System for measuring the thickness of a first surface layer of a multi-layer structure and its non-destructive measuring method

Abstract

System for measuring the thickness of a first layer on the surface of a multi-layer structure and its non-destructive measuring method One aspect of the invention relates to a system for measuring the thickness of a first layer (10) comprising a spectral inversion radar and a capacitive probe (C2), an acquisition and control unit (UAC) for determining signals, an inverse processing unit (UTI) for evaluating a real dielectric permittivity ε'rhf and imaginary ε''rhf of the medium, and a calculation unit configured to calculate a real dielectric permittivity ε'rbf, determine whether the medium is dispersive or non-dispersive and calculate the value of the real dielectric permittivity ε''rhf evaluated by the inverse processing unit if non-dispersive,then calculate a propagation time and a propagation speed of electromagnetic waves in the first layer from the value of the real dielectric permittivity ε'rbf or ε'rhf and a thickness from the propagation time and the calculated propagation speed. Figure to be published with the abstract: Figure 2,

Description

Title of the invention: System for measuring the thickness of a first layer on the surface of a multi-layer structure and its non-destructive measuring method TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of a system for measuring the thickness of a first layer on the surface of a multi-layer structure of civil engineering (roadways, structures and other public works) or of buildings or infrastructures in the railway sector and its non-destructive measurement method.

[0002] The present invention therefore relates to a system and a method for measuring the thickness of a first surface layer of a multi-layer structure, thus making it possible to obtain a non-destructive diagnosis and evaluation of a surface layer thickness of a multi-layer structure. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] It is known in the fields of Civil Engineering, building or roadway or even railway for the construction of a multi-layer structure to use an external coating layer on an internal layer of the multi-layer structure. For example, the multi-layer structure is a wall or a floor or even a roadway. The multi-layer structure comprises this coating layer and one or more internal layers forming a support for the multi-layer structure, coated by the external coating layer.

[0004] There are thicknesses to be respected in standards and / or specifications of the coating layer to be placed on the internal layer of the support of the multi-layer structure.

[0005] This thickness of coating layer to be respected, perhaps to limit physical, thermal and water transfers etc. between the internal environment and the external environment and vice versa which may undergo changes of state.

[0006] For example, a roadway consists of several layers implemented on a terraced ground called the supporting ground. The roadway comprises on the supporting ground a base base comprising different layers, in particular foundation and base, and a surface base generally comprising a binding layer and a wearing course. Compliance with standards in terms of the thickness of these layers, for example the wearing course, is important to ensure the durability of the road infrastructure, its safety and that of its users.

[0007] In the example of a wall, it is the same, the wall can comprise a base, comprising for example an internal support layer for example concrete or bricks etc... and generally an external coating layer, a coating which may include a binder and aggregates to limit, for example, water infiltration into the support layer, but also for aesthetic reasons and to ensure the coating layer lasts over time.

[0008] To ensure the safety and / or quality of the structure, it is therefore necessary to be able to control (measure) that a layer has a thickness within a value range defined by the standard or specifications. A known solution is coring, however this solution has the disadvantage of being destructive which requires refilling which can lead to an irregularity of the layer thus reducing one of the mechanical properties of the structure and is not very aesthetic because a refilling mark is often visible to the naked eye. In addition, coring only allows control at a specific point, in fact a few centimeters from the core, the layer can be outside the value range.

[0009] Several methods for measuring coating layer thicknesses non-destructively are known, the first is the measurement of the geometry of a part when possible, before and after fixing the coating layer on another prior layer.

[0010] However, this requires taking a measurement between the two layers upstream, which is inoperable on existing infrastructures. The second method is to use known electromagnetic processes, in particular: • the measurement of the magnetic permeability of the pre-layer by a probe placed on the coating layer and is therefore distant from the pre-layer by an “air gap” distance (also known in the field as “lift-off”) corresponding to the thickness of the coating layer; however, these measurements cannot be used when the pre-layer has a very low magnetic susceptibility, because the magnetic field used in the magnetic permeability measurement is not disturbed by the pre-layer and / or the coating layer is magnetic. It is therefore not possible to detect a variation in the magnetic field as a function of the distance of the probe from the pre-layer and it is therefore not possible to deduce the thickness of the coating layer; • measurement using eddy currents generated in the pre-layer when the coating layer is electrically insulating and the pre-layer is electrically conductive. • measurement by electromagnetic wave radar, but the accuracy of these measurements is too dependent on conditioning linked to the environment, such as humidity, particularly when the structure is outside.

[0011] However, these measures do not work with just any structural layers. Such methods therefore do not work with most civil engineering and building structures, since the coating layer, for example a coating, is made with water and can also be more or less porous and therefore absorb more or less water (through rain or through absorption of water in humid air), which modifies its electrical conductivity.

[0012] There is therefore a need for precise measurement of thin thicknesses (especially below 3 cm) of coating layers subject to humidity variations during manufacture and / or over time of use, in a non-destructive manner in the field of civil engineering, in particular for structures or roadways, or for construction (BTP) or infrastructure in the railway sector. Summary of the invention

[0013] The invention provides a solution to the problems mentioned above, by making it possible to determine a layer thickness on the surface of a multi-layer structure by two non-destructive monitoring techniques, the first also making it possible to adjust the second monitoring technique.

[0014] One aspect of the invention relates to a system for measuring the thickness of a first layer of a medium comprising at least two layers of two different materials, comprising: • a measuring device comprising: • a spectral inversion radar having: • an electromagnetic radiation antenna device in ultra-wide frequency band transmission / reception • an electromagnetic wave generator connected to the antenna device to emit electromagnetic waves at different high frequencies between 100 MHz and 100 GHz, • a capacitive probe comprising: • at least one pad comprising two electrodes, • an alternating current generator connected to the skate to emit an electric current from a resonant RLC circuit at a low frequency between 30 and 40 MHz, • an acquisition and control unit connected to the measuring device configured for: • controlling the electromagnetic wave generator to emit and propagate electromagnetic waves in the medium, via the antenna device, and processing the reception of the signals measured via the antenna device to determine signals in the time domain comprising at least a first surface time echo and a second interface time echo between the two layers, • control the alternating current generator of the capacitive probe to emit and propagate an alternating current in the medium and record a resonant frequency of the electrical circuit, an inverse processing unit connected to the measuring device for carrying out inverse processing of part of the signals received by the spectral inversion radar measurements, to evaluate the real dielectric permittivity e'rhf and imaginary dielectric permittivity e"rhf of the medium, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium, as well as extrapolating values ​​of the real low-frequency dielectric permittivity e'bf and imaginary dielectric permittivity e"bf determined from the real high-frequency dielectric permittivity e'rhf and imaginary dielectric permittivity e"rhf and from a model pre-recorded in the memory, a computing unit configured to: • calculate a real dielectric permittivity e'rbf from the recorded resonance frequency of the electrical circuit • calculate a quality factor Q between extrapolated values ​​of the determined real low-frequency dielectric permittivity e'bf and imaginary e"bf, • determine a related study category by comparing the quality factor Q to a threshold value A, if the quality factor Q is below the threshold value A, the medium is considered as the first non-dispersive category and if the quality factor Q is above the threshold value A, the medium is considered as the second dispersive medium category, • receive a selection of a study category from a first non-dispersive category or as a second dispersive medium category, • when the study category is determined or selected as the first non-dispersive category, • receive and record at different locations of the first layer, at each location the first surface time echo and the second interface time echo deduced by measurements by the spectral inversion radar, • calculate and save at each different location: • a propagation time corresponding to a time difference between the first and second echoes received, • the real low-frequency dielectric permittivity e'rbf from the recorded resonance frequency of the electrical circuit, • a propagation speed of electromagnetic waves in the first layer from the value of the calculated real dielectric permittivity e'rbf and • a thickness from the propagation time and the calculated propagation speed, • when the study category is determined or selected as the second dispersive category: • receive and record at different locations of the first layer, at each location: • the first surface time echo and the second interface time echo deduced by measurements using spectral inversion radar, • the value of the real dielectric permittivity e”rhf evaluated by the inverse processing unit, • calculate and save at each different location: • a propagation time corresponding to a time difference between the first and second echoes received, • a propagation speed of high-frequency electromagnetic waves Va emitted by the spectral inversion radar in the first layer from the value of the actual dielectric permittivity e'rhf measured and • a thickness from the propagation time and the propagation speed of electromagnetic waves Va calculated.

[0015] Thanks to the invention, it is possible to determine a thickness of a surface layer on another layer in a non-destructive manner that is more reliable and more precise than with conventional tools such as geo-radar used in direct and qualitative approach. Indeed, the system makes it possible to select according to a choice of the user or determine by comparing the calculated quality factor to a threshold value, between a first and second category of study of the layer, that is to say either respectively non-dispersive or dispersive.

[0016] The alternating electrical signal oscillating according to the low frequency generated in the medium by the capacitive probe makes it possible to induce in a portion of the first layer between the two electrodes, a planar capacitor dependent on this electrical supply signal, thus allowing the probe to measure a resonance frequency of the RLC circuit so that the calculation unit can calculate from the resonance frequency received from the acquisition and control unit, the real dielectric permittivity e'rbf of the layer.

[0017] Thus, if the medium is categorized (by selection or determination) as non-dispersive, the system does not use the inverse processing unit, which allows much faster and therefore less energy-consuming data processing. As mentioned, the thickness is calculated from a propagation time obtained by a time difference between the first surface time echo and the second interface time echo deduced by measurements by the spectral inversion radar and from a calculated propagation speed.

[0018] The calculated propagation speed is according to the dielectric permittivity of the first layer which is calculated using a measurement of the resonance frequency measured by the capacitive probe. Thus, the calculation of the dielectric permittivity makes it possible to specify the calculation of the propagation speed of the spectral inversion radar wave in order to calculate its thickness (compared to the prior art).

[0019] However, if the medium is categorized (by selection or determination) as dispersive, the permittivity value is greatly increased at low frequency (due to the presence of free charges due to a high volumetric water content for example), the evaluation of this parameter using the capacitive probe (low frequency device) is therefore not relevant, but remains possible at high frequency.

[0020] Thus, in this case of a medium categorized as second category (by selection or determination) as dispersive, the inverse processing unit system connected to the measuring device carries out (which is more energy-consuming and costly in terms of calculation time compared to the case of first category (non-dispersive) an inverse processing of a part of the signals received by the measurements of the spectral inversion radar, to evaluate the variation in frequency of the real dielectric permittivity e'rhf and imaginary e"rhf of the medium, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium.

[0021] The steps of calculation, estimation, recording, acquisition etc.... are carried out using a processor or several processors of a single or different devices. The predetermined values, functions or tables etc.... are in stored in an internal memory. The calculated, deduced, evaluated, determined values ​​are stored in a RAM or / and internal memory.

[0022] In addition to the characteristics which have just been mentioned in the preceding paragraph, the measuring system according to one aspect of the invention may have one or more additional characteristics among the following, considered individually or according to all technically possible combinations: • According to one embodiment, the radar is frequency hopping. • According to a variant of the previous embodiment, the radar is pulsed. • According to one embodiment, the wide frequency band extrapolation model is a Jonscher model pre-recorded in the memory, and in that the extrapolated values ​​of the real low frequency dielectric permittivity e'bf and imaginary e"bf are determined by extrapolation from the 4p variant of the Jonscher model and the values ​​of the real and imaginary dielectric permittivity, e'rhf and e"rhf, evaluated by the processing unit in a frequency range between 1.5-2 GHz. • According to an example of this embodiment, when the study category is determined or selected as the second dispersive category, the calculation unit is configured to: • receive in at least one of the different locations a value of the real dielectric permittivity e'rhf evaluated in the frequency range [1.5 - 2 GHz] by the inverse processing unit from measurements from the spectral inversion radar, • evaluate at at least one of the different locations, respectively, the values ​​of the real and imaginary dielectric permittivity, e'bf and e"bf in a low frequency range (between 30 and 40 MHz) by extrapolation from the 4p variant of the Jonscher model from at least the value of the real dielectric permittivity e'rhf, evaluated by the processing unit, • compare at least one of the different locations, the value of the real dielectric permittivity at the second frequency e'bf, evaluated by extrapolation and the value of the real low frequency dielectric permittivity e'rbf calculated, • calculate the low-frequency quality factor Q from the values ​​of the real and imaginary dielectric permittivity e"bf and e'bf evaluated by extrapolation to validate the selection of the study category or modify the selection of the study category. • According to a particularity of this example, when the study category is terminated as second dispersive category, the computing unit is configured to further receive in at least one of the different locations a value of the imaginary dielectric permittivity e"rhf. According to one embodiment, the signal measured via the antenna device is in the frequency domain and the acquisition and control unit applies an inverse Fourier transform of the measured signals to determine the signals in the time domain. According to a variant of this embodiment, the signal measured via the antenna device is in the time domain. According to one embodiment, when the category is determined as first category (non-dispersive), the system is configured to calculate a corrected thickness from a predetermined value of a witness by calculating: • a simulated propagation time Atb equal to a predetermined coefficient K multiplied by the square root of the value of the calculated low-frequency dielectric permittivity e'rbf, • a propagation time difference value Atc between the calculated propagation time and the simulated propagation time Atb, • a difference in thickness to be corrected AEc equal to the value of the difference in propagation time Atc multiplied by the calculated speed of propagation of high-frequency electromagnetic waves Va, • a difference thickness ED to be corrected equal to the difference thickness to be corrected AEc / 2, • a corrected thickness Ec of the first layer by adding a predetermined thickness value ET to the calculated difference thickness ED. This embodiment makes it possible to correct and therefore improve the accuracy of the thickness calculation. Indeed, when using the radar, a measurement error due to the dielectric nature of the medium being monitored is produced by the emission of electromagnetic waves by the radar transmitting antenna which also radiates towards the receiving antenna of the UWB antenna causing a time shift. The predetermined coefficient K (calculated in the laboratory on the standard) is equal to a propagation time Ata' (round trip) calculated on a standard of the same material as that of the first layer, divided by the square root of the real dielectric permittivity of the witness eiT, i.e. .A11L. The predetermined value ^eïT of the thickness ET is the thickness value of the same witness (measured). According to an example of this embodiment, the system comprises a plurality of coefficient K values ​​associated with a thickness ET value and a material recorded in a memory, for example in table form, and in that the system is configured to display an input request listing the different materials stored and in that the K value and the thickness ET are according to the corresponding selected material. • According to one embodiment, the system comprises a plurality of pads of different electrode sizes and in that the calculation unit is configured to check the measurement consistency per pair of pads by calculating deviations between actual dielectric permittivity values ​​e'rbf calculated with different pairs of pads and comparing each calculated deviation with a predetermined deviation value and thus detecting an inconsistency if one of the deviations is beyond the predetermined deviation value. • According to an example of this embodiment, the calculation unit transmits to a human-machine interface of the system data to display a size of a recommended pair of pads after the consistency check, the size of the recommended pair of pads being the pair of pads having the largest size among the pairs of pads having allowed the deduction of an actual dielectric permittivity e'rbf while having a deviation less than a predetermined value. • According to one embodiment, the system comprises a human-machine interface for displaying the results of calculation values ​​including that of the thickness E and / or the corrected thickness EC (according to the embodiment described previously).

[0023] Another aspect of the invention relates to a method of measuring by a system according to one of the preceding claims a thickness of a first layer of a first material in a medium further comprising a second layer of a second material distinct from the first material, the second layer being coated by the first layer, comprising: • a step of determining or selecting the category of the medium from a first non-dispersive category and a second dispersive category, in which if the first step is a determination, the first step includes the sub-steps of • controls the electromagnetic wave generator to emit and propagate electromagnetic waves in the medium, via the antenna device • measuring the signals generated by the propagation of electromagnetic waves through the antenna device and processing them to determine signals in the time domain including at least minus a first surface time echo and a second interface time echo between the two layers, • evaluation of the real dielectric permittivity e'rhf and imaginary e"rhf of the medium by carrying out an inverse processing of a part of the processed signals, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium, • extrapolation of values ​​of the real low-frequency dielectric permittivity e'bf and imaginary e”bf from the real high-frequency dielectric permittivity e'rhf and imaginary e"rhf and from a 4p variant of a Jonscher model pre-recorded in the memory, • calculation of a quality factor between the extrapolated values ​​of the real dielectric permittivity e'bf and imaginary e"bf • determination of the study category by comparing the quality factor to the threshold value when the study category is determined or selected as the first non-dispersive category, a step of calculating and recording a thickness of a first layer of a non-dispersive medium at different locations and for each different location comprising the sub-steps: • control of the alternating current generator of the capacitive probe to emit and propagate an alternating current in the medium and record a resonance frequency of the electrical circuit, • calculation of a real dielectric permittivity e'rbf from the measured resonance frequency of the electrical circuit, • controlling the electromagnetic wave generator to emit and propagate electromagnetic waves in the medium, via the antenna device, • measuring the signals generated by the propagation of electromagnetic waves via the antenna device and processing them to determine signals in the time domain comprising at least a first surface time echo and a second interface time echo between the two layers, • calculation and recording: • a propagation time corresponding to a time difference between the first and second echoes received, • a propagation speed Va of electromagnetic waves in the first layer from the value of the per- actual dielectric mittivity e'rbf calculated and • the thickness of the first layer from the propagation time and the calculated propagation speed Va, when the study category is determined or selected as the second dispersive category, a step of calculating and recording a thickness of a first layer of a dispersive medium at different locations and for each different location comprising the sub-steps: • controlling the electromagnetic wave generator to emit and propagate electromagnetic waves in the medium, via the antenna device, • to measure signals generated by the propagation of electromagnetic waves via the antenna device and process them to determine signals in the time domain comprising at least a first surface time echo and a second interface time echo between the two layers • evaluation of the real high-frequency dielectric permittivity e'rhf of the medium by carrying out inverse processing of part of the processed signals, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium, • calculation and recording: • a propagation time corresponding to a time difference between the first and second echoes of the processed signal, • a propagation speed of high-frequency electromagnetic waves in the first layer from the value of the real high-frequency dielectric permittivity e'rhf obtained by the inverse processing unit UTI and • the thickness of the first layer from the propagation time and propagation speed of electromagnetic waves emitted by the spectral inversion radar. According to one embodiment, the sub-step of evaluating the real dielectric permittivity e'rhf of the medium by the inverse processing unit is evaluated over an entire frequency range, in particular between 1.5 and 2 GHz, by controlling the spectral inversion radar in this frequency range, and in which the step of calculating and recording a thickness of the first layer when the study category is selected as the second dispersive category, further comprises the following sub-steps if the dielectric permittivity varies beyond a predetermined percentage: • Evaluation at at least one of the different locations, respectively, of the values ​​of the real dielectric permittivity e'rhf and imaginary, e'bf and e"bf in the low frequency range (between 30 and 40 MHz) by extrapolation from the 4p variant of the Jonscher model from at least the value of the real dielectric permittivity e'rhf, evaluated by the processing unit, • Controlling the alternating current generator of the capacitive probe to emit and propagate an alternating current in the medium and record a resonant frequency of the electrical circuit, calculating an actual dielectric permittivity e'rbf from the measured resonant frequency of the electrical circuit, and comparing at least one of the different locations, the value of the actual dielectric permittivity at the second frequency e'bf, to the value of the calculated actual dielectric permittivity e'rbf, • Calculation of the low frequency quality factor Q from the values ​​of e"bf and e'bf of the real and imaginary dielectric permittivity evaluated by extrapolation to enable validation of the study category selection or modification of the study category selection. • According to one embodiment, the step of calculating and recording a non-dispersive second category layer thickness further comprises a sub-step of calculating the corrected thickness EC comprising the calculation: • a simulated propagation time Atb between a first and second echo, by multiplying a predetermined coefficient K by the square root of the value of the calculated low-frequency dielectric permittivity e'rbf, • a propagation time difference Atc by subtracting the calculated propagation time from the simulated propagation time Atb, • a difference in thickness to be corrected AEc by multiplying the value of the difference in propagation time Atc by the calculated speed of propagation of high-frequency electromagnetic waves Va • a difference thickness ED to be corrected by dividing by 2 the difference in thickness to be corrected AEc, • a corrected thickness Ec of the first layer by adding a predetermined thickness value ET to the calculated difference thickness ED.

[0024] The invention and its various applications will be better understood upon reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0025] The figures are presented for information purposes only and in no way limit the invention.

[0026] [Fig.l] shows a schematic representation of the steps of a method of a system according to an embodiment of the invention.

[0027] [Fig.2] shows a measuring system according to one embodiment;

[0028] [Fig.3] shows a capacitive probe of the measuring system according to one embodiment.

[0029] [Fig.4] shows a step of the method of [Fig.l] with the capacitive probe of [Fig.3].

[0030] [Fig.5] shows a step of the method of [Fig.l] with a spectral inversion radar of the system of [Fig.2].

[0031] [Fig.6] shows the method step of [Fig.5] with another example of a radar of a system according to another example of this embodiment.

[0032] [Fig.7] schematically represents an example of measurement by the radar of [Fig.5], according to the fourth step of the method of [Fig.l]. DETAILED DESCRIPTION

[0033] The figures are presented for information purposes only and in no way limit the invention.

[0034] [Fig.l] shows a schematic representation of the steps of a method of a thickness measurement system according to an embodiment of the invention of a first layer 10 of a medium 1 represented by a block diagram in [Fig.2]. The medium 1 is a multi-layer structure comprising a first layer 10 made of a first material coating a second layer 11 made of a second material distinct from the first material. Here the first layer 10 is shown transparent (white) and the second layer 11 in black. The invention makes it possible to non-destructively measure a thickness of a first layer of a multi-layer medium, here the layer 10 of the medium 1. The first layer 10 of a medium 1 is a layer of which an external surface is visible from the outside, in other words the layer coating a surface of a second layer from the outside.

[0035] The invention therefore relates to both the system and the method for measuring the thickness of a first layer of a first material, coating a second layer of a second material distinct from the first material.

[0036] The thickness measurement system comprises a UWBE measurement device comprising a spectral inversion radar C1 and a capacitive probe C2. The spectral inversion radar C1 and the capacitive probe C2 may be removable and independent of each other or may comprise common parts.

[0037] The spectral inversion radar Cl comprises an antenna device with electromagnetic radiation in ultra-wide frequency band UWB transmission / reception (UWB English acronym for Ultra Wide Band meaning ultra-wide frequency band). According to a first example, the spectral inversion radar Cl is a pulsed ground penetrating radar and the electromagnetic radiation antenna device in ultra-wide frequency band transmission / reception UWB comprises a block comprising a transmitting antenna 30 and a receiving antenna 31. According to a second example shown schematically in [Fig.6], the spectral inversion radar Cl' can be a frequency hopping radar and the UWB' transmitting / receiving antenna device is then a frequency hopping antenna.

[0038] The spectral inversion radar Cl further comprises an electromagnetic wave generator clO comprising for example a microcontroller or a processor and a memory configured for controlling the UWB, UWB' transmitting / receiving antenna device at different high frequencies also called by the acronym HF, between 100 MHz and 100 GHz in order to propagate electromagnetic waves and for reading and analyzing these electromagnetic waves received by the UWB, UWB' transmitting / receiving antenna device.

[0039] The capacitive probe C2 comprises at least one pad 2 comprising at least two electrodes 20, 21 and an alternating current generator c20 connected to the pad 2 to emit an electric current from a resonant RLC circuit at a low frequency (LF) between 30 and 40 MHz. The capacitive probe C2 may comprise several pads comprising different sizes of electrodes. In addition, a pad designates only the block may comprise a single (in which case the probe will comprise at least two pads) or more than two electrodes. The alternating current generator c20 comprises for example a microcontroller or a processor and a memory configured to emit an electric current from a resonant RLC circuit at a low frequency (LF) between 30 and 40 MHz in order to propagate this electric current in the medium 1 by forming an electric circuit and reading and analyzing a resonant frequency of the electric circuit formed by the emission of the electric current.

[0040] The system further comprises an acquisition and control unit UAC connected at least by computer to the UWBe measuring device, an inverse processing unit UTI connected at least by computer to the UWBe measuring device and a calculation unit UC connected at least by computer to the acquisition and control unit UAC and to the inverse processing unit UTI. These three units can be implemented within the same computer (on one or more processors of the computer) or within separate computers and configured to communicate with each other.

[0041] The configurations and steps of the measurement method using the system shown schematically in [Fig. 1] will now be described, as well as some of these steps in Figures 3, 4, 5, 6 which represent different steps of an example of this method. schematically on the medium 1.

[0042] The measuring method comprises a step E1 of determining or selecting the category of the medium 1 from a first non-dispersive category and a second dispersive category.

[0043] The selection can be made by a human-machine interface by one of the units by the user's choice.

[0044] The user can also select the determination of the category of environment 1 via this human-machine interface.

[0045] In this case, this first step E10 comprises several sub-steps described below: • a control sub-step elO, by the acquisition and control unit UAC, of ​​the electromagnetic wave generator clO to emit and propagate electromagnetic waves S1 in the medium 1 in a frequency range, for example between 1.5-2 GHz, by means of the UWB antenna device, as visible in [Fig.5]; • a sub-step el 1 of measuring the signals generated by the propagation of the electromagnetic waves SI by means of the antenna device and processing them to determine signals in the time domain comprising at least a first surface time echo EC1 visible in [Fig.5] and a second interface time echo EC2 between the first layer 10 and the second layer 11 also visible in [Fig.5]; In other words the electromagnetic background echo SI on the surface of the second layer 11 covered by the first layer 10; The measured received signal can be time or frequency, in the case of measurement of frequency signals, the acquisition and control unit UAC applies an inverse Fourier transform of the signals measured in the frequency domain to determine the signals in the time domain; • a sub-step e 12 of evaluation of the real dielectric permittivity e'rhf and imaginary e"rhf of the medium, by the inverse processing unit UTI, by carrying out an inverse processing of a part of the processed signals received by the measurements of the radar Cl, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium; the inverse processing unit UTI can carry out the processing under a computer environment such as Matlab, Python, C++ etc.; This model and this method are for example described in the literature [Ihamouten A., Bosc F., Guan B., Le Bastard C., Fauchard C., Lambot S., Dérobert X. (2018). Full-waveform inversion using a stepped-frequency GPR to characterize the tack coat in hot-mix asphalt (HMA) layers

[0046]

[0047]

[0048]

[0049] of flexible pavements; NDT & E international, Elsevier, 95, pp. 17-25]. The processing can for example be only on a precise part of the high-frequency radar signal, preferably beyond 2 GHz in order to extract the real dielectric permittivity e'rhf and imaginary e"rhf; • a sub-step el3 of extrapolation, by the inverse processing unit UTI, of values ​​of the real low-frequency dielectric permittivity e'bf and imaginary e"bf, in a second frequency for example in the range of 30 and 40 MHz, from the real high-frequency dielectric permittivity e'rhf and imaginary e"rhf and from a model pre-recorded in the memory. In particular in this example, the model is a Jonscher model pre-recorded in the memory, and the extrapolation is carried out from a 4p variant of this model. In this example, the processing unit carries out this extrapolation in a frequency range between 1.5-2 GHz. The model could also be a pre-recorded algorithm depending on the material of the first layer. • a calculation sub-step el4 by the calculation unit UC of a quality factor Q between extrapolated values ​​of the real low-frequency dielectric permittivity e'bf and imaginary e"bf; • a sub-step el5 of determining the study category by comparing the quality factor Q to the threshold value A, if the quality factor Q is below the threshold value A the medium is considered as the first non-dispersive category and if the quality factor Q is above the threshold value A the medium is considered as the second dispersive category. In this example, the extrapolation sub-step el3, by the inverse processing unit UTI, is carried out by the following formula; ... x .. . / M\l (ÙJ) — I 1 ] I — COt | । I "h Gx> — 7---------------. where the effective electrical susceptibility, n the dispersion parameter, e oo the instantaneous dielectric response of the medium and oDC the contribution of the electrical conductivity at zero frequency. The measuring method comprises after this step E1, depending on the determination or selection of the study category in the first step E1, either a step of calculating and recording a thickness E of a first layer 10 of a non-dispersive medium E2 (when the study category is determined or selected as the first non-dispersive category), or a step of calculating and recording a thickness E of a first layer 10 of a dispersive medium E3 (when the study category is determined or selected as the second dispersive category).

[0050] The step of calculating and recording a thickness E of a first layer 10 of a non-dispersive medium E2, hereinafter called step E2, at different locations and for each different location, comprises a sub-step of controlling e20, by the acquisition and control unit UAC, the alternating current generator c20 of the capacitive probe C2 to emit and propagate an alternating current in the medium and record a resonance frequency of the electrical circuit.

[0051] The method may comprise in this sub-step e20, represented by [Fig.3] a control of the alternating current generator c20 of the capacitive probe C2 connected by two cables 25, 24, to a pad 2 chosen by the user, comprising in this example two electrodes 20, 21 for emitting an oscillating current according to a predetermined frequency in the air, then measuring one or more resonant frequencies in the air f air per electrode and recording by the acquisition and control unit UAC of this resonant frequency. The resonant frequency of the device transmitting the electrical signal is between 30 MHz and 40 MHz (more particularly between 33 MHz - 34 MHz).The alternating current generator c20 of the capacitive probe C2 thus transmits the alternating electrical signal oscillating at a predetermined frequency to the electrodes 20, 21 and measures the resonance frequency in the air transmitted to the acquisition and control unit UAC which records it.

[0052] In the case of the optional embodiment in which the pad 2 comprises more than two electrodes, the alternating current generator c20 is configured to calculate the average of the measured signals and records the resonance frequency in the air according to this calculated average.

[0053] In this sub-step e20, the user can have several choices of pad 2 comprising different sizes of electrodes and number of electrodes. The choice of pad by the user is a function of the presumed thickness of the first layer to be measured. For example, a second pad 2 comprises smaller electrodes than that of the pad 2 shown in this example, the user knows that the second pad 2 makes it possible to evaluate the average permittivity of the surface up to 5-7 mm deep, while the pad 2 of this example comprising average electrodes go up to 15 mm deep.The choice of the pad 2 can further be selected by the user on a human-machine interface of the system to allow the calculation unit Uc to select a predetermined linear law corresponding to the chosen pad comprising coefficients of this linear law which can be determined in a spreadsheet or stored in a memory, for example in a non-volatile memory. The linear law of each pad can be determined by calibration using different blocks of reference material whose capacity is known, or a capacitor.

[0054] In this sub-step e20, the user, as can be seen in [Fig.4], positions the pad 2 against the external surface of the first layer 10 of the structure 1 such that external surfaces of the electrodes 20, 21 of the pad 2 are in contact with this external surface of the first layer 10.

[0055] The user can thus, via a human-machine interface, select a request for deduction of the real low-frequency dielectric permittivity e'rbf of the first layer 10. The acquisition and control unit UAC thus controls the electromagnetic wave generator c20 to emit an oscillating current according to a predetermined frequency and to propagate via the electrodes 20, 21 pads 2 an alternating current in the medium 1. The electromagnetic wave generator c20 measures one or more resonant frequencies of an RLC electrical circuit formed by the first layer 10 as a planar capacitor and the resonant frequency of the air between the two electrodes and components of the electromagnetic wave generator c20. The acquisition and control unit UAC records this resonant frequency of the RLC circuit.The electrical frequency of the device transmitting the electrical signal is between 30 MHz and 40 MHz (more specifically between 33 MHz - 34 MHz). As shown in [Fig.4], part of the resonant frequency of the RLC circuit includes the resonant frequency of air.

[0056] This step E2 of this method further comprises a sub-step e21 of deducing a real dielectric permittivity e'rbf, by the acquisition and control unit UAC, from the recorded resonant frequency of the RLC electrical circuit and the recorded resonant frequency of the air. Indeed, there is a relationship between the difference in resonant frequency and the capacitance of the medium, the calculation unit UC can therefore calculate or deduce a relative permittivity of the medium. The calculation or deduction of the real dielectric permittivity e'rbf can be carried out from the difference in resonant frequencies between the measurement in the air and the medium is possible via a linear law of the form: e = ax (fosc - fair) + b; a and b being constants characteristic of the system and of the pad used and determined during a calibration carried out on control materials.

[0057] The user can use different pairs of pads and perform steps e20 and e21 with each pair of pads to verify value consistency. The verification can be performed by the calculation unit by calculating the deviations between the calculated actual dielectric permittivity values ​​e'rbf and comparing each calculated deviation with a predetermined deviation value and thus detecting an inconsistency if one of the deviations is beyond the predetermined deviation value.

[0058] In the case where the user needs to know the correct size of the pad 2, in other words that he does not know a thickness range of the first layer 10 or that he hesitates between two choices of pads, the calculation unit is configured to calculate a real dielectric permittivity e'rbf, from several measurements made with the probe C2 with the different pads, and by choosing the real dielectric permittivity e'rbf deduced by a measurement carried out with the pad having the largest size among the pads having allowed the deduction of a real dielectric permittivity e'rbf having a calculated value deviation lower than the predetermined deviation value.

[0059] This step E2 further comprises a sub-step e22 of control, by the acquisition and control unit UAC, of ​​the electromagnetic wave generator C20 to emit and propagate electromagnetic waves SI in the medium 1, by means of the UWB antenna device of the radar C1. The user can thus place, as in the example shown in [Fig.5], the UWB antenna device of the radar C1 against the outer surface of the first layer 10 and request a thickness measurement by a human-machine interface.

[0060] This step E2 further comprises a sub-step e23 of measuring the signals generated by the propagation of the electromagnetic waves SI via the UWB antenna device and processing, by the acquisition and control unit UAC, the reception of the measured signals to determine signals in the time domain comprising at least a first surface time echo EC1 and a second interface time echo EC2 between the two layers 10 and 11, like sub-step el 1. In a manner identical to sub-step el 1, the measured received signal may be time-domain or frequency-domain, in which case the acquisition and control unit UAC applies an inverse Fourier transform to determine the signals in the time domain.

[0061] Finally, step E2 includes a calculation and recording sub-step e24, by the calculation unit UC: • a propagation time Ata corresponding to a time difference between the first and second echo EC1, EC2 received and recorded at different locations of the first layer 10, • a propagation speed of electromagnetic waves in the first layer 10 from the value of the real low-frequency dielectric permittivity e'rbf deduced in sub-step e21 received from the acquisition and control unit UAC, • the thickness E of the first layer from the propagation time Ata and the calculated propagation speed.

[0062] For example, if step E2 is carried out at several locations on the external surface of the first layer E10. The sub-steps e20, e21, can be carried out only once each for a set of different locations on the external surface of the first layer E10, while the sub-steps e22 to e24 are carried out at each location where it is necessary to know the thickness of the layer el0 for the user.

[0063] The step of calculating and recording a thickness E of a first layer 10 of a non-dispersive medium E2, hereinafter called step E2, at different locations and for each different location, includes a control sub-step e20, by the acquisition and control unit UAC, of ​​the alternating current generator c20 of the capacitive probe C2 to emit and propagate an alternating current in the medium and record a resonance frequency of the electrical circuit.

[0064] To improve the measurement accuracy of the thickness E of this step E2, step E2 of the method may further comprise a sub-step of calculating the corrected thickness e25 comprising the calculation: • a simulated propagation time Atb equal to a predetermined coefficient K multiplied by the square root of the value of the calculated low-frequency dielectric permittivity e'rbf, • a propagation time difference value Atc between the calculated propagation time and the simulated propagation time Atb, • a difference in thickness to be corrected AEc equal to the value of the difference in propagation time Atc multiplied by the calculated high-frequency electromagnetic wave propagation speed Va, • a difference thickness ED to be corrected equal to the difference thickness to be corrected AEc / 2, • a corrected thickness Ec of the first layer by adding a predetermined thickness value ET to the calculated difference thickness ED.

[0065] The values ​​are known by actual measurements on the standard. For example, a standard material T of the same material as that of layer 10 has a real control dielectric permittivity e'rT = 3.9 (pre-recorded value) and a thickness ET of 19.0 mm (the thickness is also pre-recorded). The electromagnetic wave propagation speed propagates at a speed of 15.19 cm.ns-1 (pre-recorded value) (air speed, 30 cm.ns-1, divided by the square root of the dielectric permittivity). The propagation time Ata' (round trip) calculated and stored beforehand by the calculation unit UC between the first echo EC1 and the second echo EC2 measured by the same radar C2 is 0.286 ns.

[0066] A non-precise calculated thickness ET1 is therefore 15.19 * 0.286 / 2 or 21.7 mm whereas in reality it is 19 mm (i.e. an error of approximately 14% greater). This bias (measurement error) is linked to the emission of electromagnetic waves by the transmitting antenna 30 of the radar C2 which further radiates in the material of the first layer 10 towards the receiving antenna 31 of the UWB antenna causing a shift in the arrival time of the wave. The corrected thickness calculation sub-step e25 makes it possible to calculate a corrected thickness EC of the first layer 10 from the measurements of this witness T.

[0067] For example, the first layer 10 comprises a material with a real low-frequency dielectric permittivity e'rbf deduced in sub-step e21 = 6.1 for which it has A propagation time Ata of 0.172 ns was calculated between the two echoes EC1; EC2 in the first layer. The speed Va of the wave propagation in the layer is therefore 12.15 cm.ns-1 according to the equation:

[0068] Va= = 12.1466 = or 30 / . / 6 j or rounded to 12.15 rbf V

[0069] (c being the speed of light (therefore of waves) in a vacuum, i.e. c equals approximately 30 cm per nanosecond.)

[0070] The calculation unit UC therefore calculated an imprecise thickness E of 12.15 * 0.172 / 2 = 1.04 cm according to the formula E = va*Ata / 2.

[0071] The calculation unit comprises in the corrected thickness calculation sub-step e25, the calculation of simulated propagation time Ata' between the first and second echo EC1, EC2, by applying the value of the real low-frequency dielectric permittivity e'rbf, here 6.1 of the first layer 10 to a function f corresponding to the witness T or replacing the value of the real low-frequency dielectric permittivity witness e'rT = 3.9 by the real low-frequency dielectric permittivity e'rbf deduced in the sub-step e21 = 6.1 in the model of the standard material T. The calculation unit can thus calculate in a first step the simulated propagation speed of the witness vts of EM background in the standard with the real low-frequency dielectric permittivity e'rbf and the thickness of 19.0 mm which would then be a speed of 12.15 cm.ns-1 according to the equation:

[0072] v / s =

[0073] Let a simulated propagation time Atb corresponding to the round trip of the EM ground in the standard material, which is 0.358 ns. according to the formula: ET = —*Ata' = —-^=*Atb or Atb = Jeïbf ATT *0.286 =0.35778 ns rounded to 0.358 ns rbf ««««««. To your

[0074] The simulated propagation time Atb is therefore equal to a predetermined coefficient K (calculated in the laboratory on the standard) equal to AA_ multiplied by the square root of the value of the low-frequency dielectric permittivity e'rbf calculated

[0075] The time difference Atc between Ata= 0.172 and Atb=0.358 is -0.186 ns, with the speed Va of the wave propagation in layer 10 calculated from the witness which is equal to 12.15 cm.ns-1, the difference in thickness (outward and return) AEc to be corrected calculated by the calculation unit UC is -22.6 mm corresponds to the multiplication of Atc * Va or -0.172 ns multiplied by 12.15 cm.ns-1 between the witness T and layer 10 of structure 1, or a difference thickness ED = AEc / 2 or (22.6 mm / 2), or in this example 11.3 mm less and layer 10 of structure 1 compared to the standard E. Layer 10 of structure 1 therefore has a corrected thickness Ec = ET - ED or 19 mm (control thickness) -11.3 mm or 7.7 mm or 0.77 cm instead of 1.04 cm of calculated thickness E.

[0076] Thus this operation makes it possible to improve the calculation precision of the corrected thickness Ec of the layer 10 by adding a predetermined thickness value ET (thickness in this example equal to 19mm of the witness) to the calculated difference thickness ED.

[0077] According to an example of this embodiment, the system comprises a plurality of coefficient values ​​K associated with a thickness value ET and a material recorded in a memory, for example in the form of a table, and in that the system is configured to display an input request listing the different materials stored and in that the value K and the thickness ET are according to the corresponding selected material.

[0078] The step of calculating and recording a thickness E of a first layer 10 of a dispersive medium E3, hereinafter called step E3, at different locations and for each different location, comprises different sub-steps described hereinafter including a sub-step of control e30, by the acquisition and control unit UAC, of ​​the electromagnetic wave generator c30 of the radar C3 to emit electromagnetic waves SI in the medium 1, at different locations as represented for example by the different arrangements dO-dlO of [Fig.5], in this case by means of the transmitting antenna 30 of the UWB antenna device. Step E3 further comprises a sub-step e31 of measuring, in this case by means of the receiving antenna 31, the signals S2 generated by the propagation of the electromagnetic waves SI (of course this can also be done by controlling the spectral inversion radar Cl' of [Fig.6] with its frequency-hopping UWB transmitter / receiver antenna device).

[0079] This measurement sub-step further comprises the processing by the acquisition and control unit UAC, the reception of the measured signals S2 to determine signals in the time domain as represented in [Fig.7], comprising at least a first surface time echo EC1 and a second interface time echo EC2 between the two layers 10 and 11, like sub-step el 1 as well as the recording of the processed signal. In a manner identical to sub-step el 1, the measured received signal can be time or frequency, in which case the acquisition and control unit UAC applies a Fourier transform to determine the signals in the time domain.

[0080] Step E3 further comprises a sub-step e32 of evaluating the real dielectric permittivity e'rhf of the medium 1 by carrying out an inverse processing of a part of the processed signals S2, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium. This sub-step e32 is like the evaluation sub-step el2.

[0081] Optionally, step E3 may comprise: • an evaluation sub-step e34 identical to the extrapolation sub-step el3, Thus • that a sub-step of controlling the alternating current generator of the capacitive probe to emit and propagate an alternating current in the medium and record a resonance frequency of the electrical circuit, of deduction of an actual dielectric permittivity e'rbf from the measured resonance frequency of the electrical circuit, of comparison e35, at at least one of the different locations, of the value of the actual dielectric permittivity at the second frequency e'bf, evaluated by extrapolation over the entire frequency range, with the value of the deduced actual dielectric permittivity e'rbf, and • calculation e36 of the low frequency quality factor Q' from the values ​​of e"bf and e'bf of the real and imaginary dielectric permittivity evaluated by extrapolation to enable validation of the selection of study category or modification of the selection of the study category.

[0082] Step E3 further comprises a sub-step of calculation and recording e33; by the calculation unit UC, of ​​the thickness E from a propagation time Ata calculated by the calculation unit, corresponding to the time difference between the first and second echoes EC1, EC2 received and of the speed of propagation of high-frequency electromagnetic wave in the first layer 10 calculated by the calculation unit UC from the value of the real dielectric permittivity e'rhf obtained in the evaluation sub-step e32.

[0083] Unless otherwise specified, the same element appearing in different figures has a single reference.

Claims

Claims

1. System for measuring the thickness of a first layer (10) of a medium (1) comprising at least two layers of two different materials, comprising: - a measuring device (UWBe) comprising: • a spectral inversion radar (Cl) having:

1. an antenna device (UWB) with electromagnetic radiation in ultra-wide frequency band transmission / reception 2. an electromagnetic wave generator (clO) connected to the antenna device (UWB) to emit electromagnetic waves at different high frequencies between 100 MHz and 100 GHz (HF), • a capacitive probe (C2) comprising:

1. at least one pad (2) comprising two electrodes (20, 21), 2. an alternating current generator (c20) connected to the pad (2) to emit an electric current from a resonant RLC circuit at a low frequency (LF) between 30 and 40 MHz, • an acquisition and control unit (UAC) connected to the measuring device (UWBE) configured to: • controlling the electromagnetic wave generator to emit and propagate electromagnetic waves in the medium, via the antenna device, and processing the reception of the signals measured via the antenna device to determine signals in the time domain comprising at least a first surface time echo (EC1) and a second interface time echo (EC2) between the two layers (10, 11), • control the alternating current generator of the capacitive probe to emit and propagate a current alternating in the medium and record a resonance frequency of the electrical circuit, an inverse processing unit (UTI) connected to the measuring device (UWB e) for carrying out inverse processing of a part of the signals received by the radar measurements (UWB), to evaluate the real dielectric permittivity e'rhf and imaginary e"rhf of the medium, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium, as well as extrapolating values ​​of the real low-frequency dielectric permittivity e'bf and imaginary e"bf determined from the real high-frequency dielectric permittivity e'rhf and imaginary e"rhf and from a model pre-recorded in the memory, a calculation unit configured to: • calculate a real dielectric permittivity e'rbf from the recorded resonance frequency of the electrical circuit, • calculate a quality factor Q between extrapolated values ​​of the determined real low-frequency dielectric permittivity e'bf and imaginary e"bf, • determine a study category by comparing the quality factor Q to a threshold value A, if the quality factor Q is below the threshold value A the medium is considered as the first non-dispersive category and if the quality factor Q is above the threshold value A the medium is considered as the second dispersive category, receive a selection of a study category from a first non-dispersive category or as a second dispersive category, when the study category is determined or selected as the first non-dispersive category, 1. receive and record at different locations of the first layer, at each location the first surface time echo (EC1) and the second echo interface time (EC2) deduced by measurements by the spectral inversion radar (Cl), 2. calculate and record at each different location: a. a propagation time (Ata) corresponding to a time difference between the first and second echoes received, b. the real low-frequency dielectric permittivity e'rbf from the recorded resonant frequency of the electrical circuit c. a propagation speed of electromagnetic waves in the first layer from the value of the calculated real dielectric permittivity e'rbf and d. a thickness (E) from the propagation time (Ata) and the calculated propagation speed, when the study category is determined or selected as the second dispersive category is determined:

1. receive and record at different locations of the first layer (10), at each location: a. the first surface time echo (EC1) and the second interface time echo (EC2) deduced by measurements by the spectral inversion radar (Cl), b. the value of the actual dielectric permittivity e'rhf evaluated by the inverse processing unit, 2. calculate and record at each different location: a. a propagation time (Ata) corresponding to a time difference between the first and second echoes (EC1, EC2) received, b. a high-frequency electromagnetic wave propagation speed Va emitted by the spectral inversion radar (Cl) in the first layer from the value of the actual dielectric permittivity e'rhf received and c. a thickness (E) from the propagation time (Ata) and the calculated electromagnetic wave propagation speed Va.

2. The system of claim 1, wherein the radar is frequency hopping.

3.

4. System according to claim 1, wherein the radar is pulsed. System according to one of the preceding claims wherein the extrapolation model is a Jonscher model pre-recorded in the memory, and in that the extrapolated values ​​of the real low-frequency dielectric permittivity e'bf and imaginary e"bf are determined by extrapolation from the 4p variant of the Jonscher model and the values ​​of the real and imaginary dielectric permittivity, e'rhf and e"rhf, evaluated by the processing unit in a frequency range between 1.5 -2 GHz.

5. System according to claim 4, wherein when the study category is determined or selected as the second dispersive category, the calculation unit is configured to: - receive in at least one of the different locations a value of the real dielectric permittivity e'rhf evaluated in the frequency range [1.5-2 GHz] by the inverse processing unit from measurements from the spectral inversion radar (Cl), - evaluate at the at least one of the different locations, respectively, the values ​​of the real and imaginary dielectric permittivity, e'bf and e"bf in a low frequency range (between 30 and 40 MHz) by extrapolation from the 4p variant of the Jonscher model from at least the value of the real dielectric permittivity e'rhf, evaluated by the processing unit, - compare at the at least one of the different locations, the value of the real dielectric permittivity e'bf at the low frequency,evaluated by extrapolation and the value of the real low-frequency dielectric permittivity e'rbf calculated, - calculate the low-frequency quality factor Q from the values ​​of the real and imaginary dielectric permittivity e"bf and e'bf evaluated by extrapolation to enable validation, study category selection or change study category selection.

6. System according to the preceding claim, in which when the study category is determined as a second dispersive category, the calculation unit is configured to further receive in at least one of the different locations a value of the imaginary dielectric permittivity e"rhf.

7. System according to one of the preceding claims, in which the signal measured via the antenna device is in the frequency domain and in that the acquisition and control unit applies an inverse Fourier transform of the measured signals to determine the signals in the time domain.

8. System according to one of claims 1 to 6, wherein the signal measured via the antenna device is in the time domain.

9. System according to one of the preceding claims, wherein when the category is determined as first category (non-dispersive), the system is configured to calculate a corrected thickness from a predetermined value of a witness by calculating: - a simulated propagation time Atb equal to a predetermined coefficient K multiplied by the square root of the value of the calculated low-frequency dielectric permittivity e'rbf, - a propagation time difference value Atc between the calculated propagation time (Ata) and the simulated propagation time Atb, - a thickness difference to be corrected AEc equal to the value of the propagation time difference Atc multiplied by the calculated high-frequency electromagnetic wave propagation speed Va - a difference thickness ED to be corrected equal to the thickness difference to be corrected AEc / 2,- a corrected thickness Ec of the first layer by adding a predetermined thickness value ET to the calculated difference thickness ED.,

10. Method of measurement by a system according to one of the preceding claims- preceding a thickness of a first layer (10) of a first material in a medium further comprising a second layer (11) of a second material distinct from the first material, the second layer (11) being coated by the first layer (10), comprising: - a step (El) of determining or selecting the category of the medium from a first non-dispersive category and a second dispersive category, in which if the first step is a determination, the first step (El) comprises the sub-steps of: • controls (elO) the electromagnetic wave generator to emit and propagate electromagnetic waves (SI) in the medium (1), by means of the antenna device, • measuring (el 1) the signals generated by the propagation of electromagnetic waves (SI) via the antenna device and processing them to determine signals in the time domain comprising at least a first surface time echo (EC1) and a second interface time echo (EC2) between the two layers, • evaluation (el2) of the real dielectric permittivity e'rhf and imaginary e"rhf of the medium by carrying out an inverse processing of a part of the processed signals, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium, • extrapolation (el3) of values ​​of the real low-frequency dielectric permittivity e'bf and imaginary e”bf from the real high-frequency dielectric permittivity e'rhf and imaginary e"rhf and a 4p variant of a Jonscher model pre-recorded in the memory, • calculation (el4) a quality factor Q between the extrapolated values ​​of the real dielectric permittivity e'bf and imaginary e"bf, • determination (el5) of the study category in comparing the quality factor Q with the threshold value A, - when the study category is determined or selected as the first non-dispersive category, a step of calculating and recording a thickness (E) of a first layer (10) of a non-dispersive medium (E2) at different locations and for each different location comprising the sub-steps: - control (e20) of the alternating current generator of the capacitive probe to emit and propagate an alternating current in the medium and record a resonance frequency of the electrical circuit, - calculation of a real dielectric permittivity e'rbf (e21) from the measured resonance frequency of the electrical circuit, - control (e22) of the electromagnetic wave generator for emitting and propagating electromagnetic waves (SI) in the medium (1), by means of the antenna device, - measuring (e23) the signals generated by the propagation of electromagnetic waves (SI) via the antenna device and processing them to determine signals in the time domain comprising at least a first surface time echo (EC1) and a second interface time echo (EC2) between the two layers, 1. calculation and recording (e24): a. a propagation time (Ata) corresponding to a time difference between the first and second echoes (EC1, EC2) received, b. of a propagation speed of electromagnetic waves in the first layer from the value of the calculated real dielectric permittivity e'rbf and c. the thickness of the first layer (E) from the propagation time (Ata) and the calculated propagation speed, 2. when the study category is determined or selected as the second dispersive category, a calculation step and recording a thickness E of a first layer (10) of a dispersive medium (E3) at different locations and for each different location comprising the sub-steps: • control (e30) of the electromagnetic wave generator (c30) for emitting and propagating electromagnetic waves (SI) in the medium (1), by means of the antenna device, • to measure (e31) signals (S2) generated by the propagation of electromagnetic waves (SI) via the antenna device and process them to determine signals in the time domain comprising at least a first surface time echo (EC1) and a second interface time echo (EC1) between the two layers (10, 11) • evaluation (e32) of the real dielectric permittivity e'rhf of the medium by carrying out an inverse processing of a part of the processed signals, from a direct model recorded in a memory of the system, based on a Green function for the propagation of waves in a semi-infinite stratified medium, • calculation and recording (e33):

1. a propagation time (Ata) corresponding to a time difference between the first and second echoes (EC1, EC2) of the processed signal, 2. of a propagation speed of high-frequency electromagnetic waves in the first layer from the value of the real dielectric permittivity e'rhf obtained by the inverse processing unit UTI and 3. the thickness of the first layer (E) from the propagation time (Ata) and the propagation speed of the electromagnetic wave emitted by the spectral inversion radar (Cl).

11. Measuring method according to the preceding claim in which the sub-step of evaluation (e32) of the real dielectric permittivity e'rhf of the medium by the inverse processing unit is evaluated over an entire frequency range, in particular between 1.5 and 2 GHz, by controlling the spectral inversion radar (Cl) in this frequency range, and in which the step of calculation and recording (E3) of a thickness of the first layer (E) when the study category is selected as the second dispersive category, further comprises the following sub-steps if the dielectric permittivity varies beyond a predetermined percentage: - Evaluation (e34) at at least one of the different locations, respectively, of the values ​​of the real dielectric permittivity e'rhf and imaginary, e'bf and e"bf in the low frequency range between 30 and 40 MHz, by extrapolation from the 4p variant of the Jonscher model from at least the value of the real dielectric permittivity e'rhf, evaluated by the processing unit, - Control of the alternating current generator of the capacitive probe to emit and propagate an alternating current in the medium and record a resonance frequency of the electrical circuit, calculation of a real dielectric permittivity e'rbf from the measured resonance frequency of the electrical circuit, and comparison (e35) at at least one of the different locations, the value of the real dielectric permittivity e'bf at the frequency carried out in the extrapolation sub-step (el3), with the value of the calculated real dielectric permittivity e'rbf, - Calculation (e36) of the low frequency quality factor Q from the values ​​of e"bf and e'bf of the real and imaginary dielectric permittivity evaluated by extrapolation to enable validation of the study category selection or modification of the study category selection.

12. Measuring method according to the preceding claim in which the step of calculating and recording (E2) a thickness (E) of a non-dispersive second category layer, further comprises a sub-step of calculating the corrected thickness EC (e25) comprising the calculation: of a simulated propagation time Atb between a first and second echo, by multiplying a predetermined coefficient K by the square root of the value of the calculated low-frequency dielectric permittivity e'rbf, of a propagation time difference Atc by subtracting the calculated propagation time from the simulated propagation time Atb, of a thickness difference to be corrected AEc by multiplying the value of the propagation time difference Atc by the calculated high-frequency electromagnetic wave propagation speed Va a difference thickness ED to be corrected by dividing by 2 the difference thickness to be corrected AEc, a corrected thickness Ec of the first layer by adding a predetermined thickness value ET to the calculated difference thickness ED.

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