Communications: directive radio wave systems and devices (e.g. – Transmission through media other than air or free space
Reexamination Certificate
1998-06-03
2002-04-23
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Transmission through media other than air or free space
C342S021000, C342S118000, C342S134000, C342S175000, C342S176000, C342S179000, C342S190000, C342S191000, C342S195000
Reexamination Certificate
active
06377201
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of imaging and materials identification, and, more particularly, to a radar for imaging a subsurface area of interest and for classifying the material composition of subsurface objects, and a method therefor. Although the present invention is subject to a wide range of applications, it is especially suited for use in a ground-penetrating radar (GPR) system radar, and will be particularly described in that connection.
BACKGROUND OF THE INVENTION
GPR uses radio waves to detect buried objects in non-metallic material and can penetrate soils, rock, and man-made structures. GPR is used to map the interior of objects penetrable by radio waves, similar to the way X-rays can image the inside of a human body. It us used to discriminte metallic and nonmetallic materials, utility lines, voids, bed rock layers, rebar spacing, concrete floor thickness, and other subsurface anomalies or debris.
The depth of exploration and image definition depend on the radio frequency (RF) used. Low frequencies are used for deep geological mapping. High frequencies are used for high definition imaging.
A typical ground-pulse radar (GPR) system for investigating a subsurface area of interest is shown in FIG.
1
.
The function of the transmitter
204
is to generate a known waveform of RF energy to probe the subsurface area of interest
102
. This energy waveform is typically a sinusoidal pulse, 0.5 to 2.0 nanoseconds seconds in duration. Use of such a narrow pulse improves the GPR systems ability to distinguish small subsurface features while simultaneously limiting the depth to which it can probe.
The narrow pulse formed by transmitter
204
is routed to an antenna
202
where it is radiated into the subsurface area of interest
102
. For ease of illustration and explanation, the subsurface area of interest
102
is shown as a multilayer system. One of ordinary skill will appreciate that the thickness of the layers can be varied as well as the shape of the layers. As the electromagnetic pulse leaves the antenna it becomes a transmitted signal
209
, traveling though free air until it strikes the first surface
102
. A fraction of the transmitted signal
209
passes through the first surface and a fraction is reflected in other directions away from the air-first surface interface as shown in element
210
A. The process of signal transmission and reflection is repeated at each layer-layer interface as the transmitted signal
209
A continues to propagate into the subsurface area of interest. At each interface the transmitted portion of the signal becomes weaker as indicated by the relative widths of signals
209
B and
209
C.
Layers within the subsurface area of interest are defined in terms of their electromagnetic properties such as their dielectric constant. Thus, any time two materials with different electromagnetic properties abut, there will be an interface between them that affects the transmission of a propagating RF signal.
During RF pulse transmission, an isolator
203
connects the transmitter
204
to the antenna
202
. Shortly after transmission, the isolator
203
breaks this connection, making a connection between the antenna
202
and receiver
205
. The function of the isolator
203
is to protect the receiver's
205
input components from damage from the high energy output of the transmitter
204
.
Using one antenna for both RF pulse transmission and reception of a reflected signal
210
is called monostatic operation. If two antennas are used, one for transmission and one for reception, the operation is called bistatic. Both systems are functionally and structurally equivalent.
The task of the receiver
205
is to capture weak reflected signals
210
A-C and amplify them for subsequent processing. Following reception, the captured and amplified reflected signal are passed to a signal processor
206
. The specific signal processing performed depends upon the primary application of the GPR system but would, in almost all cases, include digitization so that the received signal could be placed in a digital storage device
207
.
The final GPR system component is the display unit
208
, the purpose of which is to present the reflected signal in a format useful to the human operator. The display unit
208
is typically a CRT screen or computer monitor. Typically, a processing unit (not shown) will analyze the digitized reflected wave signal before displaying it.
Ground-penetrating radar works according to a pulse-echo principle of clocking the two-way time of flight of an electromagnetic pulse. This type of ground-penetrating radar is called impulse radar because an unmodulated or baseband pulse is radiated rather than the usual sinusoidal burst found in conventional radar. The pulses are a sequence of impulses; there is no carrier. There is no specific frequency associated with this radar; rather, its frequency spectrum is related by the Fourier transform of the pulse. In conventional impulse radar, the free-space radiated pulse is a Gaussian-shaped impulse about 200 picoseconds wide.
A major advantage to impulse radar is that its spectrum has frequency components located close to DC, where attenuation of the signal amplitude by the medium it traverses is the lowest. For example, for the case where the transmitted signal wave front is planar with respect to a flat surface, the propagation of a transmitted wave in a homogeneous medium along the axis perpendicular to the flat surface has its signal amplitude governed by the equation:
E=E
0
e
−az
e
−j&bgr;z
, (1)
where E
0
is the amplitude of the electric field vector in volts/meter, z is the distance along the direction of propagation in meters, and &agr; is a frequency-dependent attenuation parameter and &bgr; is a frequency-dependent phase parameter related to the two material properties magnetic permeability (&mgr;) and permittivity (∈) in Farads/meter.
Equation (1) indicates that 1) the magnitude of the electric field of the transmitted signal, E, decreases as it propagates into the homogenous medium and 2) that its pulse shape is distorted because of the nonlinear phase term, &bgr;z.
While the assumptions do not hold in the strictest sense, standard practice in GPR analysis holds that they are acceptable simplifications for purposes of field use and theoretical development.
A GPR pulse propagating inside the subsurface area of interest
102
will undergo transmission and reflection events whenever it encounters an interface between different layers. The propagation impedance in free space is governed by the following equation:
Z
0
(space)={square root over ( )}(&mgr;
0
/∈
0
) (2)
The propagation impedance in a material (wood) having ∈
r
=2 is governed by the following equation:
Z
r
(wood)={square root over ( )}(&mgr;
0
/∈
r
∈
0
)=Z
0
(space)/{square root over ( )}(∈
r
)=Z
0
(space)/{square root over ( )}2 (3)
The free space propagation impedance is 377 ohms and the propagation impedance of wood is 266 ohms. This difference in impedance causes a difference in the reflection magnitude at the air-wood interface.
In a one dimensional analogy to propagation along a transmission line, which can be equated to time domain reflectometry (TDR), reflections off the wood layer become equivalent to reflections from a transmission line discontinuity. The reflection coefficient, R, defined as (Y−1)/(Y+1) where Y=Z(wood)/Z(space), can be applied to determine what fraction of the radiated pulse is returned.
For example, wood with an ∈
r
=2, the reflection magnitude, relative to 377 ohms, is 0.17. Thus the difference in reflection magnitude between the presence and absence of a wood layer is 0.17. If the layer were metal, the reflection would be total, or 1.0. Thus, metal is easily discerned from wood by a 5.9 times greater reflection magnitude.
The parameters which determine the amount of energy that is reflected (away from the signal&
Gregory Bernarr E.
Science Applications International Corporation
Sidley Austin Brown & Wood LLP
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