AU2009227997A1 - System, apparatus and method for measuring soil moisture content - Google Patents

Description

WO 2009/117784 PCT/AU2009/000380 SYSTEM, APPARATUS AND METHOD FOR MEASURING SOIL MOISTURE CONTENT Field of the Invention This present invention relates to a system, method and device for measuring soil 5 moisture content and more particularly, but not exclusively, to a technique and electrode configuration for measuring soil moisture profiles and/or deep drainage in three dimensions. Background of the Invention It is necessary to understand soil characteristics to determine how best to irrigate an 10 agricultural field. For example, knowing the moisture content and the deep drainage characteristics of the agricultural field greatly assists in determining when and how much water to use in an irrigation event. In addition, such information can be utilised for scheduling irrigation events based on predicted weather conditions. 15 Various techniques have been proposed for measuring soil moisture content. One such technique involves utilising a neutron probe. Neutron probes operate by measuring the energy loss of neutrons projected into a region of interest. Due to the radioactive nature of neutron probes, great care must be taken when handling the device. In addition, the sphere of influence of neutron probes varies with the drying cycle of soil and therefore the accuracy of the device is 20 reduced the closer the measurements are taken to the soil surface. Another technique involves installing a grouping of lysimeters around the region for providing a direct measurement of deep drainage. Lysimeters collect the mobile soil solution which can subsequently be analysed to determine the soluble constituents in the drainage water. 25 However, persons skilled in the art will appreciate that installing lysimeters can be very time consuming and frequent sampling is inconvenient. Summary of the Invention In a first aspect the present invention provides a system for determining soil moisture 30 content, the system comprising: two or more probes arranged to be located in boreholes spaced about a region of interest, each probe of the two or more probes including a plurality of electrodes spaced at known intervals along the length of the probe; and a control unit operable to: 35 apply a measured current between selected electrode pairs and to measure the resultant potential difference between other selected electrode pairs to take electrical resistivity measurements for the region, each electrode of the electrode pairings being located on different WO 2009/117784 PCT/AU2009/000380 -2 ones of the two or more probes; and process the electrical resistivity measurements to determine the soil moisture content. 5 In an embodiment the control unit determines the soil moisture content by calculating the apparent anisotropy index (AAI) of the soil from the resistivity measurements, the AAI being correlated against a data set to determine the soil moisture content. In an embodiment the data set comprises data associated with various soil characteristics for the 10 soil profile that are related to soil moisture content. In an embodiment the other electrode pairs are located adjacent to the electrode pairs. In an embodiment the electrode pairs are arranged in a square configuration about the region of 15 interest. In an embodiment the control unit is operable to determine a bulk electrical resistivity of the region by taking the same measurements over a defined period of time, the temporal nature of the electrical resistivity providing an indication of the change in soil moisture content. 20 In an embodiment the measurements are taken at various depths to generate a three dimensional resistivity profile. In an embodiment the probes are located about a root structure of a plant and whereby the depth of the boreholes corresponds with the depth of the root structure. 25 In accordance with a second aspect the present invention provides a method for determining soil moisture content, the method comprising the steps of: providing two or more probes arranged to be located in boreholes spaced about a region of interest, each probe of the two or more probes including a plurality of electrodes spaced at 3 0 known intervals along the length of the probe; taking electrical resistivity measurements for the region by applying a measured current between selected electrode pairs and measuring the resultant potential difference between other selected electrode pairs, each electrode of the electrode pairings being located on different ones of the two or more probes; and 35 processing the electrical resistivity measurements to determine the soil moisture content.

WO 2009/117784 PCT/AU2009/000380 -3 In an embodiment the soil moisture content is determined by calculating the apparent anisotropy index (AAI) of the soil from the resistivity measurements, the AAI being correlated against a soil data set to determine the soil moisture content. 5 In an embodiment the soil data set comprises data associated with various soil characteristics related to soil moisture content for the soil profile. In an embodiment the other electrode pairs are located adjacent to the selected electrode pairs. 10 In an embodiment the measurements are taken at various depths to generate a three dimensional resistivity profile. In an embodiment the electrode pairs are arranged in a square configuration about the region of interest. 15 In an embodiment a bulk electrical resistivity measurement of the region is obtained by taking the same measurements over a defined period of time, the temporal nature of the electrical resistivity measurements providing an indication of the change in soil moisture content. 20 In an embodiment the probes are located about a root structure of a plant and whereby the depth of the boreholes corresponds with the depth of the root structure. In accordance with a third aspect there is provided a method for determining soil moisture 25 content and/or deep drainage of a soil profile, the method comprising the steps of: measuring cracking intensity of the soil profile at various depths; and comparing the cracking intensity measurements against a data set comprising known soil characteristics for an associated soil profile to determine the soil moisture content and/or deep drainage. 30 In an embodiment the cracking intensity of the soil profile is determined by calculating the apparent anisotropy index (AAI) of the soil profile. In an embodiment the AAI is calculated utilising the system in accordance with the first aspect. 35 In accordance with a fourth aspect there is provided computer program code which, when executed by a processor, implements the method in accordance with the second aspect.

WO 2009/117784 PCT/AU2009/000380 -4 In accordance with a fifth aspect there is provided a computer readable medium comprising the program code in accordance with the fourth aspect. 5 In accordance with a sixth aspect there is provided a data signal comprising the computer program code in accordance with the fourth aspect. Brief Description of the Drawings Embodiments of the present invention will now be described, by way of example only, 10 with reference to the accompanying drawings, in which: Figure 1 is a schematic showing a resisitivity measuring technique in accordance with an embodiment of the present invention; 15 Figure 2a is a schematic of a probe configuration in accordance with an embodiment; Figures 2b & 2c are a perspective and plan view respectively of one of the Figure 2a probes; 20 Figure 3 is a flow chart illustrating method steps for implementing the invention, in accordance with an embodiment; Figure 4 illustrates three independent electrode configurations utilised to calculate the Apparent Anisotropy Index (AA) using the square probe array of Figure 2; 25 Figure 5 is a schematic of a well tank experimental setup used for testing embodiments of the present invention; Figure 6 is a graph showing AAI measurements output from the experimental setup of 30 Figure 5; Figures 7a & 7b are graphs showing AAI measurements for a field experimental setup, taken at four sample measurement times; 35 Figure 8 is an electrical resistivity distribution plot for the same four measurement times used in Figure 7; and WO 2009/117784 PCT/AU2009/000380 -5 Figure 9 is a deep drainage plot measured from a mini-lysimeter set-up. Detailed Description of the Preferred Embodiment Through extensive research and testing, the present inventors have discovered that it is 5 possible to effectively determine soil moisture content and/or the likelihood of deep drainage by inspecting the cracking intensity of soil located in an area of interest. The present inventors have also found that determining cracking depth and intensity can readily be achieved by measuring the heterogeneity of current flow in the soil (in one embodiment, utilising Anisotropy index measurements). Embodiments described herein illustrate such techniques and 10 also show simple system configurations for carrying out the same. Embodiments of the present invention also relate to techniques for determining soil moisture content utilising bulk electrical resistivity measurements. Bulk electrical resistivity of soil is influenced by properties of the soil matrix as well as by the properties of the soil solution. is In general, the properties of the soil solution show higher variations with time than the properties of the soil matrix. Using such techniques short-term temporal changes in bulk electrical resistivity can be related back to changes in the soil solution or the soil moisture content. 20 Principles Basic operation for determining the electrical resistivity of a soil sample in two dimensions can be achieved utilising the electrode structure shown in Figure 1. In basic terms, low electrical resistivities relate to moist soil, while high resistivities relate to drier soil. This is because, as moisture is removed from the soil, electric current will find fewer continuous 25 pathways in which to pass thus causing the resistivity to increase. In Figure 1, four electrodes are distributed along a plane, although more electrodes may be provided depending on a desired level of resolution. Two of the electrodes 102a, 102b, are "current carrying" electrodes operable to carry a constant measured current produced by an electrical source 106. The source 106 may either be an AC or DC voltage source. The total voltage required to be output by the source 106 30 will ultimately depend on the maximum spacing between the current carrying electrodes 102. The other two electrodes (i.e. "voltage detecting" electrodes 104a, 104b) are utilised to measure to the voltage drop (i.e. potential difference) between the current carrying electrodes 102. A voltmeter 108 is connected to the voltage detecting electrodes 104 to measure the voltage drop. The ratio of the measured current to the detected voltage, normalised by a geometric factor that 35 incorporates the distance between the electrodes "a", provides a one dimensional measure of the apparent conducitivity in an area beneath the center of the electrode array. To determine the true conductivity from the apparent conductivity distribution in the subsurface, the dataset needs to WO 2009/117784 PCT/AU2009/000380 -6 be inverted. A software package suitable for performing such two-dimensional inversion routines is the "RES2DITNV" package (available for download from the following URL: www. http://www.geoelectrical.com). The approximate depth of this zone/area is half the electrode separation. A two-dimensional measure is constructed by varying the location of the four s electrodes. The depth can also be altered by adjusting the electrode separation, as will be understood by persons skilled in the art. Relationship Between Electrical Resistivity and Soil Moisture Content In sediments with low surface conductance that contain a solution of low resistivity the 10 relationship between moisture content and resistivity can be expressed by Archies law (for unsaturated conditions) as shown in Equation 1 below: (Eq.1) Where op, is the conductivity of the partly saturated soil, S is the fraction of the soil pores filled 15 with water, n is an empirical constant, ow is the conductivity of the soil solution, 1 is the porosity and m is the cementation factor. If clay or organic matter is present within the sample, as is the case in agricultural fields, the relationship is more complex. One example equation which can be utilised is shown 20 in Equation 2: (Y" C" + gl G 01 ) (Eq. 2) Where ob is the conductivity of the soil and og is the grain surface conductivity. 25 Following the approach in Equation 1, Equation 2 can be expressed for unsaturated conditions as follows: = S- (C,.0 + 0 ag (I1- 0 (Eq. 3) 30 Persons skilled in the art will appreciate that Equation 3 does not include a measure of crack intensity, which as air filled cracks act as isolators, will influence the electrical conductance in soil. The measurement of the Apparent Anisotropy index (AAI) may therefore be included to cater for this variable. Utilising AAI measurements for determining soil 35 moisture content and deep drainage is described in detail in subsequent paragraphs.

WO 2009/117784 PCT/AU2009/000380 -7 A schematic illustration of a system 200 utilised to carry out three-dimensional bulk resistivity measurements in accordance with the above-described embodiment, is shown in Figure 2a. Utilising the proposed system 200 a constant resolution can be maintained throughout both the width and depth of soil profile. The system 200 includes four probes 202a, 5 202b, 202c and 202e (see Figures 2b & 2c for more detail of the probe configuration) arranged to surround the region of interest. Each probe 202 is located in one of four boreholes 214 of known separation surrounding the region (e.g. a individual plant of a crop that is to be irrigated as part of a mass irrigation). Resistivity measurements are carried out including electrodes on two or four probes at a time. 10 As described above, the bulk resistivity readings can provide an indication of the soil moisture content at various depths surrounding the plant's root structure which may in turn be utilised to determine when and by how much to irrigate. The readings can also advantageously be utilised to determine when future irrigations should take place by taking into consideration 15 predicted future climate conditions. Each probe 202 comprises a plurality of electrodes 204 in the form of annular stainless steel discs. The annular discs 204 are provided at known intervals along the length of a non conductive tubular body 206; in this embodiment 16 electrodes along a 1.2 meter length of PVC 20 piping. Again, any number of electrodes can be disposed on the body 206, however for ease of illustration, this embodiment shows only four electrodes per probe. Each electrode is connected to a control unit 210 by way of a connecting wire, as shown in Figure 2a. The control unit 210 is operable to control the supply of current to selected electrode 25 pairs and to measure the resultant potential difference between other electrode pairs. In this embodiment currents do not exceed 20niA, as higher currents may cause heat production. If the spacing of boreholes and electrodes are increased, however, higher currents may be preferable. To achieve this functionality, the control unit 210 comprises a LUND ES 10-64 automatic electrode selector connected to an ABEM SAS4000 Terrameter. Two types of electrode 30 configurations may be used for carrying out the bulk resistivity measurements. In one embodiment, the apparent resistivity is measured along six planes that can be drawn between the four probes 202. If the probes are assigned the numbers 1,2,3 and 4, six planes may be drawn between the probes as follows: 1+2, 2+3, 3+4, 4+1,1+3, 2+4. A measurement along each plane consists of one current and one potential electrode on each of the two probes 35 involved in the measurement. In an embodiment, the current electrodes are located above the potential electrodes. Measurements may be carried out with three to six electrode spacings between the current and the potential electrodes. Each spacing is measured with the electrodes WO 2009/117784 PCT/AU2009/000380 -8 on both probes at the same height, as well as with height differences of up to six electrode spacings between the electrodes on the two probes. An example polling technique to achieve the desired measurements is shown in Table 1 of Annexure A. Multiple channel measurements may be utilised to reduce the acquisition time. In an embodiment, shorter measurement routines s can be achieved following the same principal described above but skipping certain electrode spacings. While being slightly less accurate, such routines may be favourable where transient processes need to be monitored. A method for determining soil moisture content utilising the system 200 of Figure 2a is 10 shown in the flow diagram 300 of Figure 3. In a first step 302, the method comprises providing two or more of the probes 202 in boreholes spaced about the region of interest. At step 304, electrical resistivity measurements for the region are taken by applying a measured current between selected electrode pairs and measuring the resultant potential difference between other selected electrode pairs. The measurements may be taken for each associated plane (dependent is on the number of probes), as outlined in the preceding paragraph. Each electrode of the electrode pairings are located on different ones of the two or more probes. In a final step, the electrical resistivity measurements are processed using, for example, the processing techniques outlined in preceding paragraphs, to determine the soil moisture content (step 306). The processing may be carried out by the control unit 210 or, in an alternative embodiment, by some 20 other processing or computing means located remotely from the in-situ probe configuration 200. An alternative electrode configuration includes utilising the AAI index to provide an indication of soil moisture content and/or deep drainage. Again measurements are carried out 25 with all electrodes at the same height in the probe as well as with height differences of up to four electrodes between the two current and the two potential electrodes. This technique is described in more detail below with additional reference to Figure 4. As discussed above, this technique is suitable for readily determining soil moisture content and/or deep drainage by taking advantage of the relationship between the Anisotropy index and cracking intensity. 30 Three independent electrode configurations for a square array are depicted (in top view) in Figure 4. In Figure 4, "C" is used to denote a current carrying electrode, while "P" is used to denote a voltage potential electrode. In homogeneous ground, the apparent resistivity measured with the electrode configuration "A" is equal to that measured with electrode configuration "B". 35 In inhomogeneous ground apparent resistivity depends on the location and orientation of the current source relative to the studied area. Thus, the apparent resistivity measured with electrode configurations A and B would in most cases differ.

WO 2009/117784 PCT/AU2009/000380 -9 The Apparent Anisotropy Index (hereafter "AAI") is the ratio between the apparent resistivity (pa) measured with electrode configuration A and the apparent resistivity measured with electrode configuration B and can be expressed as follows: S AAI=paA/paB (Eq. 4) As will be apparent from the experimental results section below, Equation 4 can be utilised to provide a measure of crack intensity with depth. To do so electrodes in each of the four probes 202 are used (two potential and two current electrodes). The square array configuration A, B 10 and C is measured and the AAI is calculated. Experimental Results (a) Well Tank Experimental Set-Up 15 The above-described AAI embodiment was first tested in a well tank as shown in Figure 5. The system 200 was positioned in a weighing lysimeter 502 having a depth of 45 cm filled with sand to carry out square array measurements at various depths in the sand profile. A 1 cm thick electrical isolator in the form of a plastic sheet 504 was inserted between the probes 20 202 to simulate an air filled soil crack. A resistivity protocol using the three square-array configurations shown in Figure 4 was programmed into the control unit 210. Measurements were carried out with all electrodes at one depth, as well as with a depth difference of up to three electrode spacing's between the two current and the two potential electrodes. A total of 270 measurements were taken for each square configuration. 25 Figure 6 shows the AAI measurements for two different positions of the plastic sheet 504. In both cases the plastic sheet 504 was inserted to a depth of 20 cm, which is half the depth of the sand profile. In the first plane, the plastic sheet only reached half way into the square formation, while in the fourth plane the plastic sheet passed through the entire square. 30 The resistivity protocol programmed into the control unit 210 first addressed the electrodes at the top of the profile, then moved to the bottom of the profile and then up again (explaining the increase in AAI towards the middle of the x-axis (which corresponds to the bottom of the profile) where the influence of the plastic sheet 504 at the surface is decreased). 35 The AAI for the fourth plane shows a larger divergence from 1, thus indicating larger in-homogeneity in the profile. This is consistent with the larger horizontal extension of the inserted plastic sheet.

WO 2009/117784 PCT/AU2009/000380 - 10 (b) Field Experimental Set-Up Four vertical boreholes were drilled around a Sorghum plant in an agricultural field in s Boggabri, NSW, Australia. The protocol programmed into the control unit 210 discussed above was run during four different times in the growing season, explained as follows: e Sample "290": taken in November at the beginning of the growing season. Moist soil with no evident cracking was observed. The plants were noted to be between 5-10 cm 10 high. " Sample "314": taken in December just before second and last irrigation of the year. Minor surface soil cracking was observed. * Sample "319": taken the day after the last irrigation. A wet soil profile with no surface cracks was observed. is * Sample "328": taken in mid January ten days before harvest. A very dry soil profile with severe surface cracking was observed. Figure 7a shows the AAI measurements for each of the four observation times described above for all electrodes on one depth level. The AAI profile measured for the "328" 20 sample (i.e. a number of weeks after the last irrigation) shows the largest AAI divergence from 1, thus indicating the largest in homogeneity in horizontal layers. Figure 8 is a corresponding bulk electrical resistivity distribution plot for each of the four measurement times (described in more detail under the heading experimental set-up "c"). 25 It can be seen from Figure 8 that sample "328" was taken in a very dry soil profile (which corresponds with the observed conditions). The AAI with the next largest divergence from 1 is found in sample "314", which is the second driest profile. The AAI observed in the top of the profile is most probably influenced by the east west direction of the irrigation furrows. An AAI of 1 for homogenously wetted non-cracked conditions would therefore not be expected. 30 Figure 7b shows the AAI for the entire protocol. In other words, Figure 7b includes the measurements for all electrodes on one level as well as the measurements with potential and current electrodes at different depths. The high divergence from an AAI of 1 at measurement point number 49 is due to a protocol programming error for that measurement. The occasional 35 outlier for sample 319 is most likely due to a measurement error as the protocol used in the example set-up was run in time-lapse mode without repeated measurements for quality control.

WO 2009/117784 PCT/AU2009/000380 - 11 Overall the dominant AAI for Figure 7b can be seen to lie above 1, which may be due to slight divergence from a perfect square in the borehole location. To quantify changes in layer homogeneity it might therefore be more significant to use the changes in AAI relative to the homogeneously wetted uncracked condition. Just as in Figure 7a, the AAI for sample 328 5 shows the largest divergence from 1. Toward the middle of the 100 resistivity measurements, the AAI increases to almost 1. As can be seen in the electrical resistivity plots of Figure 8, the area of increased AAI corresponds with resistivity measurements that are carried out in the lower half of the soil profile. The lower the electrodes that are involved in the measurement, the closer the AAI for sample 328 gets to 1. This increased in-homogeneity with depth is a 10 promising indicator that determining the degree of cracking with the AAI is possible. However the in-homogeneity could also be introduced by uneven drying patterns in the soil profile. Figure 8 shows that the largest in-homogeneity in soil moisture is found at a depth of 0.45 meters, while the largest homogeneity is found further up in the soil profile. This data 15 suggests that it is not differences in drying patters that rule the AAI, but rather the soil cracking. The high variation of recorded deep drainage within the sample field emphasises the high special variability of hydrological processes and the need for measuring methods that give spatial resolutions. The proposed system and apparatus offers such a method and the 20 experimental results from the growing experimental period show that the theoretical concepts of the proposed system were successfully applied in the field. (c) Bulk Resistivity Experimental Set-Up 25 The same probe set-up used to obtain the AAI for the experimental results (a) and (b), was also utilised to measure the bulk resistivity distribution in three dimensions. Measurements were carried out along the six planes between the four boreholes (as indicated in table 1 of Annexure A) and inverted with RES3DINV. The resistivity was measured at several times throughout the growing period and results were compared to deep drainage collected in a mini 30 lysimeter apparatus located adjacent to the electrode configuration 200. Graphs 314 and 319 of Figure 8 show the resultant electrical resistivity distribution before and after the second irrigation event. The fact that the electrical resistivity at a depth of 1.2 metres shows only small changes indicates that no or only minor occurrence of deep drainage occurred at this depth. The result matches the absence of recorded deep drainage water in the adjacent mini-lysimeter, 35 which records water moving past the depth of 1.2m. Aside from measuring soil moisture content and deep drainage, embodiments of the WO 2009/117784 PCT/AU2009/000380 - 12 present invention may also be utilised to: * Monitor water movement in soil/deep drainage (i.e. water movement past the root zone) * Monitor resistivity changes in the soil profile, which we can be related to moisture s changes. The calibration to quantify these moisture changes includes measurement of soil porosity, cation exchange capacity, soil salinity and cracking intensity of soils. * Monitor changes in cracking intensity over depth profile, by measuring the AAI of the soil at different depth. This information can then be used for the calibration of the soil moisture-bulk electrical resistivity relationship. It can also be used to determine risk of 10 deep drainage (cracks are pass ways for preferential water movement in soil and thus for deep drainage. The depth of a crack system, which can be measured with this method, is an indicator for the depth of preferential flow). * Monitor salt leaching. As the salt content of a soil influences its bulk electrical resistivity. Changes in bulk electrical resistivity can be used to track salt movement in 15 a soil profile. In general a farmer applies some excess irrigation water to remove salts that have accumulated in the soil profile. This instrument could be used to monitor the need for such an excess application. Any reference to prior art contained herein is not to be taken as an admission that the 20 information is common general knowledge, unless otherwise indicated. Finally, it is to be appreciated that various alterations or additions may be made to the parts previously described without departing from the spirit or ambit of the present invention.

WO 2009/117784 PCT/AU2009/000380 - 13 Annexure A Example probe polling technique for measuring the plane between probe 1 and 2: 5 Assuming four probes with 14 electrodes each (as seen in figure 1) the beginning of the longest electrode configuration is shown in table 1. Pobe rb 14 29 14 29 Schematic of 4 Probes with 14 electrodes each installed in one of the four boreholes 10 Table 1: current and potential electrode numbering for measuring the plane between probe 1 and probe 2 current 1 current 2 potential 1 potential 2 1 15 4 18 2 16 5 19 3 17 6 20 4 18 7 21 5 19 8 22 6 20 9 23 WO 2009/117784 PCT/AU2009/000380 - 14 7 21 10 24 8 22 11 25 9 23 12 26 10 24 13 27 11 25 14 28 1 15 5 19 2 16 6 20 3 17 7 21 4 18 8 22 5 19 9 23 6 20 10 24 7 21 11 25 8 22 12 26 9 23 13 27 10 24 14 28 1 15 6 20 2 16 7 21 3 17 8 22 4 18 9 23 5 19 10 24 6 20 11 25 7 21 12 26 8 22 13 27 9 23 14 28 1 15 7 21 2 16 8 22 3 17 9 23 4 18 10 24 5 19 11 25 6 20 12 26 7 21 13 27 8 22 14 28 1 15 8 22 2 16 9 23 3 17 10 24 4 18 11 25 5 19 12 26 WO 2009/117784 PCT/AU2009/000380 - 15 6 20 13 27 7 21 14 28 1 16 5 20 1 17 5 21 1 18 5 22 1 19 5 23 1 20 5 24 1 21 5 25 2 15 6 19 2 17 6 21 2 18 6 22 2 19 6 23 2 20 6 24 2 21 6 25 2 22 6 26 3 15 7 19 3 16 7 20 3 18 7 22 3 19 7 23 3 20 7 24 3 21 7 25 3 22 7 26 3 23 7 27 4 15 8 19 4 16 8 20 4 17 8 21 4 19 8 23 4 20 8 24 4 21 8 25 4 22 8 26 4 23 8 27 4 24 8 28 5 15 9 19 5 16 9 20 5 17 9 21 5 18 9 22 5 20 9 24 WO 2009/117784 PCT/AU2009/000380 - 16 5 21 9 25 5 22 9 26 5 23 9 27 5 24 9 28 6 15 10 19 6 16 10 20 6 17 10 21 6 18 10 22 6 19 10 23 6 21 10 25 6 22 10 26 6 23 10 27 6 24 10 28 7 15 11 19 7 16 11 20 7 17 11 21 7 18 11 22 7 19 11 23 7 20 11 24 7 22 11 26 7 23 11 27 7 24 11 28 8 16 12 20 8 17 12 21 8 18 12 22 8 19 12 23 8 20 12 24 8 21 12 25 8 23 12 27 8 24 12 28 9 17 13 21 9 18 13 22 9 19 13 23 9 20 13 24 9 21 13 25 9 22 13 26 9 24 13 28 WO 2009/117784 PCT/AU2009/000380 - 17 10 18 14 22 10 19 14 23 10 20 14 24 10 21 14 25 10 22 14 26 10 23 14 27

Claims (22)

1. A system for determining soil moisture content, the system comprising: two or more probes arranged to be located in boreholes spaced about a region of 5 interest, each probe of the two or more probes including a plurality of electrodes spaced at known intervals along the length of the probe; and a control unit operable to: apply a measured current between selected electrode pairs and to measure the resultant potential difference between other selected electrode pairs to take electrical resistivity 10 measurements for the region, each electrode of the electrode pairings being located on different ones of the two or more probes; and process the electrical resistivity measurements to determine the soil moisture content. 15

2. A system in accordance with claim 1, wherein the control unit determines the soil moisture content by calculating the apparent anisotropy index (AAI) of the soil from the resistivity measurements, the AAI being correlated against a data set to determine the soil moisture content. 20

3. A system in accordance with claim 2, wherein the data set comprises data associated with various soil characteristics for the soil profile that are related to soil moisture content.

4. A system in accordance with any one of the preceding claims, wherein the other electrode pairs are located adjacent to the electrode pairs. 25

5. A system in accordance with any one of the preceding claims, wherein the electrode pairs are arranged in a square configuration about the region of interest.

6. A system in accordance with any one of the preceding claims, wherein the control unit 30 is operable to determine a bulk electrical resistivity of the region by taking the same measurements over a defined period of time, the temporal nature of the electrical resistivity providing an indication of the change in soil moisture content.

7. A system in accordance with any one of the preceding claims, wherein the WO 2009/117784 PCT/AU2009/000380 - 19

8. A system in accordance with any one of the preceding claims, wherein the probes are located about a root structure of a plant and whereby the depth of the boreholes corresponds with the depth of the root structure. 5

9. A method for determining soil moisture content, the method comprising the steps of: providing two or more probes arranged to be located in boreholes spaced about a region of interest, each probe of the two or more probes including a plurality of electrodes spaced at known intervals along the length of the probe; taking electrical resistivity measurements for the region by applying a measured current 10 between selected electrode pairs and measuring the resultant potential difference between other selected electrode pairs, each electrode of the electrode pairings being located on different ones of the two or more probes; and processing the electrical resistivity measurements to determine the soil moisture content. 15

10. A method in accordance with claim 9, wherein the soil moisture content is determined by calculating the apparent anisotropy index (AAI) of the soil from the resistivity measurements, the AAI being correlated against a soil data set to determine the soil moisture content. 20

11. A method in accordance with claim 10, wherein the soil data set comprises data associated with various soil characteristics related to soil moisture content for the soil profile.

12. A method in accordance with any one of the preceding claims 9 to 11, wherein the other 25 electrode pairs are located adjacent to the selected electrode pairs.

13. A method in accordance with any one of the preceding claims 9 to 12, wherein the measurements are taken at various depths to generate a three-dimensional resistivity profile.

14. A method in accordance with any one of the preceding claims 9 to 13, wherein the 30 electrode pairs are arranged in a square configuration about the region of interest.

15. A method in accordance with any one of the preceding claims 9 to 14, wherein a bulk electrical resistivity measurement of the region is obtained by taking the same measurements over a defined period of time, the temporal nature of the electrical resistivity measurements 35 providing an indication of the change in soil moisture content.

16. A method in accordance with any one of the preceding claims 9 to 15, wherein the WO 2009/117784 PCT/AU2009/000380 - 20 probes are located about a root structure of a plant and whereby the depth of the boreholes corresponds with the depth of the root structure.

17. A method for determining soil moisture content and/or deep drainage of a soil profile, 5 the method comprising the steps of: measuring cracking intensity of the soil profile at various depths; and comparing the cracking intensity measurements against a data set comprising known soil characteristics for an associated soil profile to determine the soil moisture content and/or deep drainage. 10

18. A method in accordance with claim 17, wherein the cracking intensity of the soil profile is determined by calculating the apparent anisotropy index (AAI) of the soil profile.

19. A method in accordance with claim 18, wherein the AAI is calculated utilising the is system in accordance with any one of claims 1 to 8.

20. Computer program code which, when executed by a processor, implements the method of any one of claims 9 to 19. 20

21. A computer readable medium comprising the program code of claim 20.

22. Data signal comprising the computer program code of claim 21.