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Description  |
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BACKGROUND
The present invention generally relates to a method and apparatus for detecting the presence of rock during coal or ore mining operations.
A more effective way to control solid mineral mining equipment, or miners, has been greatly desired by the mining industry. Many concepts have already been tried, over a period of many years, to improve mining controls to increase the amount of
coal, or other mineral, cut by the mining equipment and to decrease the amount of undesirable rock cut by the mining equipment. Many of these concepts involve "guidance" systems that direct or point the miner where to cut, based on predictions or
assumptions related to the location of the mineral-rock interface. These predictions or assumptions are typically based on data or information obtained from the experience of the mining equipment from previous cuts.
One seemingly simplified approach employs repetitive cycles. A computer is instructed by the miner operator to perform specific cycles or the control system is programmed to memorize operator actions over a cycle and duplicate them. This
approach does not work well because of the high variability of the rock and mineral formations and operational considerations. This approach is particularly ineffective when applied to continuous miners, because the miner rides on the floor that has
been cut resulting in cutting errors (e.g., leaving an excessive layer of coal on the floor, or cutting excessively down into the rock on the floor) for one cut tending to be amplified for subsequent cuts.
In the case of long-wall mining there is some opportunity to utilize what has been learned on one pass along the mineral face to improve upon cutting strategy for the next pass along the face. One approach utilizes a memory system to log the
profiles of the rock face at the floor and roof on one pass and then to use this knowledge to influence the cutting as the cutters pass along the same face, going in the opposite direction. This approach has been of only limited success because the rock
face profile on one pass does not exactly reflect the needed rock face profile of the next pass and because there is much variability in the formations and mining operations. Consequently, such equipment and operation are limited in their efficiency in
cutting to the rock-coal interface using guidance strategy.
Gamma detectors have, over the years, shown promise in detecting the location of the rock-wall interface for both continuous miners and long wall miners, but typically have not been effective because they have been installed so as to measure
where the mining equipment has been rather than where the cutter is going. One reason that gamma detectors have often been used in a non-effective manner is that the detectors could not physically survive if subjected to the environment in locations
where they would be most effective.
Numerous other approaches have already been conceived and tested over the years for directing or guiding mining equipment. Most of these concepts have not proven to be commercially successful due to technical deficiencies, implementation
problems, and cost. Many types of sensors have been incorporated into control systems to monitor the shape, profile and characteristics of the formations through which the mining equipment is cutting and to make cutting decisions on where to point
subsequent cuts based on this information. Thus, these approaches fail not only due to practical implementation problems but also because of a fundamental flaw with the concept. Knowledge about the shapes, profiles, or characteristics of the formation
being passed through does not provide accurate information about the formation just ahead, for which the cutting decisions must be made.
In most of the examples above, the control systems employed have been complex and expensive. A typical approach is to -use a gravity-referenced or inertial-referenced control system, with various other sensors added. Some of these control
concepts have been referred to as "horizon control systems." A horizon control system typically uses the gravity-referenced sensors or inertial-referenced sensors that keep track of the orientation of the continuous miner and the profile of the roof and
floor.
In principle, the horizon control system approach is to control the mining equipment by use of guidance systems adapted to mining applications. However, as discussed above, guidance systems cannot generate accurate information about the
formation to be cut because the historical information that they log in detail is not a valid indicator of what is ahead. Moreover, these guidance systems are complex and costly.
It is described in co-pending U.S. application Ser. No. 09/811,781 that in underground coal mining, a properly designed and properly positioned, forward-looking armored gamma detector, in combination with a suitable control system, can be
effective for reducing the amount of rock taken while extracting an increased amount of coal or other mineral. A mining control system that incorporates such forward-looking detectors is referred to as a "rock avoidance system." The use of rock
avoidance systems can help cut the floor of the mine very smoothly and simplify the job of the operator. Rock avoidance systems allow continuous miner operators to be positioned further from the coal face, thus reducing health hazards.
However, even when used with forward-looking rock detectors as described in co-pending U.S. application Ser. No. 09/811,781, these horizon control systems do not utilize the data generated by the rock detectors as fully as it could be used,
because the systems are conceived and designed to guide or point, determining the direction to move, rather than being appropriately responsive to sources of external intelligence such as armored gamma detectors. In addition, inertial or gravity
referenced systems are not typically designed to provide precision and timely measurements of cutter movements that will allow a rock detector to achieve maximum sensing accuracy.
Rock avoidance systems that rely upon complex guidance systems are costly and, complicated and have some inherent inefficiency resulting from their methodology. A need now exists to provide an accurate rock avoidance system that is simple,
economical and easy to install and operate. There is also a need for such a rock avoidance system for use on long-wall mining equipment as well as continuous mining equipment.
SUMMARY
These deficiencies are alleviated to an extent by the present invention which in one aspect provides a rock avoidance system for solid mineral mining using a forward looking rock/mineral interface detector and controlling the miner to cut to the
detected rock/mineral interface.
In another aspect, vertical movements of the cutting mechanisms are measured for the purpose of being used by the rock detector to make more accurate mathematical calculations of the location of the coal-rock interface.
In another aspect, a method is provided for improving accuracy by incorporating a device within an armored rock detector to sense angular movements of the cutter boom and to correlate changes in gamma radiation to the angular movements, within
selected energy ranges. An armored rock detector, so configured, can make effectively accurate cutting decisions under a wide range of mining conditions without support from complex control systems. Cutting decisions from the rock detector are
transmitted directly to the miner control system to slow or stop the movement of the cutter toward the coal-rock interface or to a control and display panel where other constraints and logic may be applied.
In another aspect, the change in attenuation is determined, and the thickness of the remaining coal is calculated by measuring the rate at which the gamma radiation increases. Greater accuracy in the calculations is achieved by measuring the
relative changes in gamma counts for various energy levels. Quick response is achieved so that the cutter of a continuous miner moving toward the rock on each cut may be stopped before reaching the rock by employing curve-fitting techniques that
correlate the gamma ray measurements with incremental movements of the cutters. The rock detector is outfitted with the required logic elements and algorithms.
In yet another aspect, a method of geosteering is provided on a continuous miner is for a shearing down to be slowed slightly as the floor is approached. Control of the shearing is accomplished by signals from the rock detector which operate the
solenoids that control the hydraulic system. Following the shearing stroke, the miner is placed in reverse for a short distance in order to remove the small cusp left behind the cutter. During this backing up, the rock detector will maintain the boom
at constant angle so that the floor will be cut level. Next, the operator moves the miner forward slowly, simultaneously shearing up, to sump to approximately fifty percent the diameter of the cutter. If a rock detector is used at the roof, it will
slow the cut slightly before reaching the rock interface and then stop the cut. While the boom is being held at a constant angle by the rock detector, the operator drives the miner forward to a fill sump. At this point, the operator is ready to start
the shear down to repeat the cycle.
In another aspect, the rock detector is placed near the cutter on a continuous miner, so that it can detect the radiation passing through the coal in front of the advancing cutter. When cutting at the floor, the detector moves with the advancing
cutter such that the angular size of the field of view is not reduced as the cutter moves down toward the bottom portions of the miner.
In another aspect, the rock detector is placed near the cutter on a long-wall miner When geosteering the trailing drum, the divergence rock detector is positioned within a few feet of the bottom edge of the picks so that a divergence between the
tips of the picks and the rock will be detected before coal is left unmined. Also, the divergence rock detector is positioned close to the picks so that the cutter can be biased toward divergence without concern for leaving coal unmined. In another
aspect, a convergence rock detector is used on the trailing drum, and positioned close enough to the cutter to be able to detect rock that is being mined and then mixed with the coal. In a preferred embodiment a geosteering system is provided that
includes an armored rock detector, positioned on the boom of a continuous miner to view the area where coal is being cut, to measure the changes in gamma radiation as a result of the coal being cut away, to correlate the changes in gamma radiation with
incremental changes in the position of the cutter, and to make logical decisions when to slow and/or to stop the cutter before cutting into the rock. In order to obtain precise measurements of rotation of the cutter boom or of the vertical movements of
the cutter, an accelerometer is incorporated into the rock detector.
In another preferred embodiment, the geosteering system includes a control and display panel that keeps the operator informed about the cutting progress, particularly in regard to cutting at the roof. This panel accepts data and decisions from
the rock detectors and also displays the position of the cutter relative to the most recent cuts at the floor. A solid-state accelerometer, in the form of a micro-chip, is included as part of the electronics. This accelerometer acquires additional
information on the instantaneous motion of the continuous miner and sends that information to the rock detector so that the rock detector can subtract errors resulting from motion of the miner from the measured incremental movement of the cutter and rock
detector. In a typical application, gamma data is correlated to the incremental movements of the cutter and this information is retained within the control and display panel for at least ten cutting cycles. Detailed, automatic analysis of this data
allows refinement of the logical decisions to be made for future cutting cycles.
In another embodiment, an encoder and/or a potentiometer are provided to instantly measure and report to the rock detector, the movement of the boom, on which the cutter is attached. Such substantially instant, precise data allows the rock
detector to make fast, accurate measurements. When rock detectors are being used for controlling cutting at the roof, in addition to controlling cutting at the floor, such auxiliary devices provide supporting information to the rock detector, to the
miner control system, and to the operator. This preferred embodiment includes a cutter motion indicator, containing an optical encoder and a potentiometer, at the pivot point of the boom. By combining this precise, high-speed data with the expanded
computational capabilities of other preferred embodiments, advanced automation at higher speeds of operation are made possible.
In yet another embodiment, rock detectors are used to steer the cutting of a long-wall mining system. In some applications, both the leading drum and the trailing drum of a long-wall shearing system are geo-steered by use of rock detectors.
Whenever the mining equipment reverses direction, the leading drum becomes the trailing drum. The armored rock detector is placed near the bottom of the cowl for the trailing drum and allows direct view of the surface being cut by the drum. The rock
detector begins by slowly raising the drum until the rock detector determines that coal is being left unmined. Raising and lowering of the drum by the rock detector is accomplished by operating the solenoids that control the hydraulic system. Upon
recognition that a small amount of coal is being left over the rock, the rock detector quickly lowers the drum by approximately two inches. The amount that the drum is lowered will depend upon the miner and mining conditions. In one aspect, the rock
detector continues to steer the drum so that the cutting operation cycles between three conditions (1) removal of only a small amount of rock, (2) preferable removal of all coal and no rock, and (3) leaving up to one or two inches of coal over the rock.
In the case where the coal bonds well to the rock, typically fire clay, the maximum amount of coal occasionally left will preferably be less than two inches. The preferable result is that for most of the cut along the face, almost no floor rock is mined
and very little coal is left unmined. For the case where soft coal is not bonded to the fire clay, preferably substantially all of the coal will be removed substantially all of the time.
These and other objects, features and advantages of the invention will be more clearly understood from the following detailed description and drawings of preferred embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a continuous miner including a pair of rock detectors constructed in accordance with a preferred embodiment of the invention.
FIG. 2 is a graph showing a typical equilibrium energy spectrum for a homogenous rock formation above and below a coal vein.
FIG. 3 is a graph showing the effects of coal on a typical equilibrium energy spectrum for a homogenous rock formation.
FIG. 4 is a partial cross-sectional view of one of the armored rock detectors of FIG. 1.
FIG. 5 is a cross-sectional view of one of the rock detectors of FIG. 4.
FIG. 6 is a view taken along section line VI-VI of FIG. 5, at the scintillation element.
FIG. 7 is a view taken along section line VII-VII of FIG. 5, at the photo-multiplier tube.
FIG. 8 is a view taken along section line VIII-VIII of FIG. 5, at the accelerometer.
FIGS. 9a and 9b are graphs of gamma ray counts versus time and versus change of cutter boom angle.
FIG. 10 is a schematic drawing of a logic element used with a rock detector constructed in accordance with an embodiment of the invention.
FIG. 11 is a schematic drawing of a logic element and digital signal processor used with a rock detector constructed in accordance with an embodiment of the invention.
FIG. 12 is a schematic drawing of a logic element and digital signal processor used with a pair of rock detectors constructed in accordance with an embodiment of the invention.
FIG. 13 is a schematic drawing of a junction box and cables used in an embodiment of the invention.
FIG. 14 is a schematic drawing of a control and display panel and cables used in an embodiment of the invention.
FIG. 15 is a schematic drawing of a control and display panel, accelerometer and cables used in an embodiment of the invention.
FIG. 16a is a view of a cutter motion indicator used with a rock detector in accordance with an embodiment of the invention.
FIG. 16b is a cross-sectional view of the cutter motion indicator of FIG. 16a.
FIG. 17 is a cross-sectional view of a linkage mechanism used with cutter motion indicator of FIG. 16a.
FIG. 18 is a schematic view of a longwall shearing system in accordance with an embodiment of the invention.
FIG. 19 is a schematic of a pair of rock detectors on the trailing shear of the long-wall miner of FIG. 18.
FIG. 20 is a graph of predicted and measured floor depth versus distance traveled.
FIG. 21 is a graph of detected gamma ray counts versus coal/rock interface depth.
FIG. 22 is a graph like FIG. 21.
FIG. 23 is a graph like FIG. 21.
FIG. 24 is a cross-sectional view of a rock detector constructed in accordance with another embodiment of the invention.
FIG. 25 is a cross-sectional view taken along line XXV-XXV of FIG. 24.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides a more accurate and faster solid mineral mining by use of a rock avoidance system that applies a new methodology called geosteering to solid mineral mining.
Geosteering techniques have been used in oilfield applications as exemplified in U.S. Pat. No. 5,230,386, RE 035,386, and U.S. Pat. No. 5,812,068. With geosteering, the distance to the oilfield bed boundary is measured while in the
formation, and the drill string is steered by direct measurements of the formation so that it stays in the mineral vein. This technology has advanced to the point where horizontal wells in excess of one mile are routinely drilled. Further, these wells
can now be drilled with the drill string staying in the reservoir formation throughout the horizontal section. Such geosteering for oilfield applications was recognized as an important new methodology and a substantial advance over directional drilling
techniques exemplified by U.S. Pat. Nos. 3,982,431 and 4,905,774.
The "directional drilling" approach to horizontal drilling in oil and gas wells is somewhat analogous to currently-used "horizon control" that has been used for mining applications. In both cases of directional-based controls, for oil and for
coal, independent attitudinal and/or inertial reference systems provide the basis for guiding or pointing the machinery. In each application, the extent and profile of a solid mineral vein to be mined is not predictable. Indeed, the problem is more
critical in coal mining than in oil well drilling, because the mining operation needs to be accurate to within inches compared to the accuracy of feet typically required in oil wells.
Guidance or pointing based on an inertial or gravity based reference system does not provide the intelligence needed to accurately make the next cut. The control functions at any moment must be accomplished by signals from sensors that are
measuring relevant parameters for the formation just ahead, where the cutting will occur. Directional control systems, such as horizon control, used in solid mineral mining have not produced the successes achieved with directional drilling in horizontal
oil wells. Thus, implementation of geosteering to solid mineral mining represents an even greater opportunity for improvement than did the implementation of geosteering for drilling oil and gas wells.
The principle of geosteering for continuous miners is to keep the cutter moving between the boundaries of the coal vein and letting the continuous miner follow the cutter through the geologic formation. Geosteering is more straightforward than
conventional approaches, and is fundamentally simpler in concept. The actual profile of the tunnel being cut through the earth during mining, the vertical excursions of the tunnel, and the slope of the floor and roof of the tunnel are not primary the
primary objective of geosteering. These parameters can be derived from data acquired while performing geosteering, and may be of some interest, but such data are the consequence of geosteering rather than being the guide for cutting.
Coal is located in a formation between other materials, generally classified as rock. An example would be a coal seam having black marine shale at the roof and fire clay, another form of shale, at the floor. In this example, the shale has a
significantly higher level of natural radiation than the coal. As the shale radiation passes through the coal from the rock, it is attenuated. The thickness of the coal is reduced as a continuous miner removes the coal. Reduction in the thickness of
the coal results in less attenuation so that the gamma radiation reaching the detector increases as the coal is cut away. At the point of contact between the cutter and the rock, there is no attenuation by coal and the gamma radiation is at a maximum.
By measuring the rate at which the gamma radiation increases, the change in attenuation can be determined, and the thickness of the remaining coal can be calculated.
Greater accuracy in the calculations is achieved by measuring the relative changes in gamma counts for various energy levels. Quick response is required because the cutter of a continuous miner is moving rapidly toward the rock on each cut and
should be stopped before reaching the rock. Since the cutter picks are on a rotating drum, the advancing face of the cutter is a curve. As the first picks along the centerline of the drum begin to enter the rock, bare rock is exposed and pieces of rock
are cut away and dragged on top of the coal pile behind the cutter. If the cutters actually enter the rock, it is desirable to immediately stop the advance of the cutter to save wear on the picks and avoid cutting undesirable rock. To achieve faster
response and higher accuracy, curve-fitting techniques are employed by correlating the gamma measurements with incremental movements of the cutters. The system includes associated logic elements and algorithms.
Geosteering, which relies primarily upon measurements of natural gamma radiation, can only be properly implemented by understanding the physics of the processes and physical phenomena involved in making and interpreting the gamma measurements.
Physical characteristics of the formations and their radiation properties are reviewed below. The logic elements included in the preferred embodiments have been created to accomplish the required decision-making, taking advantage of this understanding
of the physics involved, within the confines of the protected environment provided within the rock detector.
Radiation flux from coal/rock usually originates from trace levels of radioactive potassium, uranium, or thorium that are within the rock. In a typical case, a discrete spectrum of gamma rays is produced by the radioactive decay of the trace
elements. These gamma rays are transported through the formation, losing energy through Compton scattering (and possibly pair production), until they are finally photo-electrically absorbed. Within the rock, an equilibrium spectrum is soon established
reflecting a balance between the production of gamma rays in radioactive decays, the downscattering of gamma rays to lower energy, and the absorption of gamma rays through photoelectric absorption.
When the flux enters the coal region, this equilibrium is upset. The production of gamma rays in coal is much lower, reflecting a significantly lower level of potassium, uranium, and thorium. Since the higher energy regions of the radiation
flux are not replenished, the spectrum shifts to lower energies as the gamma rays are down-scattered and decreases in magnitude as the gamma rays are absorbed.
The inverse of this process is observed as coal is mined. First, the gamma flux is low in magnitude and energy, reflecting the extensive absorption by the thick layer of coal. Then, as coal is removed, the magnitude of the flux increases, and
the mean energy of the flux increases.
A typical equilibrium spectrum for a homogeneous rock formation above and below a coal vein is shown in FIG. 2. The broad peak at about 100 kev is the down-scatter peak. Most of the gamma radiation under this peak has lost energy through
Compton scattering. If Compton scattering were the only physical process involved, a 1/E.sup.2 distribution would be seen, instead of the down-scatter peak. However, as gamma rays lose energy, their cross-section for photoelectric absorption increases. This absorption results in the gamma radiation having the lower energy, producing the backscatter peak that is observed in FIG. 2.
The formula for the photoelectric cross-section is given as:
.times..times..times..times..times..times..times..times..times. ##EQU00001## where Z is the average atomic number of the formation. The denominator in this formula shows the strong energy dependence of the cross-section, and explains the
existence of the backscatter peak. The numerator gives the dependence of the cross-section on the lithology of the formation.
An oilfield convention for describing this dependence is to consider the photoelectric cross section at E=30.6 kev. At this energy, the numerator=0.01 and we have:
.times..times..times..times. ##EQU00002##
Using this convention, the photoelectric cross-section of coal is found to range from about 0.1 to about 0.3 barnes/electron, while the rock above and below the coal typically ranges from 2-5 barnes/electron. As a result, of this difference in
the photoelectric cross-section, the down-scatter peak for the rock above and below the coal is at a higher energy than the down-scatter peak for coal.
It is somewhat easier to visualize these parameters by starting with only rock and adding coal on top of the rock, as happens when steering the trailing shearing drum of a long-wall miner. If the drum is raised, a thin layer of coal is added on
top of the rock and the spectrum is shifted to lower energies. Gamma rays from the rock lose energy as they are Compton-scattered in the coal. The higher energy regions of the flux are not replenished, because the natural radioactivity of the coal is
much lower than that of the rock. As more coal is added, the gamma rays are shifted to sufficiently low energies to allow absorption to be a significant factor again. The reverse of this description then applies to the removal of coal by the cutters on
a continuous miner.
FIG. 3 shows an example of this phenomenon, presenting the spectrum at the surface of bare rock (0 cm) and at the surface of a coal layer on top of that rock at distances of 10 cm and 20 cm from that rock. From the plots on FIG. 3, it is clear
that the percent of flux per energy unit is greater at the rock face than that observed through a layer of coal.
Geosteering accomplishes the steering for solid mineral mining through direct measurements made on the formation in the region where the cutting is being performed. Inertial reference systems, attitudinal reference systems or guidance systems
are not required for geosteering. The steering is accomplished using rock detectors that follow the mineral formation.
In conventional systems, the vertical movements of the cutter are controlled to be in conformance to a complex profile of the movements and/or attitudinal parameters of the continuous miner and of the tunnel through which it is moving.
Conventional systems have been arranged primarily to track where the miner has been, and then attempts to adjust the direction and actions, and point the cutter based on what is learned during cutting. Geosteering, in contrast, simply follows the
mineral vein within the formation.
Another preferred embodiment includes increasing the computational capabilities within the rock detector so as to be able to perform more complex calculations for making better cutting decisions. Statistical analyses are performed to determine
the probable accuracy of the decisions made by the rock detector. Data from this expanded capability supports higher level analyses. This is depicted in FIG. 20. FIG. 20 shows the estimates of the position of the coal/rock interface at the floor for
previous cutting cycles, as well as predictions for the next cutting cycle. This prediction is used as the "0" reference for the next measured cycle. The position of the regular measurement of the counts is given in terms of the distance to the
predicted coal/rock interface. A typical measurement is depicted in FIG. 21. It shows the counts measured in a time interval of 0.25 seconds as a function of depth. (This time interval is not unique but is given as a typical example.) When these data
points are analyzed, the predicted rock interface is at -1.67 inches, not 0.0 inches. However, that is not an error. To illustrate the ability of this technique to pick out changes in slope, the model formation incorporated a change in slope at 275
inches, which resulted in the coal/rock interface being 1.5 inches lower than predicted. The measured data were sufficient to determine this change.
This measurement will be added to the earlier measurements, the expanded set of measurements will be fitted, and a prediction will be made for the next cut. Also, the measurement can be used to extend the present cut to the newly measured
boundary. Immediate use within a pass requires quick decision-making during the sweep down, since an entire sweep down can occur in just two or three seconds. The processing capability described in this invention (including PICs and a DSP) have the
speed and capability needed to determine the boundary in sufficient time to affect the cut.
Another feature that should be noted is the ability of such a system to "learn" from previously obtained data. An example of this would be the observed count rates as a function of the distance to the interface. As long as the radiation from
the rock above and below the coal is constant, and the thickness of the coal vein is constant, this function will remain the same. But, as these variables change, so will the function.
Typically, these changes occurs at a much slower rate than the change in the position of the floor. Thus, over the interval used to predict the next floor position, the response function can be assumed to be a constant. But, over longer
periods, a change in this function can be noted. Generally, it can be assumed to be constant over about ten to fifteen mining passes, which should be sufficient to determine the position of the boundary at the next cut. But, over longer intervals, such
as a day of making cuts, the coal thickness and or the level of radioactivity in the rock above and below the coal can vary.
The change in the response pattern produces a signal that can be distinguished from the signal produced by changes in the position of the coal/rock interface. There are two ways in which this difference can be observed. First, the ratio of the
count rates in various energy regions changes with the distance to the boundary. An increase in the level of radioactivity will have minimal effect on this ratio. Second, there is a unique signature when the miner breaks through the coal/rock interface
and start mining into the rock. This signature will be considered in some detail in the next example.
When a change in the thickness of the coal, or the level of radioactivity in the formation above or below the coal crosses a threshold of significance, the system is capable of performing two actions. First, it can alert the person supervising
the mining activities of the change in the conditions. This is done through the use of the control and display panel. This affords him the opportunity to manually change the actions of the miner. Second, it can alter the pattern it uses to determine
the interface to reflect the new conditions.
Another preferred embodiment involves a system with two detectors: one for the roof and one for the floor. An example is pictured in FIG. 1. In this example, the roof rock is five times as hot as the floor rock. Examples of the relative
signals for the roof and the floor are shown in FIG. 22, which gives the count rate as a function of the distance from the miner to the floor.
The response of the floor detector is much flatter than the response of the roof detector, as well as much flatter than the floor detector response in the prior example. This is a result of the heightened background cause by the roof being five
times as radioactive as the floor.
Even with shielding, the floor detector still has some sensitivity to the radiation from the roof. When, as in the prior example, the roof radiation is comparable to the floor radiation, the effects of this sensitivity are relatively small.
But, when the roof is five times as hot as the floor, the effects become noticeable.
Note that the background radiation level from the roof is not a constant. As the process of mining down towards the floor rock begins, the boom containing the cutter and the armored rock detectors is typically level or tilted slightly upwards.
As the mining progresses, it tilts down towards the floor. With this motion, there is maximum sensitivity to the roof radiation at the start of the process, and a reduction in sensitivity as the miner tilts toward the floor. This results in a decrease
in the count rates due to the roof radiation, which partially offsets the increase in the count rate that result from the removal of coal from the floor and the flattened response seen in FIGS. 22-23.
This reduction in signal combined with an increase in the statistical uncertainty due to the higher background from the roof results in significantly greater uncertainty in determining the floor coal/rock interface from measurements made while
cutting coal than from establishing the roof coal/rock interface from measurements made while cutting coal. Given this difference, one might think that the floor detector will not add to the accuracy of the measurement.
There is, however, a very significant bed boundary signal that is unique to the floor detector. It is a significant rise in the count rate as the miner reaches the floor. An example of this is shown in FIG. 23, which shows a step function
change in count rate at the coal/rock interface.
The reason for this change is that, when the miner reaches the boundary, it starts mining the radioactive rock instead of the coal. The surface of the coal pile is quickly covered with shale. Since the coal pile is very close to the detector,
the higher radiation from this region results in a significant increase in the detector count rate.
A similar signal is not seen at the roof. When the miner breaks through the coal/rock interface at the roof, the shale falls to the floor. The roof armored rock detector is shielded from the floor signal, so it does not show a marked increase
right at the boundary.
Armored rock detectors may be used for geosteering at the floor and at the roof of a mining operation. FIG. 1 shows a continuous miner 10 that has been configured with two armored rock detectors 20, 120. The primary function of these detectors
20, 120 is to determine when the cutter picks 14 are approaching the coal-rock interface 15, 16, to slow the movement of the boom 11, and to stop the movement of the boom 11 whenever all of the coal 18 has been removed.
Space for installing a gamma detector on a continuous miner is very limited since the detector must be positioned in a specific location in order to be in view of the coal to rock interface. The presence of armor, which is required to protect
the detector, further limits the available space. An explosion-proof housing takes up even more of the available space, and often results in reducing the diameter of the photomultiplier tube. As the diameter of the photomultiplier tube is reduced, the
efficient transfer of light to the tube becomes more critical. The dynamic support system for the scintillation element 110 preferably should be effective for a sodium iodide (NaI) crystal having a high length to diameter ratio since NaI crystals are
easily fractured by vibration, shock, shear or bending forces.
Mine safety requirements dictate that electrical and electronic equipment be housed in enclosures that are explosion-proof in order to prevent ignition of dust or gas that may be around the detector. One unique feature of the illustrated
embodiment is that the detectors 20, 120 are configured so that the explosion-proof requirement is met at the detector. Having the explosion-proof housing 59 at the detector allows the electronics to be at the detector so that the sensitive, low level
signals do not have to be transmitted outside the protective structures to electronics which have been located at some distance away, often many feet. All this has been achieved in such a way so as to not require a large space, the small volume making
it possible for the detector to be strategically placed near the target stratum. Explosion-proof boxes typically used to protect electrical systems on miners are so large that they generally do not survive in those locations.
Each of these detectors 20, 120 has been strategically positioned to allow it to receive gamma radiation from the rocks at the coal-rock interface 15, 16 in front of the advancing cutter picks 14, as well as directly behind the cutters. To reach
the rock detectors 20, 120, some of the radiation 28 passes between the picks 14. In the event that the cutter picks 14 overshoot the interface 15 at the floor, and enter the floor rock 26, the picks will throw rock on top of the coal pile 21 behind the
cutter. This sudden exposure of the rock surface and the loose rock added on top of the coal pile 21 behind the cutter gives an immediate rise in gamma counts, an indication that the cutter 12 has gone too far and the shearing is stopped before a
significant amount of rock 26 is removed. By making the rock detectors 20, 120 faster and more accurate, the cutter 12 can be stopped before cutting into the coal-rock interface 15. A variety of techniques are employed to increase the accuracy and
speed of the detectors 20, 120.
Many functional elements are required to make effective the rock detectors 20, 120. As can be seen in FIGS. 1 and 4, the rock detectors 20, 120 are protected by armor 70 that surrounds, shields, and supports them at a critical location near the
cutter picks 14. A challenge in designing the armored rock detector 20, 120 is the simultaneous provision of effective protection from the harsh environment and of an unobstructed path for the gamma rays 28 to enter the scintillation element 50 with as
little attenuation as possible. Windows are provided in each portion of the structure to prevent obstruction | | |
