<p>The northeast Thelon Basin, Nunavut, is a rapidly developing albeit poorly studied, frontier exploration domain for unconformity-associated uranium deposits. Critical criteria for unconformity –associated uranium deposit models are knowledge of the basement geology, fault history and depth to unconformity surface. This thesis sets forth to derive working geological and geophysical models for the northeast Thelon Basin through the implementation of integrated geophysical techniques.</p> <p>A physical rock property database is compiled defining average density and susceptibility values for key map units for integration into subsequent modelling iterations. Forward and inverse potential field modelling using these petrophysical with geological controls define the structure and geometry of the Shultz Lake intrusive complex and northeast Amer Belt, both of which are present below the Thelon sedimentary cover. Implementation of a new source edge detection technique estimates fault development and location within the basin. This provides a quantitative analysis of fault timing, identifying potentially reactivated faults which have an increased likelihood to serve as conduits to transport uranium-rich fluids and focus deposition. Corroborating the geophysical signatures on the aeromagnetic map with petrophysical properties and outcrop observations, a predictive geological map is developed for area beneath the sedimentary cover sequences. Inversion and interpretation of several discrete aeromagnetic anomalies provides local source depth estimates. Knowledge of fault locations is employed to delineate fault block boundaries. Interpreted geological model profiles include abrupt changes in sediment thickness with faults while abiding with the integrated source depth estimates. Integrating the results from a number of profiles provides a pseudo-3D rendition of the unconformity surface and its relationship to known faults. The geological-geophysical models which are presented herein incorporate all currently available data while providing a framework for the inclusion of future information as the knowledge gap for this remote region diminishes.</p> / Doctor of Science (PhD)

A 3D velocity model and 3D wave propagation code have been employed to simulate long-period ground motions in the upper Mississippi embayment. This region is exposed to seismic hazard in the form of large earthquakes in the New Madrid seismic zone and observational data are sparse, making simulation a valuable tool for predicting the effects of large events. These simulations were undertaken in order to estimate ground-motion characteristics and to investigate the influence of the 3D embayment structure and finite-fault mechanics. There are three primary fault zones in the New Madrid seismic zone, each of which was likely associated with one of the three main shocks of the 1811-1812 earthquake sequence. For this study, three simulations have been conducted on each major segment, evaluating the effects of different epicentral locations and rupture directions on ground motions. The full wave field up to a frequency of 0.5 Hz was computed on a 200 × 200 × 50 km3 volume, and up to a frequency of 1.0 Hz on a 100 × 100 × 50 km3 volume, using a staggered-grid finitedifference code. Peak horizontal velocity, bracketed durations, and pseudospectral accelerations were calculated at the free surface. Animations showing the evolution of peak horizontal velocity through time at the free surface were also generated. The New Madrid seismic zone simulations indicate that for the considered bandwidth, finite-fault mechanics such as fault proximity, directivity effect, and slip distribution exert the most control on ground motions. The 3D geologic structure of the upper Mississippi embayment also influences ground motion, with indications that the bedrock surface acts as a wave guide, trapping waves in shallow, low-velocity parts of the embayment.

Tilt and strain meters were installed on the Portland State University campus in the summer of 1982 and data was collected for 4 months. Instrument selection, operation, installation and performance are discussed. Suggestions that could enhance data quality and data collection efficiency are presented. An analysis procedure is suggested and an example of this procedure for an interval of data is discussed. The influence of the temperature, pressure, rainfall and solid earth tides on the signal is investigated, as well as the correlation between similar channels of the different tilt instruments. The temperature, rainfall and solid earth tides were all determined to have an influence on the instruments. A statistical test of the influence of the barometric pressure on the signal revealed no significant influence. A very low correlation between similar components of the different tilt instruments was observed aside from their thermal dependence. It was concluded that in order to obtain high quality data for the use in quantitative calculations, the temperature influence on the raw record must be minimized.

Before raw seismic data records are interpretable by geologists, geophysicists must process these data using a technique called migration. Migration spatially repositions the acoustic energy in a seismic record to its correct location in the subsurface. Traditional migration techniques used a transpose approximation to a true acoustic propagation operator. Conventional least squares migration uses a true inverse operator, but is limited in functionality by the large size of modern seismic datasets. This research uses a new technique, called stabilized least squares migration, to correctly migrate seismic data records using a true inverse operator. Contrary to conventional least squares migration, this new technique allows for errors over ten percent in the underlying subsurface velocity model, which is a large limitation in conventional least squares migration. The stabilized least squares migration also decreases the number of iterations required by conventional least squares migration algorithms by an average of about three iterations on the sample data tested in this research.

An Enhanced Geothermal System (EGS) allows for the generation of electricity using the Earth's heat by improving ('enhancing') the fracture permeability of rock and flowing fluid through the optimized medium. The complex behavior of EGS fracture systems and heat flow processes are being studied at various scales to determine the practical capabilities of EGS technology. The EGS collaborative (Collab) project is focused on experimentation of intermediate-scale (i.e., 10's of meters) EGS reservoir generation processes and model validation at crystalline rock sites. A key phase of the project involves seismic characterization of a rock mass intended to be representative of EGS reservoir rock. A suite of boreholes was drilled from inside a mine drift on the 4850-foot (~1.5 km) level of the Sanford Underground Research Facility (SURF) in Lead, South Dakota. The boreholes, comprised of one stimulation (injection) well, one production (extraction) well, and six monitoring wells, were each nominally drilled approximately 200 feet (~60 meters) deep into the surrounding crystalline rock formation near the location of a previous experiment at this site (kISMET). Active source seismic data were collected using an electrical sparker source and an electro-mechanical impulse source to generate compressional (P-) wave and shear (S-) wave energy, respectively, at varying depths in the stimulation well. Seismic receivers were deployed in the sub-parallel production well, in addition to receivers installed in the monitoring wells, to detect P- and S-wave arrivals. Over the summer, I picked all the P-wave arrivals and helped generate initial tomographic models. The 3D P- and S-wave velocity models associated with these are presented here with a discussion of the elastic parameters they imply. The rock is found to be more complicated and heterogeneous than expected. Velocity and calculated elastic moduli values are reasonable for crystalline rock. These elastic parameters are used for modeling and monitoring seismic hypocenters that are associated with fracture propagation during EGS stimulation activities.

An abnormally high number of explosion quakes were noted during the monitoring effort for the 2007 eruption of Pavlof Volcano on the Alaskan Peninsula. In this study we manually counted the explosion quakes from their characteristic ground-coupled air waves. This study makes an effort at better quantifying the number of explosion quakes and how the characteristic ground-coupled air waves are affected by wind direction and wind speed. Additionally this study investigates how the ground coupled air waves might be used in a monitoring or analysis effort by calculating energy release and gas mass release. Over 3.2x104 quakes were recorded. It was found that wind direction affects the travel time of the air wave by up to 0.7 seconds depending on station location and wind direction. Wind direction and speed, however, are demonstrated not to cause an appreciable difference in ground-coupled air wave frequencies or amplitude ratios. The energy release from the explosions is calculated to be 3.04x1011 J. and the total gas mass (assuming 100% water) released was 729 metric tons. These values are compared to other volcanoes in the literature and found to be somewhat lower. Nevertheless, the tracking of explosion quakes has the potential to become a valuable member of the seismic monitoring arsenal.

Seismic first-arrival tomography is a technique currently experiencing a renaissance in popularity due to the simplicity of implementation and promising results for delineating a variety of subsurface targets. The purpose of this study is to investigate seismic first-arrival tomography in a variety of settings and applications, and thus to provide a solid framework for future work. The study largely consists of two separate themes, hydrogeophysics and low-velocity anomaly detection. Hydrogeophysics is an emerging field whereby measured geophysical properties are used as proxies for physical properties of the subsurface. This study represents one of the first high-resolution hydrogeophysical investigations in the upper three meters of the subsurface using seismic first-arrival tomography, and consists of detecting shallow high-velocity zones that are interpreted to be perched water bodies on the basis of geophysical and hydrologic evidence. The delineation and imaging of the perched water bodies is further advanced using trend-analysis techniques. A second theme of this dissertation is the optimization of methods for delineating low-velocity anomalies at depth using seismic first-arrival tomography. In order to locate a low-velocity zone at Oak Ridge, Tennessee, multiple seismic lines were collected and correlated with site-wide geology. The integration of geologic and geophysical data-sets assisted in developing a comprehensive transport conceptual model, and provided a predictive framework for future geophysical investigations at Oak Ridge. As a second component of this theme, a systematic methodology for detecting and delineating shallow low-velocity zones is developed.

Karst environments are unique landscapes that contain important resources, including freshwater aquifers and specialized ecosystems, which are easily disturbed due to the interconnected nature of the surface and subsurface. The anthropogenic impacts on karst are deleterious to the ecosystems that are dependent on the karst environment and also to groundwater supplies. The Karst Disturbance Index (KDI) is a holistic tool used to measure anthropogenic impacts associated with karst environments, which has been applied and refined through studies performed in Florida and Italy, yet still remains untested and susceptible to modification for other areas. Application of the KDI in Arecibo, Puerto Rico, which is geographically isolated, and highly vulnerable due to its sensitive karst resources, provides an opportunity to test the index in an island setting. This research resulted in two KDI scores for the study area using both the original and recently modified methods. The scores reflect a significant to severe disturbance to the municipality’s karst environment of 0.54 and 0.68, respectively. Issues regarding the KDI were found from the application and comparison of these methodologies and revealed the need for adding additional indicators, including Mogote Removal and Coastal Karst, as well as several additional refinements and recommendations pertaining to scale, weighting, and incorporating the two methods together to create a single, more practical KDI tool. The disseminated results of the assessment of the area using the KDI will educate and help to foster stewardship of this vital resource in Puerto Rico.

The Central United States Seismic Observatory (CUSSO), is a fifteen-element array of three-component accelerometers and seismometers in the New Madrid seismic zone, within the upper Mississippi embayment. Its location within the thick (up to 1 km) sequences of unlithified sediment comprising the embayment, make CUSSO a unique array in its ability to directly measure seismic wave propagation, including ground motion site effect; however, before the observational data can be used for analysis, the orientation and instrument response of the CUSSO array must be defined. This study used cross-correlation and direct comparisons to filter out the instrument response and determine the instrument orientation, making CUSSO data ready for analysis, and making CUSSO a viable calibration site for other free-field sensors in the area. The corrected waveforms from five far-field earthquakes with magnitudes ranging between 2.5 and 4.7, which were recorded at CUSSO, were used to validate the site’s proposed dynamic soil model. The corrected bedrock motions were also numerically propagated through the CUSSO soil profile (transfer function) and compared, in terms of both peak acceleration and amplitude spectra, to the recorded surface observations.

Regional-scale complete Bouguer gravity anomalies underlying the Lassen and Shasta -Medicine Lake regions in northern California and southern Oregon are associated with subduction of the Gorda plate beneath North America. These generally negative anomalies reflect where underplating has deepened to form the mantle wedge, and where subduction has given rise to a series of Quaternary volcanoes comprising the southernmost end of the Cascade range. Multiple conductive bodies were identified by Park and Ostos (2013) in their magnetotelluric (MT) study of the broader Lassen volcanic region. Their broadband and long period measurements were conducted along a 250 km profile spanning from the California-Nevada border, to just west of the Great Valley in California. Utilizing their MT conductor geometries as a starting point, a forward gravity model was generated along the same profile, and agrees well with what they interpret to be the locations and depths of mid-crustal magma bodies in the Lassen and surrounding regions. The excess mass and volume of modeled anomaly (a) - most closely attributed to underlying Lassen Peak - were estimated at -2 x 1014 kg and 7 x 1011 m3, respectively.