Geophysical tomography allows for the measurement of stress-induced density changes inside of a rock mass or sample by non-invasive means. Tomography is a non-destructive testing method by which sensors are placed around a sample and energy is introduced into the sample at one sensor while the other sensors receive the energy. This process is repeated around the sample to obtain the desired resolution. The received information is converted by a mathematical transform to obtain a tomogram. This tomogram shows a pixelated distribution of the density within the sample. Each pixel represents an average value at that point. The project discussed in this paper takes the principle of ultrasonic tomography and applies it to geomechanics. A new instrumentation system was designed to allow rapid data collection through varying sample geometries and rock types with a low initial investment. The system is composed of sensors, an ultrasonic pulser, a source switchbox, and analog to digital converters; it is tied together using a LabVIEW virtual instrument. LabVIEW is a graphical development environment for creating test, measurement, and other control applications. Using LabVIEW, virtual instruments (VIs) are created to control or measure a process. In this application LabVIEW was used to create a virtual instrument that was automated to collect the data required to construct a tomogram. Experiments were conducted to calibrate and validate the system for ultrasonic velocity determination and stress redistribution tomography. Calibration was conducted using polymethylmethacrylate (PMMA or Plexiglas) plates. Uniaxial loads were placed on limestone and sandstone samples. The stress-induced density contrasts were then imaged using the acquisition system. The resolution and accuracy of the system is described. The acquisition system presented is a low-cost solution to laboratory geophysical tomography. The ultimate goal of the project is to further the ability to non-invasively image relative stress redistribution in a rock mass, thereby improving the engineer's ability to predict failure. / Master of Science

Time-lapse passive seismic velocity tomography was conducted utilizing data from three underground longwall coal mines to produce a better understanding of the processes that lead to ground failure in mines, especially large, violent failures, such as bumps. Two of the datasets, US Western I and US Western II, were collected at bump-prone underground longwall coal mines in the Western United States using surface mounted receiver arrays, while the third data set was collected at an underground longwall coal mine in Australia utilizing an underground array. The Australian mine was experiencing problems with periodic caving and subsequent wind blasts, rather than bumps. Seismic velocity tomography allows for non-invasive imaging of a rock mass and inference of stress redistribution from the velocity images. These tomograms are unique as they are generated using source data that was collected remotely and the sources are mining-induced. Tomograms were generated using three inversion methods: simultaneous iterative reconstructive technique (SIRT), double difference least squares event relocation, and least squares event relocation. The three methods were compared and contrasted to determine if one is superior and if event relocation improves the image. Also, the tomograms were analyzed to determine if passive seismic velocity tomography is an appropriate technology for the study of stress in mines and assistance in forecasting of bumps. The tomograms were compared with known roof events, face advance, and fall locations at the mines to establish if expected stress features can be imaged with velocity. Finally, synthetic tomograms were generated using a starting velocity model that approximates the predicted "true" model for each mine to determine if the velocity images produced correlate with the theoretical stress state at the mine. Results indicate that high velocity zones correlating with high stress abutment regions can be imaged for the US Western I data set with all three inversion methods, but the SIRT method provided the best agreement when the synthetic tomogram was generated. Additionally, a low velocity zone that correlates with the gob is consistently imaged. These features also redistribute with face advance. The US Western II data set was not as densely sampled as the US Western I data set. A low velocity region was consistently present in the gob area and redistributed with face advance, but abutment stress features were not evident. Additionally an unexplained high velocity feature was evident on several of the tomograms. Synthetic tomography indicated that the double difference least squares event relocation method is most appropriate for this data set. Finally, the Moonee Colliery results, which were also not as densely sampled as US Western I were uncertain. While velocity anomalies were often present in the vicinity of a fall, the anomalies were not reliably high or low. Again, synthetic tomography indicated that the double difference damped least squares event relocation method was most appropriate for this data set. The tomograms presented indicate that source-receiver configuration and density and variable gridding are extremely important in the application of passive seismic velocity tomography to mines. The source-receiver configuration and density determine how well various areas of a model are constrained, and the variable gridding allows areas that are not well sampled to still be adequately constrained. As a result of this work several things can be drawn about requirements that must be met in order to utilize seismic velocity tomography for inference of stress in underground mines. First, typical longwall stress abutment patterns can be inferred from velocity images of underground coal mines. Second, synthetic tomography and analysis of this tomography, in addition to some knowledge of the general location and frequency of microseismic events, is necessary prior to designing receiver arrays for passive seismic velocity tomography. Suboptimal source-receiver configurations may be used for passive seismic velocity tomography, but there is a minimum threshold for the number of raypaths that must be met that is unique to each site. Finally, a good understanding of the mechanics of stress and failure at the site is necessary to interpret the tomograms. / Ph. D.

Underground mining has one of the highest fatal injury rates among any of the industries in the United States, which is more than five times the national average of the other industries (MSHA). Many of these incidents take place due to stress redistribution resulting from mine workings. Thus it is very important to develop some tools to predict this failure in advance and prevent any fatalities arising from the failure. The current study uses two tools — numerical modeling and laboratory tomography - to image the stress distribution in laboratory rock samples as they are uniaxially loaded. The discrete element code, PFC3D, is used. The laboratory properties of the rock sample need to be converted to the micro-properties of the particles in the model. Currently no theory exists for this conversion. In the current study an equation has been developed for this process. Based on the users' input, the equation determines the micro-properties for the model. Further, various techniques to study the stress redistribution from these models at the particle level are discussed. Tomography is a non-destructive technique through which the interior of a body can be imaged without penetrating the surface by any physical means. In the current study sensors were attached around the rock sample and tomograms were obtained at certain intervals of the load. Initially, an indentation load was applied on a rectangular block to study the comparison between the stress and the velocity in two dimensions. In the last part of the study three-dimensional tomograms were obtained from the rock samples as they were loaded to failure. / Ph. D.

Velocity tomography is a noninvasive technology that can be used to determine rock mass response to ore removal. Velocity tomography is accomplished by propagating seismic waves through a rock mass to measure velocity distribution of the rock mass. Tomograms are created by mapping this velocity distribution. From the velocity distribution relative stress in the rock mass can be inferred, and this velocity distribution can be mapped at specific time intervals. Velocity tomography is an appropriate technology for the study of rockbursts. Rockbursts are events that occur in underground mines as a result of excessive strain energy being stored in a rock mass and sometimes culminating in violent failure of the rock. Rockbursts often involve inundation of broken rock into open areas of the mine. They pose a considerable risk to miners and can hinder production substantially. The rock mass under investigation in this research is the strata surrounding an underground coal mine in the western United States, utilizing longwall mining. The mine has experienced rockbursts. Seismic data were collected over a nineteen day period, from July 20th, 1997 to August 7th, 1997, although only eighteen days were recorded. Instrumentation consistsed of sixteen receivers, mounted on the surface, approximately 1,200 feet above the longwall panel of interest. The system recorded and located microseismic events, and utilized them as seismic sources. The data were analyzed and input into a commercial program that uses an algorithm known as simultaneous iterative reconstruction technique to generate tomograms. Eighteen tomograms were generated, one for each day of the study. The tomograms consistently display a high velocity area along the longwall tailgate that redistributes with face advance. Numerical modeling and mine experience confirm that the longwall tailgate is subject to high stress. Additionally, microseismic events are correlated with the velocity tomograms. Velocity tomography proves to be an effective method for the study of stress redistribution and rockburst phenomena at underground longwall coal mines, because it generates images that are consistent with prior information about the stress state at the mine and with numerical models of the stress in the mine. / Master of Science

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. / Master of Science

Ein zentraler Baustein zur Reduktion von CO2-Emissionen ist der Ausbau der erneuerbaren Energien, insbesondere der Windenergie. Forschungsbedarf besteht dabei bei der ressourceneffizienten Herstellung der Turmstrukturen. Bei Nabenhöhen von über 100 Metern sind Hybridtürme aus vorgespannten Stahlbetonsegmenten die geeignetste Konstruktion. Hierfür ist jedoch eine genaue Kenntnis des Ermüdungsverhaltens von Beton erforderlich. In der Literatur existieren überwiegend Untersuchungen an kleinformatigen zylindrischen Probekörpern, deren Ergebnisse nur bedingt auf die großmaßstäblichen Bauteile übertragen werden können. Im Rahmen dieses Vorhabens wurden daher zum einen Großversuche mit zyklisch biegebeanspruchten, vorgespannten Betonbalken sowie Begleitversuche an zylindrischen Probekörpern und zum anderen numerische Simulationen der Balkenversuche durchgeführt. Das numerische Materialmodell wurde aufbauend auf einem additiven Dehnungsmodell im Finite-Elemente-Programm ANSYS Mechanical in einem iterativen Berechnungsablauf implementiert. Die Betondehnungen setzen sich hierbei aus vier Anteilen zusammen, einem elastischen, einem plastischen, einem viskosen und einem Temperaturdehnungsanteil. Somit konnte der kombinierte Einfluss der Anteile auf das Ermüdungsverhalten von Beton dargestellt werden. In den Großversuchen konnte bei den Balkenprobekörpern ein Ermüdungsversagen der Betondruckzone erzeugt werden, das sich an dieser Stelle durch Risse parallel zur Drucknormalspannung sowie teilweises Abplatzen der Betondruckzone, die der größten Spannungsschwingbreite ausgesetzt war, einstellte. Es zeigte sich, dass dies erst nach deutlich mehr Lastwechseln eintrat als bei den axial beanspruchten Betonzylindern in den zyklischen Begleitversuchen mit derselben Spannungsschwingbreite. Dies ist auf die Spannungsumlagerung zurückzuführen, die im Querschnitt aufgrund der ermüdungsbedingten Materialdegradation und Steifigkeitsverringerung der stark beanspruchten Randbereiche stattfand. In den Begleitversuchen wurden die Materialparameter für das numerische Modell ermittelt, mit dem im Anschluss die Balkenversuche nachgerechnet wurden. Es konnten die in den Versuchen beobachteten Effekte der Steifigkeitsdegradation und Spannungsumlagerung und die daraus resultierende Lebensdauerverlängerung nachgebildet werden. Das Modell kann somit für weitergehende Lebensdaueruntersuchungen von ermüdungsbeanspruchten Betonbauteilen verwendet werden.:ABSCHLUSSBERICHT 1 Allgemeine Angaben 2 Zusammenfassung / Summary 3 Wissenschaftlicher Arbeits- und Ergebnisbericht 4 Veröffentlichte Projektergebnisse