EP1360526A1

EP1360526A1 - Method for processing georeferenced electrical resistivity measurements for the real-time electrical mapping of soil

Info

Publication number: EP1360526A1
Application number: EP02701409A
Authority: EP
Inventors: Michel Dabas; Jeanne Tabbagh; Sébastien FLAGEUL
Original assignee: Universite Pierre et Marie Curie Paris 6
Current assignee: Geocarta
Priority date: 2001-02-07
Filing date: 2002-02-06
Publication date: 2003-11-12
Also published as: CA2437625A1; FR2820509B1; FR2820509A1; US20040158403A1; WO2002063344A1

Abstract

The invention relates to a method for the georeferenced measuring of electrical resistivity for the electrical mapping of soil. The area of soil to be mapped is divided up into a fine network of points. According to the invention, measuring means are used to obtain n parameters (n being at least equal to 3) characterising the electrical resistivity at a given point at n different depths. In addition, an absolute position measurement and k relative displacement measurements are obtained using positioning means for a number k of points measured. The three measurements obtained are subsequently sent to a microcontroller which synchronises the acquisitions. The data sent by the microcontroller are digitally processed in real time on a computer. Furthermore, two different display windows are used to view, simultaneously and in real time, a profile showing the sequential variations for a given depth in the resistivity of the soil across the area studied and a map showing the position of the measuring means. The invention is suitable for use in precision farming and for prospecting archaeological sites.

Description

Method for processing georeferenced electrical resistivity measurements for electrical mapping of soil in real time

In recent years, a new approach to agriculture has emerged, Precision Agriculture. This approach is based on an adequacy of agricultural treatments to the local conditions of the environment.

This approach responds not only to a search for maximizing the yields of cultivated soils and reducing costs, but also to a more demanding respect for the environment and therefore to a more parsimonious use of the doses of inputs used (seeds, fertilizers, products phytosanitary).

In order to optimize the yield of cultivated soils, it is important to know both the texture of a given soil or the size of the constituent particles of this soil and the depth of cultivable surface soil. Thus, a high clay content, a high salinity can affect the yield of a plot. It is therefore beneficial to carry out a recognition of the soils of an agricultural plot and to establish homogeneous zones. A specific and appropriate treatment can then be established for a given area in order to maximize its yield.

One method then consists of performing direct measurements, i.e. auger drilling and digging pits. In addition to the punctual aspect of such measures, they have the disadvantage of being destructive, costly and of modifying the structure of the zone studied after sampling (irreversible effect). This type of measurement does not allow the establishment of a reliable and sufficiently detailed mapping of the homogeneous areas of an agricultural plot for Precision Agriculture.

Another method is to use the data provided by airborne or satellite means. However, these data correspond to minimum tracts of land much larger than the current dimensions of agricultural plots in Europe. They are therefore not usable for Precision Agriculture which requires an accuracy of the spatial knowledge of soils of the order of ten meters.

A method is also known for defining homogeneous zones by a system for measuring the electrical resistivity of soils described in particular in a communication by Dabas et al [Société des Electriciens et des Electroniciens - Le 05 Février 1987], in an article by Panissod et al [Geophysical Prospecting; 45 (1997) 983] and in US Pat. No. 5,841,282. This direct physical measurement is correlated to the properties and structure of the soils measured (porosity, water resources, clay content, etc.) and therefore makes it possible to define the homogeneous zones of a plot. These measurements are made continuously by injecting a current into the soil and by measuring the resulting potential using other electrodes in contact with the soil to be characterized. These measurements are absolutely georeferenced (GPS). This measurement system suffers, however, from a double disadvantage. The measurements recorded by said system are not processed and therefore viewable in real time by an on-board computer. The measurements are recorded and then processed at a time later than the measurement of the agricultural plot. It is therefore not possible, for example, to couple this measurement system to spreading means and to adapt in real time the dose of inputs required for a specific area to be treated.

The objective of the present invention is therefore to propose a method for processing georeferenced electrical resistivity measurements for electrical mapping of soils in real time. This simple method in its design and in its implementation should allow, in addition to a technical advance, a significant reduction in the costs linked to soil mapping. This should result in a much more extensive practice of Precision Agriculture in the agricultural world with its inherent benefits for the environment.

To this end, the invention relates to a method for processing georeferenced electrical resistivity measurements for electrical mapping of soils in which:

- the area of a soil to be mapped is divided into a network of points defined by the repetition of the same elementary mesh,

- measurement means are moved in the area to be mapped, - at least one measurement of electrical resistivity is acquired at each point during the movement of the measurement means,

- positioning means are used to geographically and absolutely reference the measurement associated with each point, - the recorded data are digitally processed,

- a map is displayed showing the variations for a given depth of the electrical resistivity of the soil.

According to the invention, - the electrical resistivity is recorded for each point at n different depths, n being at least equal to 3,

- it also acquires by said positioning means and for each point measured a measurement of relative displacement with respect to the previous point, - the three sets of measurements obtained are sent to a microcontroller which synchronizes the acquisitions,

In different particular embodiments each having its particular advantages and capable of numerous possible technical combinations: • the data sent by the microcontroller to a computer are processed digitally and in real time,

• simultaneously viewed in real time on two different display windows, a profile showing the sequential variations for a given depth of the resistivity of the ground across the area studied and a map showing the positioning of the measurement means,

Before the said measurements are collected, the scale of the map of the second display window is defined, by performing a locating tour of the area to be mapped, which is recorded on the computer by a particular programmed procedure,

A guidance system is used to control the displacement of the means for measuring the relative displacements, of electrical resistivity and of absolute positioning between the points,

• the measurement of the relative displacements is obtained by a doppler radar,

• the measurement of the relative displacements is obtained by an incremental encoder,

The measurement of the relative displacements is obtained by a system capable of delivering TTL pulses as a function of the displacement of the measuring means, • the sets of resistivity measurements and relative positioning measurements between two absolute coordinates are statistically processed at the level of the computer in order to eliminate the erroneous resistivity values and to refine the positioning measurements, “the resistivity is measured at constant current.

The invention will be described in more detail with reference to the accompanying drawings in which:

- Figure 1 is a schematic representation of the successive steps a), b), c), d) and e) leading to the display of a map of resistivity and positioning measurements and its storage, according to the invention;

- Figure 2 shows schematically the measuring means according to the invention;

- Figure 3 is a map. presenting the path of the measurement means on a particular plot of soil;

- Figure 4 is an assembly of electrical resistivity maps obtained for a set of soil plots comprising the plot, object of Figure 3;

FIG. 5 shows a typical example of display windows in real time: a profile showing the sequential variations for a given depth of the resistivity of the ground across the area studied and a map showing the positioning of the measurement means.

The first step of the method represented in FIG. 1, consists in acquiring a set of measurements at given points of a plot of soils to be mapped. These points are defined by the repetition of the same elementary mesh thus cutting the area of this plot into a network of points. Said network of points is therefore defined as a regular arrangement of points in the plane of the surface of the plot of soils. Each point being connected to another in a direction given by the length of the elementary mesh and in a direction perpendicular thereto, by the width of said elementary mesh. The dimensions of the elementary mesh in the plane of the surface are typically 0.1 m by 8 m. However, the length of this mesh, or no sampling, can be reduced to a few centimeters in the direction of movement of the measuring means. The network of points being defined, measurement means 1 are moved in the area to be mapped. There is then acquired continuously during the displacement of the measurement means, n electrical resistivity measurements 2 at each point. By resistivity measurement 2 is meant either a galvanic resistivity measurement or an electrostatic resistivity measurement. The current measuring means include a towed alternating current resistivity meter made up of k articulated 3-6 axles. A quad 7 can, for example, be used to tow the measuring means 1. The term "quad" 7 means a motorcycle with four wheels. One of the axles 3 allows the injection of a preferentially regulated current, that is to say of constant intensity, emitted by a source 8 in the soil while the other axles 4-6 measure the resulting potentials by wheel electrodes. The respective dimensions and location of said axles, and therefore the very structure of the measuring means, ensure the measurement of the resistivity for a given point in n different depths. The value of n is greater than 3. The value of the current injected into the soil varies according to the nature of the soils studied but is between 0.1 and 20 mA. ^: .

The electrical resistivity measurements 2 are georeferenced. Each resistivity measurement is therefore associated with a pair of coordinates making it possible to locate said measurement geographically in the plane of the surface of the plot of soil to be mapped. These resistivity measurements are in fact triggered by a measurement of the relative position of the measuring means at said point. This relative position measurement can be carried out by a doppler radar, an incremental encoder or any system 9 capable of delivering pulses, preferably TTL, as a function of the movement of the vehicle. The triggering of the resistivity measurements by a positioning measurement implies that the resistivity measurements are carried out as a function of the distance traveled and not on a fixed time basis. As a result, whatever the speed of movement of the measurement means in the area to be mapped, the measured points are regularly spaced. The density of measured points is therefore homogeneous.

The relative position measurement is further coupled to an absolute position measurement. The absolute system is a GPS 10, differential or not. The implementation of an absolute differential positioning system (dGPS) advantageously allows any movement of the measurement means 1 in the area of soils to be mapped. Current absolute positioning systems 10 allow the acquisition of measurements every second or so. Depending on the speed of movement of the measuring means 1, the relative positioning system 9 provides more measurements than the absolute positioning system 10. In an embodiment presented in FIG. 3, a number of ten relative measurements varies according to speed generally between 1 and 30 is obtained between the acquisition of two absolute measurements. The acquisition of electrical resistivity measurements 2 being triggered by a measurement of the relative position, the number of resistivity measurements is thus greater.

The three basic inputs of the system, obtained synchronously, therefore correspond to the acquisitions of the voltages on the n potential channels, the acquisitions of relative positioning measurements and finally, the acquisitions of absolute positioning measurements.

These inputs are processed by a microcontroller 11 (step 2, Fig. 1 b)) synchronously. In a particular embodiment, the microcontroller 11 receives at its input the electrical signal sent by the relative positioning system 9 and produces an output signal. This output signal is sent to the microcontroller 11. This signal triggers said measurements. The signals from the measurements arrive at the input of the microcontroller and are acquired synchronously. The microcontroller 11 then sends data at its output which are representative of the signals received at its input. This data is finally sent in real time, by the microcontroller 11 to a computer 12 (step 3, Fig. 1 c)). They are then processed digitally by software. We also perform a statistical treatment of the different sets of measurements between two absolute coordinates. The oversampling of the resistivity and relative positioning measurements compared to the absolute positioning measurements allows this processing. False resistivity values resulting, for example, from the loss of contact of one of the electrodes with the ground are thus eliminated. The positioning measurements are also refined. In one embodiment preferred, the median algorithm is implemented for its speed of execution and for the fineness of control of the threshold beyond which the data are rejected.

The software allows to visualize (step 4, Fig. 1 d)) simultaneously and in real time on two different display windows (Fig. 4), a first sequence showing the variations for a given depth of the resistivity of the ground along of the studied area and a second window showing the positioning of the measurement points. The direct control of these measurements by visualization makes it possible to assess the validity of the measurements. A special procedure has been programmed to determine the scale of the plot and therefore to be able to fix the dimensions of the window visually representing the location of the measurement means (second window). This particular procedure requires the completion of a tracking tour prior to the acquisition of any measurement. This turn consists of a continuous displacement of the measuring means 1. The tracking turn is also an opportunity to assess the area of variation of the resistivity. In a preferred embodiment, the positioning window makes it possible to view the positions in the French Lambert system after conversion of the absolute positioning measurements (satellite coordinates).

The first graphics make it possible to directly view the resistivity measurements as a function of the displacement because the calibration curves of the resistivity meter have been integrated in order to be able to pass from the potentials measured for a regulated current given to the resistances and resistivities.

The sets of measurements and profiles are then stored on the computer 12 (step 5, Fig. 1 e)).

The continuous acquisition during the displacement of the measurement means, of n electrical resistivity measurements at each point of a plot of soils requires the implementation of devices and of a measurement chain whose response time is compatible with the speed of movement of said measuring means. This same speed is limited by the nature of the ground, the distance to be traveled between two measurements (the length of the elementary mesh), and by the response time of external devices, for example, spreading means, possibly coupled to said measuring means. The processing in real time of the data collected, required by these external devices, is fast enough not to limit the speed of the entire device. The computer 12 directly controls these external devices (step 5, Fig. 1 e)). In one embodiment, spreading means are coupled to the georeferenced measurement means. The information on the nature of the soil processed by the computer 12 makes it possible to adapt, in real time, the dose of inputs required for a specific area to be treated.

The parallelism of the measurements during the continuous movement is ensured by means of orientation, for example, a guidance system. This guidance system linked to absolute differential positioning measurements (dGPS) guarantees the acquisition of a homogeneous density of measurements over the entire plot of soils to be mapped.

Figures 3 and 4 are an example of maps obtained during the study of an agricultural operation in Champagne Berrichonne du Cher, south of Bourges. This involved studying four plots 21, 22, 23, 24 with a total area of 120 hectares. The dimensions of the elementary mesh in the plane of the surface to be studied are 1 m by 12 m. A reinterpolation with a mesh of 6m by 6m was carried out during the digital processing of the data. The regulated current used was 20 mA due to the conductive nature of the terrain. The average speed of data acquisition was of the order of 1.2 to 1.5 m / s. FIG. 3 represents the displacement of the measurement means on one of the plots 21 called "Les Bois Forts". The perimeter 25 of the plot defines the external limits of the area to be mapped. The starting point 26 of the measurement means is identified by its coordinates in the Lambert system 27 and 28. The lines 29 represent either a forward or return movement of the measurement means on the plot. FIG. 4 represents the resistivity signal measured as a function of the displacement for an integrated depth of 0.5 m. The results obtained for the four plots 21, 22, 23, 24 were assembled. The cumulative acquisition time to obtain Figure 3 is 17 hours. This card corresponds to 305,000 measurements.

Figure 5 shows a typical example of display windows as they can appear in real time to the user during acquisition. of measures. The graph 30 shows the variations of the electrical resistivity of the soil, 31, for a given depth as a function of the relative displacement of the measuring means, 32. The graphs 30, 33 and 34 correspond to resistivity measurements at different depths of the soil , which are generally between 0 and 2m. Graph 35 shows the absolute positioning in real time of the measurement means as described in FIG. 3.

This method can advantageously be used in Precision Agriculture (A.P.). Indeed, associated with spreading and sowing means, this method should make it possible to adapt in real time the dose of inputs required for a specific area to be treated. This results in significant time savings and lower costs. It should also guarantee better respect for the environment. This method can also be advantageously used in the context of prospecting archaeological sites.

Claims

1. Method for processing georeferenced electrical resistivity measurements for electrical soil mapping in which:

- measurement means are moved in the area to be mapped,

- at least one measurement of electrical resistivity is acquired continuously during the movement of the measurement means, at each point, - positioning means are used to geographically and absolutely reference the measurement associated with each point,

- the recorded data are digitally processed,

- a map is displayed showing the variations for a given depth of the electrical resistivity of the soil. characterized in that:

- the electrical resistivity for each point is recorded in n different depths, n being at least equal to 3,

by means of said positioning means and for each point measured, a measurement of relative displacement relative to the previous point is also acquired,

- the three sets of measurements obtained are sent to a microcontroller which synchronizes the acquisitions.

2. Method for processing electrical resistivity measurements according to claim 1, characterized in that the data sent by the microcontroller to a computer is processed digitally and in real time.

3. Method for processing electrical resistivity measurements according to one of claims 1 and 2, characterized in that a profile showing the sequential variations for a given depth is displayed simultaneously and in real time on two different display windows. the resistivity of the soil across the study area and a map showing the positioning of the measurement means.

4. Method of processing from electrical resistivity measurements according to any one of claims 1 to 3, characterized in that one defines before collecting said measurements, the scale of the map of the second display window, by locating the area to be mapped, which is recorded on the computer by a specific programmed procedure.

5. Method for processing electrical resistivity measurements according to any one of claims 1 to 4, characterized in that a guidance system is used to control the displacement of the means for measuring relative displacements, of electrical resistivity and of positioning absolute between points,

6. Method for processing electrical resistivity measurements according to any one of claims 1 to 5, characterized in that the measurement of the relative displacements is obtained by a doppler radar,

7. Method for processing electrical resistivity measurements according to any one of claims 1 to 5, characterized in that the measurement of the relative displacements is obtained by an incremental encoder,

8. Method for processing electrical resistivity measurements according to any one of claims 1 to 5, characterized in that the measurement of the relative displacements is obtained by a system capable of delivering TTL pulses as a function of the displacement of the measurement means ,

9. Method for processing electrical resistivity measurements according to any one of claims 1 to 8, characterized in that the sets of resistivity measurements and relative positioning measurements between two coordinates are statistically processed. absolute to eliminate erroneous resistivity values and refine positioning measurements.

10. Method for processing electrical resistivity measurements according to any one of claims 1 to 9, characterized in that the resistivity is measured at constant current.