BACKGROUND

1. Field of the Invention

The present invention is in the general technical field of buried object detectors, and more particularly, it relates to detecting non-ferrous materials such as wires, pipes, hollow cavities, and other buried objects,

2. Discussion of the Prior Art

Several techniques for detecting buried objects are known, such as active illumination, induction coils of various types, and ground penetrating radar. None of these approaches is effective at detecting small, buried non-ferrous materials such as copper wire, bronze, hollow cavities, and dielectric materials.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In one embodiment, a method for detecting the presence of a dielectric boundary includes transmitting a series of electromagnetic signals from an antenna having a generally linear configuration into the surface of a region, wherein successive signals in the series have different orientations that rotate at a scan frequency. A series of electromagnetic signals are received after being reflected back to the antenna from the surface of a region. The complex reflection coefficient of the received signals is measured and an anisotropy vector is calculated representing differences in the measured complex reflection coefficients in response to the different orientations of the electromagnetic signals. The magnitude of the anisotropy vector is used to determine the presence of a dielectric boundary embedded in the region.

In another embodiment, a system for detecting a dielectric boundary includes an antenna disposed to radiate electromagnetic radiation toward the surface of a region and a transmitter for generating a series of signals for transmission by the antenna, the antenna generating a series of electromagnetic signals having an orientation that rotates at a scan frequency. Also, the system includes a receiver for processing signals received by the antenna, and a vector network analyzer for measuring the complex reflection coefficient of the received signals and for calculating an anisotropy vector representing differences in the measured complex reflection coefficients in response to the different rotating orientations of the electromagnetic signals, wherein the magnitude of the anisotropy vector may be used to determine the presence of a dielectric boundary in the region..

In a further embodiment, a system for detecting objects embedded in the ground includes an antenna disposed to radiate electromagnetic radiation toward the surface of ground and a transmitter for generating a series of signals for transmission by the antenna, the antenna generating a series of electromagnetic signals having an orientation that rotates at a scan frequency. The system also includes a receiver for processing signals received by the antenna, and a vector network analyzer including a detection processor for measuring the complex reflection coefficient of the received signals and for calculating an anisotropy vector representing differences in the measured complex reflection coefficients in response to the different rotating orientations of the electromagnetic signals, wherein the magnitude of the anisotropy vector may be used to determine the presence of an object in the ground.

BRIEF DESCRIPTION OF THE DRAWING

The objects, advantages, and features of embodiments of the invention will become more apparent from the following detailed description, when read in conjunction with the accompanying drawing, in which:

Fig. 1 is a perspective view of a mechanically rotated single antenna embodiment of the present invention;

Fig. 2 is a perspective view of a mechanically rotated dual antenna embodiment of the present invention;

Fig. 3 is a top view of an electronically scanned three-antenna embodiment of the present invention;

Fig. 4 is a top view of an electronically scanned eight- antenna embodiment of the present invention;

Fig. 5 is a simplified representation of a vector network analyzer for use with embodiments of the invention; Fig. 6 is a simplified Smith Chart, which relates to the vector network analyzer of Fig. 5;

Fig. 7 is a simplified representation of the vector network analyzer of Fig. 5, with an auto-tuner incorporated between the sensor antenna assembly and the vector network analyzer; and

Fig. 8 is a simplified Smith Chart, which relates to the vector network analyzer of Fig. 7.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In a broad characterization, the present invention can be described as an antenna sensor with support electronics specifically tailored to the detection of buried objects by measurement of microwave anisotropy. Depending on the nature of the target, size constraints, intended search depth, and concept of operations the sensor is either hand-held, mounted on a robot, or mounted on a vehicle.

An embodiment of the present invention is a buried object sensor that exploits the electromagnetic anisotropy of ground caused by the presence of foreign objects having electromagnetic properties which are different than the surrounding ground. Specifically, electromagnetic anisotropy is understood as a measurable property of the characteristic impedance of ground which is not uniform in all directions. This measurement is made using either one or more mechanically rotated or electronically scanned antennas above ground which interact with the complex reflection coefficient of the air-ground interface. S parameter measurements taken at the sensor antenna (or antennas), which antenna is close to and electromagnetically coupled to ground, is representative of the complex reflection coefficient of the air-ground interface. By rotating (or electronically switching) a sensor antenna (or antennas) and extracting the now modulated anisotropy vector from the main reflection (the air-ground interface) the properties of the anisotropy vector are revealed.

A feature of this invention is the ability to measure and exploit these properties to reveal 1) the presence of a buried object, 2) its orientation with respect to the scanning sensor, and 3) the depth of the object.

The characteristic impedance of any electromagnetic medium is given as follows; where:

Z is the characteristic impedance of the medium; μ is the magnetic permeability; ε ₀ + jε ^* is the complex dielectric constant (the displacement component);

is the conductive component;

ε is the real portion of the displacement term of the complex electric permittivity; ε ^* is the imaginary (loss) portion of the complex electric permittivity; σ is the bulk conductivity of the dielectric; ω is the frequency in radians per second; and j = V-T which characterizes the complex property of this relationship.

As frequency increases, the displacement term in the denominator (G ₀ + jε ^* ) dominates the conduction term ( σj jω ). hi practice, the conduction term can be ignored in electromagnetic propagation studies for most common boundaries above 200 MHz, including sea water and ground. The exception to this approximation is where a very highly conductive material is present, such as a conductive metal (copper and aluminum are examples). Here the conduction term is dominant over a much wider frequency range and its contribution to the denominator cannot be ignored.

In the case of the air-ground interface, we observe an impedance mismatch by measuring the complex reflection coefficient, 1 , where

Eq. 2

^ati satϋε

where: ^ _so u _rce == 377 + _/0 , the characteristic impedance of free space; and

μ

^toad is the complex impedance of ground as defined above.

. * σ ε + jε H

When the displacement term of the electric permittivity of ground has anisotropic (directional) properties, such as for a wire or the edge of a metal object, the dielectric constant of the medium (in this case, ground) also demonstrates measurable anisotropic properties. Specifically, the complex reflection coefficient,

1 , of the air-ground interface is a combination of a large reflection from the generally isotropic air-ground interface and a small reflection from an anisotropic buried object, when present. This anisotropic property is modulated by the rotation of the sensor antenna and has a frequency equal to twice the antenna rotation frequency. It is separable from the main reflection by way of a band pass filter whose frequency is centered at twice the rotational frequency of the scanning antenna.

The modulation of the reflection coefficient is related to the linear polarization of the rotating sensor antenna and the generally linear polarization of the target. When these polarizations are orthogonal to each other, there is no interaction. When these polarizations are in line with each other (parallel) then the interaction is maximized. A rotating antenna thereby modulates the reflection coefficient at twice the frequency of the rotating antenna. The complex reflection coefficient of the air- ground interface is thereby observed to move between its fiducial component (no coupling to the target) to another value which incorporates the target coupling.

In the present embodiment, the modulated complex reflection coefficient of the air-ground interface is measured using a mechanically rotated antenna (or an electronically scanned facsimile) using circuitry which is equivalent to a vector network analyzer. It is the measurement of this complex reflection coefficient, the isolation of the anisotropy vector, and its exploitation as an indication of the presence, orientation, and depth of a buried object that is a focus of this invention.

The imaging of abrupt boundaries (edges) of dissimilar dielectric materials of buried objects, as compared to the generally homogenous dielectric constant of ground, applies equally to air cavities in ground since their dielectric properties are likewise distinct, presenting anisotropic properties along the edges of the buried cavity. It also follows that the directional properties of this anisotropy are also useful so as to indicate the orientation (or direction) of the object as well as its depth. The above description applies to all measurements of the complex reflection coefficient where a single rotating antenna or a scanned antenna array as an equivalent facsimile is used to measure anisotropy.

With reference now to the drawings, and more particularly to Fig. 1 thereof, there is shown a mechanically rotated single antenna version having RF connector 1, counterpoise 2, balun 3, cable 4, and antenna 5. These are suspended above a target wire 6, which is either on the surface or buried in ground 7. Antenna 5 is suspended between counterpoise 2 and ground 7. The diameter of the circular area scanned under the antenna is approximately equal to the antenna length.

S parameters (SIl, S 12, S21, and S22) referred to herein are known as Scattering Parameters and represent microwave measurement techniques and principles which are uniformly used by microwave designers to describe the interactions of electromagnetic waves and the effects of impedance discontinuities, such as those as described herein. In the implementation of the invention as shown in Fig. 1 the antenna is connected to a vector network analyzer (described hereunder) whereby measurement of the complex reflection coefficient of the antenna, is made. It is assumed that, for a well-matched antenna, S 11 is functionally equivalent to the complex reflection coefficient, 1 , of the air-ground interface beneath the antenna.

The complex reflection coefficient of the antenna, Sl 1 (or 1 ), is measured by way of RF connector 1, connecting cable 4, and balun 3, using a vector network analyzer for which a simplified circuit is shown in Fig. 5.

For the purpose of this description, Sl 1 is generally displayed on a Smith Chart (Fig. 6), which will be described in more detail below. As sensor antenna 5 is rotated above the air-ground interface 7, S 11 will vary in the presence of a long conducting wire 6. As shown on Smith Chart 47, this variation is represented as the difference vector 50, being the vector difference between the measured vectors 48 and 49.

Refeπing now to the embodiment of the invention of Fig. 2, there is shown a mechanically rotated dual antenna version having connectors 8 and 9, counterpoise 10, two cables 11, two baluns 12, and two antennas 13 which are in line with each other.

In the Fig. 2 embodiment, the entire assembly is rotated. Connectors 8 and 9 are connected to a vector network analyzer whereby measurement of the complex transmission coefficients of any antenna pair (Sl 1, S 12, S21, and S22) is made and, by extension, the complex reflection coefficient of the air-ground interface beneath the antenna is also measured. With the pair of mechanically rotated antennas 13, measurement of Sl 1 , S 12, S21, and S22 are made by a vector network analyzer. These antennas 13 are rotated between antenna αxunterpoise 10 and air-ground interface 15. The directional properties of Sl 1, S12, S21, and S22 reveal the presence of a surface (or buried) wire 14. In this case, the anisotropy of Sl 1 and S22 as well as the anisotropic properties of the S12/S21 pair are available for use.

When using two antennas, more information is available for analysis and exploitation. There are now four parameters available for analysis; SI l, S12, S21, and S22. While S 12 and S21 should be identical, variations in the magnitude of the S 12/S21 pair with azimuth are strongly affected by the presence of a high aspect ratio conductive object between the two elements, such as a buried wire or pipe that electromagnetically couples any two antennas. This variation in the complex transmission coefficient directly reveals anisotropy of the complex reflection coefficient of the air-ground interface. In this case, electromagnetic coupling is greatest when the E fields of the two sensor antennas are in line with the major axis of the buried object (such as a wire).

Using now SIl and S22, we have the means to confirm anisotropy as measured by the S 12/S21 pair with anisotropy as measured by Sl 1 and/or S22, This is a significant tool in discriminating against the effects of rocks, sticks, shrapnel, and other debris which do not demonstrate these properties.

Using the techniques of embodiments of the invention, measuring and exploiting these parameters provides discrimination advantages against local clutter by relying on the peculiar broadband RF signature of wires to confirm whether the detected object is, in fact, a wire or is some other type of object. For example, with a two antenna implementation of this invention, the modulation of Sl 1 (or 1 ) in the presence of in the measurement field (rocks and debris) occurs at the same frequency as the rotating sensor antenna (one interaction per rotation) while the modulation of SI l (or 1 ) for a long, thin target will occurs at twice this frequency. Clutter is thereby removed by filtering the AC component of SI l using a band-pass filter.

With reference to Fig. 3, counterpoise 15 is configured with three diametric dipole antennas 16, 17, and 18 which are connected to commutated RF relay or commutation switch 19 which sequentially selects the antennas by external command 21. The selected antenna is connected at output 20 to the circulator 38, shown in Fig. 5, or circulator 54 shown in Fig. 7, which is then connected to a vector network analyzer whereby measurement of the complex reflection coefficient of the antenna, SI l, is made and, by extension, the complex reflection coefficient of the air-ground interface beneath the antenna is also measxired. This is an electronically scanned sensor antenna array and no mechanical rotation of the assembly is required.

While only three antennas are shown in Fig. 3, this description is intended to be representative of an electronically scanned antenna array of any number of antennas where only one antenna at a time is measured. The rapid electronic scanning of these antenna elements is broadly equivalent to the mechanically scanned antenna of Fig. 1 except that there are no moving parts.

As shown in Fig. 4, counterpoise 22 is configured with eight radial dipole antennas 23-30 which are connected to commutation switches or commutated RF relays 31 and 32 which sequentially select any two antennas by external command 35. The selected antenna pair is through outputs 33 and 34 to the circulator 38, shown in Fig. 5, or circulator 54 shown in Fig. 7, which is then connected to a vector network analyzer, whereby measurement of the complex transmission coefficient of the antenna pair, S21, is made and, by extension, measurement is also made of the complex reflection coefficient of the air-ground interface beneath the antenna. As with the Fig. 3 embodiment, no mechanical rotation of the assembly is required.

While only eight antennas are shown in Fig. 4, this description is intended to be representative of an electronically scanned antenna array of any number of antennas where two or more antennas at a time are measured, The rapid scanning of these antenna elements is broadly equivalent to the mechanically scanned antenna of Fig. 2 except that there are no moving parts.

The system shown in Fig. 5 includes rotating antenna 36, rotary joint 37, circulator 38, and a vector network analyzer 70. The vector network analyzer 70 includes a two-way zero-degree divider 39, system oscillator 40, two mixers 41 and 42, two band-pass filters 43 and 44, and two analog to digital converters 45 and 46, and a detection processor 72. This is only one of many possible implementations of a vector network analyzer and it is shown only to demonstrate the operating principles of the invention. For example, in some embodiments, a microwave reflectometer may be used instead of the circulator 38. It is by means of mis or an equivalent circuit that the complex reflection coefficient of the circuit is measured and converted to a usable vector format by which anisotropy of the complex reflection coefficient of the air- ground interface can be computed. This is one of many possible implementations of this function and is shown for reference only.

A conventional vector network analyzer is an instrument widely used by microwave engineers to measure the complex reflection coefficients in distributed microwave systems, The vector network analyzer 70 shown in Fig. 5, which includes a detection processor 72, also includes a digital computer and software programs to implement the calculations described herein. The network analyzer 70 also provides a reference signal of known phase and amplitude to a microwave reflectometer where the transmitted and received signals are separated. The received signal, the reflection from the point of meas^^rement _> is thereby compared to the reference signal and both the amplitude and phase of the received signal is computed and referenced to the measurement plane of the instrument (in this case, the antenna feedpoint), and the result displayed in a usable format. The detection processor 72 may also include user interfaces and displays (not shown) for providing user input and for displaying outputs such as Smith Charts, and other user-perceptible visible or auditory indications of the presence, location, orientation, and depth of a buried object.

Smith Charts (a variation of Polar Charts) are commonly known to practitioners of the art of microwave impedance measurements and are widely used as a means of displaying complex (vector) microwave properties. In the Smith Chart 47 shown in Fig. 6, there is vector 48 which represents the complex reflection coefficient SI l of the sensor antenna for the case of ground alone. There is also shown vector 49 which represents the complex reflection coefficient SIl where the E field of the sensor antenna is parallel to the orientation of a wire or other buried object below the sensor antenna. Vector 50 is the computed difference vector of vectors 48 and 49 and is a representative measurement of the anisotropy of the air-ground interface beneath the sensor antenna or antennas. When the polarization of the buried target is orthogonal to the polarization of the sensor antenna, the magnitude of vector 50 is zero. When the polarization of the buried target is parallel to the polarization of the sensor target, vector 50 is maximized. As the rotation angle of the sensor antenna is a known quantity, the modulation of vector 50 with respect to the sensor antenna position reveals the orientation of the buried target. Likewise the angle of vector 50 with respect to vector 48 reveals the depth of the target,

Vector 50 is an equivalent measurement of electromagnetic anisotropy of the complex vector reflection coefficient of the air-ground interface below the sensor antenna. In particular, vector 50 has three important components: a. Magnitude: by which the degree of anisotropy is generally known; b. Phase Angle: by which the depth of the object is generally known; and c. Modulation Orientation: by which the direction of the anisotropy is generally known as the sensor antenna is rotated so that it is either co- polarized with the anisotropy being observed (maximum return) or cross- polarized (null return).

A feature of embodiments of this invention is that the transmitter and receiver may use a common frequency reference as well as a quadrature receiver, a configuration common to all vector network analyzers, so that the measured results may be recorded and processed in a vector form rather than scalar. Another feature is that the signal received from the microwave circulator (or reflectometer) may be band-pass filtered so as to isolate the anisotropy vector from the main reflection, noting that the modulation frequency of the anisotropy vector is twice the rotating frequency of the scanning antenna.

The system shown in Fig. 7 includes rotating antenna 51, rotary joint 52, auto- tuning circuit 53, and vector network analyzer 74. The vector network analyzer 74 includes a circulator 54, two-way zero-degree divider 55, system oscillator 56, two mixers 57 and 58, two low pass filters 59 and 60, two analog to digital converters 61 and 62, detection processor 76, and input means 63 for controlling the auto-tuning circuit 53. The auto-tuning circuit 53 is used to center the anisotropy vector to the middle of the Smith Chart, which is equivalent to impedance matching. The detection processor 76 includes a digital computer and software programs to implement the calculations described herein. This is one of several possible implementations of a vector network analyzer with an auto-tuner and is shown only to demonstrate the operating principles of an embodiment of the invention. It is by means of this or equivalent tuning circuit that the average complex input impedance of the rotating antenna is tuned to the approximate center of the Smith chart to produce an optimized response, as explained below.

Smith Chart 64, as shown in Fig. 8, shows vector 65, which represents the complex reflection coefficient of the sensor antenna for the cases of no wire or other buried object beneath the sensor antenna, or for the case where a wire or other buried object is perpendicular to the E field of the sensor antenna. Vector 66 represents the complex reflection coefficient where the E field of the sensor antenna is parallel to the orientation of a wire or other buried object below the sensor antenna. Vector 67 is the computed difference vector of vectors 66 and 67. Taken as a whole, these vectors are more or less centered to the middle of Smith Chart 64 by auto -tuner 53.

The magnitude of vector 67 is an equivalent measurement of anisotropy of the complex reflection coefficient of the air-ground interface below the sensor antenna in a tuned and optimized condition. The magnitude of vector 67 may be referred to as "anisotropy."

Ground properties may vary over a wide range from one location to another. Furthermore, the frequency of operation of the sensor may vary over a wide range depending on the intended application and the intended depth of penetration. A tuning circuit may be used to produce an optimized condition whereby the full range of SIl measurements are centered to the middle of the Smith Chart. The difference vector, the measurement of anisotropy, is maximized in the unique case of an auto- tuned condition and produces an ideal modulation of the magnitude of Sl 1 such that the amplitude modulation frequency is exactly twice the sensor rotation frequency, making it more suitable to narrow band or synchronous detection.

While the foregoing written description of the invention enables one of ordinary skill to make and xise what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.