Method for constructing vertical images of anomalies in geological formations

Abstract

Apparatus and method for detecting geological anomalies occuring in geological formations. The instrumentation comprises a medium frequency continuous wave narrowband FM transmitter and receiver pair. Two instrument configurations are downhole instruments for insertion into boreholes. Survey procedures are provided to detect anomalies through signal attenuation, path attenuation and signal phase shift. Continuity measurements at different depths in the drillholes provide data to determine the existence of anomalies. Tomographic techniques are employed to provide a visual image of the anomaly. Computer aided reconstruction techniques provide such visual images from the generated data.

Description

BACKGROUND OF THE INVENTION

This application is a continuation-in-part of patent application Ser. No. 06/812,625, filed Dec. 23, 1985, now U.S. Pat. No. 4,691,166, which is a divisional of Ser. No. 06/731,741, filed May 6, 1985, now U.S. Pat. No. 4,577,153, issued Mar. 18, 1986.

FIELD OF THE INVENTION

This invention relates generally to instrumentation and procedures for detecting geological anomalies in geological formations and more specifically to continuous-wave low and medium frequency radio imaging techniques combined with computer aided reconstruction to provide graphic radiogenic images of seam anomalies.

DESCRIPTION OF THE PRIOR ART

Coal seams or deposits occurring in layered formations have been distorted by many different types of geological mechanisms. Differential compaction occurring in surrounding layers causes faults, twists and rolls to occur in the seam. Ancient streams have washed coal from beds leaving sand and rock deposits. These deposits, known as fluvial channel sand scours, can cause washouts and weak roof. Such stream distortions and rock deposits are physical barriers to mining equipment. Two types of underground mining techniques are extensively used in the coal mining industry. One type, referred to as room and pillar, or continuous mining, can mine around many of these barriers. The continuous mining technique is less expensive and requires less manpower. For example, set-up generally requires three shifts of eight people. Continuous mining, however, produces only approximately 300 tons per shift. Longwall mining, the other widely used technique, is much more efficient in uniform coal beds. This method yields production rates averaging 1500 tons per shift.

In the United States, the Mining Safety and Health Administration requires that retreating, rather than advancing longwalls be used. On the other, in Europe, advancing longwalls are extensively used. Retreating longwalls are set up to mine in the direction of the main entry, whereas advancing longwalls mine away from the main entry. Continuous mining techniques are employed to set up the retreating longwall. From the main entry, entry ways are mined at right angles to the main entry and on either side of the longwall panel. These entry ways, the head gate entry and tail gate entry respectively, extend the length of the longwall panel. At the end of the panel, a crosscut is made between the head gate and the tail gate entries. The wall of the crosscut facing the main entry is the longwall face. The longwall machine is set up along the face with a heading toward the main entry. As the longwall moves forward, the roof caves in over the mined out area. A barrier block of unmined coal is left at the end of the run to support the roof over the main entry.

The high yield of longwall mining makes it economically advantageous to use where a long panel can be mined. A typical longwall panel contains from 500,000 to 1,000,000 tons of coal. The initial investment and set-up cost of longwall mining are high. Equipment cost averages many millions of dollars. Longwall set-up requires thirty days minimum, at three shifts per day, with twelve to fourteen men per shift. Thus, set-up expenses are very large as a result and to achieve the low cost production advantage of the longwall method, a uniform coal seam is necessary to ensure a long production run. Seam anomalies such as faults, washouts, interbeddings and dikes can cause premature termination of the longwall production run. In many instances, longwalls become "ironbound" after encountering an anomaly. Removal of such "ironbound" equipment requires blasting which can damage equipment and exposes miners to extreme danger. Accordingly, if seam anomalies could be detected and analyzed in advance of mining, the mining techniques could be planned for minimum production cost. Where the survey discloses a long continuous coal seam, the low cost longwall technique can be employed. If barriers to longwall mining are discovered, the mine engineering department can use continuous mining to mine around the barriers.

Geological surveys for potentially productive coal formations use many well known procedures. These procedures employ a wide variety of technologies. Satellite imaging and photography provide global data for use by mine geologists. However, because of the broad overview of the data they are of no value in determining the mineability of a coal seam. Macrosurvey (foot prospecting) of surface strata and outcrop features enable geologists to forecast formation characteristics based upon prior knowledge. Surface based seismic and electromagnetic wave propagation procedures are extensively used in geophysical surveys for valuable deposits including oil and gas. The microsurvey techniques, however, are not reliable in examining the detailed structure in a coal seam.

Various microsurveying in-seam seismic techniques are currently employed to yield useful data concerning seam anomalies. A technique under development in Europe comprises firing shots of sixteen points into a block of 120 geophone groups, each consisting of thirty-six geophones. Computerized processing of the seismic data results in the detection of faults. To date, the procedure requires placing charges at five foot intervals and requires the installation of extensive cabling. Seismic techniques are primarily intended for advancing, rather than retreating longwalls. Further, this method has not proven to have the capability of resolving channel sand anomalies, especially for partial washouts and smaller, less significant anomalies, nor can they detect roof/floor rock conditions. The emerging of the surface based spectral magnetotelluric method with controlled sources may have the capability of seeing into the earth's crust. This method appears to be useful in detecting major faults in layered formations, but cannot resolve detailed seam structure.

Downhole drilling has been used to probe longwall blocks. A ten-twelve hole pattern drilled six-hundred feet into the panel provides samples of the coal in the seam. This method, however, has the disadvantage of covering only a small percentage of the block. Because of this limited coverage, this technique is not useful to detect and resolve seam anomalies that may exist in the seam between the boreholes. Surface core drilling and logging remains the most reliable source of seam information. Core sampling provides useful data in mapping stratified mediums. Logging enables probing of the formation in the vicinity of the drillhole. None of the currently used logging methods can detect and resolve seam anomalies that may exist in the seam between the bore holes over distances greater than about fifty feet. In-seam horizontal drilling can detect seam anomalies, but is subject to the same coverage limitations of vertical drilling. Horizontal drilling, additionally, is very expensive, averaging twenty cents per ton of coal produced.

Electromagnetic technologies have been investigated in an attempt to provide a geophysical method to see within the coal seams. Conventional and synthetic radar techniques have been reported in the literature. Because of the high frequency of the radar, it is exceedingly useful in investigating the geological structure in near proximity to the borehole. Deep seam penetration, however, requires very high transmit power in order to maintain any sort of useful resolution. This is because high frequency signals are attentuated very rapidly with distance in the seam. Accordingly, present radar methods cannot see deep into the seam.

Publications by R. J. Lytle, Cross Borehole Electromagnetic Probing to Locate High-Contrast Anomalies, Geophysics, Vol. 44, No. 10, October 1979; and Computerized Geophysical Tomography. Proceedings of the IEEE, Vol. 67, No. 7, July 1979, have described a method of imaging coal seams using continuous wave (CW) signals. His method proposed only tomographic imaging between nearby boreholes. The method of Lytle had limited range and resolution, because of the limited spatial measurements that could be taken using downhole probes. To satisfy the requirements for tomography, Lytle used a higher frequency range, thus achieving less range. Further, the conductivity of rock was found to be much greater than the conductivity of coal. Where the difference conductivity (contrast) is large, the tomography algorithm will diverge rather than converge, resulting in no image.

A study conducted by Arthur D. Little, Inc. for the U.S. Bureau of Mines investigated continuous-wave medium-frequency signal propagation in coal. The results, published by Alfred G. Emslie and Robert L. Lagace, Radio Science, Vol. II, No. 4, April 1976, dealt with the use of electromagnetic waves for communication purposes only. Additionally, errors may be present in the wave propagation equations employed.

Other electromagnetic techniques suffer similar range and resolution problems. None of the prior art recognized the existence of a coal seam transmission window in the 100-800 kHz range. Accordingly, none of the prior art achieved a long range, high resolution imaging of geological anomalies.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly, it is an object of the present invention to provide instrumentation and procedures for in-seam and surface imaging of geological formation anomalies with a range sufficient to image an entire longwall panel.

It is a further object of the present invention to provide instrumentation and procedures to image coal seam anomalies with resolution sufficient to detect faults, full and partial washouts, fluvial channel sand scours, dikes and interbeddings.

It is a further object of the present invention to minimize production costs by surface mapping of fault directions, and providing longwall headings where appropriate.

It is another object of the present invention to minimize production costs by in-seam imaging of longwall panels, after the panel headings have been developed.

An additional object of the present invention is to mitigate or eliminate hazards to miners resulting from unexpected geological anomalies.

Briefly, a preferred embodiment of the present invention includes a continuous wave low and medium frequency transmitting with FM capabilities and equipped with a directional loop antenna, a continuous wave medium frequency receiver equipped with a directional loop antenna (short magnetic dipole) and capable of accurately measuring and recording the received signal amplitude and phase shift of the transmitted signal, and data processing means for producing a pictorial representation of the coal seam from the raw data generated. Both the transmitter and the receiver are portable and are designed in two configurations: a cylindrical configuration, referred to as a sonde, for insertion down boreholes adjacent to a coal seam, and a portable, or entry configuration adaptable for in-seam use.

The invention further includes survey procedures for imaging structures in coal seams. Two methods of seam imaging are provided with procedure selection dependent on terrain and seam depth. Surface based seam imaging with downhole continuity instruments is expected to be used in moderately shallow beds with good surface drilling conditions. In-seam imaging and tomographic techniques will be used when a clear picture of the seam structure is required.

The preferred embodiment utilizes continuous wave low and medium frequency (MF) signals to achieve high resolution imaging of geophysical anomalies in coal seams with relatively low output power. Because the coal seam is bounded above and below by rock with a differing conductivity, at certain signal frequencies electromagnetic energy becomes trapped and will propagate over great distances. This transmission window, or coal seam mode, is excited by the turned loop (short magnetic dipole) antennas employed in the preferred embodiment causing the MF signals to travel several hundred meters in the coal seam. Seam anomalies create regions with different electrical constitutive parameters relative to the coal. This electrical contrast between the coal and the anomalous structure gives rise to the imaging method. The contrast will change the wave propagation constant in the region whereby the wave received on the far side of the region can be analyzed to determine structure between the transmitter and receiver.

The low and MF in-seam continuity and tomography instruments further employ MF signals with a narrow occupied spectrum bandwidth. The receiving instruments detect and measure the signals with phase-locked-loop (PLL) techniques. PLL receivers extend the signal detection threshold well into the noise, thus enhancing operating range. The continuity imaging procedure is used where relatively large electrical contrast between the coal seam and anomalous structure is present. Tomography is applicable when a small electrical contrast exists. Tomography instrumentation can improve resolution by making more spatial measurements, thus overcoming the inherent radar range limitations.

The downhole procedure will require a drilling plan that will enable the medium frequency signal to propagate in the seam between boreholes. In this procedure, a plurality of holes are drilled on either side of the seam. The transmitter and receiver probes are inserted into the boreholes on opposite sides of the seam and signal attenuation is measured across the seam. A series of data points is generated by communicating with a receiver and a transmitter at differing levels within downholes to establish vertical profiles and varying the location of the transmitter and receiver across the series of boreholes to establish a horizontal profile. Signal attenuation, path attenuation, and phase shift are measured and compared with calculated values to determine if seam anomalies are present. Additionally, this data can be reconstructed by computer assisted imaging techniques to provide a pictorial representation of the seam. When a fault is detected additional boreholes are drilled bisecting boreholes in the original drilling plan and further readings are taken to localize the fault. The in-seam imaging technique is carried out in a similar manner to the surface based imaging technique, except the transmitter and receiver instruments are located in the head and tail gate entries adjacent to the seam.

Additionally, the instrumentation can be used to improve mining safety by detecting coal seam fire headings. Fire in the seam affects the coal seam's conductivity and will thus be detected in the same manner as anomalies are detected. Where a fire is known to exist, the in-seam detection methods are used to localize it so it can be controlled. In another application, communications with trapped miners can be established by equipping the miners with small receivers or transceivers. By drilling in the suspected area of the trapped miner, the downhole instrument can excite the coal seam mode and be used to communicate with the trapped miner.

It is an advantage of the present invention that graphical representation of geological formation anomalies are developed by the imaging technique.

It is an advantage of the present invention that vertical scanning can be accomplished to establish a vertical image of the geological area under study.

It is another advantage of the present invention that production costs can be minimized by selecting the appropriate mode of mining the coal.

It is a further advantage of the present invention that the imaging can be carried out using a minimum of equipment and a minimum of boreholes.

It is a further advantage of the present invention that imaging can be accomplished using a relatively low transmitter power.

It is a further advantage of the present invention that mining safety can be improved through the detection of geological anomalies in the working phase.

It is yet another advantage that partial washouts caused by fluvial channel sand scour can be detected by the present invention.

It is a further advantage that roof/floor rock conditions can be determined by the present invention.

It is yet another advantage of the present invention that the instrumentation can be used to communicate with trapped miners, thus increasing mining safety.

It is a further advantage that coal seam fire headings can be determined with the present invention.

These and other objects and advantages of the present invention will no doubt become obvious to those of oridinary skill in the art after having read the following detailed description of the preferred embodiments as illustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is a front elevational view of an in-seam receiver for use in the present invention;

FIG. 2 is a front elevational view of an in-seam transmitter for use in the present invention;

FIG. 3 is a partial cut-away view of a downhole probe for use with the present invention;

FIG. 3a is a front elevational view of a downhole probe fitted with a borehole probe centralizer;

FIG. 4 is a schematic representation of the modular components of the downhole receiver and transmitter probes of FIG. 3;

FIG. 5 is an idealized cut-away view of a coal seam showing the location of the transmitter of FIG. 2 and receiver of FIG. 1 for in-seam tomography;

FIG. 6 is an idealized cut-away view of a coal seam showing boreholes adjacent to the coal seam with downhole probes and surface equipment in position;

FIG. 7 is illustrates a tunnel study area of a geological formation wherein the receiver communicates with transmitters from two different locations;

FIG. 8 is a graphical reading of measured field signal strength versus radial distance from radiating antenna;

FIG. 9 is a diagrammable illustration of vertical imaging for a geological formation;

FIGS. 10A and 10B illustrate vertical RIM cross-hole survey results with FIG. 10A illustrating the tomography phase constant depends upon the conductivity (mhos per meter) of the media. Below 300 KHz the attenuation rate phase constant is very sensitive to the conductivity of the media. The tomography algorithm determines the attenuation rate in the vertical survey planes shown in FIG. 10B.

The attenuation rate data is used in a contouring program to draw contours of constant attenuation in the plane of the image, in increments of one db per 100 feet. The contour lines reconstruct the image of the geologic structure as illustrated in the isopach map of FIG. 10B. The image cells may be 5 ft.×3.9 ft. and 5 ft.×13 ft. in the 30 and 100 meters scans, respectively. FIG. 10B illustrates the contours of constant attentuation for the ore zone on the left side of the image and three tunnels, one over the other, are shown on the right.

It may be useful to briefly describe some aspects of the electromagnetic wave propagation theory in a slightly conducting medium. The EM radiating antenna is an electrically short magnetic dipole. The magnetic dipole antenna produces a farfield toroidal antenna pattern and the plane of the physical antenna lies in the x-y plane. The magnetic moment vector (M) is normal to the plane of the physical antenna coincident with the z axis. The antenna produces a radial magnetic field component (H_r), an azimuthal magnetic field component (H_φ) and an electric field component (E_θ). In free space the field equations are given by: ##EQU7## where B=2π/λ (radians/meter). k=B-jα (i.e. k=the complex wave number); B is the phase constant in radians/meter; and α is the attenuation rate in nepers/meter.

As r becomes large, the terms involving r-1 become 20 predominate and the farfield magnetic field terms follows from equation 5 as: ##EQU8##

The bracket term mathematically represents the constant term in equation (4).

In the near vicinity of the borehole antenna, the predominate magnetic field terms are magnetic. Therefore, the antenna near field energy is stored in its magnetic fields. By way of contrast, if the antenna structure was a short electric dipole, the dual of equations 7 through 9 would mathematically define the near field components E₁00, E_r and H₁00 H_θ , The electric field terms E_φ and E_r would dominate. These terms would cause a current density J=6E J=σE to flow in the medium in close proximity to the antenna causing a substantial dissipation loss of energy thereby resulting in a very short unacceptable imaging system. We thereby see the advantage of using the short magnetic dipole antenna in subterranean imaging.

FIG. 11 illustrates the range electrical property values of a typical geological material expected with an imaging frequency of 300 KHz. For comparison purposes, the attenuation rate (α) of a plane EM wave in the lake water, schist, and coal mediums were calculated from Equation 6 and illustrated in Table A.

TABLE A
______________________________________
PLANE EM WAVE ATTENUATION RATE IN A
HOMOGENEOUS MEDIUM AT 300 KHz
Conductivity in mhos/meter
dB/100 ft.
______________________________________
10-2 (lake water)
26.48
4 × 10-3 (schist)
18.6
4 × 10-4 (coal)
7.88
______________________________________

The table shows that the attenuation rate in schist should be near 18.6 dB/100 ft. The phase rate constant may be determined from: ##EQU9## The wavelength ##EQU10## and skin depth ##EQU11## of plane waves propagating in coal and schist mediums are illustrated in Table B.

TABLE B
______________________________________
WAVELENGTH AND SKIN DEPTH IN A
HOMOGENEOUS MEDIUM
Conductivity Wavelength (λ)
Skin Depth (δ)
(mhos/meter) (meter/feet) (meter/feet)
______________________________________
4 × 10-3 (schist)
93.22/305.7  14.2/46.6
4 × 10-4 (coal)
395.2/1296   33.6/110.2
______________________________________

Magnetic field strength (H₁00) measurements are made at various location in the tunnel. The measurements are made when the transmitting antenna is in drillhole 101. The measurements are made with a tuned loop antenna and portable field strength meter. The antenna output voltage is measured. For example, Table C illustrates the antenna output voltage which is within a constant of the field strength inside a tunnel.

TABLE C
______________________________________
MEASURED FIELD STRENGTH INSIDE
OF TUNNEL (dB re 1 nanovolt)
Field Strength
Measuring   Appoximate  Center of Tunnel
Station     Distance (ft)
(dB per nanovolt)
______________________________________
1           300         85.6
2           250         82
3           200         86
4           150         101
5           100         111
6            50         123
7            50         131
______________________________________

The measured data (20 log₁0 H_φ) was plotted in FIG. 8 (long dash-dot curve). Assuming farfield radiation conditions, the measured data should correspond to the logarithmic form of equation 4 given by: ##EQU12##

Two additional curves are constructed in FIG. 8 to enable the determination of c and α from the measured data. The upper curve (plus-dash) was constructed by adding the spherical spreading factor (20log₁0 r) to each measured data point. The constructed curve should be linear for measurements made in excess of: ##EQU13## from the radiating antenna.

The constructed line is approximately linear with a slope of 16.8 dB/100 ft. when r>100'. This seems to confirm strong spherical spreading of the RIM image signal in the schist medium. The measured attenuation rate is in agreement with the expected schist attenuation rate given in Table A. The constant factor is seen to have a value of 185.0 dB per nanovolt above one nanovolt.

In the homogenous schist medium, the constant c may be analytically determined by substituting equation 10 into the bracket term in equation 8. The antenna coupling constant is mathematically represented by: ##EQU14## where σis the conductivity of the medium;

μ_o is the magnetic permeability;

ε is the permitivity; and

is the frequency of the imaging signal.

In non-magnetic rocks, the magnetic permeability remains constant at the free space value of 4π×E-07. The imaging signal frequency (f) and the antenna magnetic moment (M) are also constants. The coupling constant is directly proportional to the conductivity (σ) of the schist medium. It can also be shown that the constant depends upon the inverse square of the signal wavelength in the mediums. The wavelength ratio relative to a coal medium is given by k=395m/93m=4.24. This would produce a 20log₁0 k=25.12 dB increase relative to that found in a coal seam. The measured c value in coal is 160; therefore, an expectation of 185 is in close agreement with the measured schist value.

Vertical imaging, i.e. measuring attenuation rates in a vertical plane, as taught herein allows for the vertical reconstruction of tomography images of faults. This is also an important discovery for hard rock mining since chemical mineralization occurs in these areas in a subterranean environment.

The vertical imaging method may also be applied to gas and oil reservoir analysis. As illustrated in FIG. 13, since the conductivity of a sand reservoir decreases when the communicating pores are filled with oil or natural gas and increases when filled with water, the conductivity contrasts between the petroleum reservoir and water reservoir cause the attenuation and phase shift rates to marketly change the ray paths intersecting different mediums.

Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such 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 disclosure. Acordingly, 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.

Claims

1. A method for imaging geologic anomalies in a vertical plane in underground geological formations comprising:

drilling a plurality of downholes about said formation and at locations remote from each other;

placing within a first of said downholes a transmitter having continuous wave transmit capabilities in a medium frequency range of approximately 100 KHz to approximately 800 KHz, the transmitter including a short magnetic dipole antenna for propagation of waves through said seam;

placing within a second of said downholes a receiver having continuous wave receive capabilities in a medium frequency range of approximately 100 KHz to approximately 800 KHz, the receiver including a vertical tuned resonant loop antenna for receiving waves propagated by said transmitter, said receiver further including measuring and recording means for measuring and recording a plurality of characteristics of said received propagated waves;

successively changing the elevation of said transmitter in said first hole to different transmitter stations;

at each transmitter station, transmitting a plurality of transmissions of continuous wave medium frequency waves with spherical spreading and an azimuthal magnetic field component propagated toward and received by said receiver antenna;

measuring a plurality of signal transmission characteristics of the received azimuthal magnetic field components;

calculating a plurality of expected signal transmission characteristics of signals propagated through said formations; and

comparing said calculated signal transmission characteristics with said measured signal transmission characteristics and generating, by tomography reconstruction, geographical representation of said vertical formation therefrom.

2. The method of claim 1 including the further step of,

with said transmitter at each transmitter station, successively changing the elevation of said receiver in said second hole to different receiver stations.

3. The method of claim 1 including the further step of,

predetermining the distance between each transmitter station and successively moving said transmitter to said predetermined transmitter stations.

4. The method of claim 2 including the further step of,

predetermining the distance between each receiver station and successively moving said receiver station to said predetermined receiver stations.

5. The method of claim 2 wherein,

each successive change in elevation of the transmitter is substantially equal to each successive change in elevation of the receiver.

6. The method of claim 2 including the further steps of,

removing the transmitter from the first of said downholes;

placing the transmitter within a third of said downholes and successively changing the elevation of said transmitter in said third hole to different transmitter station;

at each transmitter station within said third hole, transmitting a plurality of transmissions of continuous wave medium frequency waves with an azimuthal magnetic field component propagated toward and received by said receiver antenna;

measuring a plurality of signal transmission characteristics of the received azimuthal magnetic field components;

calculating a plurality of expected signal transmission characteristics of signals propagated through said formation; and

comparing said calculated signal transmission characteristics with said measured signal transmission characteristics and generating, by tomography reconstruction, a geographical representation of said vertical formation.

7. The method of claim 1 wherein, the short magnetic dipole antenna is a vertical tuned resonant loop antenna.