WO2012161645A1

WO2012161645A1 - Ground penetrating radar system comprising a magnetoresistive sensor

Info

Publication number: WO2012161645A1
Application number: PCT/SE2012/050543
Authority: WO
Inventors: Anders Persson
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-05-20
Filing date: 2012-05-18
Publication date: 2012-11-29
Anticipated expiration: 2013-11-20
Also published as: WO2012161645A8; SE1150476A1; SE536435C2

Abstract

A ground penetrating radar (GPR) system (1) and a method of detecting a feature (9) embedded in a solid or liquid medium (10) from outside of that medium are disclosed. The GPR system comprises a transmitter (2) for generating and transmitting a radar signal (8) towards the solid or liquid medium comprising the feature, and a receiver (3) for receiving the radar signal after reflection on the feature, the transmitter comprising a signal generator (4) and at least one magnetic dipole antenna (5). The receiver comprises at least one magnetoresistive sensor, which is used to detect the reflected radar signal.

Description

GROUND PENETRATING RADAR SYSTEM COMPRISING A

MAGNETORESISTIVE SENSOR

TECHN ICAL FI ELD OF TH E I NVENTION AND BACKGROUN D ART

The present invention relates to a ground penetrating radar (GPR) system.

The GPR technique is used in many different areas, such as archaeolog ical surveys, mineral exploration , and mine detection . It offers a rapid and non-destructive method for imaging and analysing soil and rock layers from a couple of metres down to several kilometres. GPR systems are typically based on electric field detection with pulses in the mega- to gigahertz range. Such pulses are emitted by a transmitter antenna, reflected on interfaces between materials with different dielectric permittivity, and detected by the transmitter antenna, or an additional receiver antenna.

This kind of setup suffers from some major disadvantages.

Firstly, the use of resonant antennas limits the bandwidth of the system. This is disadvantageous since information on the material content of the studied soil can be derived from the frequency dependence of the dielectric permittivity and a limited bandwidth thus limits the amount of derivable information .

Secondly, the stratigraph ical information that can be derived from an electromagnetic GPR system is difficu lt to interpret, since almost every change in material , density, humidity, etc. will give rise to an echo. Even though the result is rich in information , the interpretation of it is difficult and interesting features can often be hidden by uninteresting ones. Often , a high ly skilled and experienced expert is required to interpret the data. Third ly, the different relative dielectric permittivity of air and soils causes part of the transmitted signal to be reflected directly off the surface of the ground . Fourthly, resonances in the transmitting and receiving antennas create ringing effects that taint the detected signal.

In order to overcome some of the problems, attempts have been made to use antennas operated in the magnetic near-field regime for transmitting and receiving radar signals, see for example Bellett et a/. , Proceedings of the URSI-F Commission F Triennium Open Symposium, 2004. In this way it is possible to avoid the problem of the transmitted signal being reflected directly off the ground . However, these systems rely on the use of resonant antennas and are still subject to ringing effects and limited bandwidth .

SUM MARY OF THE I NVENTI ON An object of the present invention is therefore to provide a GPR system which in at least some aspect overcomes the drawbacks present in common GPR systems. This object is ach ieved by a GPR system according to claim 1 . Another object of the present invention is to provide a simplified and in at least some aspect improved method of detecting a feature in the ground . This object is achieved by a method accord ing to claim 8.

The ground penetrating radar system according to the invention comprises a transmitter for generating and transmitting a radar signal and a receiver for receiving a radar signal . The transmitter comprises a signal generator and at least one magnetic dipole antenna. The system is characterized in that the receiver comprises at least one magnetoresistive sensor. Since the GPR system accord ing to the present invention uses a magnetoresistive sensor for signal detection instead of a resonant antenna, the bandwidth is increased so that signals from kilo- u p to gigahertz frequencies can be detected . This enables both time and frequency domain analysis of the relative permittivity and magnetic permeability of the materials in the investigated volume. Moreover, the magnetoresistive sensor is not based on resonance and is therefore not subjected to interference, ringing or cou pling to the transmitting antenna. Using a magnetoresistive sensor, the receiver may also be min iaturized .

According to one embodiment of the invention , the transmitter and/or the receiver comprises a shield for shielding the electric field component of a generated radar signal. In th is embodiment, the signal is easier to interpret than in a common GPR system.

According to another embodiment of the invention , the shield is comprised in the magnetic dipole antenna. In this embodiment, the GPR system is configured to operate in the magnetic near- field regime. The data is easier to interpret than data from a common GPR system relying on the electric components of radar signals. Additionally, better ground penetration is achieved and problems related to signal reflection off the air-soil interface are avoided . According to another embodiment of the invention , the magnetic dipole antenna is a shielded loop antenna. The GPR system according to this embodiment is operated in the magnetic near- field regime. According to another embodiment of the invention , the shield is comprised in the receiver. In th is embod iment, the electric field component is removed at the receiver. This enables detection of signals arising from far-field interaction of the radar signal with a feature. According to one embodiment of the invention , the magneto- resistive sensor is a sensor based on anisotropic magneto- resistance, the planar Hall effect, giant magnetoresistance, or tunnelling magnetoresistance. A sensor based on anisotropic magnetoresistance offers a low noise level. A sensor based on the planar Hall effect offers a low noise level and also a high linearity. A sensor based on giant magnetoresistance offers a large dynamic range, while a sensor based on tunnelling magnetoresistance offers a strong signal .

According to one embodiment of the invention , the magneto- resistive sensor comprises magnetoresistive sensing elements configured in a Wheatstone bridge. This embod iment offers a high signal-to-noise ratio, and an improved thermal stability.

According to a second aspect of the present invention , the invention relates to a method of detecting a feature embedded in a solid or liquid medium from outside of that medium . The method comprises the steps of: generating a radar signal; transmitting the generated radar signal towards the solid or liqu id medium using at least one magnetic dipole antenna; and detecting a reflected radar signal from the solid or liquid medium, which reflected radar signal was generated upon interaction of the transmitted radar signal with a feature. The method is characterized in that a magnetoresistive sensor is used in the step of detecting the reflected radar signal.

According to one embodiment of the second aspect of the present invention , a further step of removing an electric field component from the transmitted and/or the reflected radar signal is added .

According to one embodiment of the second aspect of the present invention , the magnetic dipole antenna is operated in continuous mode. In this embodiment, target detection in the magnetic near-field regime is simplified . According to another embod iment of the second aspect of the present invention , the magnetic dipole antenna is operated in pulse mode. In this embod iment, more advanced time and frequency domain analysis can be performed .

BRI EF DESCRI PTION OF THE DRAWI NGS The present invention will now be described in detail with reference to the attached drawings:

Figure 1 is a schematic illustration of the GPR system accord ing to the invention , and

Figure 2 shows a radar image of an iron disc at a depth of 1 m acquired using the GPR system.

DETAI LED DESCRI PTI ON OF EMBODI M ENTS OF THE I NVEN- TION

The ground penetrating radar (GPR) system according to the invention is schematically shown in figure 1 . The GPR system 1 comprises a transmitter 2 and a receiver 3. The transmitter comprises a signal generator 4 and one or more magnetic dipole antennas 5. The receiver comprises one or more magnetoresistive sensors. The receiver is connected to an amplifier 6 and a data acquisition unit 7. In operation , a radar signal 8 is generated by the signal generator 4. In order to detect a feature 9 embedded in a solid or liqu id medium 10, the radar signal is transmitted by the magnetic dipole antenna 5 towards that medium. The transmitted radar signal is then reflected on interfaces between materials within the medium with different magnetic permeability and/or different relative permittivity. The reflected signal is detected by the magnetoresistive sensors comprised in the receiver 3 as a variation in magnetic field strength and/or direction . After detection , an outgoing signal from the sensor is amplified by the amplifier 6 and passed on to the data acquisition unit 7 for further analysis. A radar image of an iron disc with a diameter of 27 cm, buried in the ground at a depth of 1 m, which was acquired using a GPR system according to the present invention , is shown in figure 2. The magnetoresistive sensor used for detecting the reflected radar signal comprises sensing elements with a resistance dependent on the magnetic field strength , direction and/or variation . The magnetoresistive sensor used in the present invention may for example be a sensor based on anisotropic magnetoresistance (AMR), the planar Hall effect (PHE), giant magnetoresistance (GMR), or tunnelling magnetoresistance (TMR). Such sensors are highly sensitive and can detect signals rang ing from kilohertz up to gigahertz frequencies. Both time and frequency domain analysis of the relative permittivity and magnetic permeability of an investigated volume can therefore be carried out. In general, sensors based on AMR offer low noise levels, but unfortunately also low signals. Sensors based on PHE exh ibit low noise and good linearity. Sensors based on GMR, also termed spin valves, typically exhibit higher noise levels than sensors based on AMR and PHE , but a very broad dynamic range. Sensors based on TMR, also termed magnetic tunnel junctions, give high output signals, but unfortunately also relatively high noise levels. In one embodiment of the invention , the magnetoresistive sensor is comprised of several sensing elements, which sensing elements can be magnetic tunnel junctions or spin valves. The sensing elements are arranged on a substrate so as to form a Wheatstone bridge comprising four branches. Two branches may be magnetically shielded in order to serve as reference resistances, while the two non-shielded branches serve as sensing branches. Each branch may comprise several sensing elements connected in parallel and/or series in order to optimize the signal-to-noise ratio. A common factor for all mentioned magnetoresistive sensors is that they do not rely on resonances with the transmitting antenna. This factor renders the detected signal from the GPR system according to the invention more stable than signals from a common GPR system based on resonances between a transmitting and a receiving antenna. The sensor is not subjected to interferences, ringing or coupling to the transmitting antenna, and the output data from the receiver is therefore much easier to interpret. The antenna should be a magnetic dipole antenna, which may or may not be shielded to remove the electric field component from the transmitted radar signal. In a preferred embodiment, a shield is included in the antenna, such as in a shielded loop antenna. In this embodiment, the shield in the antenna removes the electric field component from the transmitted radar signal and consequently, only the magnetic field component of the radar signal interacts with the material of the solid or liquid medium. Since the signal has no electric field component, it is only reflected on interfaces between materials with different magnetic permeability, and consequently, no signals resulting from dielectric reflections are detected by the receiver. The output data from the receiver is less rich in information than data from a common GPR system relying on dielectric reflections, but on the other hand, much easier to interpret.

Using a shielded antenna for signal transmission in the GPR system means that the system is configured to work in the magnetic near-field region, or at a distance from the antenna corresponding to a maximum of about 2λ, where A is the wavelength of the transmitted radar signal. Operation in the magnetic near-field region enables better penetration into most liqu id or solid mediums than in a common GPR system, since the entire signal is transmitted into the med ium, given that the med ium has the same magnetic permeability as the atmosphere in which the transmitter is located . The GPR system according to th is embodiment thereby avoids problems related to the electrical signal being reflected off e.g . an air-soil interface as a consequence of different relative dielectric permittivity of air and soil .

The GPR system according to the present invention can be operated in either continuous mode or pulse mode. In continuous mode, a feature is identified by studying the standing wave pattern between the transmitting antenna and the receiver. This mode is particularly useful for basic target detection and is carried out in the magnetic near-field region . A distance to the feature may be determined by transmitting radar signals of at least three orthogonal frequencies. In pulse mode, the distance to the feature is determined by measuring the time delay from transmitting the radar signal to receiving the reflected signal. The pulse mode enables more advanced time and frequency analysis than the continuous mode, and can also be used for detecting features in the electromagnetic far-field region .

The antenna may also be a different kind of magnetic dipole antenna, such as a spiral antenna. In the case where the antenna is not shielded , the receiver will, in addition , detect electromagnetic far-field reflections, and hence materials with different dielectric permittivity. A shield may instead be included in the sensor to prevent false signals from arising as a result of interaction of these far-field reflections with electronics surround ing the sensor.

Claims

CLAI MS

1 . Ground penetrating radar system (1 ) comprising a transmitter

(2) for generating and transmitting a radar signal (8) and a receiver (3) for receiving a radar signal, the transmitter comprising a signal generator (4) and at least one magnetic dipole antenna (5) ,

characterized in

that the receiver comprises at least one magnetoresistive sensor.

2. Ground penetrating radar system ( 1 ) according to claim 1 , characterized in that the transmitter (2) and/or the receiver

(3) comprises a shield for shielding the electric field component of a generated radar signal (8) .

3. Ground penetrating radar system (1 ) according to claim 2, characterized in that the shield is comprised in the magnetic dipole antenna (5) .

4. Ground penetrating radar system (1 ) according to claim 3, characterized in that the magnetic dipole antenna (5) is a shielded loop antenna.

5. Ground penetrating radar system (1 ) according to claim 2, characterized in that the shield is comprised in the receiver (3) .

6. Ground penetrating radar system (1 ) according to any of the previous claims, characterized in that the magnetoresistive sensor is a sensor based on anisotropic magnetoresistance, the planar Hall effect, giant magnetoresistance, or tunnelling magnetoresistance.

7. Ground penetrating radar system (1 ) according to any of the previous claims, characterized in that the magnetoresistive sensor comprises magnetoresistive sensing elements configured in a Wheatstone bridge.

8. Method of detecting a feature (9) embedded in a solid or liqu id medium (10) from outside of that medium, the method comprising the steps of:

- generating a radar signal (8) ;

- transmitting the generated radar signal (8) towards the solid or liquid medium (10) using at least one magnetic dipole antenna (5) ;

- detecting a reflected radar signal (8) from the solid or liquid medium (10), which reflected radar signal was generated upon interaction of the transmitted radar signal with a feature (9) ;

characterized in

that a magnetoresistive sensor is used in the step of detecting the reflected radar signal (8).

9. The method of claim 8, characterized in a further step of removing an electric field component from the transmitted and/or the reflected radar signal (8) .

10. The method of claim 8 or 9, characterized in that the magnetic dipole antenna (5) is operated in continuous mode.

1 1 . The method of claim 8 or 9, characterized in that the magnetic dipole antenna (5) is operated in pu lse mode.