Scanning near field electromagnetic probe

A method and apparatus is devised for detecting objects of interest in which frequency-scanned RF in the HF region of the electromagnetic spectrum is projected out across a given area and returns are detected and converted into image data in which phase, amplitude, range and frequency associated with the incoming data is correlated with frequency-dependent range templates to determine the existence of, the range of and the direction of the objects of interest.

FIELD OF THE INVENTION

This invention relates to a system for detecting electrically conductive or other structures and more particularly to impedance sensing and the utilization of range templates to detect the existence of and the range to the structures.

BACKGROUND OF THE INVENTION

While there are various systems for detecting subterranean structures such as for instance ground-penetrating radars, there nonetheless exists a need to detect such structures at a distance without having to be directly over the structures and project radio waves into the ground.

What this means is that some subterranean structures such as wire, cable or pipes of varying size are objects which one would like to know the existence of, length of and orientation of without having to be right on top of them. These objects exist in urban environments. It is noted that the detection of such objects at a distance, for instance at 100 meters, presents significant problems.

It would be useful to be able to aerial-survey an area to detect such objects, in which the objects in general are of varying size, permeability and dielectric constant.

These objects include mineral deposits buried beneath the earth and can include objects inside buildings as well as moving objects such as personnel within the buildings themselves.

SUMMARY OF INVENTION

The existence of, length and range to a sensed structure or object is detected through a resonance technique involving measuring impedances, in which frequency swept energy in the HF band, typically from 1.5 to 20 megahertz, is projected towards a given area and radiation returned from the area is analyzed. The transmitter used in one embodiment steps its output from 1.5 megahertz to 20 megahertz in 50 kilohertz frequency bins or steps. The returns from the exposed area are monitored using a near field scanning sensor operating in the HF band in which in its simplest implementation involves a single high Q switchable antenna which acts as an electromagnetic probe, for instance mounted on a ground-based vehicle or aerial vehicle. As the vehicle travels in a given direction, the probe collects mutual impedance data between it and the surrounding infrastructure. This data is converted to an image in which amplitude and phase are correlated with range to provide a range profile at each of the stepped frequencies.

It has been found that there is a near field region about 300 meters in diameter where fields are highly inhomogeneous. This inhomogeneity gives rise to unique features associated with the geometries of the objects of interest. One object of interest is a subterranean conductive structure such as a wire, cable or pipe. In one embodiment, the returns from a wire are analyzed in terms of frequency, phase and range, with range-correlated images being generated for each frequency bin.

The area under surveillance is previously mapped to provide frequency-dependent range-correlated image templates (range templates) which are arranged in increased frequency steps. The range-correlated images generated during the sensing are correlated against range templates, each one of the range templates being associated with a given structure and a given orientation of the structure relative to for instance, direction of travel of the vehicle carrying the antenna or antennas.

Upon correlation between the images formed from the returns and the range templates stored as a result of the survey, the stored template most closely matching the image formed from the returns is returned to describe both the existence of a wire, its length, and its range.

One way to describe the subject invention is that it is similar to synthetic aperture radars in which images of the infrastructure are gathered as a vehicle proceeds. There is a near field region about 300 meters in diameter where the fields are highly inhomogeneous and 2-D images with range as one axis and frequency as the other are formed as the vehicle proceeds. The image contains pixels that constitute complex aggregate mutual impedances, or in this case having frequency, phase, amplitude and range attributes.

In one embodiment an S-parameter receiver, in essence a network analyzer, detects the incoming signal and generates a complex reflection coefficient S-11value that is coupled to a frequency/range image generator, which is in turn coupled to an image processor. This image processor generates an image in which range is mapped with respect to frequency, phase and amplitude of the incoming signal. The amplitude and phase responses with frequency are combined into a range-dependant image, with the range dependant image correlated with surveyed and stored templates, each one designating a structure of a predetermined length and a predetermined orientation for a predetermined range. Correlation of the two images provides the identity of the stored range template which most closely correlates to the sensed data, thereby to identify 1) the existence of the structure, 2) the length of the structure, and 3) the range to the structure.

In one embodiment, 370×150 pixel template images of objects of interest are generated and incorporated into the system. Incoming data is continuously correlated against the library of objects of interest in real time. While the subject invention has been described in terms of wires and electrically conductive cables and pipes, other objects of interest having characteristic complex permeability and dielectric constants are within the scope of this invention. This includes both electrically conductive objects, mineral deposits and even people moving within a building.

In one embodiment, a Greens function-like template is used to “map” the city or town. Clutter rejection is possible by comparing or subtracting previously taken data along the same route. The above is particularly attractive since changes in the environment provide important clues as to the current situation.

Experiments have shown that with an output power of 1 milliwatt and a coherent integration time of 750 milliseconds, a 150 foot piece of wire lying on the ground at a range of 100 meters with medium urban/atmospheric daytime noise is detectable with a signal-to-noise ratio of 50 dB at 3.0 MHz. Although there is copious energy available, the effect of clutter from surrounding infrastructure can present problems. If however the images of interest are very different from the clutter, then the correlation pattern recognition is effective to determine an object of interest, namely the existence of a wire. Note that finite difference or gradient sampling is well suited to the subject invention since the near field used to scan the object of interest is rapidly changing.

In one embodiment, in order to detect the resonance associated with a wire or cable the frequency is stepped from 1.5 megahertz in 40 steps. With each step the antenna used for the transmission or receipt of the return signals is appropriately tuned.

A single antenna is used in one embodiment for both transmitting and receiving, requiring an S-11receiver.

When utilizing a second antenna as a receive antenna, the receiver is an S-21mode receiver, with the subject system looking at the transfer function between the two antennas. Here the transfer function changes in the presence of a wire such that the wire is detectable, even though the wire may be 100 feet ahead of the two antennas.

Thus, with a single antenna and the receiver operating in the S-11mode, amplitude, phase, frequency and range are measured in terms of the signal returned at the sole antenna.

In the S-21mode utilizing a transmit antenna and a receive antenna, what is measured is the transfer function between the transmit antenna and the receive antenna.

If for instance the half-wave resonance wavelength is 500 feet, one typically has an antenna length for a half-wave antenna of 160 meters or about 250 feet. With a 500 foot long length of wire, one would see a response at about 900 kilohertz. If there is a half-wave resonance at 900 kilohertz, there will also be a three half-wave resonance at 2700 kilohertz, etc. Thus when one is sweeping through the frequencies in either the S-11or S-21mode, one will see resonant peaks at one half-wave length, three half-wave lengths, or five half-wave lengths. One might also see peaks at one wavelength as well as both odd and even harmonics depending on how the wire is configured.

It is noted that there will be peaks in amplitude at given ranges. There will also be phase peaks in terms of the phase function for the difference in phase between the outgoing signal and the reflected signal. Note that the progression of the phase will be different for different frequencies, different resonances and different ranges. It has thus been found that phase is a function of range for different frequencies, enabling one to template out the effect.

Note also that with respect to amplitude, amplitude will increase in a certain manner characteristic of attenuation along the ground so that another way of obtaining range is to measure amplitude.

What has been found is that for both amplitude and phase there is a characteristic that is a function of range. Thus, the range information in both the amplitude and phase domains can be used simultaneously to obtain range.

Moreover when using the S-21receiver and two antennas, one can determine to which side of a road the structure extends. By appropriate phasing of the two antennas one can provide null and non-null directions. Thus, if there is a return from a non-null direction it can be ascertained to which side of the line of travel the structure is running.

As will be appreciated, in the S-11mode one is looking to analyze the complex reflection coefficient. The complex reflection coefficient is a result of looking at the reflected power from a load or from an antenna. As the antenna approaches an object the reflected power starts to change and by monitoring the reflected power one can sense the object in a rather sensitive manner.

Thus in the S-11mode, one is looking for changes in reflected power in terms of phase and amplitude over frequency correlated with range.

In the S-21mode, since one is looking for changes in the energy that are picked up versus that which is transmitted, the changes are measured in terms of the transfer function between the two antennas.

In one embodiment where the lowest operating frequency is 1.5 megahertz, the antennas can take the form of large coils which are tapped for tuning purposes, with the taps being set by relays or electronic switches that are tracking the transmit frequency.

In summary, a method and apparatus is devised for detecting subterranean or other objects of interest in which frequency-scanned RF in the HF region of the electromagnetic spectrum is projected out across a given area and returns are detected and converted into image data in which phase, amplitude, range and frequency associated with the incoming data is correlated with frequency-dependent range templates to determine the existence of, the range of and the direction of objects of interest.

DETAILED DESCRIPTION

Referring now toFIG. 1, a conductive structure10such as a wire, cable or pipe is located beneath a road12in an urban environment.

In one embodiment, structure10is embedded along the direction of travel18of a vehicle20having a pair of probes30and32. It is the purpose of the subject invention to detect such structures or objects.

In order to provide such detection in one embodiment, and as illustrated inFIG. 2, a pair of tunable high Q electromagnetic probes30and32, one embodiment in the form of HF coil antennas, are coupled to an S-parameter receiver34, the output of which is the unitless complex reflection coefficient S-11, namely involving amplitude, phase and frequency. The heart of the S parameter receiver is a network analyzer. Either S11or S12modes may be used. S11requires just one probe while S12requires two probes. The probes are switch tunable high Q antennas. The idea of the S11mode is to sample the mutual impedance of the probe to objects in the near field. The idea of the S12mode is to sample the transfer function between the two probes in the presence of objects in the near field.

Critical to the sensitivity of the S parameter receiver is the use of a set of reference data at all the frequencies involved. In the initialization phase, each time the receiver is turned on, the system is positioned in a near field object free zone and S11or S12data is taken over all frequencies. This set of data is used as a reference such that the working output of the receiver is S11-S11(ref) or S12-S12(ref). Thus if the system is initialized and operation is started in the near field object free zone, the output of the receiver is zero. This type of operation gives equivalent performance to a receiver with a low noise figure, dependent upon the accuracy of the above subtraction. When an A/D converter is used, the subtraction is done after the A/D converter so that the noise figure and dynamic range are limited by the number of bits. An alternative mode of operation, useful when the system is installed on a moving vehicle; is the differential mode. The differential mode subtracts sequential pairs of data sets. This mode looks for differences in S11or S12as the vehicle progresses. The differential mode requires no absolute reference set.

In the subject invention amplitude, phase and frequency are correlated with range. This means that for each range an image can be generated, the pixels of which reflect amplitude, phase and frequency.

To this end the output of the S-parameter receiver is coupled to a frequency/range image generator36which provides an image comprised of I, Q complex pixels in which for each frequency there is a range image, with the pixel intensities indicating amplitude and phase of the signal returned from the scene.

The images are coupled to an image processor38, in one embodiment provided with the position of the probe from a GPS unit40. The image from the realtime data is compared or correlated with stored image data at42comprising templates that are the result of a survey of the area in question. In one embodiment, the survey for detection of a wire includes amplitude, phase and frequency for various ranges of various length wires in various orientations with respect to the path of a vehicle.

The image processor output is correlated with the stored images at correlator44, with a correlation being output at46to apprise the operator of the vehicle that a wire exists, its range and its direction.

Note that a control48controls the tuning of the high Q EM probes30and32, as well as the stepped frequency transmission of the electromagnetic energy projected into the scene by the transmitter utilized.

Referring toFIG. 3, it can be seen that a transmitter50is frequency swept by a variable frequency oscillator52to project energy out from a tuned antenna54into the probe environment. The antenna is tuned as illustrated by56to be resonant at the requisite stepped frequency bins, thereby providing extremely good sensitivity across the stepped frequency range.

GPS40is coupled to S-parameter receiver34which detects returns from the scene and outputs amplitude60, phase62and frequency66that are utilized to generate the images described above.

As can be seen at graph70, a half-wave phase resonance plot at frequency f1is shown by dotted line72, with the plotted phase defined by the phase of the outgoing signal compared to the phase of the reflected signal. It will be seen that the phase plot at half-wave resonance has the characteristic shown at74for a wire of a particular length and orientation versus range.

Graph70also shows an amplitude plot76that has a particular shape for the one-half wave resonance that is peculiar to a wire of a particular length and orientation versus range.

As shown at76′ for an off-resonance graph, the amplitude plot takes on quite a different configuration as does the phase plot74′ indicating at least in these two graphs that there is a substantial difference between the one-half wavelength resonance response of the system and the off-resonance response.

As seen, images80are generated for frequencies f1, f2, f3. . . fnin which the images are composed of pixels having densities corresponding to a combination of phase and amplitude versus range for each frequency. These images therefore characterize the response of the sensing system for the stepped frequency bins involved.

Survey templates90involve range templates92for the frequencies f1, f2, f3. . . fnwhich are utilized to model the returns from a given length wire at a given orientation with respect to the line of travel of the vehicle. These templates are used in a correlation process carried out by correlator44, with the closest correlation defining that survey template to which the incoming data most closely approximates. This identifies the existence of a wire, the length of the wire and the range of the wire, as well as its direction when multiple phased antennas are used.

Referring toFIG. 4, the graph shows amplitude versus frequency for a given length wire and shows that there are amplitude peaks at the one-half wave resonance, three-half wave resonance and five-half wave resonance for the particular wire. Thus, there are different resonance characteristics associated with the detected wire.

It will be seen that these resonances are equally spaced in a pattern which is highly recognizable and at the very least specifies the length of the wire.

In terms of phase, and referring now toFIG. 5, the phase of the incoming signal versus the phase of the outgoing signal is plotted with respect to frequency, with an inflection100occurring at the one-half wavelength responses, here shown at 3 MHz.

FromFIGS. 4 and 5it will be appreciated that the resonance characteristic associated with a wire can be sensed both in amplitude and phase which, for a given range are uniquely determinative of the range of the wire from the sensing head.

Referring toFIG. 6, if a vehicle20is provided with phased array antennas100and102, assuming an antenna array phasing module104is employed to steer the array, and assuming that there is a null which can be steered across the scene where wire10exists, it will be appreciated that if there is a response at a non-null direction106, it will mean that the wire exists to the right of roadway12, whereas if there is no response when the beam is directed in its null direction as illustrated at108, then the right/left direction of the wire is determinable.

Referring toFIG. 7, the subject system can be characterized by a matched filter110utilized to correlate the detected data to survey-generated range templates, with the wire existence, length and range being outputted as illustrated at114.

The amplitude sensitivity of the subject system is illustrated inFIG. 8. Here in an image120in which frequency is graphed against range exhibits pixels having amplitudes122that appear to be regularly spaced along the frequency axis. This regular spacing indicates the amplitude resonance characteristic of the returned signal in which the resonances are highly recognizable.

Referring toFIG. 9, the phase response of the subject system is shown. Here, frequency is graphed against range, with the pixel densities indicating the phase angle. Note there is a recognizable phase response124which lies within the half-wavelength band126. While the phase relationships in the three-half wavelength resonance band128or the five-half wavelength resonance band130are not so highly recognizable, by correlation with the aforementioned templates recognizable patterns for the phase-related resonance effects of the wire can be ascertained. Note that the darkening of the pixels in bands128and130indicates phase inflections associated with higher resonances.

Referring toFIG. 10, image140indicates the modeled amplitude resonance of an automobile at 100 meters in front of a vehicle, in which the detected response is a smeared out increase in amplitude for the pixels shown in area142.

Referring toFIG. 11, when one considers the amplitude response of a wire, here illustrated at150, one can see the regular amplitude resonances for a wire which have a different periodicity than for instance power lines haying resonance peaks152.

Also noted is the increased density of the pixels representing the presence of a automobile. These appear in the lower right hand side of the image as illustrated at154.

Referring now toFIGS. 12 and 13, amplitude, is graphed against frequency for a 150 foot wire and a 300 foot wire. As can be seen fromFIG. 12, graph160shows amplitude resonances for the 150 foot wire at162and164, corresponding to the half and three-half wave resonances for the 150 foot wire. When detecting a 300 foot wire at the same range, as illustrated by graph170there is a one-half resonance peak at172. This confirms that the length of the wire determines the resonance characteristics and that the resonances are detectable by the subject technique.

While the subject invention has been described in terms of detecting a wire, it can be used to detect all manner of objects that have a characteristic signature to energy returned to the probe.

It can therefore be seen that the subject system has a uniquely sensitive response for detecting resonances due to the presence of an object exhibiting a characteristic permeability and dielectric coefficient, given the correlation process in which range templates are compared with realtime data.