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Archived: Interstate Technical Group on Abandoned Underground Mines
An Interactive Forum

Advanced Electromagnetic Wave Technologies for the Detection of Abandoned Mine Entries and Delineation of Barrier Pillars: Continued - Part 3/3


RIM Case Study

A RIM survey was recently conducted to investigate geological conditions and the extent of abandoned mine works ahead of proposed mine development. A mine had re-entered previously developed mine works from a highwall adit, and was extending operations down-dip into unmapped regions. Previous mine works in the area were well documented, but it was possible that some historical mine works were not shown on existing plans. In addition, the mine was affected by seam roof paleochannels that could hinder coal extraction.

The RIM survey was designed both to detect abandoned mine works and evaluate geological conditions ahead of mining. The objectives of the imaging program were to:

  • Establish the location of zones of coal that may be affected by abandoned mine works
  • Provide an objective evaluation of seam geological conditions ahead of mining
  • Make recommendations regarding an appropriate approach to managing geological and/or old mine works risk at the mine.

Case Study Survey Procedure

A plan of the survey site with borehole locations, a summary of borehole stratigraphic column, and a map of known historical mine works were provided prior to survey start-up. Aerial photographs of the survey site, borehole locations, a summary of borehole stratigraphic columns, and predicted location of the works are helpful in pre-survey planning.

In general, RIM equipment should be deployed into vertical boreholes lined with PVC casing, never metal casing. This prevents damage or loss to the system resulting from hole collapse or debris. If the geology is exceptionally competent, the probes can be used without PVC casing. The borehole depths can be a minimum of 20 ft and a maximum of 1500 ft.

The basic geology of the survey area, as well as coal seam geology, needs to be characterized in general terms prior to developing a survey plan. Drill history provides a good geologic picture, particularly in identifying seam position and thickness. The dominant bounding rock in the stratigraphy is important in determining optimal imaging frequency and borehole spacing. These rock types may include clay, shale, siltstone, and sandstone. It is also important to know if there is evidence of geological anomalies, such as paleochannels, faults, and dykes, in the survey area. Without this information, it must be assumed that the seam is homogeneous throughout the survey.

The survey utilized RIM equipment from vertical boreholes and from underground in-mine, hand-held units. A total of 44 individual ray paths were measured from 14 individual boreholes (ST-1 to ST-13, and MA92-12) and 9 underground survey stations. The boreholes were drilled to a maximum depth of 10 ft below the coal seam level. The survey layout of the boreholes and the current workings is shown in Figure 36. The survey was carried out using two frequencies: 242 kHz and 432 kHz. The lower frequency was used for the longer borehole-to-borehole shots; the higher frequency was used for closer borehole spacing and borehole-to-in-mine shots. In general, experience dictates that it is desirable to use the highest possible frequency in order to achieve higher resolution. Also, advanced RIM systems have synchronized signals enabling the collection of phase shift data, which when utilized with attenuation rate measurements provided a most superior analysis/image.

Diagram of the survey area showing downhole boring location, in-mine receiver station, and transmission ray path
Figure 36. Diagram of the survey area showing downhole boring location, in-mine receiver station, and transmission ray path

Case Study Survey Results

The establishment of a standard RIM response in continuous coal is the first goal of the survey. The RIM signal decay rate is essentially constant (once out of the near field) and any disruption to the signal can be interpreted as a RIM anomaly—in this case due to the presence of mine works affecting the ray path or geological anomalies, such as paleochannel systems. Minor variation to linear decay may occur due to slight variation in coal seam height and moisture content.

The measured signal strength of the RIM transmission was adjusted for cylindrical spreading within the seam and normalized by ray-path length to establish the attenuation rate (units of dB/100 ft). The attenuation rate was then compared for each individual ray path and for each frequency utilized during the survey. Using the location of each ray path and that path's measured attenuation rate, a contour map of in-seam signal attenuation was created for the survey area and is shown in Figure 37.

Diagram of survey area showing contour map of RIM signal attenuation rate.
Figure 37. Diagram of survey area showing contour map of RIM signal attenuation rate. The contour interval is 1 dB/100 ft.

For the purpose of analysis and discussion, five distinct regions have been designated within the survey area: Region A (northwest of existing workings), Region B (southeast of existing workings), Region C (immediately in front of workings), Region D (southwest of existing workings), and Region E (further southwest of existing workings). The important conclusions are that attenuation rates measured along the perimeter of the regions indicate "solid" coal, or no voids that could be old works.

Additionally, the survey results showed high attenuation rates around and to the northwest of borehole ST4 (north is to the top of this map). As indicated in Figure 37, and confirmed by mine management, a sand channel was present in the area of high attenuation. The paleochannel scoured within the seam and introduced extremely difficult mining conditions. However, the RIM survey results indicated that the channel did not have extensive width and depth across the section. This intelligence about he anomaly was shared with management so appropriate decisions were made to continue mining operations. Once through the channel, mine management confirmed that the survey accurately predicted the physical features of the channel. Therefore, while the RIM survey's primary purpose was to establish or confirm the barrier pillars separating the old works from active mining, management gained the added benefit of using RIM to identify and confirm adverse mining conditions.

Void Detection and Confirmation Electromagnetic Wave Instrumentation Under Development

The Mine Safety and Health Administration (MSHA), universities with mining engineering departments, machinery manufacturers, and the mining companies have dedicated manpower to improving productivity and safety in the mining industry. The West Virginia University (WVU) International Conference on Ground Control in Mining is just one of the many technical meetings dedicated to improving safety. During the annual meeting in August 1999, the concept of incorporating real-time uncut coal thickness sensors on cutting drums was presented (28) as illustrated in Figure 38.

Schematic of detecting coal-rock interface horizons.  Shows rock split, mapping geology, sandstone, potential for rock fall, interbeds, 'bad top', horizon sensing, mud stone, and 'bad floor'
Figure 38. Detecting coal-rock interface horizons

The safety benefits of the drum-mounted sensor were obvious to the industry, but the technical problems appeared to be intractable. Coal cutting machines must be operated from a remote location where man interdiction is still viable. Even at these safe distances, miners are exposed to respirable dust and high acoustic noise. As illustrated above, the science of roof control can be effectively applied in unstable roof conditions by leaving roof coal. The thin layer can prevent spalling and the cutting edge from striking fragile roof rock. The drum-mounted Horizon Sensor enables real-time uncut coal thickness measurements for selective mining. The uncut coal layer will reduce the roof fall potential, especially under the margins of paleochannels. In some seams, the thin layer of roof or floor coal has higher percentages of ash, sulfur, mercury, and other heavy metals. The benefits of selective mining vary from seam to seam.

The coal-rock interface detection (CID) problem has been extensively investigated and the more promising technologies developed into experimental hardware. In the early 1970s, the National Aeronautics and Space Administration (NASA) Marshall Space Flight Center investigation concluded that natural gamma sensors were viable for measuring uncut coal thickness (29). Bessenger and Nelson developed a gamma sensor for measuring floor coal thickness (30). Chufo (31) developed a radar with a moving antenna that proved that coal thickness and the dielectric constant could be determined from the measurement. These technical approaches were considered very successful, but could not be integrated on coal cutting drums.

During the WVU conference paper (28) question-and-answer segment, Dr. Kelvin Wu of MSHA suggested that a look-ahead capability be added to the Horizon Sensor design to enable detection of metallic gas well casing and abandoned mine entries. The concept is illustrated in Figure 39.

schematic of  cutting drum look-ahead radar sensor
Figure 39. Cutting drum look-ahead radar sensor

The cutting drum-mounted sensor makes measurements when rotated 90 degrees from the look-up position.

The technical problems to be solved include the 90-g force shock and vibration levels measured on coal cutting drums.

Because continuous miner drums do not have slip rings for electric power generation, electric power must be generated on the cutting drum by electrodynamic generators (32). Measured data must be transmitted by radio data transmission between the machine body and the cutting drum. Research and development determined that a Resonant Microstrip Patch Antenna (RMPA) could be designed to withstand the coal cutting environment when enclosed in the flameproof enclosure shown in Figure 40 (33, 34).

Photo of MSHA flameproof approved RMPA Horizon Sensor (HS-3) mounted on a coal cutting drum
Figure 40. MSHA flameproof approved RMPA Horizon Sensor (HS-3) mounted on a coal cutting drum

The Horizon Sensor HS-3 has been installed on several Joy 12CM machines and operated during the past 24 months. The RMPA sensor and its resonant impedance (observable) are illustrated in Figure 41.

Schematic of Horizon Sensor RMPA as discussed belowGraph of Horizon Sensor Response
Figure 41. Horizon Sensor Response

The Horizon Sensor RMPA generates the electric fields (magnetic fields are not shown) in the insulator between two copper plates. Along the edges of the plates, the electric fields fringe and cause a horizontally polarized electric field to travel upward through the uncut coal layer to coal-rock interface. The reflected secondary electric field returns to RMPA where the total fields and the resulting impedance are measured. The impedance is a complex number with real and imaginary parts. The HS-3 is calibrated by cutting different thicknesses of coal and measuring impedance. The sensor has a maximum thickness limitation near 40 inches. Unlike radar, the sensor measuring accuracy increases as uncut coal thickness decreases.

The Horizon Sensor instrumentation was developed and demonstrated with partial support of the Department of Energy (DOE) Mining Industry of the Future (MOF) Program. J. Michael Canty was the MOF Program Manager and Morgan H. Mosser was the acting contracting officer's representative.

The New Yorker magazine article (35) chronicled Dr. Kelvin Wu's competent engineering leadership role in the successful rescue of the nine trapped miners. If his 1999 vision of a cutting drum-mounted sensor with look-ahead radar capability could have become a reality by July 2002, the following photograph (Figure 42) could not have been taken.

Photo of Quecreek Mine breach as discussed in paper
Figure 42. Quecreek Mine breach (photograph courtesy of MSHA)

To evaluate the HS-3 look-ahead detection range limitation, a salt block barrier pillar simulation was set up in Stolar's laboratory. The laboratory barrier pillar simulation site was constructed with salt blocks (εr= 6) shown in Figure 43.

Photo of salt block simulation of a coal barrier pillar
Figure 43. Salt block simulation of a coal barrier pillar

The HS-3 Horizon Sensor was rotated at the rate of 60 rpm and the HS-3 made measurements through the salt blocks to a reflector at the simulated barrier thickness. The maximum HS-3 range-detection capability was found to be 10 ft. The HS-3 computer-controlled electronics are capable of generating an automatic machine shut-down command to prevent mining into the void.

The HS-3 electronics operate in the CWSF mode to establish the resonant frequency of RMPA. Depending on the uncut thickness requirement, the RMPA frequency can be automatically set by the embedded computer to any frequency in range from 500 to more than 2500 MHz.

The HS-3 RMPA can be augmented by a wideband (non-resonant) antenna to detect far-field reflections from voids. Instruments were set up in the laboratory simulation to demonstrate range-detection capability. The CWSF radar generated 51 equally-spaced frequency steps between 0.9 and 1.1 GHz. This corresponds to a range of at least 50 ft with a 1-ft resolution. The measured radar data were processed in an FFT with the time-domain response shown in Figure 44.

Graph of Phase (degrees) vs distance (ft)
(a)
Graph of Magnitude vs distance (ft)
(b)
Figure 44. Results from the experiments in salt: (a) phase and (b) magnitude response

Features close to the radar antenna are obscured due to the impulse response of the wideband radar. Most of the reflected signal's energy is concentrated at the beginning of the transformed signal, and time-gating is usually performed; this masks the response from the nearby scatterers. A change in the phase of the time-domain (impulse) response indicates the presence of a scatterer, whereas the magnitude of the impulse response corresponds to the size of the scatterer (15). The radar phase response is the observable at 23 ft. The laboratory tests confirm the feasibility of cutting drum void detection.

A necessary condition for void detection is the measurement of the coal anisotropic dielectric constant. The dielectric constant can be measured with a RMPA illustrated in Figure 45.

Schematic of Dielectric constant measured with the Resonant Micorstrip Patch Antenna (RMPA)
Figure 45. Dielectric constant measured with the Resonant Micorstrip Patch Antenna (RMPA)

The RMPA driving point impedance will be measured with X-directed polarized electric fields and then Y-directed polarized electric fields. The RMPA will be integrated with cutting drum look-ahead radar antennas.

Proposed In-Mine Test Site

A radar enclosed within an MSHA-approved flameproof enclosure will be used to build and demonstrate the detection of abandoned mine air-filled and water-filled entries from the working face. The demonstration will be conducted using the following in-mine demonstration site (Figure 46).

diagram of in-mine demonstration site showing mains entry drive and water container wall
Figure 46. In-mine demonstration site

The continuous mining machine would first advance toward an air-filled crosscut to demonstrate detection range to a void, then advance toward a crosscut with stacked plastic water containers representing a water-filled void.

Drillstring Radar (DSR) for In-Seam Guidance and Navigation

Governor Schweiker's report (27) recommended improvements in horizontal directional drilling. Real-time Measurements-While-Drilling (MWD) radar must be integrated with guidance and navigation to improve horizontal drilling. CONSOL Energy pioneered the development of horizontal drilling technology for de-gassing coal beds in advance of mining. The CONSOL instrumentation measures pitch, roll, and azimuth of the drill bit with great precision. There is no need to improve this measurement technology, which will be referred to as navigation technology. Integration of radar and relative dielectric constant instrumentation with navigation is new and will increase the horizontal drilling efficiency by more than 30%, enabling drilling along the center line in an undulating coal bed (32). The DSR can be applied through vertical drillholes to confirm the distance to the void or geologic anomaly from the borehole (Figure 47).

diagram of Radar mapping of voids and geologic anomalies from vertical boreholes
Figure 47. Radar mapping of voids and geologic anomalies from vertical boreholes

Abandoned mine barrier pillar verification can be achieved using directionally drilled horizontal boreholes as illustrated in Figure 48.

cross section showing abandoned mine and horizontal boreholes
Figure 48. Detection and imaging of abandoned coal mines along boreholes.

Horizontal boreholes can establish that a safe barrier pillar of coal exists between the borehole and the mine development. The boreholes would be drilled parallel to and 30 ft away from a suspected abandoned mine boundary. Mining would be allowed to approach no closer than 50 ft to the directionally drilled horizontal hole. Radar and navigation instrumentation integrated into the drillstring would be used to measure the distance to the abandoned mine voids and the mine development entries.

Surface directional drilling technology for de-gasification of coal beds in advance of mining has been achieved in Australia and Appalachian coal basins. Vertical borehole segments are drilled to a predetermined depth and then directional drilling a 90-degree curve to intercept the coal bed horizontally.

Current In-Seam Drilling Technology

Current MWD navigation systems are used to guide in-seam drilling in coal deposits, but these systems cannot directly determine seam thickness or distance to a void or changes in seam orientation (e.g., dips and rolls) without employing a time-consuming sequence of drilling to the floor and then to the roof or vice versa in the same region of the panel, commonly called sidetracks (see Figure 49). The drilling machine operator is only able to detect when the drill is on the roof or floor horizon by the evidence of rock in the cuttings. Once these situations are detected, the operator redirects the drill motor in an appropriate drillstring rotation angle to realign the drill within the seam. A down-the-hole drill motor used in horizontal drilling applications is also shown in Figure 49.

Vertical cross section of a cola bed illustration conventional 'sidetrack' trial-and-error drilling under a paleochannel as well as the down-the-hole drill motor
Figure 49. Vertical cross section of a cola bed illustration conventional 'sidetrack' trial-and-error drilling under a paleochannel as well as the down-the-hole drill motor

The current directional drilling with a bent sub has a minimum seam height limitation. A controlled gimbal would enable drilling in thinner coal beds.

Basic Structure of the Drillstring Radar (DSR) Tool

The drillstring radar (DSR) has been designed to be installed inside a conventional drillrod as shown in Figure 50.

Photo of DSR with schematic showing the electronics
Figure 50. Stolar Drillstring Radar (DSR)

The radar electronics are installed in a retractable titanium flameproof enclosure (36, 37). The enclosure has passed the MSHA flameproof investigation. A battery pack (under MSHA I/S investigation) will be replaced by a hydroturbine. An inductive radio provided data transmission along the drillstring.

Photo of MWD with the various parts labelled
Figure 51. Measurements-While-Drilling (MWD) drillstring radar

A block diagram of the DSR instrument system is given in Figure 52.

A block diagram of the DSR instrument system as discussed
Figure 52. Block diagram of the drillstring radar instrument system

The MWD subsection of the DSR technology includes: (1) radar electronics for determining the distances from the horizontal boring to the roof/floor sedimentary rock layer and void, (2) navigation electronics for mapping the direction of the boring, and (3) drillstring data transmission hardware for transmitting processed radar and navigation information to the drilling machine.

The MWD instrumentation includes a coal seam waveguide imaging signal transmitter. A companion receiver in a prior-drilled horizontal borehole acquires RIM-IV data for tomographic processing.

The DSR technology features a downhole MWD instrument docked inside a beryllium copper drillrod subsection. This drillrod subsection is next to the antenna array drillrod subsection. For surface-based drilling, an F1/F2 repeater is deployed in the middle of the vertical section of the drillstring. At the collar, a GUI will log and process measured data. The graphical display will show the vertical cross section of the coal seam, the seam height, and type of boundary rock along the drillhole. The display will also include a plan view showing the heading of the drillhole.

Major Subsystems

  1. GUI with data logging and processing software
  2. Two-way radio data transmission subsystem for communications along the drillstring
  3. MWD instrument to acquire data and transmit the data to the collar
    • Radar to map seam height and type of boundary rock along the path of the drill
    • Navigation instrument to measure heading of the horizontal drill pitch, yaw, and rotation angle of the drilling subsection
  4. RIM-IV instrumentation for crosshole measurement

Concluding Remarks

This paper has been written for decision makers concerned with enhancing safety and productivity in the coal mining industry. The mining industry has achieved impressive safety and productivity improvements in recent years. Productivity has improved between 6 and 7% per year (38-40) for many years; safety has exceeded the pace.

The deterioration in US coal reserves and the spoiling of coal beds with the aggressive cavitation method of coal bed methane (CBM) production calls into question the continuation of productivity improvement. In future years, electromagnetic (EM) wave technologies will play a vital role in continuing the safety and productivity improvement through coal seam vision. From a safety point of view, abandoned mines and gas well casings are mining hazards that impact safety in recognizable ways. Anomalous geology presents even greater safety issues. Oftentimes, paleochannels scouring into the sedimentary roof rock weaken the seam boundary rock. Channels have caused ground control problems that have indirectly caused significant loss. For these reasons, we have expanded the scope of this paper beyond the detection of voids to include geologic anomalies.

EM wave vision requires the cognitive processes of detection, confirmation, and mitigation. EM wave technologies solve the detection problem. Imaging forms a silhouette of the void or geologic anomaly, while advanced horizontal drilling is essential to the confirmation process. Imaging is a targeting technology for advanced drilling. Hazard mitigation improves because the science of ground control can be appropriately applied in anomalous geologic zones. This science, along with horizon control under the margins of paleochannels, will improve safety and productivity, and prevent future accidents such as the Quecreek mine inundation. Also, with improved barrier confirmation, pillars can be reduced eliminating some coal sterilization.

The case study proves the points being made in these remarks. The goal of the RIM barrier pillar survey was to determine the pillar integrity. An additional benefit was realized by the detection of a paleochannel ahead of mining and the better knowledge that clear coal followed the channel scours. The image and improved intelligence about the geology prevented the mine from shutting down, increasing coal recovery from the reserve.

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Acknowledgements

The advanced development of the Horizon Sensor and Radio Imaging Method (RIM) instrumentation was partially sponsored by the Department of Energy (DOE) Mining Industry of the Future (MOF) and Initiative for Proliferation Prevention (IPP) programs. Dr. Hooman M. Tehrani contributed to the development of the antennas. The authors want to thank Gerald L. Stolarczyk and Igor Bausov for their work on development of the instrumentation. Senior Geophysicist Joseph T. Duncan conducted the in-mine demonstration programs. He was supported in this effort by Bruce Ley, Randy L. Acre, and John A. Myers.

The DOE IPP drillstring radar (DSR) program is administered by Dr. Chris Baumgart, Chris Miller, and Ashot Tumagyan of the DOE's Kansas City Plant Albuquerque Office. Approximately 150 scientists and engineers are working at the MINATOM Sedakov Institute for Measuring Systems Research on development of software and associated radar electronics.

Special appreciation is given to Jerry Jones and Daniel Carreon for the development of the manufacturing and quality-control processes used in building the instrumentation. Special recognition is extended to Dan Gregory and the structural dynamic engineering staff at Sandia National Laboratories for helping solve the 100-g shock and vibration sustainability of the drum-mounted Horizon Sensor. Glenn Rightmire, retired Professor of Mechanical Engineering at Columbia University, contributed to the solution of the shock and vibration problem.

The authors want to extend their special thanks to Glenn G. Wattley for managing the product launch and maintaining mining industry relationships during the field test program. Randy L. Acre provided valuable assistance in developing the sales and field support strategies for the in-mine installations.

The underground demonstration of the Horizon Sensor and RIM technologies was in part sponsored by the DOE MOF program. J. Michael Canty was the Program Manager and Morgan H. Mosser was the acting contracting officer's representative. The Horizon Sensor received the 2002 R&D 100 Award. The Chief Executive Officers of the NMA developed the hierarchy of technology needs for the DOE MOF program. The technical staff of the following NMA mining companies participated in the in-mine demonstrations: InterWest Mining, RAG American Coal Company, CONSOL Energy, and West Elk, a unit of Arch Coal, Inc. The following non-NMA companies also participated: Oxbow Coal Mining, The Ohio Valley Coal Company, Exxon Monterey Coal, and Blue Mountain Energy.

The authors want to thank Jeannie Pomeroy for preparing the manuscript, and Steve Bertola for the illustrations.

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