EVALUATION OF SHAFT-MOUNTED TDT READINGS IN DISTURBED AND UNDISTURBED MEDIA

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1 EVALUATION OF SHAFT-MOUNTED TDT READINGS IN DISTURBED AND UNDISTURBED MEDIA G.C. Topp 1, D.R. Lapen 1, G.D. Young 2, M. Edwards 1 1 Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada Ottawa, Ontario, Canada, K1A 0C6; toppc@em.agr.ca 2 E.S.I. Environmental Sensors Inc., Suite 100, 4243 Glanford Ave. Victoria, British Columbia, Canada, V8Z 4B9; jyoung@esica.com ABSTRACT TDR/TDT (time domain reflectometry/transmissometry) measurements using insertable probes are made in the modified soil. The major factors affecting the interpretation of the TDR/TDT signal are water and solid redistribution in the soil during probe insertion and how far the electromagnetic field reaches into the soil. Nevertheless, the consequence of local soil disturbance on the TDR/TDT reading is not precisely known. Field and laboratory measurements were made with a helical wrapped parallel pair transmission line configured as a single probe (15 mm diam.) and operated in TDT mode on the Terra.Point penetrometer shaft. Ambient water content were measured using the Terra.Point TDT instrument and with independent multi-pronged (3 and 6 mm diam.) TDR probes. TDT readings were calibrated against liquids of known permittivity. In soil columns of silty clay loam and sandy loam textures both TDR and TDT estimates of water content were compared at a range of uniform water contents. Similar measurements comparing the two instruments were made in the field. The TDT instrument was shown to provide a reliable measure of relative permittivity and thus of soil water content with a resolution of ± 0.02 m 3 m -3 The apparent water content increase arising from insertion of the TDT probe was estimated at about 0.01 m 3 m -3 in the field but somewhat larger in the sandy loam soil in the laboratory. Liquid and air referencing periodically of the TDT estimates was useful in substantiating the performance of the TDT instrument. 1. INTRODUCTION TDR is widely used for soil water measurement both for water content and bulk soil conductivity (Topp and Ferré, 2000). The broad applicability of TDR has been enhanced by the flexibility of probe configurations that allow the TEM (transverse electromagnetic) wave propagation of the signal. Ferré and Topp (2000) have reviewed some of the findings from a variety of probes. In all of these, the goal behind probe design was to obtain an improved measure of water content for samples of specific size and shape. The same flexibility of probe design for TDR increases the possibility of combining TDR with other techniques to provide measurement of water content with other soil parameters. A number of soil parameters and processes are highly dependent on water content. This thinking led to the combined soil penetrometer and water content sensor (Young et al., 2000; Vaz and Hopmans, 2001). The cone, used to sense the cone resistance or soil strength, was followed into the soil by a water content sensor, so that both soil parameters were measured in the same soil in an in situ manner. Soil water content must be accounted for in evaluating resistance measures. Although mounting TDR sensors on a single probe has advantages for accessing soil the sensitivity to water content is more confined than with multiple probes. In addition, the transmission line for signal propagation in soil results in a more complex reflected wave form to interpret. Young et al. (2000) used successfully time domain transmissiometry (TDT) on a single probe arrangement, where travel

2 time is determined by comparing transmit time delay of a similar sharp rise time pulse. The shape of transmitted pulses, not complicated by a returning reflection, is often an easier signal to interpret. TDT and TDR are based on the same principle of measuring travel time of propagation velocity as a measure of relative dielectric permittivity. The evaluation of combination probes adds complexity, in that, interactions between parameter measurements must be removed or taken into account. One interaction is the effect of friction imposed by the water content sensor on cone resistance values. Vaz and Hopmans (2001) used a water content sensor of equal diameter as the cone on a cone penetrometer while Young et al (2000) used a water content sensor of slightly larger diameter than the cone to help insure good water content sensor-soil contact as the probe moved vertically through the soil. Irrespective of either approach, both techniques measure soil dielectric properties that have been disturbed, as water content measurements are made after the passage of the cone. The precise physical nature of such a disturbance is unclear, but appears strongly related to the soil physical condition and the manner of probe insertion into the soil. Roth et al.(1997) evaluated the impact on TDR measurements of the disturbance caused by insertion of varying diameter rods. They concluded that soil should be removed before insertion of rods > 10 mm in diameter to alleviate the effect of soil disturbance around the rods on the water content measurement. With diameter > 12.7 mm, the combination water content/penetrometer probe (Young et al, 2000; Vaz and Hopmans, 2001) were expected to show effects of disturbance that have not been detected and quantified. Topp et al. (2001) showed possible disturbance effects which implied greater concentration of soil and water adjacent to the probe. Vaz and Hopmans (2001) detected effects of disturbance for only one soil type, a sand, and disturbance was toward less dense soil and water adjacent to the probe. Vaz and Hopmans (2001) indicated how difficult it was to confirm and quantify independently the soil disturbance because of the limited extent of the disturbed soil around the probe. In this paper, comparisons were made between probe calibrations in liquids (non-deformable medium) of known relative permittivity with those derived from soils with known relative permittivity and physical properties. Differences between liquid and soil calibration results provides a means for quantifying, in a relative manner, the effect of soil disturbance imposed by the single penetrometer probe on water content readings. The sensor on the penetrometer was operated in TDT mode and compared with TDR using conventional triple pronged probes 2. MATERIAL AND METHODS 2.1. Description of the Terra.Point Penetrometer Combination Instrument The general operation of the Terra.Point penetrometer with both water content and cone resistance measurement capability was described by Young et al. (2000) and Topp et al. (2001) (Fig. 1). The Terra.Point system uses a lightweight, twelve-volt electric motor driven, screw jack assembly to provide a steady velocity for probe insertion of approximately 28 mm/sec. The length of travel is 400 mm. The drive mechanism is mounted on a supporting frame made of lightweight materials which also houses the sensor signal conditioning electronics and a datalogger/controller. The frame is supported vertically by three horizontal legs on which the operator(s) stands to hold the mechanism down during operation. The power for the whole system, including the drive motor, is supplied by a 12V, gel-cell battery carried in a backpack The helical-wrapped TDT probe Topp et al. (2001) desribed briefly the helical parallel pair water content sensor, which is similar to that of Vaz and Hopmans (2001) which they called coiled. Our sensor used an additional 50 O coaxial cable connected to the lower end of the parallel pair helix, to convey the transmitted signal after it passed along the helix to the TDT circuit for analysis. The time delay in the return of the transmitted signal caused by soil or other media surrounding the helical probe was converted to a relative voltage. This voltage relates directly to propagation velocity and thus to water content. The water content sensor consists of a stainless steel tubular core in which cables pass from the water content sensor and cone force sensor (Fig 1). The tubular core is surrounded by epoxy resin in which the helical pair is embedded (Sealtronic 21AC-7Vfrom Industrial Formulators, Burnaby, BC). The fabrication sequence was to machine the stainless steel tube (i.d.=4.5 mm) exterior from 15.8 mm to 9 mm diameter for a length of 65 mm for the water content sensor. The portion removed by machining was replaced by an epoxy casting resin. Two parallel helical grooves (square in x-section) were cut to a depth of 1.5 mm and separated by 3 mm. This was accomplished by machining a double-start thread of 4 threads per inch (25.4 mm).

3 Stainless steel rod circular in cross-section, of 1.5 mm diameter, was formed into two helical springs with 12.5 mm i.d. and 6 mm separation along the axis. Each spring was threaded into each helical groove in the epoxy cast and retained in place with additional epoxy in the groove and around the rod. After hardening, the assembly was machined to 15 mm diameter to provide a smooth exterior surface and exposing the stainless steel helical pair for direct contact with soil. Water Content Sensor Force Sensor (a) (b) (c) Figure 1 (a) Terra.Point penetrometer probe with a part of the insertion frame, (b) the probe identifying the sensor components, and (c) schematic details of the TDT helical parallel pair transmission line, not to scale. The length of the sensing region was 6 cm. At each end of the helical pair, a hole was drilled through to the central cavity, allowing insertion of 50 O cables into and along the central core of the probe to the TDT circuit. At the ends of the helical sensor, the shield and centre conductor of the 50 O cables were separated. The shields were connected (soldered) to each end of one helical rod. The centre conductors were soldered to the ends of the other helix. These connections were carefully embedded in epoxy so they were not exposed to abrasion during insertion of the probe into the soil Calibration of the TDT Sensor Reference calibration in liquids of known relative permittivity A series of organic liquids of known relative permittivity, that spanned the range of relative permittivity values encountered in soils (Table 1), were used to calibrate the sensor. The gain and zero of the TDT circuit were set so that voltage output was well within its operating range (0-5V) when in air (minimum relative permitivity) or immersed in water (maximum permittivity). Voltage readings were recorded when the probe was in air and when immersed in each of the chosen liquids (Table 1). The voltage difference associated with each medium was a direct result of the increase in propagation velocity

4 along the helical probe caused by the interaction of the EM wave with the liquid. Table 1: Relative dielectric permittivity of liquids Medium Reference e r ε r Cyclohexane Benzene Amyl acetate Ethyl acetate octanol butanol butanol propanol Ethanol Ethanol:water (2:1) Ethanol:water (1:1) Ethanol:water (1:3) Water Laboratory and field calibration in soils of known relative permittivity A series of soil columns were prepared, at a sequence of water contents starting from air-dry. Two soils of differing texture, North Gower SiCL and Matilda SL, were used. Soil samples, whether air-dried or wetted, were sieved (2 mm) and packed by hand into a cylindrical plastic container, 25 cm long by 15 cm diam. The sieved soil was added to the container in pre-weighed increments to fill 5 cm. Each increment was hand-packed by tapping as needed to force a 15 cm diam. plunger to the top of the 5 cm incremental height. The intended bulk density for the soils was 1.20 Mg m -3 for the SiCL soil and 1.55 Mg m -3 for the SL. The wetting sequence used was from initial air-dry to fully wetted with a series of incremental additions of tap water. Wetting of the soil was carried out by adding approximately 150 ml of tap water to the required soil. About 1/3 of the soil was spread evenly on a tray to about 20 mm deep. A portion of the water was sprinkled over the surface of the spread soil; another 1/3 of the soil was added and water sprinkled on it and so on until all soil and water had been added. Soil and liquid were mixed thoroughly in the tray and passed through a series of sieves to break into about 2 mm sized particles and packed uniformly as described above. The upper limit on volumetric water content was determined by the limit on manipulation and mixing of the wetted soil to allow uniform packing. A 1502C cable tester (Tektronix Inc., Beaverton, OR) and 20 cm wave guides (three-pronged type) were used to estimate the bulk relative permittivity of the soil in the column (20 cm depth) and at 6, 12, and 18 cm depths to evaluate the potential for water content gradients in the columns. An in-field calibration of the Terra.Point water content sensor was carried out by comparing TDR measured water contents (TRASE model 6050, Soilmoisture Equipment Corporation, Santa Barbara, CA) over the top 15 cm to the average of the outputs of the Terra.Point sensor for the same depth interval. The TDR determinations were made within 10 cm of the location of insertion of the Terra.Point probe, using a 3-pronged probe of 6 mm prongs separated 35 mm. Reference

5 determinations with the Terra.Point probe were made in air, tap water and 2-propanol after approximately every 30 readings in the soil to check for zero and calibration drift. The Terra.Point outputs were used to develop a calibration from field soil to compare with that obtained in laboratory Reference Liquid Calibration of TDT Probe 3. RESULTS AND DISCUSSION The use of reference liquids achieved two purposes, firstly, to assess the TDT method for the measurement of relative permittivity where the liquid has ideal physical properties to assure good contact with the sensor and secondly, to provide data against which to compare equivalent readings from soil. The reference liquids (Table 1) showed a highly linear response between square root of relative permittivity ( ε r ) and TDT voltage output, Fig. 2, with an r = The liquids provided a complete seal around the probe and therefore the liquids were considered effectively undisturbed or idealdeformable media with the desired contact and uniformity exterior to the probe surface. A linear regression = V provides a very convenient calibration relationship by which to convert V from TDT to e r. These ε r voltages readings in Fig. 2 are all relative to readings taken in air. Thus the intercept ( ε ) gives relative r = 126. permittivity higher than that of air because the epoxy material in the helical transmission line forms part of the measured dielectric, resulting in an over-estimate for which corrections could be made. These corrections would use mixing laws and follow the procedure given by Vaz and Hopmans (2001). The high linearity of the relationship in Fig. 2 and the lack of any consistent deviation or bias from linearity indicates this correction would have a minor impact on the relationship SQRT (g r ) TDT (V) Figure 2: The square root of relative permittivity of selected liquids,, (Table 1) as a function of the output ε r voltage (V) recorded from the TDT sensor. The line is the linear regression reported in the text TDT Probe Measurements in Soil Columns The uniformity of soil packing in the columns was assessed using TDR. Variable length wave guides inserted into the column soil immediately after TDT measurements showed uniform vertical soil water content profiles. Similarly, lateral uniformity was ascertained from randomly located replicate measurements between the central TDT location and the

6 column wall. Therefore, the apparent relative permittivity,, for the bulk column, as determined via TDR using the 20 ε ra cm wave guides (usually average of three readings around the TDT probe insertion locale), was related to TDT (V) measures at the three depth positions for each soil column. The chosen depth increments were the length of the sensor, 6 cm, starting immediately after complete immersion of the sensor at the soil surface. For all depth positions (0-6, 6-12 and cm), the North Gower SiCL columns (Fig.3) showed linear relationships between by TDR and V by TDT. In addition, the linear relationships were consistent with that for the liquid reference ε ra materials, having slopes of 2.23, 2.31 and 2.42 V -1 for the depths 0-6, 6-12 and cm, respectively. Although these slopes are similar to the value 2.39 V -1 for the reference liquids, there is an indication from Fig. 3 that 0-6 cm depth measurements were closer to the reference line, while at the other depths the recorded TDT voltages were slightly larger. This leads to the suggestion that with progressive movement of the probe through the column, there was greater potential for the TDT to overestimate true soil water content within the probe s area of influence. In soil water content terms, the maximum deviation from the liquid reference line, as derived from the bottom (12-18 cm) depth measurements, was.0.02 m 3 m -3. Overestimation was not entirely unexpected since insertion of the probe displaces water and soil toward the probe perimeter. The pressure gradients imposed by the probe would likely retain more water in the soil next to the probe, rather than vice versa. 6 SQRT (g ra ) TDT (V) Figure 3: North Gower silty clay loam soil column data. Apparent relative permittivity, ε, data derived ra from TDR versus TDT voltage, for 0-6 cm depth (solid circle), 6-12 cm depth (hollow triangle), and bottom reading (12-18 cm depth) (crosshair). The solid line is the liquid reference line from Fig. 2. A similar but more exaggerated pattern was shown by the Matilda SL columns (Fig.4). Again, the 0-6 cm depth relationship was linear, having a slope of 2.52, and for the most part similar in form to those for the liquids and the silty clay loam soil. There is a tendency for a slightly lower TDT voltage in the range of ε = 3.5 to 4.5, indicating TDT gave an under- ra estimate in the mid-water content range. For the deeper depths, however, the TDT gave an over-estimate of ε in the ra range from 2 to 4 for the sandy loam soil. The relationship between ε and V for the 6-12 and cm depths is also ra

7 non-linear. The degree of overestimation, expressed in terms of soil water content, did not exceed 0.05 m 3 m -3. The soil column data converge with the liquid line at around = 4 which suggests that the factors that augmented water next the ε ra probe were effectively eliminated at soil water contents approaching saturation. 6 SQRT (g ra ) TDT (V) Figure 4: Matilda sandy loam soil column data, ε ra from TDR versus TDT voltage, for 0-6 cm depth (solid circle), 6-12 cm depth (hollow triangle), and bottom reading (12-18 cm depth) (crosshair). The line is the liquid reference line from Fig Field Calibration of the TDT Sensor The field calibration of the TDT probe provided unexpectedly good fit to the reference line for the liquids (Fig.5). On a whole, there was the expected slight overestimation in the TDT field results, however, in this case it was quite small at < 0.01 m 3 m -3. The soil water content range from about 0.14 to about 0.36 m 3 m -3, covers the most dynamic and usual range encountered in most field soils. The field soils were similar to the soils used in the soil columns. The linear correlation coefficient was 0.95 and there was a high degree of linearity. The linear regression equation = V was similar to that given for the reference liquids. As expected, the scatter or variability about the line was greater for the field results as the data include spatial variability. The TDR probe was inserted near but not coincident with the TDT probe. Even with the variability given in Fig. 5 a SEE of was obtained, which corresponds to a water content resolution of ±0.02 m 3 m -3 for the TDT based estimates. ε ra Hence a calibration relationship which could be functionally used by an operator has the form, = V θ v (1) where? v is the volumetric water content and V is the TDT voltage output relative to that obtained with the probe suspended in air. Eqn (1) makes use of the linear equation given by Topp and Reynolds (1998) between? v and ε ra to convert the field-based relationship (dashed line, Fig. 5) between TDT and TDR to water content.

8 As with any instrument, the TDT has shown two kinds of drift over time. A change of zero, i.e. the reading in air, may occur with no change in sensitivity, which is zero drift. This occurred, particularly, during warm-up. In addition, some changes in gain or sensitivity were also observed but not related to any specific causal factor. Although we have not characterised or quantified these drifts we have followed a referencing procedure where periodic measurements were made in reference liquids, water and 2-propanol. From these measurements, and those in air, we were able to compensate the TDT voltage output fully for the drifts which occurred to our instrument during field measurements. The wide scope of measurements using the Terra.Point penetrometer in the field have been carried out at ambient temperatures from 0 to +30EC. The use of this referencing procedure was necessary to assure the excellent resolution capability we obtained and is similar to the rigour of measurement that we used in the early development of TDR for field applications. 6 SQRT (g ra ) TDT (V) Figure 5: Field soil data, ε by TDR versus TDT voltage, for 112 locations taken 0-15 cm depth (solid ra circle). The solid line is the liquid reference line from Fig. 2. The dashed line is the linear regression through the field data as given in the text. 4. CONCLUSIONS We used liquids of different reference relative permittivity to confirm the TDT system as a reliable approach for measurement of relative permittivity, giving values equivalent to TDR. The use of TDT depends on signal travelling through the sensor transmission line using separate send and retrieve cables, making single probe format more usual. The likelihood of soil disturbance within the measured zone was high for a 15 mm diameter probe. We were able to identify probable soil disturbance effects on the TDT measurements of < 0.05 m 3 m -3 in a sandy loam soil column and < 0.03 m 3 m -3 for a silty clay loam column. In the field calibrations, the increased concentration of water around the probe that was attributed to disturbance from probe insertion was < 0.01 m 3 m -3. The water content resolution of ± 0.02 m 3 m -3 is equivalent to TDR which was used as the reference method in this study. The periodic referencing procedure, using air, water and 2-propanol, was very beneficial for monitoring the consistency of instrument performance and highly recommended, particularly, when calibrating or evaluating instrument performance. The TDT single probe has been shown to be straight-forward to use and to provide estimates of soil water content that are equivalent to those obtained from TDR. More detailed studies are needed to elucidate the depth of measurement region around the TDT probe.

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10 5. REFERENCES Ferré, P.A. and Topp, G.C., 2000, Time-domain reflectometry sensor techniques for soil water content measurements and electrical conductivity measurements. In Sensors Update vol. 7, H. Baltes, W. Gopel and J. Hesse Ed.s, Wiley-VCH, Weinheim Roth, A., Weis, W.,Matthies, D., Hess, U., and Ansorge, B., 1997, Changes in soil structure caused by insertion of time domain reflectometry probes and their influence on the measurement of soil moisture. Water Resour. Res. 33, Topp, G.C. and Reynolds, W.D., 1998, Time domain reflectometry: a seminal technique for measuring mass and energy in soil. Soil and Tillage Res., 47, Topp, G.C. and Ferré, P.A., 2000, Measuring Water Content in Soil using TDR: A State-of-the-Art in In Comparison of soil water measurement using the neutron scattering, time domain reflectometry and capacitance methods. International Atomic Energy Agency, Vienna, Austria. IAEA-TECDOC-1137, Topp, G.C., Adams, B.A., Young, G.D., and Lapen, D.R., 2001, Practical considerations for the use of a combination penetrometer and water content sensor. In Proc. Soil Structure/ Carbon Workshop, Leamington, ON, Aug , 1999 (in Press). Vaz, C.M.P. and Hopmans, J.W., 2001, Simultaneous measurement of soil penetration resistance and water content with a combined penetrometer-tdr moisture probe. Soil Sci. Soc Amer. J. 65, Young, G.D., Adams, B.A. and Topp,G.C., 2000, A portable data collection system for simultaneous cone penetrometer force and volumetric soil water content measurements. Can. J. Soil Sci. 80,