginary part of the measured impedance and describes the solid part of the medium (texture, structure, etc.). The Z-meter 2 device allows measurement of both parts of the impedance and it works with different kinds of probes. The probes considered in this study, called picket type, are made of steel with 10 mm in diameter and 120 mm in length with a conical bit 15 mm long. These probes were chosen because they are easily installed in rock without damage the probes. 2.2 Rock samples preparation The rock used for the calibration tests is a calcarenite, a marine sedimentary rock of Plio-Pleistocene age, with a granular skeleton constituted by irregularly rounded carbonate lithoclasts and bioclasts embedded in a matrix. The calcareous matrix and the sparry cement are entirely subordinate to the granular skeleton and the degree of cementation is low. Particularly, the samples of calcarenite used for the tests came from an active quarry, located in Canosa of Puglia, a City in the south of Italy (Fig.1). The blocks of calcarenite, quarried still now, are used as building materials. Fig. 1 (a) Location of Canosa City; (b) quarry from which the rock samples have been collected. Two blocks of Calcarenite, hereafter called E1 and E2, collected from the quarry, were used in order to verify the repeatability of measurements. The dimensions of two samples are lightly different: 25x16x15 cm and 25x15x15 cm for E1 and E2 respectively. The main physical characteristics of samples are reported in Tab

5 Tab. 1 Physical characteristics of the calcarenite samples tested. Sample E1 Sample E1 Sample E2 Volume (cm 3 ) Dry weight (kg) Saturated weight (kg) Bulk density (Mg/m 3 ) Porosity (%) For the installation of the picket probes, a guide hole was drilled with approximately 10 mm diameter. The probes were then inserted into the sample so that a good contact between the rock and the sensor was ensured. The Fig. 2 shows the position of the probes installed in the sample. The lateral surface of the samples were sealed using a bi-component epoxy resin in order to ensure a unidimensional (vertical) water evaporation process. The samples were saturated, using a vacuum bell jar, from the bottom avoiding both air entrapment and uneven water distribution. Deionised water and KCl solutions at 0.01 M and 0.03 M were used for saturation of the samples in order to verify how the different electrical conductivity (EC) of water solution could influence the output of the device. using the system for the automated data collection, ZScan 2A version 2.0, the measurements were performed by choosing a voltage of 1000 mv and different frequencies: 100, 500, 1000, 3000, 5000, 8000, Hz. For each frequency the measures were repeated 10 times in order to verify the accuracy and precision of the output of the device (Ohm). The entire cycle of measurements was repeated 2-3 times per day except that for the frequency 10000Hz for which the measures were automatically recorded every 30 minutes (Fig. 3); contemporarily the weight loss of the rock sample, due to the evaporation, was registered by means of electronic balance (Fig. 3); the test was stopped when the sample s weight reached approximately it's dry weight; volumetric water content was determined, from the gravimetric one after the oven drying, by the equation (2), ρb θ = θ (2.2) v m ρ w where θ is the volumetric water content (m 3 m -3 ), v θ is the gravimetric m water content (kg kg -1 ), ρ is the bulk density of the rock, and b ρ is the w density of water solution (Mg m -3 ); the test was repeated for different solution concentrations used to saturate the samples. Fig. 2 Scheme showing the position of the probes in the sample. 2.3 Experimental tests The tests consisted of finding the correlation between the output probe of the Z-meter 2 device and the rock water content gravimetrically determined. This provides an indirect measure of rock water content. In order to avoid the influence of temperature on the test results, the experimental measurements have been performed in thermostatic room with an average air temperature of 25 ±1 C provided with a ventilation system. The experimental procedure has comprised different steps: the rock sample was saturated following the procedure described in theprevious paragraph; Fig. 3 Experimental set up for testing probes of Z-meter 2 with the rock sample

6 3 Results and Discussion Testing the probes in the calcarenite, made it possible to investigate both the real component of impedance (resistance R) and on the imaginary component (capacitive reactance X). The experimental results relative to sample E2 saturated with KCl solutions at 0.03 M, were omitted because the test is still ongoing. The weight loss of samples E1 and E2, due to evaporation during the calibration test, is shown in the graph of Fig. 4a for each solution used to saturate samples. The unequal starting point of weight for the sample E1 in the different experiments was due to the fact that a fully saturation was reached only for the KCl solutions at 0.01 M. Anyway this is not significant to the results of the test. It is evident from the graph that the weight decreases differently in time. In fact, the evaporation from a soil or rock may be modeled as a twostage process. The first stage corresponds to high water content so that the evaporation rate is governed by environmental conditions. This is followed by the falling rate stage when the water content has decreased to a critical value and evaporation is controlled by the ability of the soil or rock to transmit water [Brutsaert,1982; Jury et al., 1991]. Fig. 4b more clearly shows this different behaviour: the first part of experimental data, corresponding to higher water content values, follows a linear relation, while the tail of data is better fit by an exponential function as the type: x y = a + bexp (3.1) c where a is the dry weight of sample, a+b is its satured weight and c is the only coefficient that varies to have the best fit of the experimental data. The passage from the first to the second stage occurs for water content about 6 to 9%. The analysis of resistance data (Rx output probe) confirms the independence of the real component of impedance on the frequency applied and the strong relationship between resistance and water content. This concept is better highlighted in Fig. 5 that shows the output probe for all the tested measurement frequencies relative to sample E1, saturated with KCl solutions at 0.01 M. Generally, the output values vary within the range of 102 Ohm to 106 Ohm. The higher the output value the lower the water content; the output values decline steeply over three order of magnitude with increasing the water content. Moreover, the output for the all tested frequencies are very similar to each other, meaning that the device has high accuracy and precision. Changing the frequencies has no effect on the resistance values that are therefore related with the water content only. As a consequence, it was possible to fit the data on mean values of the whole dataset, comprising the outputs for all frequencies tested, and to sketch only one calibration curve for a known solution used to saturate the samples. Fig. 4 a) Weight loss of the samples, E1 and E2, saturated with the different water solutions - the dotted blue and red lines indicate the dry weight of the samples; b) Weight loss and fit functions of the sample E1 saturated with 0.03M KCl solution

7 Fig. 5 Outputs of the device vs volumetric water content, for different applied frequencies. Additionally, the output of device is very affected by changing the EC value of the solution used for the sample saturation, as shown in Fig.6. The different curves have the same shape, so that they seem to differ only by a constant factor. This behaviour is observed only for water content higher than about 5%. In fact for lower values, the resistance increases rapidly and the curves tend to converge. This evidence could suggest a direct correlation between the resistance measured by device and the EC of the solution saturating the rock at least in a wide range of water contents (higher than 5%). However this aspect should be better investigated specially for low water content. Fig. 7 show the experimental data, expressed as mean values of the outputs of device for all frequencies tested, and the interpolation curve obtained for each solution used to saturate both sample E1 and E2. It is clear that, the fitting of each curve is very good in all cases while some differences appear comparing the samples E1 and E2. Additional testing is necessary to explore these differences. Fig. 6 Changing in Rx output probe vs volumetric water contents for different solution used to saturate calcarenite. a) E1; b) E2. The difference between a) and b) in terms of density of experimental data are due to the availability of the automatic balance only during the E2 test

8 With regard to the capacitive reactance (Xx output probe), the analysis of data shows interesting results. It strongly depends on the applied frequency because, as known, it is inversely proportional to the frequency. This behaviour is well shown in Fig.8 where the Xx output is reported both for deionized water (Fig 8a) and for KCl solution at 0.01M (Fig 8b). Nevertheless, it s very interesting to notice that the Xx outputs are not recorded for the same water contents at each frequency. In more detail, the greater the frequency applied the higher the correspondent water contents at which the Xx output is recorded. Moreover, in the case of deionized water (Fig 8a) the reactance appears for water contents slightly lower than saturation, while it appears at lower water contents in the case of KCl solution at 0.01M. Fig. 7 Exponential interpolation curves for each solution used to saturate both. Fig. 8 Changing in Xx output probe vs volumetric water contents for both sample E1 and E2. a) deionized water; b) KCl solution at 0.01M

9 This phenomenon is better shown in the Fig. 9 where the data are for the E2 sample only with reactance values at frequency of 10000Hz. The gap of data for deionized water (between water content of 0.25 and 0.16) is not due to an absence of signal but to a power outage which stopped the automated recording of data. The data show an inverse correlation between the reactance and water content because of dependence of reactance on capacitance which in turn depends on dielectric constant of the investigated medium. The latter, as know, is strongly affected by water content so that it can be used to obtain indirect measurement of water contents. Nevertheless, the precision of device seems to be affected by water content so that the reactance output are more dispersed for greater values of water content and tends to behave linearly for lower values. The evidence seems to suggest some considerations: 1. some recommended frequencies could be used for better investigate the reactance (higher than 5000 Hz); 2. the reactance could be influenced, in some way, by the EC of the solution used to saturated the sample; 3. the precision of any indirect measure of water content starting from reactance values seem to be higher for lower values of water content. All these aspects should be further investigated by continuing the experimentation not only for the same medium (calcarenite) but also for different rock. Fig. 9 Changes in Xx output probe at 10000Hz vs volumetric water contents for sample E2 both for deionized water and KCl solution at 0.01M. 4 Conclusions Laboratory experimental tests were carried out in order to verify that the Z-meter 2 device allows measurement of water content in porous rock; more specifically on samples of sedimentary rock, known as Calcarenite, from a quarry located in Canosa (South of Italy). The tests were aimed at understanding the effects of the applied frequency and the electrical conductivity of KCl solution, used to saturate the samples, on the device outputs and on the calibration curves. A wide range of frequencies (from 100Hz to 10000Hz) was investigated and different water solutions with different electrical conductivity values were used to saturated the rock samples. As the Z-meter 2 device is based on the electrical impedance spectrometry method, the results of the tests reported in this study are related both with the real part of the impedance (resistance) and the imaginary part (reactance) which were evaluated separately. The preliminary results of this study indicate that the probes, connected with the device, work well in the calcarenite and thus allow measurement of the impedance without any gap effects observed. The resistance outputs correlated with water content directly measured by gravimetric method provide a fit function of exponential type with a high correlation coefficient. The discontinuous recording of reactance outputs, suggests a strong dependence on applied frequency, on the electrical conductivity of the solution used to saturate the sample, as well as on the water content. This evidence, together with the scatter in the reactance vs water content data, suggests that the reactance is not useful as an indirect measurement of water content. Although the experimentation is not concluded, the preliminary results reported in this note is encouraging for further studies which could provide more information on the capacitive reactance and its dependence by water content and physical properties of the investigated medium. It would also be of great interest to apply the same methodology on different types of rocks. 5 References [1] BAUMHARDT, R.L., LASCANO R.J., and EVETT S.R., Soil Material, Temperature, and Electrical Conductivity Effects on Calibration of Multisensor Capacitance Probes, Soil Science Society of American Journal, vol. 64, pp [2] BOGENA H.R., HUISMAN J.A., OBERDOSRSTER C.A. and VEREECKEN H., Evaluation of a Low-Cost Soil Water Content Sensor for Wireless Network Application, Journal of Hydrology, vol. 344, pp [3] BRUTSAERT W., Evaporation into the Atmosphere: Theory, History and Applications, Kluwer Academic, Hingham, Mass

10 [4] JONES S.B., and OR D., Frequency Domain Analysis for Extending Time Domain Reflectometry Water Content Measurements in Highly Saline Soils, Soil Science Society of American Journal, vol. 68, pp [5] JURY W., W. R. GARDNER and W. H. GARDNER,1991. Soil Physics, 5th ed., John Wiley, New York. [6] KIZITO F., CAMPBELL C.G., COBOS D.R., TEARE B.L., CARTER B., AND HOPMANS J. W., Frequency, Electrical Conductivity and Temperature Analysis of a Low-Cost Capacitance Soil Moisture Sensor, Journal of Hydrology, vol. 352, pp [7] ROBINSON D.A., JONES S.B., WRAITH J.A., OR D. and FIREDMENA S.P., A Review of Advances in Dielectric and Electrical Conductivity Measurements in Soils Using Time Domain Reflectometry, Vadose Zone Journal, vol. 2, pp [8] SEYFRIED M.S. and MURDOCK M. D., Measurement of Soil Water Content with a 50-MHz Soil Dielectric Sensor, Soil Science Society of American Journal, vol. 68, pp