EP1252534A1 - Method for obtaining underground imagery using a ground-penetrating radar - Google Patents

Method for obtaining underground imagery using a ground-penetrating radar

Info

Publication number
EP1252534A1
EP1252534A1 EP00993445A EP00993445A EP1252534A1 EP 1252534 A1 EP1252534 A1 EP 1252534A1 EP 00993445 A EP00993445 A EP 00993445A EP 00993445 A EP00993445 A EP 00993445A EP 1252534 A1 EP1252534 A1 EP 1252534A1
Authority
EP
European Patent Office
Prior art keywords
signals
antennas
radar
reflected
basement
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP00993445A
Other languages
German (de)
French (fr)
Inventor
Jean-Jacques Berthelier
Richard Ney
Alain Meyer
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Centre National de la Recherche Scientifique CNRS
Original Assignee
Centre National de la Recherche Scientifique CNRS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Centre National de la Recherche Scientifique CNRS filed Critical Centre National de la Recherche Scientifique CNRS
Publication of EP1252534A1 publication Critical patent/EP1252534A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V9/00—Prospecting or detecting by methods not provided for in groups G01V1/00 - G01V8/00
    • G01V9/02—Determining existence or flow of underground water
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
    • G01S3/02—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using radio waves
    • G01S3/14—Systems for determining direction or deviation from predetermined direction
    • G01S3/143—Systems for determining direction or deviation from predetermined direction by vectorial combination of signals derived from differently oriented antennae
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/024—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00 using polarisation effects
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
    • G01V3/12—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation operating with electromagnetic waves
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/885—Radar or analogous systems specially adapted for specific applications for ground probing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/88—Radar or analogous systems specially adapted for specific applications
    • G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A90/00—Technologies having an indirect contribution to adaptation to climate change
    • Y02A90/30—Assessment of water resources

Definitions

  • the present invention relates to a method for obtaining imagery of the basement using a ground penetration radar. It relates more particularly to a process allowing, from a fixed point, the exploration of the geological structures of a subsoil as well as the detection of obstacles buried at a shallow depth below the surface (pipelines in the field of civil engineering , mines in a military application ). According to another aspect of the invention, it relates to a ground penetration radar implementing this method of obtaining imagery of the subsoil from a fixed point. For planetary missions, and for Mars in particular, such ground penetrating radars indeed appear as the instruments best suited for exploring the subsoil; this is due in particular to their good adaptation to the strong constraints imposed by such missions on the mass and power of the measuring instruments.
  • the present invention therefore aims to overcome such drawbacks by proposing a method for obtaining imagery of the basement comprising a ground penetration radar which allows to determine the direction of propagation of the waves reflected or backscattered by the inhomogeneities of the subsoil and therefore to measure not only the distance of the reflectors or diffusers but also their direction relative to the transmitter without using a mobile instrument; obtaining such data then making it possible to produce a three-dimensional image of the underground reflectors or diffusers using processing and analysis algorithms.
  • the method of obtaining a subsoil imagery according to the invention, using a ground penetration radar comprising means for transmitting, receiving and processing signals is characterized in that:
  • - signals are transmitted from a fixed point relative to the basement and using at least two electrical antennas; - We receive reflected or backscattered signals by reflectors or diffusers of said basement using said electrical antennas and three magnetic antennas;
  • FIG. 1 represents a diagram of the electronic device of the radar according to the invention
  • - Figure 2 shows a radiation diagram of an electric antenna used in ground penetration radar
  • - Figures 3a and 3b show radiation patterns of 3 electrical antennas used simultaneously with a phase difference of 120 °;
  • the method of obtaining imagery of a basement uses a fixed ground penetration radar comprising a transmitter, a receiver, a set of at least two antennas electric and three magnetic antennas and an electronic command and control device to manage the signals transmitted and received. It is a stationary radar where the transmitter and the receiver are placed in the same place and where transmission and reception of the signals must be separated in time. After the end of the transmission, the receiver measures the echoes. In addition, electrical antennas are used to transmit these signals and all of the electrical and magnetic antennas when received.
  • polarized waves are emitted by said radar antennae in a plane, and thanks to the measurement of three magnetic components of the reflected or backscattered waves, we obtain, using algorithms for processing and analyzing Maxwell's equations which the electric and magnetic fields satisfy, the direction of their propagation vectors.
  • two or three electric antennas can be used for transmitting signals which operate simultaneously with a phase difference suitably chosen between the signals transmitted.
  • this device advantageously comprises five subsystems: a wave generator and amplifier, a receiver with an electric antenna, a receiver with an antenna magnetic, a radar control unit and a digital unit.
  • the signals emitted by the wave generator are first filtered to eliminate unwanted spectral lines from the bandwidth of the radar, and then amplified. In addition, there are switches at the inputs of the amplifiers so as to isolate the latter when they are not in use.
  • the signals emitted by the wave generator are either in the form of a single pulse, or in the form of a coded pulse train. This produces a coding of the transmitted signals which allows better recognition of the reflected or backscattered signals and which has the advantage of being able to carry out measurements with different time periods of the signals.
  • the electric antenna receiver has three switches Cl-E, C2-E and C3-E installed to protect the receiver from an overload that could occur during the signal transmission period.
  • the CST-E sensitivity control device consists of a variable attenuator whose attenuation decreases when time elapses during the period of reception of the signals so as to approximate approximately the anticipated variation of the amplitude of the electric field with the depth survey.
  • an FPB-E bandpass filter eliminates all unwanted signals in the useful frequency band, which may arise, for example, from electromagnetic interference.
  • the time sensitivity control CST-E and the bandpass filter FPB-E must advantageously be configured so as to adapt to the impedance of the antenna during the signal reception period.
  • the radar used implementing the method of the invention being stationary, the reflected electromagnetic field is also stationary, and the three antennas are connected in sequence to the amplifier so as to achieve a complete measurement whatever the plane of polarization of the waves.
  • the signals from the magnetic antennas AMI, AM2 and AM3 are measured in a similar way: an antenna (AM) is selected by a switch (CM) and then connected to a preamplifier (PA-M ); the outgoing signal then feeds an amplifier (A2) via a time sensitivity control device (CST-M) and a bandpass filter (FBP-M).
  • an amplifier can be used for each antenna.
  • the digital unit includes an analog-to-digital ADC converter, a controller and a memory.
  • the signal selected by a CDE double input switch undergoes digital conversion and coherent integrations are then carried out to take advantage of the immobility of the radar and from there, improve the signal to noise ratio.
  • the function of the radar control unit is to continuously generate all the signals necessary for the various operations. A preferred embodiment of the electrical and magnetic antennas used in the radar will now be described.
  • the electrical antennas are typically half-wave dipoles whose optimal length is equal to a quarter of the wavelength used; the profile of the electrical resistance along these being adjusted so as to attenuate the internal reflections of the transmitted signals and to dampen the natural resonances which prevent the observation of the shallow layers of the basement.
  • quarter wave monopolies can also be used.
  • the radiation pattern in the plane of the electric field of such an antenna transmitting a linearly polarized wave is characterized by a lobe structure: a main lobe facing the ground, two secondary lobes and a lobe weaker directed upwards.
  • the electric antenna only illuminates (or sends energy) significantly in a sector of angle less than 60 °.
  • FIGS. 3a and 3b also represent the variations in radiated power as a function of the direction of propagation of 3 electric antennas used simultaneously and phase-shifted by 120 ° between them. These antennas thus emit waves polarized elliptically in a plane.
  • the radiation patterns 3a in plane no. 1 and 3b in plane no. 2 also show a structure with 3 lobes facing the ground.
  • the magnetic antennas are of the high frequency flux return type; they each have a sensor and are connected to a preamplifier.
  • the three sensors associated with the preamplifier are used to detect the three magnetic components of the signals reflected on the radar.
  • these magnetic antennas are arranged so as to form an orthonormal trihedron which will serve as a benchmark for measuring the magnetic components of the reflected or backscattered signals; each component being detected by one of these sensors associated with the preamplifier.
  • each of the sensors comprises a coil made up of turns wound on a ferromagnetic core. The material of this ferromagnetic core is chosen according to the frequency range.
  • the ferromagnetic core is made up of either a ferrite rod or a bar.
  • the primary and secondary turns of each sensor are produced by an enamelled copper wire and are directly wound on the core. Their structure is configured so as to minimize parasitic capacities.
  • Each sensor is then placed in an epoxy resin structure, the outer surface of which is covered with a conductive layer which protects the sensor against external electric fields and which improves the thermal balance of the system.
  • the three magnetic antennas of the radar implementing the method of obtaining imagery of a basement can be replaced by a single magnetic antenna which pivots in space so that to position themselves correctly to measure the three magnetic components of the reflected or backscattered waves.
  • the method of obtaining imagery of a subsoil is used on board an automatic station deposited on the surface of Mars in order to study the underground geological structures of this planet and especially to check for the presence of ice and / or water under liquid form.
  • the landing module on Mars includes deployable solar panels, three magnetic antennas orthogonal to each other and three electric antennas angularly spaced 120 °.
  • a fixed radar working frequency is chosen in the vicinity of 2 MHz so as to probe the subsoil to a depth of approximately 3 kilometers.
  • the optimal length of the antennas equal to a quarter of the wavelength used is therefore about 35 meters; the propagation losses in the subsoil decrease with the wave frequency and the spatial resolution requires a large bandwidth.
  • daytime measurements will be made at a frequency lower than the critical frequency of the ionosphere which, under these conditions, acts as a shield against galactic electromagnetic noise.
  • the device implementing the method which is the subject of the invention can be used in geology, for the detection of resources in a subsoil (groundwater ...) or for the exploration of geological structures, in the field of civil engineering for the detection of buried obstacles at shallow depth (pipes, etc.) or even for military applications (mine detection, etc.), or even for aerial radar applications .
  • a working frequency suited to the dimensions of the object or obstacle to be detected and to the depth of the survey.
  • the present invention offers multiple advantages; in particular, the use of such a method to obtain an image of the three-dimensional structure of the basement and of the buried obstacles makes it possible, while limiting the grid therefore the time and the difficulty of measurement, to obtain by inversion more precise results since they are based not only on echo propagation time measurements but also on knowledge of their direction of arrival.

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  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Remote Sensing (AREA)
  • General Physics & Mathematics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • Geophysics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Hydrology & Water Resources (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Electromagnetism (AREA)
  • Environmental & Geological Engineering (AREA)
  • Geology (AREA)
  • Radar Systems Or Details Thereof (AREA)
  • Geophysics And Detection Of Objects (AREA)

Abstract

The invention concerns a method for obtaining underground imaging, using a ground-penetrating radar comprising means for transmitting, receiving and processing signals. The invention is characterised in that it consists in: transmitting signals from a fixed point relative to the underground zone and using at least two electrical antennae; receiving the signals reflected or backscattered by reflectors or diffusers from said underground zone through said electrical antennae and three magnetic antennae; and processing said reflected or backscattered signals with an algorithm to obtain said underground imaging.

Description

Procédé d'obtention d'une imagerie du sous-sol utilisant un radar à pénétration de sol Method for obtaining basement imagery using ground penetrating radar
La présente invention est relative à un procédé d'obtention d'une imagerie du sous-sol utilisant un radar à pénétration de sol. Elle vise plus particulièrement un procédé permettant, à partir d'un point fixe, l'exploration des structures géologiques d'un sous-sol ainsi que la détection d'obstacles enfouis à faible profondeur sous la surface (canalisations dans le domaine du génie civil, mines dans une application militaire...) . Selon un autre aspect de l'invention, elle vise un radar à pénétration de sol mettant en oeuvre ce procédé d'obtention d'une imagerie du sous-sol à partir d'un point fixe. Pour les missions planétaires, et pour Mars en particulier, de tels radars à pénétration de sol apparaissent en effet comme les instruments les mieux adaptés à l'exploration du sous-sol ; ceci tient notamment à leur bonne adaptation aux fortes contraintes imposées par de telles missions sur la masse et la puissance des instruments de mesure. L'utilisation sur Terre de radars pour l'exploration des structures géologiques du sous-sol requiert en général un instrument mobile qui permet de quadriller le terrain en effectuant de nombreuses mesures et l'utilisation d'algorithmes d'inversion pour retrouver la structure du terrain sous-jacent. En effet, de tels instruments enregistrent uniquement la distance des réflecteurs ou des diffuseurs sans mesure de leur direction par rapport à l'émetteur et ne permettent donc d'obtenir d'images en trois dimensions du sous- sol qu'à condition d'être déplacés au dessus de la surface à sonder. Or, au cours de missions planétaires comme celle prévue sur la planète Mars, une station automatique sera déposée sur la surface de la planète et les mesures seront donc faites à partir d'un point fixe, 1 ' atterriseur . On connaît également comme instruments de détection, des radars embarqués à bord de satellites placés en orbite autour de la surface de la planète à sonder. A l'heure actuelle, de tels radars présentent l'inconvénient majeur de ne pas être suffisamment puissants car trop éloignés de la surface pour permettre une détection au-delà de 100 mètres de profondeur environ.The present invention relates to a method for obtaining imagery of the basement using a ground penetration radar. It relates more particularly to a process allowing, from a fixed point, the exploration of the geological structures of a subsoil as well as the detection of obstacles buried at a shallow depth below the surface (pipelines in the field of civil engineering , mines in a military application ...). According to another aspect of the invention, it relates to a ground penetration radar implementing this method of obtaining imagery of the subsoil from a fixed point. For planetary missions, and for Mars in particular, such ground penetrating radars indeed appear as the instruments best suited for exploring the subsoil; this is due in particular to their good adaptation to the strong constraints imposed by such missions on the mass and power of the measuring instruments. The use on Earth of radars for the exploration of the geological structures of the subsoil generally requires a mobile instrument which makes it possible to grid the ground by carrying out many measurements and the use of inversion algorithms to find the structure of the underlying land. Indeed, such instruments record only the distance of the reflectors or diffusers without measuring their direction relative to the transmitter and therefore do not allow obtaining three-dimensional images of the basement unless they are moved above the surface to be probed. However, during planetary missions such as that planned on the planet Mars, an automatic station will be deposited on the surface of the planet and the measurements will therefore be made from a fixed point, 1 lander. Also known as detection instruments are radars on board satellites placed in orbit around the surface of the planet to be probed. At present, such radars have the major drawback of not being powerful enough because they are too far from the surface to allow detection beyond about 100 meters in depth.
La présente invention vise donc à pallier de tels inconvénients en proposant un procédé d'obtention d'une imagerie du sous-sol comportant un radar à pénétration de sol qui permet de déterminer la direction de propagation des ondes réfléchies ou rétrodiffusées par les inhomogénéités du sous-sol et donc de mesurer non seulement la distance des réflecteurs ou des diffuseurs mais aussi leur direction par rapport à l'émetteur sans avoir recours à un instrument mobile ; l'obtention de telles données permettant alors de réaliser une image en trois dimensions des réflecteurs ou diffuseurs souterrains à l'aide d'algorithmes de traitement et d'analyse.The present invention therefore aims to overcome such drawbacks by proposing a method for obtaining imagery of the basement comprising a ground penetration radar which allows to determine the direction of propagation of the waves reflected or backscattered by the inhomogeneities of the subsoil and therefore to measure not only the distance of the reflectors or diffusers but also their direction relative to the transmitter without using a mobile instrument; obtaining such data then making it possible to produce a three-dimensional image of the underground reflectors or diffusers using processing and analysis algorithms.
A cet effet, le procédé d'obtention d'une imagerie du sous- sol selon l'invention, utilisant un radar à pénétration de sol comportant des moyens d'émission, de réception et de traitement de signaux, se caractérise en ce que :To this end, the method of obtaining a subsoil imagery according to the invention, using a ground penetration radar comprising means for transmitting, receiving and processing signals, is characterized in that:
- on émet des signaux à partir d'un point fixe par rapport au sous-sol et à l'aide d'au moins deux antennes électriques ; - on reçoit des signaux réfléchis ou retrodiffuses par des réflecteurs ou des diffuseurs dudit sous-sol à l'aide desdites antennes électriques et de trois antennes magnétiques ;- signals are transmitted from a fixed point relative to the basement and using at least two electrical antennas; - We receive reflected or backscattered signals by reflectors or diffusers of said basement using said electrical antennas and three magnetic antennas;
- et on traite lesdits signaux réfléchis ou retrodiffuses à l'aide d'un algorithme afin d'obtenir ladite imagerie dudit sous-sol.- And we process said reflected or backscattered signals using an algorithm to obtain said imagery of said basement.
D'autres caractéristiques et avantages de la présente invention ressortiront de la description faite ci-après, en référence aux dessins annexés qui en illustrent un exemple de réalisation dépourvu de tout caractère limitatif. Sur les figures : la figure 1 représente un diagramme du dispositif électronique du radar selon 1 ' invention ;Other characteristics and advantages of the present invention will emerge from the description given below, with reference to the appended drawings which illustrate an embodiment thereof devoid of any limiting character. In the figures: FIG. 1 represents a diagram of the electronic device of the radar according to the invention;
- la figure 2 représente un diagramme de rayonnement d'une antenne électrique utilisée dans le radar à pénétration de sol ; - les figures 3a et 3b représentent des diagrammes de rayonnement de 3 antennes électriques utilisées simultanément avec une différence de phase de 120° ;- Figure 2 shows a radiation diagram of an electric antenna used in ground penetration radar; - Figures 3a and 3b show radiation patterns of 3 electrical antennas used simultaneously with a phase difference of 120 °;
- la figure 4 est une vue en perpective d'un module déployé à la surface de Mars comprenant un radar selon l'invention. Selon un mode préféré de réalisation de l'invention, le procédé d'obtention d'une imagerie d'un sous-sol utilise un radar fixe à pénétration de sol comportant un émetteur, un récepteur, un ensemble d'au moins -deux antennes électriques et de trois antennes magnétiques et un dispositif électronique de commande et de contrôle permettant de gérer les signaux émis et reçus. Il s'agit d'un radar stationnaire où l'émetteur et le récepteur sont placés au même endroit et où émission et réception des signaux doivent être séparées dans le temps. Après la fin de l'émission, le récepteur effectue la mesure des échos. De plus, on utilise les antennes électriques à l'émission de ces signaux et l'ensemble des antennes électriques et magnétiques à leur réception.- Figure 4 is a perspective view of a module deployed on the surface of Mars comprising a radar according to the invention. According to a preferred embodiment of the invention, the method of obtaining imagery of a basement uses a fixed ground penetration radar comprising a transmitter, a receiver, a set of at least two antennas electric and three magnetic antennas and an electronic command and control device to manage the signals transmitted and received. It is a stationary radar where the transmitter and the receiver are placed in the same place and where transmission and reception of the signals must be separated in time. After the end of the transmission, the receiver measures the echoes. In addition, electrical antennas are used to transmit these signals and all of the electrical and magnetic antennas when received.
Selon une caractéristique avantageuse de l'invention, pour déterminer la direction des ondes réfléchies ou rétrodiffusées et détectées par le récepteur, on émet au moyen desdites antennes électriques du radar des ondes polarisées dans un plan, et grâce à la mesure de trois composantes magnétiques des ondes réfléchies ou rétrodifusées, on obtient, à l'aide d'algorithmes de traitement et d'analyse des équations de Maxwell auxquelles satisfont les champs électrique et magnétique, la direction des vecteurs de propagation de celles-ci.According to an advantageous characteristic of the invention, in order to determine the direction of the reflected or backscattered waves detected by the receiver, polarized waves are emitted by said radar antennae in a plane, and thanks to the measurement of three magnetic components of the reflected or backscattered waves, we obtain, using algorithms for processing and analyzing Maxwell's equations which the electric and magnetic fields satisfy, the direction of their propagation vectors.
De manière à obtenir de telles ondes polarisées circulairement ou elliptiquement dans un plan, on peut utiliser deux ou trois antennes électriques pour l'émission des signaux qui fonctionnent simultanément avec une différence de phase convenablement choisie entre les signaux émis .In order to obtain such waves circularly or elliptically polarized in a plane, two or three electric antennas can be used for transmitting signals which operate simultaneously with a phase difference suitably chosen between the signals transmitted.
Lorsque cela est possible, pour des applications terrestres par exemple, il est avantageux pour améliorer la précision du procédé, de disposer de 3 antennes électriques qui permettent alors la mesure de six composantes des ondes réfléchies.When possible, for land applications for example, it is advantageous to improve the precision of the process, to have 3 electrical antennas which then allow the measurement of six components of the reflected waves.
Selon un autre mode d'opération, il est également possible d'émettre de façon indépendante des ondes polarisées linéairement le long des 3 directions correspondant à chacune des 3 antennes électriques ; 1 ' ensemble complet des 3 antennes électriques et des 3 antennes magnétiques étant utilisé pour réceptionner et déterminer leur direction d'arrivée. En effet, la possibilité d'opérer avec différents schémas de polarisation présente un intérêt significatif pour le procédé puisqu'il permet d'étudier plus en détails les propriétés de diffusion des réflecteurs du sous-sol.According to another mode of operation, it is also possible to independently transmit linearly polarized waves along the 3 directions corresponding to each of the 3 electric antennas; 1 complete set of 3 electric antennas and 3 magnetic antennas being used to receive and determine their direction of arrival. Indeed, the possibility of operating with different polarization schemes is of significant interest for the method since it makes it possible to study in more detail the scattering properties of the reflectors in the subsoil.
On décrira maintenant le dispositif électronique, qui est une façon de réaliser le procédé selon l'invention. Comme illustré sur la figure 1, ce dispositif comprend avantageusement cinq sous-systèmes : un générateur et amplificateur d'ondes, un récepteur à antenne électrique, un récepteur à antenne magnétique, une unité de commande du radar et une unité digitale .The electronic device will now be described, which is one way of carrying out the method according to the invention. As illustrated in FIG. 1, this device advantageously comprises five subsystems: a wave generator and amplifier, a receiver with an electric antenna, a receiver with an antenna magnetic, a radar control unit and a digital unit.
Les signaux émis par le générateur d'ondes sont, dans un premier temps filtrés pour permettre d'éliminer les lignes spectrales indésirables de la largeur de bande du radar, et dans un second temps, amplifiés. De plus, on dispose des interrupteurs aux entrées des amplificateurs de façon à isoler ces derniers lorsqu'ils ne sont pas utilisés.The signals emitted by the wave generator are first filtered to eliminate unwanted spectral lines from the bandwidth of the radar, and then amplified. In addition, there are switches at the inputs of the amplifiers so as to isolate the latter when they are not in use.
Selon une caractéristique avantageuse de l'invention, les signaux émis par le générateur d'ondes se présentent soit sous la forme d'une impulsion unique, soit sous la forme d'un train d'impulsions codé. On réalise ainsi un codage des signaux émis qui permet une meilleure reconnaissance des signaux réfléchis ou retrodiffuses et qui présente l'avantage de pouvoir effectuer des mesures avec différentes périodes de temps des signaux.According to an advantageous characteristic of the invention, the signals emitted by the wave generator are either in the form of a single pulse, or in the form of a coded pulse train. This produces a coding of the transmitted signals which allows better recognition of the reflected or backscattered signals and which has the advantage of being able to carry out measurements with different time periods of the signals.
Le récepteur d'antenne électrique comporte trois commutateurs Cl-E, C2-E et C3-E installés pour protéger le récepteur contre une surcharge qui pourrait survenir pendant la période de transmission des signaux. On dispose d'un unique amplificateur Al en association avec les antennes électriques AE1, AE2 et A3 et ce dernier est connecté à chacune d'entre elles par l'intermédiaire d'un commutateur C-E couplé à un dispositif de commande CST-E qui fait varier la sensibilité en fonction du temps. En variante, on peut également utiliser un amplificateur pour chaque antenne. Le dispositif de commande de sensibilité CST-E consiste en un atténuateur variable dont l'atténuation diminue lorsque le temps s'écoule pendant la période de réception des signaux de façon à harmoniser approximativement la variation anticipée de l'amplitude du champ électrique avec la profondeur de sondage. De plus, un filtre passe-bande FPB-E élimine tous les signaux indésirables de la bande de fréquence utile qui peuvent provenir, par exemple, d'interférences électromagnétiques. La commande de sensibilité de temps CST-E et le filtre passe-bande FPB-E doivent être avantageusement configurés de manière à s'adapter à l'impédance de l'antenne pendant la période de réception des signaux.The electric antenna receiver has three switches Cl-E, C2-E and C3-E installed to protect the receiver from an overload that could occur during the signal transmission period. There is a single amplifier Al in association with the electric antennas AE1, AE2 and A3 and the latter is connected to each of them by means of a switch CE coupled to a control device CST-E which makes vary the sensitivity as a function of time. Alternatively, an amplifier can also be used for each antenna. The CST-E sensitivity control device consists of a variable attenuator whose attenuation decreases when time elapses during the period of reception of the signals so as to approximate approximately the anticipated variation of the amplitude of the electric field with the depth survey. In addition, an FPB-E bandpass filter eliminates all unwanted signals in the useful frequency band, which may arise, for example, from electromagnetic interference. The time sensitivity control CST-E and the bandpass filter FPB-E must advantageously be configured so as to adapt to the impedance of the antenna during the signal reception period.
Le radar utilisé mettant en oeuvre le procédé objet de l'invention étant immobile, le champ électromagnétique réfléchi est également stationnaire, et les trois antennes sont connectées en séquence à l'amplificateur de manière à réaliser une mesure complète quelque soit le plan de polarisation des ondes .The radar used implementing the method of the invention being stationary, the reflected electromagnetic field is also stationary, and the three antennas are connected in sequence to the amplifier so as to achieve a complete measurement whatever the plane of polarization of the waves.
Au niveau du sous-système récepteur à antenne magnétique, les signaux provenant des antennes magnétiques AMI, AM2 et AM3 sont mesurés de manière similaire : une antenne (AM) est sélectionnée par un commutateur (C-M) puis connectée à un préamplificateur (PA-M) ; le signal sortant alimente alors un amplificateur (A2) par l'intermédiaire d'un dispositif de commande de sensibilité du temps (CST-M) et d'un filtre passe- bande (FBP-M) . De même que pour les antennes électriques, on peut utiliser un amplificateur pour chaque antenne.At the magnetic antenna receiver subsystem, the signals from the magnetic antennas AMI, AM2 and AM3 are measured in a similar way: an antenna (AM) is selected by a switch (CM) and then connected to a preamplifier (PA-M ); the outgoing signal then feeds an amplifier (A2) via a time sensitivity control device (CST-M) and a bandpass filter (FBP-M). As with electrical antennas, an amplifier can be used for each antenna.
Enfin, l'unité digitale comprend un convertisseur analogique numérique CAN, un so mateur et une mémoire. Ainsi, le signal sélectionné par un commutateur à double entrée CDE subit une conversion digitale et des intégrations cohérentes sont alors réalisées pour profiter de l'immobilité du radar et à partir de là, améliorer le rapport signal sur bruit. L'unité de commande du radar a pour fonction de générer de manière continue tous les signaux nécessaires aux différentes opérations. On décrira maintenant un mode de réalisation préféré des antennes électriques et magnétiques utilisées dans le radar.Finally, the digital unit includes an analog-to-digital ADC converter, a controller and a memory. Thus, the signal selected by a CDE double input switch undergoes digital conversion and coherent integrations are then carried out to take advantage of the immobility of the radar and from there, improve the signal to noise ratio. The function of the radar control unit is to continuously generate all the signals necessary for the various operations. A preferred embodiment of the electrical and magnetic antennas used in the radar will now be described.
Les antennes électriques (AE1, AE2 et AE3) sont typiquement des dipôles demi-onde dont la longueur optimale est égale au quart de la longueur d'onde utilisée ; le profil de la résistance électrique le long de celles-ci étant ajusté de manière à atténuer les réflexions internes des signaux transmis et à amortir les résonances naturelles qui empêchent l'observation des couches peu profondes du sous-sol. En variante, on peut également utiliser des monopoles quart d'onde. Comme représenté par la figure 2 , le diagramme de rayonnement dans le plan du champ électrique d'une telle antenne transmettant une onde polarisée linéairement, se caractérise par une structure à lobes : un lobe principal tourné vers le sol, deux lobes secondaires et un lobe plus faible dirigé vers le haut. Ainsi, l'antenne électrique n'éclaire (ou n'envoie de l'énergie) de manière significative que dans un secteur d'angle inférieur à 60°. Les figures 3a et 3b représentent également les variations de puissance rayonnee en fonction de la direction de propagation de 3 antennes électriques utilisées simultanément et déphasées de 120° entre elles. Ces antennes émettent ainsi des ondes polarisées elliptiquement dans un plan. Les diagrammes de rayonnement 3a dans le plan n°l et 3b dans le plan n°2, montrent également une structure à 3 lobes tournés vers le sol .The electrical antennas (AE1, AE2 and AE3) are typically half-wave dipoles whose optimal length is equal to a quarter of the wavelength used; the profile of the electrical resistance along these being adjusted so as to attenuate the internal reflections of the transmitted signals and to dampen the natural resonances which prevent the observation of the shallow layers of the basement. Alternatively, quarter wave monopolies can also be used. As shown in Figure 2, the radiation pattern in the plane of the electric field of such an antenna transmitting a linearly polarized wave, is characterized by a lobe structure: a main lobe facing the ground, two secondary lobes and a lobe weaker directed upwards. Thus, the electric antenna only illuminates (or sends energy) significantly in a sector of angle less than 60 °. FIGS. 3a and 3b also represent the variations in radiated power as a function of the direction of propagation of 3 electric antennas used simultaneously and phase-shifted by 120 ° between them. These antennas thus emit waves polarized elliptically in a plane. The radiation patterns 3a in plane no. 1 and 3b in plane no. 2 also show a structure with 3 lobes facing the ground.
Selon une caractéristique avantageuse de l'invention, les antennes magnétiques (AMI, AM2 et AM3) sont du type à retour de flux à haute fréquence ; elles comportent chacune un capteur et sont connectées à un préamplificateur. Les trois capteurs associés au préamplificateur sont utilisés pour détecter les trois composantes magnétiques des signaux réfléchis sur le radar. En effet, ces antennes magnétiques sont disposées de manière à former un trièdre orthonormé qui servira de repère pour mesurer les composantes magnétiques des signaux réfléchis ou retrodiffuses ; chaque composante étant détectée par l'un de ces capteurs associés au préamplificateur. Pour ce faire, chacun des capteurs comporte une bobine constituée de spires enroulées sur un noyau ferromagnétique . Le matériau de ce noyau ferromagnétique est choisi selon la gamme de fréquence. Ainsi, pour une mission sur Mars où cette fréquence de travail est avantageusement choisie aux environs de 2 MHz, le noyau ferromagnétique est composé, soit d'une tige en ferrite, soit d'un barreau. De plus, les spires primaires et secondaires de chaque capteur sont réalisées par un fil de cuivre émaillé et sont directement enroulées sur le noyau. Leur structure est configurée de manière à minimiser les capacités parasites. Chaque capteur est alors placé dans une structure en résine d'époxy dont la surface externe est recouverte d'une couche conductrice qui protège le capteur contre les champs électriques externes et qui améliore l'équilibre thermique du système.According to an advantageous characteristic of the invention, the magnetic antennas (AMI, AM2 and AM3) are of the high frequency flux return type; they each have a sensor and are connected to a preamplifier. The three sensors associated with the preamplifier are used to detect the three magnetic components of the signals reflected on the radar. In fact, these magnetic antennas are arranged so as to form an orthonormal trihedron which will serve as a benchmark for measuring the magnetic components of the reflected or backscattered signals; each component being detected by one of these sensors associated with the preamplifier. To do this, each of the sensors comprises a coil made up of turns wound on a ferromagnetic core. The material of this ferromagnetic core is chosen according to the frequency range. Thus, for a mission to Mars where this working frequency is advantageously chosen around 2 MHz, the ferromagnetic core is made up of either a ferrite rod or a bar. In addition, the primary and secondary turns of each sensor are produced by an enamelled copper wire and are directly wound on the core. Their structure is configured so as to minimize parasitic capacities. Each sensor is then placed in an epoxy resin structure, the outer surface of which is covered with a conductive layer which protects the sensor against external electric fields and which improves the thermal balance of the system.
Selon une autre caractéristique avantageuse de l'invention, les trois antennes magnétiques du radar mettant en oeuvre le procédé d'obtention d'une imagerie d'un sous-sol peuvent être remplacées par une unique antenne magnétique qui pivote dans 1 ' espace de manière à se positionner correctement pour mesurer les trois composantes magnétiques des ondes réfléchies ou rétrodiffusées .According to another advantageous characteristic of the invention, the three magnetic antennas of the radar implementing the method of obtaining imagery of a basement can be replaced by a single magnetic antenna which pivots in space so that to position themselves correctly to measure the three magnetic components of the reflected or backscattered waves.
Selon une application de l'invention, le procédé d'obtention d'une imagerie d'un sous-sol est utilisé à bord d'une station automatique déposée sur la surface de Mars afin d'étudier les structures géologiques souterraines de cette planète et notamment pour rechercher la présence de glace et/ou d'eau sous forme liquide. Pour ce faire et comme représenté sur la figure 3, le module d'atterrissage sur Mars comporte des panneaux solaires déployables, trois antennes magnétiques orthogonales entre elles ainsi que trois antennes électriques angulairement espacées de 120°. De plus, on choisit une fréquence de travail du radar fixe au voisinage de 2 MHz de manière à sonder le sous- sol jusqu'à une profondeur de 3 kilomètres environ. La longueur optimale des antennes égale à un quart de la longueur d'onde utilisée est donc d'environ 35 mètres ; les pertes en propagation dans le sous-sol diminuent avec la fréquence d'onde et la résolution spatiale requiert une large bande passante. De préférence, on effectuera les mesures de jour à une fréquence inférieure à la fréquence critique de l'ionosphère qui, dans ces conditions, joue le rôle de bouclier contre le bruit électromagnétique galactique.According to an application of the invention, the method of obtaining imagery of a subsoil is used on board an automatic station deposited on the surface of Mars in order to study the underground geological structures of this planet and especially to check for the presence of ice and / or water under liquid form. To do this and as shown in Figure 3, the landing module on Mars includes deployable solar panels, three magnetic antennas orthogonal to each other and three electric antennas angularly spaced 120 °. In addition, a fixed radar working frequency is chosen in the vicinity of 2 MHz so as to probe the subsoil to a depth of approximately 3 kilometers. The optimal length of the antennas equal to a quarter of the wavelength used is therefore about 35 meters; the propagation losses in the subsoil decrease with the wave frequency and the spatial resolution requires a large bandwidth. Preferably, daytime measurements will be made at a frequency lower than the critical frequency of the ionosphere which, under these conditions, acts as a shield against galactic electromagnetic noise.
Selon d'autres applications non limitatives de l'invention, le dispositif mettant en oeuvre le procédé objet de l'invention peut être utilisé en géologie, pour la détection de ressources dans un sous-sol (nappes phréatiques...) ou pour l'exploration des structures géologiques, dans le domaine du génie civil pour la détection d'obstacles enfouis à faible profondeur (canalisations...) ou encore pour des applications militaires (détection de mines...), voire pour des applications en radar aérien. Pour ce faire, il suffit de choisir une fréquence de travail adaptée aux dimensions de l'objet ou de l'obstacle à détecter et à la profondeur de sondage .According to other nonlimiting applications of the invention, the device implementing the method which is the subject of the invention can be used in geology, for the detection of resources in a subsoil (groundwater ...) or for the exploration of geological structures, in the field of civil engineering for the detection of buried obstacles at shallow depth (pipes, etc.) or even for military applications (mine detection, etc.), or even for aerial radar applications . To do this, it suffices to choose a working frequency suited to the dimensions of the object or obstacle to be detected and to the depth of the survey.
La présente invention telle que décrite précédemment offre de multiples avantages ; en particulier, l'emploi d'un tel procédé pour obtenir une image de la structure en trois dimensions du sous-sol et des obstacles enfouis permet, tout en limitant le quadrillage donc le temps et la difficulté de mesure, d'obtenir par inversion des résultats plus précis puisque fondés non seulement sur des mesures de temps de propagation des échos mais aussi sur la connaissance de leur direction d'arrivée.The present invention as described above offers multiple advantages; in particular, the use of such a method to obtain an image of the three-dimensional structure of the basement and of the buried obstacles makes it possible, while limiting the grid therefore the time and the difficulty of measurement, to obtain by inversion more precise results since they are based not only on echo propagation time measurements but also on knowledge of their direction of arrival.
Il demeure bien entendu que la présente invention n'est pas limitée aux exemples de réalisation décrits et représentés ci- dessus, mais qu'elle en englobe toutes les variantes. It remains to be understood that the present invention is not limited to the exemplary embodiments described and shown above, but that it encompasses all variants thereof.

Claims

REVENDICATIONS
1 - Procédé d'obtention d'une imagerie du sous-sol, utilisant un radar à pénétration de sol comportant des moyens d'émission, de réception et de traitement de signaux, caractérisé en ce que :1 - Method for obtaining imagery of the subsoil, using a ground penetration radar comprising means for transmitting, receiving and processing signals, characterized in that:
- on émet des signaux à partir d'un point fixe par rapport au sous-sol et à l'aide d'au moins deux antennes électriques ; on reçoit des signaux réfléchis ou retrodiffuses par des réflecteurs ou des diffuseurs dudit sous-sol à l'aide desdites antennes électriques et de trois antennes magnétiques ;- signals are transmitted from a fixed point relative to the basement and using at least two electrical antennas; receiving reflected or backscattered signals by reflectors or diffusers from said basement using said electrical antennas and three magnetic antennas;
- et on traite lesdits signaux réfléchis ou retrodiffuses à l'aide d'un algorithme afin d'obtenir ladite imagerie dudit sous-sol . 2 - Procédé selon la revendication 1, caractérisé en ce que pour déterminer la distance et la direction desdits réflecteurs ou diffuseurs dudit sous-sol, on émet à l'aide desdites antennes électriques des ondes polarisées linéairement, circulairement ou elliptiquement dans un plan et on mesure les composantes du vecteur de propagation des signaux réfléchis ou retrodiffuses .- And we process said reflected or backscattered signals using an algorithm to obtain said imagery of said basement. 2 - Method according to claim 1, characterized in that to determine the distance and the direction of said reflectors or diffusers of said basement, using said electric antennas waves linearly, circularly or elliptically polarized in a plane and on measures the components of the propagation vector of the reflected or backscattered signals.
3 - Procédé selon la revendication 2, caractérisé en ce que pour obtenir des ondes polarisées circulairement ou elliptiquement dans un plan, on émet lesdits signaux à l'aide de deux ou trois antennes électriques fonctionnant simultanément avec une différence de phase convenablement choisie entre lesdits signaux émis.3 - Method according to claim 2, characterized in that to obtain circularly or elliptically polarized waves in a plane, said signals are transmitted using two or three electric antennas operating simultaneously with a phase difference suitably chosen between said signals issued.
4 - Procédé selon la revendication 2, caractérisé en ce que pour obtenir des ondes polarisées linéairement, on émet lesdits signaux de façon indépendante le long des directions respectives de chacune des desdites antennes électriques.4 - Method according to claim 2, characterized in that to obtain linearly polarized waves, said signals are transmitted independently along the respective directions of each of said electrical antennas.
5 - Radar pour la mise en oeuvre du procédé selon l'une quelconque des revendications précédentes, caractérisé en ce qu'il comporte au moins deux antennes électriques (AE) utilisées à la fois pour l'émission de signaux et la réception de signaux réfléchis ou retrodiffuses par des réflecteurs ou diffuseurs du sous-sol, trois antennes magnétiques (AMI, AM2 et AM3) utilisées pour la réception desdits signaux réfléchis ou retrodiffuses, et un dispositif électronique pour commander et traiter ces signaux. 6 - Radar selon la revendication 5, caractérisé en ce que lesdites antennes électriques (AE) sont des dipôles demi-onde ou des monopoles quart d'onde.5 - Radar for the implementation of the method according to any one of the preceding claims, characterized in that it comprises at least two electric antennas (AE) used both for the transmission of signals and the reception of reflected signals or backscattered by reflectors or diffusers in the basement, three magnetic antennas (AMI, AM2 and AM3) used for the reception of said reflected or backscattered signals, and an electronic device for controlling and processing these signals. 6 - Radar according to claim 5, characterized in that said electrical antennas (AE) are half-wave dipoles or quarter-wave monopoles.
7 - Radar selon l'une des revendications 5 ou 6, caractérisé en ce que lesdites antennes magnétiques (AMI, AM2 et AM3) sont remplacées par une unique antenne magnétique pivotante dans 1 'espace.7 - Radar according to one of claims 5 or 6, characterized in that said magnetic antennas (AMI, AM2 and AM3) are replaced by a single magnetic antenna pivoting in one space.
8 - Radar selon l'une des revendications 5 à 7, caractérisé en ce qu'il est utilisé dans le domaine militaire pour la détection de mines ou analogues.8 - Radar according to one of claims 5 to 7, characterized in that it is used in the military field for the detection of mines or the like.
9 - Radar selon l'une des revendications 5 à 7, caractérisé en ce qu ' il est utilisé dans le domaine du génie civil pour la détection d'obstacles enfouis à faibles profondeurs tels que notamment des canalisations . 10 - Radar selon l'une des revendications 5 à 7, caractérisé en ce qu'il est utilisé dans le domaine de la géologie pour l'exploration des structures géologiques d'un sous-sol, notamment pour la détection des ressources du sous-sol. 9 - Radar according to one of claims 5 to 7, characterized in that it is used in the field of civil engineering for the detection of obstacles buried at shallow depths such as in particular pipelines. 10 - Radar according to one of claims 5 to 7, characterized in that it is used in the field of geology for the exploration of the geological structures of a subsoil, in particular for the detection of the resources of the sub- ground.
EP00993445A 1999-12-14 2000-12-12 Method for obtaining underground imagery using a ground-penetrating radar Withdrawn EP1252534A1 (en)

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FR9915768A FR2802303B1 (en) 1999-12-14 1999-12-14 METHOD FOR OBTAINING BASEMENT IMAGING USING GROUND PENETRATION RADAR
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