Patent Application
20140043183
1. Field of the Invention
The present invention relates to radar devices and methods, and more particularly to imaging underground tunnels and bores by using the stress fields that typically surround their peripheries to mix and radiate heterodynes of intense acoustic tones injected from nearby vantage points.
2. Description of the Prior Art
Remote sensing into the earth to find, characterize, and image deeply buried objects and features has always been difficult. Ground penetrating radars have been developed that depend on measuring the delays, attenuations, and phase shifts imposed on reflections of radiowaves to characterize and image what lies beneath. Companies like Stolar, Inc. (Raton, N. Mex.) have gotten quite good at sorting out the carrier frequencies, modulation schemes, synchronous detection techniques, and antenna construction needed to look deep into the earth to find coal deposits, mining hazards, trapped miners, and even smugglers' tunnels.
Heterodyning is generally a radio signal processing technique in which new frequencies, specifically the sum and difference frequencies (F1+F2, |F1−F2|), are generated by combining two frequencies (F1, F2) in a mixer. EH Armstrong used this phenomenon to great effect when he developed the first heterodyne receivers.
The principal characteristic of mixers and why they can mix is that they are non-linear. Linear circuits will not produce heterodynes. The most common non-linear electronic devices are vacuum tubes, transistors, and diodes.
It just so happens that acoustic waves traveling through solid media will heterodyne when two or more intense tones are passed through non-linear materials. Loads and stress in rock and other natural deposits will produce non-linearities able to support acoustic heterodyning. Tunnels and boreholes in the earth are naturally surrounded by stress fields in the supporting, surrounding media and the stresses tend to concentrate at corners or arches.
The determination of the initial stress patterns in rock masses is an important problem in engineering rock mechanics. It is also an important basis for the stability analysis of the rock surrounding underground openings, high rock slopes, arch dam shoulders, dam foundations, and the study of reservoir induced earthquakes. Ma Qichao, Department of Hydraulic Engineering, Tianjin University, published a paper on the subject titled, “The Cause of Formation of the Initial Stress Field in Engineering Rock masses and the Rule of Stress Distribution in the Field”, Chinese Journal of Rock Mechanics and Engineering, 1986-04.
Researchers have generally identified that stress fields inherently surround even well bores and rectangular tunnels. See, Investigation Study of the Stress Field Surrounding a Well Bore and a Rectangular Tunnel, by Biao Qiu and Yi Luo of the Department of Mining Engineering, West Virginia University, published as Stress Fields around Underground Openings, 2011. More often than not, the conventional concerns about the stress fields surrounding boreholes and tunnels is the stresses can cause breakouts, fragment spalling, and other failures.
Acoustic waves can travel long distances and to great depths in the earth. This then makes the use of acoustic waves to scan deeply buried objects very attractive, maybe more so than using radiowaves.
Briefly, acoustic heterodyne radar embodiments of the present invention use accurately surveyed or otherwise known locations to repetitively launch at least two, intense acoustic tone soundwaves (F1, F2) into an underground area of search. An acoustic receiver is tuned to receive either the sum (F1+F2) or difference (|F1−F2|) heterodynes and is configured to measure and log the overall relative attenuation and roundtrip travel times of the soundwaves, like a typical radar. Any acoustic heterodynes received are assumed to be the work of non-linearities and stresses in the search area. A full-waveform three dimensional tomography algorithm is applied by a graphics processor to the collected and logged data to generate maps and profiles of objects beneath the ground which are interpreted to have produced the acoustic heterodynes.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.
Tunnels and boreholes driven into natural media create non-linear stress fields surrounding the void. The logarithmic pressure field distribution in a one-dimensional radial distance from a circular locus of points with radius (Rc) and pressure (Pc) to a concentric well bore with effective radius (rb) and face pressure (Pb) can be represented by,
The natural logarithm (ln) power series expansion is mathematically given by,
A narrow band near the borehole experiences most of the pressure differential. For example, Rc≈100 m, and rb≈0.1 m, more than one-third of the pressure differential occurs across the 1 meter nearest to the borehole core. More than one-half of the pressure differential occurs across a zone with a radius of Rc≈3 m. The situation is even more pronounced for boreholes with smaller radii, rb.
In general, the stress field can be represented by a Taylor series expansion. When two or more sinusoidal seismic S (i.e., slow traverse) waves, seismic P (i.e., fast longitudinal) waves, or acoustic frequency signals travel along a refraction path crossing through a non-linear stress field, the heterodyne of the two signals generates at least a sum and difference frequency signal given by
{circumflex over (f)}=nf
1
±mf
2.
When the stress field is strongly follows a square law, the best product frequencies {circumflex over (f)} are predominately the sum (upper heterodyne) and the difference (lower heterodyne) frequency. The magnitudes of the generated signals depend on the coefficient of the power series expansion.
The object is to locate any deeply buried boreholes and/or tunnels 114. The pressures cause by the overburden will naturally cause stresses to develop in the solid materials immediately surrounding the boreholes and tunnels 114. Such overburden or lithostatic pressure imposes stresses proportional to the weight of overlying materials, for example:
p(z)=p0+g∫0zρ(z)dz
where, ρ(z) is the density of the overlying rock at depth z and g is the acceleration due to gravity, p0 is the datum pressure, like the pressure at the surface. The depths involved here are a very small fraction of the Earth's radius, so “g” is placed outside of the integral for most near-surface applications.
Stresses which cause non-linearities surrounding boreholes and/or tunnels 114 are represented in
Tools 102 include an acoustic receiver able to filter through the heterodynes and measure their relative times of arrival and attenuation. These measurements are collected in real-time for use in post processing.
A measurements device 216 is included to determine the travel times and attenuation of any said heterodynes returned from said underground area of search to the acoustic receiver based on when and where audio tones F1, F2 were launched and where the acoustic receiver was then located.
A log 218 is configured to collect and store data in real time produced by the measurement device 216, and to carry such to post processing. A computed tomography (CT) processor 220 uses an algorithm to translate the data in log 218 into three dimensional images. A graphics controller 222 presents these to users in the form of maps 224 and profiles 226 of any tunnels and/or boreholes (e.g., 114) that may be situated in the underground area of search (e.g., 108).
Most of the acoustic heterodynes arriving and being measured at the receiver 212 are assumed to be the work of non-linearities and stresses in the underground area of search that naturally surround and outline tunnels and/or boreholes. Other anomalies and computational idiosyncrasies will produce image artifacts that will need to be ignored or scrubbed.
Receiver geophones can be built with magnetic wire coils surrounding a permanent magnetic. The coil is mounted to an Earth contact plate. The mounting configuration can be on three orthogonal axes. The media movement along each axis generates an electromotive force (EMF) voltage measured by instruments. The transmitter may be a piezoelectric ceramic radiator driven by a series of short time domain pulses that are synchronized to a direct digital synthesizer and controllable in frequency steps from 3-kHz to 30-kHz, which receives the spectra components including the transmitted frequencies ω1 and ω2 and the non-linear stress field heterodyne frequencies. Each of the frequency components may be a unique spectrum for each individual source. The received heterodyne signals can be re-heterodyned in electronic circuits to create a common intermediate frequency enabled by a detection process described in
Clutter caused by near field generation of heterodynes at the first interface can be suppressed by adapting the Bausov method described in U.S. Pat. No. 7,656,342, issued Feb. 2, 2010, and titled, DOUBLE-SIDEBAND SUPPRESSED-CARRIER RADAR TO NULL NEAR-FIELD REFLECTIONS FROM A FIRST INTERFACE BETWEEN MEDIA LAYERS. Instead of using pairs of radio frequency continuous wave (CW) transmissions, pairs of acoustic tones are substituted.
The near field return of blinding signal clutter is represented in
Using non-linear stress fields 120-124 to find and identify an otherwise unknown tunnel 114 has been described above. But when tunnel 114 is already known, and it is the extent and severity of the non-linear stress fields 120-124 that are unknown, then tool 102, tool 202, and post processor 204 can be usefully employed. Conventional methods of characterizing and measuring the stresses surrounding boreholes and tunnels have not employed acoustic heterodynes.
Each fault is assumed to be enveloped in a stress field 416-420 that manifests as a non-linearity able to mix acoustic tones and radiate heterodynes. Stress-fields 416-420 will mix and produce sum (F1+F2) and difference (|F1−F2|) heterodynes when intense audio tones (F1, F2) reach each of them respectively. Their corresponding times of travel and relative attenuation as seen by a receiver can be used to reveal the likely locations of the stress-fields 416-420 that produced them. Embodiments of the present invention interpret such heterodynes as having come from underground cracks, fissures, and unconsolidated sediments or semi-consolidated sedimentary rocks deposited in ancient, long-inactive river and stream channels, e.g., paleo-channels.
Tools 404-406 include at least one acoustic receiver amongst them able to filter through the heterodynes and measure the relative times of arrival and attenuation. These measurements are collected in real-time for use in post processing, e.g., as in
The information obtained is used in mining operations that follow later to avoid the faults or to drill ahead to stabilize the ground with injections of epoxies, cements, or other fillers and/or to install bolts and other devices. For example, TAM International Australia PTY Ltd (South Australia) markets several ground stabilization products including acrylic injection grout, colloidal silica, injection cement, and polyurethane grout for fissure grouting and injection ahead of tunnel boring machines (TBM).
The Bausov method described in U.S. Pat. No. 7,656,342 describes how deep-look ground penetrating radar gradiometers (DLRG) can overcome the problem traditional ground penetrating radars have by being blinded by overly bright near zone signals, crosstalk coupling, first interface reflections, and other clutter. Deep-look ground penetrating radar gradiometers can reject 60-dB of such clutter by transmitting double sideband (DSB) suppressed carrier frequency components to achieve greater detection depth by gradiometric suppression of arriving near zone signals.
See
in radians per second.
The ensemble of lower sideband frequency components is represented by the vector sum of phasors, each with nearly identical phase shift (i.e., ω1τN). The ensemble of upper sideband frequency components is represented by the vector sum of phasors each with nearly identical phase shift (ω2τN). The ensemble of lower sideband signals is subtracted from the upper sideband signal in the heterodyne down conversion (i.e., mixer) process. The subtraction occurs because the heterodyne process causes the lower and upper ensemble of signals to be 180° out of phase with each other. The lower and upper sideband signals reflected from the far zone interface are each shifted in phase by the radian frequency of each component multiplied by the round-trip travel times (i.e., ω1τF and ω2τF).
The reflection signals arriving from the second interface are also represented as a phasor and add to the vector sum of the lower and upper sideband phasors but with a phase difference (ωmτF) that is varied by a controller, as represented by the dashed circles at the end of each summation of phasors in
Measurements show that near zone cross talk, first interface, and clutter signals are suppressed by at least 60 dB, an improvement of 30-dB over non-double sideband processing methods. An ensemble of intermediate frequency signals is applied to a quadrature detector. The in-phase (I) and quadrature (Q) components of the intermediate frequency signal are recovered and algorithmically processed to display detection and range to an object. The quadrature detector in-phase (I) and quadrature (Q) signals are mathematically represented by,
I=cos(ωmτ+θm)cos(ωcmτ+θcm),
and,
Q=cos(ωmτ+θm)sin(ωcmτ+θcm),
where, ωcm=2πfcm is the radian frequency of the suppressed carrier signal and fcm is in Hertz.
The magnitude of quadrature detection signal,
M=|I
2
+Q
2|1/2=|cos(θm+ωmτ)|
and phase of the suppressed carrier is given by,
A controller manipulates the sideband separation frequency (ωm) to determine the range or distance to the object. Since the round trip travel time to the second reflecting interface is invariant, the change in modulation frequency (Δω) required for the I,Q signals to vary from maximum to minimum determines the range is,
where, the velocity, v, in the natural media is, for example, approximately 1.5×108 meters/second through coal.
Each heterodyne double sideband signal coherent phase difference is shifted in phase (θm) to π/2 radians, which changes the magnitude coefficients of the I, Q signals from cos(ωmτ) to the sin(ωmτ). As the Bausov suppression chart of
Each pair of received frequency components are heterodyned (down-difference or up-sum converted) to the same intermediate frequencies (IF) in superheterodyne type receiver. Each frequency component can be represented by a phasor vector whose length represents the magnitude of received frequency component. Each phasor is phase shifted 180° from the other in the heterodyne process and gradiometrically subtracted to minimize the magnitude of the IF signal. The attenuation rate and phase shift (loss tangent greater than unity) depends on the first power of frequency. The magnitude and phase shift of each received frequency component will be different.
The phase of the heterodyning signal can be used to minimize the magnitude of the IF signal input to the analog to digital converter (ADC). The full range of the ADC is needed to digitize the variations in electrical conductivity and their effects on attenuation and phase as measured by the phase coherent quadrature detection process.
Gradiometer subtraction of IF phasors is used to suppress the magnitude of the IF signal and enables automatic gain control and full dynamic rage digitization of the frequency dependent attenuation and phase shift.
The processed data enables full waveform 3D tomography reconstruction of geologic anomalies even where refraction occurs along transmission paths. A significant problem in imaging mineralized zones and anomalies in the coal seam waveguide is refraction distortion can create artifacts in the image reconstructions.
Acoustic heterodyne equipment and methods could also be usefully employed in cutterhead drums of continuous mining machines used in underground coal mining. Acoustic imaging radar promises to be able to map the fractures and cleats in the coal beds and faces ahead of the mining to help improve machine design, bit lacing patterns, performance and campaign life.
Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention.
