GB2479872A - Apparatus for radar detection of buried objects - Google Patents

Abstract

A ground penetrating radar arranged to illuminate a volume under inspection and to receive first and second substantially orthogonal return signals, and to process the signals in order to enhance the relative signal intensity from wanted objects. The system is especially relevant to the location of elongate objects such as pipes 2. The two orthogonal signals may be processed, for example subtracted from each other to highlight elongate objects.

Description

APPARATUS FOR RADAR DETECTION OF BURIED OBJECTS

FIELD OF THE INVENTION

The present invention relates to an apparatus for, and method of, radar detection of buried objects.

BACKGROUND OF THE INVENTION

It is known that civil engineering, and other applications, often require the detection of buried elongate or linear objects. In this context "buried" means hidden within a substrate, although this typically this often means buried in the ground.

An elongate object may be something whose path is not strictly linear, for example a pipe or cable whose path meanders, but which definitely has a length direction which is much longer than a width direction. Thus strict geometrical interpretation of the terms elongate or linear is not implied by use of these terms.

The capability of ground penetrating radars to identifiy objects is severely limited by "clutter", which represents echoes returned from other assorted objects within the field of view of the antenna and at the same range as the wanted objects. Another problem with ground penetrating radar is that the resultant display of the data is often ill-matched to a cross sectional image of the ground that has been traversed.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a ground-penetrating radar arranged to observe a volume under inspection at two mutually orthogonal polarisations, one substantially parallel to the orientation of the elongated object and the other substantially orthogonal to it.

When a radar antenna is polarised parallel to the axis of the elongated object, the elongated object acts like a similarly polarised antenna, for efficient reception and retransmission.

However, when the polarisation of the radar antenna, is at right angles to the axis of the elongated object, coupling to that object and coupling of the back-scatter signal to the radar are both much reduced. On the other hand, clutter has generally no preferred orientation, and so the clutter returns are substantially the same for both polarisations. Hence appropriate signal processing, such as subtraction of the two signals, yields the co-polar radar echo of the wanted elongated object with diminished potentially obscuring clutter.

The two mutually orthogonal radar signals can be obtained by sequential transmission and reception, using two mutually orthogonal antennas or from two mutually orthogonal feeds to a common collimator. Alternatively simultaneous transmission a both polarisations or, equivalently to this, a transmission polarised at 45° to both, permits the independent reception of the returns at the two orthogonal polarisations. As a further alternative, circular polarisation may be transmitted. This is equivalent to two orthogonal polarisations, with a 90° phase shift between them, and so it too permits the extraction of the two orthogonally polarised components from the return signal.

Normally the buried linear object which it is desired to locate can be assumed to be approximately horizontal and its approximate orientation is known or can be postulated.

Hence a downward looking radar may be mounted on a suitable carrier, such as a vehicle and moved at right angles to the orientation of the object across a search zone. In a simple embodiment the vehicle may be manually operated and may be similar in construction to a push-chair or cart with a sensor such as a device monitoring the rotation of a ground contacting wheel to record the distance transversed across the search zone.

The antenna or array of antennas may advantageously be kept just clear of touching the ground by individual or joint mounting on a wheeled carrier, or by using electric hover fans, or by the use of endless bands similar in shape to the tracks of a caterpillar tractor.

Advantageously the dialectic properties of such a band are similar to those of the ground.

This reduces unwanted signal returns at interfaces between different dielectrics.

Alternatively the antenna impedance could be chosen so that its ratio to that of the air boundary makes a negative reflection at the antenna to air interface substantially equal in magnitude to a positive reflection at an air to ground interface. Preferably the antenna height is then chosen to equal half a wavelength, or an integral multiple thereof, such that these reflections undergo substantial neutral destructive interference.

Advantageously to further enhance clutter discrimination a single antenna may be replaced by a linear array of several antennas parallel to the expected orientation of the target. The antennas in the array may be coupled via equal path length wave guides or cables so as to be fed in parallel from a single transmitter and to feed a single common receiver. In such an arrangement the signals from the antennas will then be coherent only when all of the antennas experience the same propagation delay to their respective points of specular reflection from the elongated target, i.e. when the target corresponds to the expected orientation. In a preferred embodiment the linear antenna array comprises three or four antennas.

Preferably so as to cover a range of alternative target orientations the antennas may operate independently. For economy this may be achieved by switching a single transmitter and receiver consecutively to the antennas and recording the received signals, possibly with range dependent gain to compensate for range dependent attenuation in the ground. The received signals may then be processed when data processing power becomes available.

This need not be done in strictly real time, or indeed in real time at all.

There are numerous possible configurations of antennas of distinct polarisations or types, switches, and duplexers (if any antennas are used simultaneously for transmission and reception) which will be evident to those skilled in the art. A preferred embodiment comprises a transmit line array of, say, 3 axially polarised antennas interleaved with 3 orthogonally polarised ones, and parallel to it a similar receive array. Two synchronised 6-way switches are then be used to connect a single common transmitter and a single common receiver consecutively to the 6 pairs of transmit antenna and receive antennas.

BRIEF DESCRIPTION OF THE DRAW1miGS

The present invention will further be described, by way of non-limiting example only, with reference to the accompanying Figures, in which: Figure 1 schematically illustrates signal paths from a plurality of antennas to an elongated reflector; Figure 2 schematically shows a ground penetrating radar constituting an embodiment of the present invention; and Figure 3 schematically illustrates a technique for cancelling reflections from interfaces.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 schematically illustrates an arrangement in which an elongate pipe, generally designated 2 is buried beneath a portion of a surface, illustrated as a patch of ground 4.

Three axially polarised antennas are shown, 10, 12 and 14, together with their respective propagation paths to and from the target object, 2. However, each transmission gives rise to a sector of a spherical wave front, irradiating not only the wanted object at its point of specular reflection, but also many other additional reflectors, such as 22 and 24, which would give rise to unwanted return signals otherwise known as clutter. When the transmissions from antennas 10, 12 and 14 are polarised parallel to the axis of the elongate pipe 2, this results in resulting strong reflection from the pipe, but transmissions polarised orthogonally to the orientation of the pipe give rise to only weak reflections. The signals returned from the reflectors 20 and 22 are not polarised. Thus by making the receive antennas sensitive to the polarisation of the returning signal it becomes possible to enhance the signal returned from the pipe 2 at the expense of the signal returned from the reflectors and 22.

Interleaved between these three antennas are three additional antennas 16, 18 and 20, polarised in the orthogonal direction, so that these additional antennas give only a weak return from the wanted target 2, but give similar returns to those received by antennas 12, 14 and 16 from the clutter targets 22 and 24. This creates the opportunity for cancelling the clutter returns with minimal loss of signal from the pipe 2.

To avoid the duplexing problems of near-simultaneous high-power transmission and low-power reception at the same antenna, a single line array of antennas may be replaced by two separate, closely spaced arrays, one for transmission and one for reception. The resultant combined array of 2N antennas can give rise to up to N(N-1) independent transmit/receive antenna pairings, but in the present application there is limited benefit in going beyond the simple scheme of N pairings.

Various techniques are known to the person skilled in the art for selecting polarisations of radiation to be transmitted to the antennas. However, as indicated above, a simple one is to transmit alternately from two orthogonal antennas or two orthogonal feeds to a common parabolic reflector or other collimator.

Figure 2 shows a survey vehicle, generally designated 30 which carries a plurality of transmit antennas which can be selectively connected via a mechanical or electronic switch to transmitter 40, and a plurality of receive antennas which can be selectively connected via similar switch to receiver 42. The receiver provides an output from each distinct pairing of one transmitting and one receiving antenna in turn to a data processor 44. One or more position sensors, optionally including a wheel sensor 46 are provided such that motion of the vehicle along the ground can be tracked such that the received signals from the antennas can be assigned to their correct spatial locations with respect to the path over which the vehicle has travelled. This enables the data processor to match the return signal to the position of the vehicle. More complex positioning arrangement may also be used, and these are well known to surveyors and hence do not need further description.

Figure 3 schematically illustrates the antenna matching scheme for use when it is impracticable to keep the antennas very close to and directly matched to the ground. The reflection at the interface between a high-impedance antenna and low-impedance air is then in anti-phase to that between low-impedance air and high-impedance soil. Making the two-way path-length difference equal to one wavelength (or an integral multiple thereof) causes destructive interference between these reflections. A suitable choice of antenna impedance can then make their magnitudes equal, giving rise to substantial cancellation, except for the first and last wavelength (or wavelengths) of a pulse.

For linear objects at orientations other than parallel to the direction of a linear antenna array there is a linear change across the aperture of the antenna array of the propagation delay to the points of specular reflection from a linear object. Hence such targets may be detected and their orientation defined by separately combining the recorded signals from the relevant observable and discriminable target orientations by applying the appropriate delay gradients across the array.

The system can generate a cross sectional image of the search zone in the form of a 2D array of "resolution cells". The width and depth of these resolution cells are proportional to the wavelength of the microwave or radio-wave radiation illuminating the ground. This wavelength is shorter than that in air, due to the higher refractive index of the ground. The resolution cells are also an inverse ftinction of the range of viewing angles within a cross sectional plane over which the cell is observed during transit of the radar over the ground.

The path delays to each resolution cell in the cross section of the search zone vary during the ground penetrating radar's transit. However with a known speed of propagation in the ground they are predictable. By equalising the delays in reading from recorded signals the echoes from any target or element of the target located in a resolution cell can be summed coherently. Following the sensor's traverse of the search zone this computation can be performed offline for all potentially relevant resolution cells to generate a complete cross sectional image.

Any linear object smaller in "width" than the resolution cell is mapped as one cell in the cross sectional image. Larger flat objects including for example the top of a reflecting liquid in a pipe are mapped as several resolution cells side by side.

The cylindrical shell of a large diameter pipe or duct may be identified as a circle, or part of a circle, of resolution cells. A solid or hard pipe primarily reflects from its upper convex outer surface, but the radiation may penetrate the outer surface of a soft pipe to be reflected primarily from the far concave inner surface. This distinction can be useful in identifying the nature of pipes when, for example, one or more pipes are relatively close to each other.

For pipes not quite large enough to form a clear circle of resolution cells each relevant and discriminable pipe radius can be postulated on a trial and error basis in relation to each potentially relevant centre line resolution cell.

For a hard i.e. reflecting pipe this requires adding twice the postulated pipe radius to the observed two-way propagation delay. The postulated radius which yields the strongest signal then identifies the radius of the pipe which can be displayed centred on that resolution cell. For a soft, i.e. transparent pipe twice the postulated pipe radius is subtracted from the observed two-way propagation delay.

Claims (10)

1. CLAIMS1. A ground penetrating radar arranged to illuminate a volume under inspection and to receive first and second substantially orthogonal return signals, and to process the signals in order to enhance the relative signal intensity from wanted objects.
2. 2. A ground penetrating radar system as claimed in claim 1, further comprising a vehicle adapted to carry the system across a search zone and to record the distance travelled.
3. 3. A ground penetrating radar as claimed in claim 1 or claim 2, further comprising antenna height control, either to keep the antennas very close to and matched to the ground, or else adjusting the height of antennas above the ground to provide for cancellation of reflection at the surface of the ground.
4. 4. A ground penetrating radar system as claimed in any one of the preceding claims, comprising a plurality of antennas supporting two orthogonal polarisations, arranged in a linear array.
5. 5. A ground penetrating radar system as claimed in any one of the preceding claims, further comprising means for combining signals from a plurality of antennas within an array, and matching the signals to identify targets.
6. 6. A ground penetrating radar system as claimed in any of the preceding claims, including means for coupling one switched transmitter to all the transmitting antennas in turn and, in synchronism with this, coupling one switched receiver to all the receiving antennas in turn and recording signals for further analysis.
7. 7. An arrangement as claimed in any one of the preceding claim, further including a data processor for performing post reception synthetic focusing.
8. 8. An apparatus as claimed in any one of the preceding claims, ftirther including means to generate a two-dimensional cross-sectional representation of an image in the ground for display to an operator.
9. 9. An apparatus as claimed in any one of the preceding claims, ftirther including means for deriving a diameter of a pipe by coherent hypothesis testing of the radius of the pipe and displaying the resulting circular cross-section thereof
10. 10. A method of detecting elongate objects, comprising illuminating a search volume with radio or microwave radiation, receiving reflected signals, the signals having first and second orthogonal polarisations, where one of the polarisations is substantially parallel with an axis of an elongate object that it is desired to detect, and processing the signals to subtract one from the other to enhance returned signals from the elongate object