Browsing by Author "Murphy, Michael M."

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    Analysis of Seismic Signatures Generated from Controlled Methane and Coal Dust Explosions in an Underground Mine
    Murphy, Michael M. (Virginia Tech, 2008-11-07)
    Examination of seismic records during the time interval of the Sago Mine disaster in 2006 revealed a small amplitude signal possibly associated with an event in the mine. Although the epicenter of the signature was located in the vicinity where the explosion occurred, it could not be unequivocally attributed to the explosion. More needs to be understood about the seismicity from mine explosions in order to properly interpret critical seismic information. A seismic monitoring system located at NIOSH's Lake Lynn Experimental Mine has monitored nineteen experimental methane and dust based explosions. The objective of the study was to analyze seismic signatures generated by the methane and dust explosions to begin understanding their characteristics at different distances away from the source. The seismic signatures from these different events were analyzed using standard waveform analysis procedures in order to estimate the moment magnitude and radiated seismic energy. The procedures used to analyze the data were conducted using self-produced programs not available with existing commercial software. The signatures of the explosions were found to be extremely complex due a combination of mine geometry and experimental design, both of which could not be controlled for the purposes of the study. Geophones located approximately 600 m (1970 ft) and over from the source collected limited data because of the attenuation of the seismic waves generated by the methane explosion. A combination of the methods used to characterize the seismic signatures allowed for differentiation between experimental designs and the size of the explosion. The factors having the largest impact on the signatures were the mine geometry, size of the methane explosion, construction of the mine seal and location of the mine seal. A relationship was derived to correlate the radiated seismic energy to the size of the explosion. Recommendations were made, based upon the limitations of this study, on methods for better collection of seismic data in the future.
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    Application of Background Oriented Schlieren (BOS) in Underground Mine Ventilation
    Jong, Edmund Chime (Virginia Tech, 2011-04-21)
    The schlieren technique describes an optical analysis method designed to enhance light distortions caused by air movement. The ability to visualize gas flows has significant implications for analyzing underground mine ventilation systems. Currently, the widely utilized traditional schlieren methods are impractical underground due to complex equipment and design requirements. Background oriented schlieren (BOS) provides a solution to this problem. BOS requires two primary components, a professional quality digital camera and a schlieren background. A schlieren background is composed of a varying contrast repetitive pattern, such as black and white stripes or dots. This background allows the camera's sensor to capture the minor light diffractions that are caused by transparent inhomogeneous gases through image correlation. This paper investigates a possible means of mitigating some of the major problems associated with surveying underground mine ventilation systems with the BOS method. BOS is an imaging technique first introduced in 1999 that allows the visualization of flowing inhomogeneous transparent media. In ventilation surveys, BOS can be used to attain qualitative data about airflows in complex areas and methane emissions from coal. The acquisition of such data would not only enhance the understanding of mine ventilation but also improve the accuracy of ventilation surveys. As an example, surveys can benefit from small scale BOS investigations around fans, regulators, overcasts, and critical junctions to identify effective data gathering positions. Regular inspections of controls and methane monitoring points could also be improved by the systematic nature of BOS. Computer programs could process images of each location identically regardless of quantity. BOS can then serve as a check to identify items that were overlooked during the routine inspection. Despite the potential of BOS for ventilation analysis, several limitations still exist. These issues are sensitivity threshold and quantification of flow data. This paper specifically examines the qualitative potential of the BOS technique for imaging various underground ventilation flows and outlines initial experimental efforts used for the evaluation. Three primary experiments were conducted to evaluate BOS as a potential qualitative analysis technique for underground mine ventilation. The first experiment used BOS to image of flow induced by an axial vane fan and an axial flow fan using an artificial background and an imitation rock background. This experiment showed that the BOS system was unable to image isothermal airflow from either fan. Heated airflow could be visualized with both fans using the artificial striped background but not with the imitation rock background. The BOS system lacked the sensitivity necessary to image isothermal airflow from the two fans. The focus of the overall BOS study was changed to explore higher pressure airflows through a regulator. The second experiment used BOS to image flow through a regulator induced by an axial flow fan using an artificial striped background. The BOS images were compared to ones produced by a traditional schlieren single mirror systems for validation of the BOS experimental design. This experiment was unable to image isothermal airflow through the regulator from either system. However, heated airflow could be visualized by both systems. The BOS and traditional schlieren systems used in this experiment lacked the sensitivity necessary to image isothermal airflow through a regulator. However, the BOS procedures were successfully validated by the ability of both the BOS and traditional schlieren systems to image heated airflows. The focus of the study was changed to explore methane gas emissions. Numerous mining industry techniques already exist to quantify methane content. However, methane content is different from the actual methane emission rate of exposed coal. Emission rates have been modeled using numerical simulation techniques, but the complexity of the methane migration mechanism still requires physical data to achieve higher accuracy. The third experiment investigated the feasibility of using the BOS technique for imaging methane flow by imaging methane emission from a porous medium. Laboratory grade methane was directly injected into a Brea sandstone core sample using a flexible tube. The BOS system was successfully able to image methane desorption in this study. A repeating pattern consisting of alternating black and white stripes served as the schlieren background for the Nikon D700 camera. The ability to image methane emission even at low injection pressures (i.e. 20 psi) demonstrates that actual methane desorption from coal can potentially be imaged. This result can only be conjectured because of a lack of research in the area of methane emission. Despite this issue, the experimental results suggest that BOS can be feasibly utilized to image methane emissions from coal in an underground mine. The results of the three experiment demonstrated that the potential for large scale implementation of BOS in underground mines does exist. Qualitative BOS information has the potential in the practical sense to optimize the procedures of ventilation surveys and design of ventilation monitoring equipment. For example, images of methane flow in active mining areas can be used to optimize the positioning of auxiliary ventilation equipment to dilute known areas of high methane concentration. BOS images could also be used to re-evaluate the placement of methane monitors on mining equipment to better facilitate the detection of dangerous methane concentrations in active mining areas. For these reasons, further investigation into the BOS technique for use in imaging underground airflows with differential temperatures and methane emissions in underground coal mines is suggested as an addendum to this study.
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    Defining Stress Changes Ahead of a Tunnel Face and Design of a Data Acquisition System
    Murphy, Michael M. (Virginia Tech, 2005-12-13)
    With increasing world population, demand for underground construction is expected to accelerate in the future. Design of tunnels in rock is still largely empirical, while rock failure in underground mines and tunnel construction continues to claim lives. A seismic method to aid in increasing safety during excavation is tomography. Seismic tomography is a non-invasive technique to map the stress changes induced by mining ahead of the active face. Seismic tomography maps the velocity distributions of elastic waves traveling through a rock mass. The velocity distributions mapped in the tomograms can relate to anomalies in the rock such as fracture zones and highly concentrated stresses. In order to develop a relationship between stress and elastic wave velocity, laboratory tests in a controlled environment are required. In the current study tomographic tests were conducted on Berea sandstone and Five Oaks limestone samples. The stress redistribution in the sandstone samples could be imaged by mapping velocity distributions. On an unconfined test the sandstone sample acted much like a coal mine pillar where the stress redistributes to the least confined area. On a sandstone test where the sample was indented by a steel platen the velocity contrast was seen directly under the load and the velocity remained almost unchanged over the rest of the sample. For the limestone tests, the stress redistribution could not be mapped in the tomograms. The ability to map the stress distribution in the tomograms were attributed to the elastic and non-elastic characteristics of the stress-strain curve. For sandstone, a porous rock, the stress redistribution could be mapped and for limestone, a stiff rock, the stress redistribution could not be mapped. A field data acquisition system to apply tomography to ground control problems in a mine was designed and calibrated. Data acquisition hardware were assembled and programmed in LabVIEW to collect seismic data in a mine. The design of a geophone array that will fit into a miniature 5.08 cm (2 in) diameter borehole is presented.
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    Monitoring Underground Mine Displacement Using Photogrammetry and Laser Scanning
    Slaker, Brent (Virginia Tech, 2015-04-15)
    Photogrammetry and laser scanning are remote sensing technologies with the potential to monitor movements of rock masses and their support systems in underground mine environments. Displacements underground are traditionally measured through point measurement devices, such as extensometers. These are generally restricted to measuring one dimension, may change behavior with installation, may obstruct mining operations, and are restricted to monitoring the behavior of a small area. Photogrammetry and laser scanning offer the ability to monitor rock mass movements at millions of points in a local area, both accurately and quickly. An improved, or augmented, method for measuring displacements underground in a practical, cost-effective manner will lead to an improved understanding of rock mass behavior. Several experiments were performed that demonstrate the applicability of these remote sensing techniques to monitoring rock mass changes. An underground mining environment presents unique challenges to using these tools for monitoring rock movements, such as: poor lighting, dust, fog, and unfavorable geometries. It is important, therefore, to demonstrate that these tools which have applications in other industries, can also be adapted to the conditions of an underground mine. The study sites chosen include two different underground limestone mines, two different underground coal mines, and the Mine Roof Simulator (MRS) at the Pittsburgh Office of Mine Safety and Health Research. Both photogrammetry and laser scanning were tested at different limestone mines to detect scaling and spalling on ribs that occurred over several weeks. Both methods were successfully used to reconstruct three-dimensional models of the limestone ribs and detect areas of rock change between visits. By comparing the reconstructed point clouds, and the triangulated meshes created from them, volumes of rock change could be quantified. The laser scanned limestone mine showed a volume of 2.3 m3 and 2.6 m3 being displaced across two ribs between visits. The photogrammetry study involved seven different pillars and at least one rib face modeled on each, with volume changes of 0.29 to 4.03 m3 detected between visits. The rock displaced from the ribs could not be measured independently of the remote sensing, but a uniform absence of rock movement across large areas of the mine validates the accuracy of the point clouds. A similar test was performed using laser scanning in an underground coal mine, where the displacement was induced by removing material by hand from the ribs. Volume changes as small as 57 cm3, or slightly larger than a golf ball, and as large as 57,549 cm3, were detectable in this environment, despite the change in rib surface reflectance and mine geometry. In addition to the rib displacement, photogrammetry was selected as a tool for monitoring standing supports in underground coal mines. The additional regulatory restrictions of underground coal may preclude the use of laser scanning in these mines where deformation is most likely to occur. The camera used for photogrammetry is ATEX certified as explosion proof and is indicative of the specifications that could be expected in an MSHA approved camera. Three different experiments were performed with this camera, including a laboratory controlled standing support deformation at the MRS and an in-mine time-lapse experiment measuring the response of a wooden crib and steel support to abutment loading. The experiment reconstructing a standing support in the MRS showed a cumulative convergence of 30.93 cm through photogrammetry and 30.48 cm as measured by the system. The standing support monitoring in the underground coal mine environment showed a steel support cumulative convergence of 1.10 cm, a wooden crib cumulative convergence of 0.62 cm, and a measured cumulative convergence on the wooden crib of 0.62 cm. These techniques explored in this report are not intended to supplant, but rather supplement, existing measurement technologies. Both laser scanning and photogrammetry have physical and regulatory limitations in their application to measuring underground mine deformations, however, their ability to provide time-lapse three-dimensional measurements of entire mine sections is a strength difficult to emulate with traditional point measurement techniques. A fast, cost-effective, and practical application of remote sensing to monitoring mine displacements will improve awareness and understanding of rock mass behavior.
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    Numerical Modeling of Room-and-Pillar Coal Mine Ground Response
    Fahrman, Benjamin Paul (Virginia Tech, 2016-03-28)
    Underground coal mine ground control persists as a unique challenge in rock mass engineering. Fall of roof and rib continue to present a hazard to underground personnel. Stability of underground openings is a prerequisite for successful underground coal mine workings. An adaptation of a civil engineering design standard for analyzing the stability of underground excavations for mining geometries is given here. The ground response curve--developed over seventy years ago for assessing tunnel stability--has significant implications for the design of underground excavations, but has seen little use in complex mining applications. The interaction between the small scale (pillar stress-strain) and the large scale (ground response curve) is studied. Further analysis between these two length scales is conducted to estimate the stress on pillars in a room-and-pillar coal mine. These studies are performed in FLAC3D by implementing a two-scale, two-step approach. This two-scale approach allows for the interaction between the small, pillar scale and the large, panel scale to be studied in a computationally efficient manner.