GB2406990A

GB2406990A - Ground penetrating radar

Info

Publication number: GB2406990A
Application number: GB0421679A
Authority: GB
Inventors: Hans-Martin Braun; Gunnar Triltzsch
Original assignee: RST GmbH
Current assignee: RST GmbH
Priority date: 2003-10-06
Filing date: 2004-09-30
Publication date: 2005-04-13
Also published as: AU2004218608A1; CA2483462A1; GB0421679D0; DE10346816A1

Abstract

A ground penetrating radar is disclosed for determining irregularities 14 in ground formations. The radar device is arranged to emit electromagnetic waves 10,12 into a ground formation and receive a reflection 11,13 from an irregularity, wherein at least two frequencies are emitted by the radar device, namely a lower frequency from the range of 400MHz to 1.6GHz, and a higher frequency from the range of 1GHz to 4.6GHz. The irregularities may be cavities or cracks in rocks, or foreign bodies in soils such as landmines. Two frequency bands may be emitted from the radar device having a stepped frequency from the lower frequency range and the higher frequency range respectively. The radar device may comprise at least two emitters and at least one receiver. The radar device may be portable.

Description

Title: Method of determining irregularities in ground formations

DESCRIPTION

The invention relates to a method of determining irregularities in ground formations, more especially cavities or cracks in rocks and foreign bodies in soils, by means of electromagnetic waves which are emitted into the ground formations, and the reflection of the irregularity thereof is received.

Prior Art

So-called ground penetrating radar is known for testing ground formations. Electromagnetic waves, with frequencies of from 1 MHz to GHz, are emitted into the ground by means of this ground penetrating radar in order to obtain information about its structure. The electromagnetic waves or radio waves are reflected at boundary layers where the dielectric properties of the material change. In order to determine the distance from the detected boundary layer of the irregularity, the time is measured between the emission and receiving of the waves. This system is also called pulse radar.

The so-called step frequency radar uses an additional technique. A sequence of electromagnetic waves of a variable stepped-down or steppedup frequency is emitted by said radar. Each frequency is emitted for a specific time interval, and then the frequency is increased or reduced until the desired spectrum is covered. The system measures the amplitude and the phase of the received echo signal for each frequency.

The use of a step frequency radar is described, for example, in GB 2 249 448 A, DE 198 47 688 At or US 5 325 095.

However, all of these methods have the considerable disadvantage that the irregularity can only be determined with a very great amount of difficulty. For example, a oracle in a rock formation cannot be distinguished front an embedded layer of a different material with the known method.

Object The basic object of the present invention is to provide a method and an apparatus of the above-mentioned type, whereby the type of irregularity in ground formations can be determined.

Achievement of the Object To achieve the object, at least two frequencies are emitted, namely a low frequency from the range of 400 MHz to about 1. 6 GHz and a high frequency from the range of about I GHz to 4.6 GHz.

The method according to the invention utilises the reflection behaviour of waves of variable wavelength at irregularities having variable spectral characteristics. It was ascertained that normal ground layers, e. g. layers of variable material, only reflect high frequencies to a very small extent, whereas, for example, cracks or cavities reflect frequency frown the low range to a very small extent. If in fact two frequencies, which lie relatively far apart from each other, are emitted into a ground formation, a conclusion may be drawn, from the quantity of reflected radiation of each frequency, as to whether a normal change of ground layer or a crack is involved.

In a preferred embodiment, the above-described step frequency method is vised for the two emitters and receivers, i.e. complete stepped-down frequency bands are respectively emitted. One frequency band belongs to the low frequency range, and another frequency band belongs to the relatively high frequency range.

The operation can be accomplished with a device in which there are at least two emitters or, however, there are at least two emitters which are also parallel with two devices. The number of receiving aerials may vary.

The signals are processed by appropriate software, which may be an integral component part of the apparatus according to the invention. The software may be so designed that utilisable images are issued which indicate to the user whether there is a normal ground layer or a crack.

Furthermore, an automatic detection means is also possible which, in the event of a crack being recognised for example, issues a warning.

The apparatus according to the invention may be disposed on a separate machine which, for example, travels through a shaft of a mine, a tunnel, a duct, etc., but an additional concept is to configure the apparatus as a hand device.

The entire method is accomplished in a contactless manner, without the emitting or receiving aerials touching the stone or the ground.

During the determination of landmines in normal ground, an adverse effect is that the ground itself is very absorbent, so that waves can only penetrate for very few centimetres. This applies, above all, to waves with a high frequency, whereby only the structure of the surface can be scanned. Only waves with a relatively low frequency penetrate here. By an appropriate filtering-out of frequencies, therefore, landmines can easily be detected by the apparatus according to the invention.

Description of the Figures

Further advantages, features and details of the invention are found in the following description of preferred embodiments, as well as with reference to the drawing; in the drawing: Figure 1 is a schematic view of the structure of a step frequency radar; Figure 2 is a schematic and diagrammatic view of the reflection behaviour of electromagnetic waves with a quasi-continuous transition of material; and Figure 3 is a schematic and diagrammatic view of the reflection behaviour of electromagnetic waves with a discontinuous transition of material.

According to the present method of the invention, a detection of discontinuous transitions of material, such as cracks and cavities in rocks for example, is effected by a so-called step frequency radar which is known front prior art. Figure 1 illustrates a structure which operates according to the step frequency principle. The basic device is a PC 1, for example a laptop. This includes the radar operation and software for processing data.

The basic device I emits commands to a frequency synthesizer 3 via a path 2, and, in turn, a reference oscillator 4 is associated with said synthesizer.

The signals, which are processed by the frequency synthesizer, are amplified and issued to a transmitting aerial 5. Furthermore, the signals are also transmitted to receivers 6.1 and 6.2 as reference signals for phase measurement purposes, which receivers are connected to a receiving aerial 7. The receiving range amplifies the echo signals and accomplishes an l/Q detection.

The amplitudes and phase signals are collected and transmitted to the basic device I via filters S.1 and S.2 and A/D converters 9.1 and 9.2. The data are processed there.

The principle of the method, according to which the step frequency radar operates, is as follows: In contrast to the conventional pulse radar system, a step frequency radar operates with continuous-amplitude radar signals. The required signal band width for the desired radar resolution is sequentially constructed instead of the constant compound spectrum in the case of pulse radar. A radar emitter sequentially supplies signals which are stepped over the desired frequency range. The stepping-down or stepping-up can be effected linearly for example.

A phase and amplitude measurement of the echo signals is accomplished in the receiving range. The result is input in a data processing means.

Ground formations are schematically illustrated in the main illustration of each of Figures 2 and 3. A diagrammatic evaluation of the material parameter c, is illustrated to the right thereof. A diagram relating to the magnitude of the reflection according to frequency is situated therebeneath.

A quasi-continuous transition of material is illustrated in Figure 2. Said transition is produced, for example, with variations in ground layers when, for example, a loam layer is situated in a sand layer. These are layers with variable spectral characteristics. An emitter 10 and a receiver 11 are indicated in the left-hand region, the emitter I O emitting electromagnetic waves with a low frequency range. An emitter 12 and a receiver 13 are situated in the right-hand region, the emitter 12 emitting electromagnetic waves of a high frequency range.

Because of the change in the material parameter Cr. which is slow compared with the wavelength, waves with a high frequency are only reflected to a small extent, since no discontinuity of material occurs, i. e. the coefficient of reflection is low. In contrast thereto, a discrete parameter change occurs for waves with a large wavelength and, in consequence, a greater proportion of the energy is reflected at the boundary layer. This is also apparent in the diagram beneath the ground formation, which diagram illustrates the development of the coefficient of reflection over the frequency. The position of the transitional region between the two illustrated modes of behaviour depends largely on the nature of the transition of material.

A clearly different reflection behaviour occurs, more especially, with discontinuous transitions of material which are small in area, such as, for example, in the case of a crack 14 in a rock formation according to Figure 3. Only a small proportion of the energy is reflected by waves, the wavelength of which is large compared with the vertical extent of the crack 14, whereas waves with a relatively short wavelength are influenced very much more strongly. This is clearly apparent in the reflection diagram.

The amplitudes of the obtained data from the variable frequency ranges are compared by means of an evaluation program in the basic device, and a decision is derived therefrom concerning the type of existing irregularity. A conclusion is drawn, from the data, as to whether there is only a transition of layers or, however, whether there is a crack. Further information concerning the nature of the boundary layer (conductivity of the irregularity) can be derived from the phase of the reflected signal, since a phase shift occurs at conductive irregularities, such as a landmine for example, which is situated in the ground formation.

Claims

1. Method of determining irregularities in ground formations, more especially cavities or cracks in rocks and foreign bodies in soils, by means of electromagnetic waves which are emitted into the ground formations, and flee reflection of the irregularity thereof is received, characterized in that the at least two frequencies are emitted, namely a low frequency front the range of 400 MHz to about 1.6 GHz and a high frequency from the range of I GHz to 4.6 GHz.

2. Method according to claim 1, characterized in that at least two frequency bands are emitted in the step frequency method from a low frequency range and a high frequency range respectively.

3. Method according to claim l or 2, characterised in that the frequencies and frequency bands respectively are emitted in parallel.

4. Apparatus for accomplishing the method according to one of claims 1 to 3, characterized in that at least two emitters are provided in a basic device for the emission of frequencies and frequency bands respectively from variable frequency ranges, as well as at least one receiver being provided therein.

5. Apparatus according to claim 4, characterized in that the basic

device is portable.

6. Method for determining irregularities in ground formations substantially as hereinbefore described and with reference to the accompanying drawings.

7. Apparatus for determining irregularities in ground formations substantially as hereinbefore described and with reference to the accompanying drawings.