Thesis (Ph. D.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (Ph. D.)--California Institute of Technology, 1983. / Includes bibliographical references.

This thesis applies three-dimensional (3-D) non-linear seismic tomography to image crustal/upper mantle structure of S.W. British Columbia and N.W. Washington. Two tomographic inversions are carried out including high-resolution imaging of upper crustal structure using controlled source data, and deeper imaging by simultaneous inversion of controlled source and earthquake data. Non-linear first arrival travel-time tomography is applied to controlled source data from the Seismic Hazards Investigation of Puget Sound (SHIPS) experiment conducted in 1998. Nearly 175,000 first arrival travel-times are inverted to obtain a minimum structure upper crustal velocity model to a depth of 12 km with a cubical cell size of 1 km. Results from checker-board tests for this velocity model indicate a lateral resolution of 20 km and above. The main geological and structural features in the study area are well defined by this velocity model. The structural outline of the sedimentary basins in the Straits of Georgia and Juan de Fuca are distinctly mapped. The Crescent Terrane is mapped beneath southern Vancouver Island with velocities up to 7 km/s that correlate well with the presence of gabbro in the subsurface. The northwest-southeast structural trend observed in the Strait of Georgia correlates with the observed seismicity. Shallow seismicity observed at the southern tip of Vancouver Island correlates with the location of the Leech River Fault. An earthquake tomography algorithm was developed for joint estimation of hypocentral and velocity parameters, and tested on a synthetic data set. Using this algorithm, tomographic inversion was performed simultaneously on earthquake and controlled source data from southwestern British Columbia and northwestern Washington. Approximately 15,000 first arrivals from 1,400 earthquakes and 40,000 first arrivals from the SHIPS experiment were simultaneously inverted for hypocentral parameters and velocity structure. Model resolution studies indicate a lateral resolution of 30 km and above. Upper-crustal earthquakes close to southern Vancouver Island correlate with the velocity contrasts associated with the Leech River, Southern Whidbey Island, and Darrington-Devils Mountain faults. Three mafic to ultramafic high velocity units are identified at approximately 25 km depth beneath the Crescent Terrane and above the subducting Juan de Fuca crust. The continental crust and subducting Juan de Fuca crust and mantle are well mapped. The transition zone to continental mantle occurs at 35 km depth beneath the eastern Strait of Georgia. The slab seismicity beneath the Strait of Georgia at depths >65 km lies below a low velocity zone mapped in the mantle wedge at depths of about 45–55 km. This low velocity zone may be indicative of the presence of fluids released during the phase change from basalt/gabbro to eclogite in the subducting slab. / Graduate

I present a general methodology for inverting seismic data in either the data or image domains. It partially overcomes one of the most serious problems with current waveform inversion methods, which is the tendency to converge to models far from the actual one. The key idea is to develop a multiscale misfit function that is composed of both a simplified version of the data and one associated with the complex part of the data. Misfit functions based on simple data are characterized by many fewer local minima so that a gradient optimization method can make quick progress in getting to the general vicinity of the actual model. Once we are near the actual model, we then use the gradient based on the more complex data. Below, we describe two implementations of this multiscale strategy: wave equation traveltime inversion in the data domain and generalized differential semblance optimization in the image domain. • Wave Equation Traveltime Inversion in the Data Domain (WT): The main difficulty with iterative waveform inversion is that it tends to get stuck in local minima associated with the waveform misfit function. To mitigate this problem and avoid the need to fit amplitudes in the data, we present a waveequation method that inverts the traveltimes of reflection events, and so is less prone to the local minima problem. Instead of a waveform misfit function, the penalty function is a crosscorrelation of the downgoing direct wave and the upgoing reflection wave at the trial image point. The time lag which maximizes the crosscorrelation amplitude represents the reflection-traveltime residual that is back-projected along the reflection wavepath to update the velocity. Shot- and angle-domain crosscorrelation functions are introduced to estimate the reflection-traveltime residual by semblance analysis and scanning. In theory, only the traveltime information is inverted and there is no need to precisely fit the amplitudes or assume a high-frequency approximation. Results with both synthetic data and field records reveal both the benefits and limitations of WT. • Generalized Differental Semblance Optimization in the Image Domain (GDSO): We now extend the multiscale physics approach to differential semblance optimization (DSO) in the image domain. That is, we identify the space-lag offset H(x, z, h) in the subsurface-offset domain as an implicit function of velocity. It describes the smoothly varying moveout H(x, z, h) of the migration image m(x, z, h) in the subsurface-offset domain, which is analogous to the smoothly varying traveltime residual ∆τ(x) of a reflection event in a shot gather. The velocity model is found that minimizes the objective function ∑x,z,h H(x, z, h)2m(x, z, h)2, where coherent noise is eliminated everywhere except along the picked curve H(x, z, h). This method is denoted as generalized DSO (GDSO) and mitigates the coherent noise problem with DSO. Numerical examples are presented that empirically demonstrate its effectiveness in providing more accurate velocity models compared to conventional DSO.

It is often claimed that the first Fresnel zone associated with the dominant frequency represents the spatial resolution limit of traveltime tomography. We show, however, that the relevant Fresnel limit for tomographic resolution is the maximum, not the dominant frequency in the data. For physically realizable causal wavelets, the maximum frequency is infinite. In practice, noise lowers the effective possible maximum frequency. To demonstrate these points, synthetic seismic data were generated for traveltime picking and inversion for a single, small velocity anomaly embedded in a homogeneous background velocity. A variety of traveltime picking techniques were tested and compared for their ability to detect the presence of objects smaller than that Fresnel zone associated with the dominant frequency. All methods produced accurate ray-theoretical (infinite-frequency) picks from noise-free seismic data for objects smaller than the dominant-frequency Fresnel zone. For the lowest dominant frequencies with Fresnel zones many times larger than the object, picking methods that focus on features along the onset of the first arrival were the most accurate, while cross-correlation with a known wavelet preformed less accurately. First-onset picking methods perform better because they take advantage of the highest frequencies in the data, whereas the correlation wavelet is typically in line with the dominant frequency. All methods successfully detected the presence of objects smaller than a wavelength. The inversion of the traveltime picks from the different picking methods always recovered the position and shape of the object. Random noise at a range of signal-to-noise ratios was then added to the seismic data and the data were repicked. Pick times with different noise realizations are statistically centered on the noise-free pick, not the time that would be recorded in the absence of the object. Trace stacking prior to picking or the averaging of many picks improves the signal-to-noise ratio and can extract signal that is not detected on an individual pick. An averaging of traveltime picks also occurs during tomographic inversion. This inherent signal-to-noise improvement allows tomography to image objects that are undetectable in individual trace picks. The resolution of tomography is limited not by the Fresnel zone associated with the dominant frequency, but by the accuracy of the traveltime picks. Resolution is further improved by dense ray coverage. / Master of Science

Siilinjärvi located in eastern Finland is presently the only phosphate mine with significant phosphorus production in western Europe. Phosphate rock and phosphorus are known as a critical raw material for the European Union due to their economic importance and being prone to the supply risks. Securing their sustainable exploration as well as extraction is hence important. At Siilinjärvi, the phosphate rocks appear within a major Archean alkaline and carbonatite system deformed by several shear zones and intruded by dike systems. By understanding their spatial and temporal relationships an improved exploration and extraction of the ore is possible, which in turn will contribute to the sustainable extraction of this critical material. In October 2018 a novel in-tunnel seismic survey was conducted in the Siilinjärvi open-pit mine. The objective of the study was to employ an in-tunnel seismic survey intersecting several major shear zones running on the eastern side of the main pit, with the idea of characterising its geometry and relationship with the mineralization. The use of the existing mine infrastructure (a water-drainage tunnel) makes the acquisition of the data quite novel in open-pit mines. The water-drainage tunnel nearly in the bottom of the pit crosscutting several major shear zones and dikes was used to enable bench-tunnel seismic data acquisition. High-quality data were acquired using 144 receivers inside the tunnel, with the sources located both inside the tunnel (Bobcat-mounted vertical drophammer) and on the surface (combined explosives and Bobcat-drophammer). Results obtained show at least two reflections interpreted to originate from subvertical shear zones intersecting the tunnel illustrating the importance of such surveys for shear-zone imaging and site characterization. Based on a careful study of a number of shot records, delay in arrival times and partial amplitude lose, these reflections are interpreted to be backscattered surface-waves generated from the shear zones. / Siilinjärvi i östra Finland är just nu den enda fosfatgruvan med en betydande fosforproduktion inom den Europeiska Unionen. Fosfater och fosfor är viktigt för EU på grund av deras ekonomiska betydelse och begränsade tillgång. Den fosforbärande bergarten i Siilinjärvi befinner sig i ett stort Arkeiskt alkalisk och karbonatit-komplex som är deformerad av flera skjuvzoner och intruderat gångsystem. En förbättring av sökandet och utvinnandet av denna viktiga malm skulle vara möjligt genom en ökad förståelse för de spatiala och temporala relationerna i komplexet. Syftet med denna studie var att tillämpa en ny seismisk undersökning, baserad på existerande infrastruktur i gruvan, för en bättre geologisk förståelse och därmed en förbättrad exploatering. En innovativ seismisk undersökning av tunnlar gjordes i Siilinjärvis dagbrott oktober 2018. En vattendräneringstunnel nästan i botten av brottet som korsar fem skjuvzoner användes för att mäta seismiskt tunnel-data. Hög kvalitativ data samlades in genom att använda 144 mottagare inuti tunneln med källor lokaliserade både inuti tunneln, i form av en vertikal dropphammare monterad på en Bobcat, och på ytan i form av en kombination av sprängning och Bobcatmoterad dropphammare. Två reflektioner tolkades att ha sitt ursprung från subvertikala skjuvzoner som korsar tunneln vilket visar på vikten av dessa typer av undersökningar för skjuvzons detektion och områdes karaktärisering.

A distância e o tempo de viagem são elementos de importância para a avaliação do acesso aos serviços de saúde, esta é uma questão que para a população do campo tem uma importância especial devido às particularidades geográficas do espaço rural. A presente pesquisa tem como objetivo desenvolver uma metodologia de mapeamento da rede de rodovias rurais com indicações de impedâncias de viagem, utilizando-se para isso dos Sistemas de Informação Geográfica (SIG) e tendo em vista a otimização do traslado da população rural aos serviços de saúde. A orientação teórica se apóia no conceito de distância na Geografia e em concepções sobre a medida da acessibilidade e mobilidade espacial. Faz-se uma ampla observação das abordagens e metodologias voltadas ao estudo da acessibilidade geográfica das populações rurais aos serviços de saúde, sobretudo, as que tratam de fatores espaciais e fazem uso dos SIG. Dados relativos às classes de rodovias rurais, declividade da paisagem, sinuosidade, visibilidade e uso da terra, são considerados na composição de uma síntese de impedâncias. A avaliação toma como parâmetro o tempo de viagem por veículo automotor e as medidas se fazem com base na rede rodoviária ponderada por impedâncias. A população rural e as unidades de atendimento à saúde são os referencias para a avaliação da acessibilidade geográfica. A metodologia foi aplicada à Microrregião de Registro (SP) e os resultados indicam que a velocidade nos percursos muda de acordo com os atributos associados às rodovias, sendo as estradas locais nãopavimentadas as que apresentam maior variabilidade para a geração de condições ótimas de viagem. A distribuição espacial dos serviços de saúde reflete na acessibilidade geográfica e algumas regiões são mais penalizadas no acesso aos serviços hospitalares. O modelo aqui apresentado orienta à montagem de um SIG para a análise da acessibilidade da população rural aos postos de saúde e hospitais e atende a duas principais aplicações: (i) o apoio ao planejamento da viagem da população rural aos postos de atendimento à saúde; (ii) e auxílio em facilitar o acesso da população rural às consultas e atendimento de ocorrências emergenciais, pela aplicação de recursos de análise de rede, tais como: caminho mínimo, matriz de origens-destinos e área de serviço. Ambas as aplicações poderão resultar num melhoramento do planejamento das áreas de influencia dos centros de saúde na região rural. / Distance and travel time are important parameters in the evaluation of access to health services. For the population living in the countryside, this has become a significant issue due to geographical particularities of rural areas. The objective of this work is the development of a GIS-based methodology for the mapping of rural road networks and the determination of travel impedances in order to optimize the travel time of the rural population to health care facilities. The theoretical orientation is based on the concept of geographical distance and on the conception of the extent of accessibility and mobility. A general overview is given of approaches and methodologies which study the geographical accessibility of health services facilities by the rural population with particular consideration of those which deal with spatial factors and make use of GIS (Geographic Information System). Data on rural roads classes, slope, sinuosity, visibility and land use are considered in the composition of the impedances synthesis. The evaluation uses the travel time by car as a parameter, and the quantifications are based on the road network with velocities adjusted by the impedances. This way, network distances, velocity and the travel time are the criteria for the evaluation of the geographical accessibility to health care facilities by the rural population. The methodology was applied to the Region of Registro (SP), and the results indicate that the velocity on the particular road sections changes depending on the attributes associated with the road classes with the observation that unpaved roads represent the worst conditions for establishing optimum travel conditions. The distribution of health service facilities is reflected in the geographical accessibility, and some regions are highly penalized regarding access to hospital services. The model presented here points out methods for the installation of a GIS for the analysis of the accessibility of health care facilities by the rural population with respect to the following applications: (i) support of the rural population when planning journeys to health care facilities; (ii) improved access to medical examination and emergency assistance by analysis of the network, i.e., by determining the minimum traveling distance, the origin-destination matrix and the service areas of the health care facilities. Both applications may improve covering and planning rural health service facilities.

A distância e o tempo de viagem são elementos de importância para a avaliação do acesso aos serviços de saúde, esta é uma questão que para a população do campo tem uma importância especial devido às particularidades geográficas do espaço rural. A presente pesquisa tem como objetivo desenvolver uma metodologia de mapeamento da rede de rodovias rurais com indicações de impedâncias de viagem, utilizando-se para isso dos Sistemas de Informação Geográfica (SIG) e tendo em vista a otimização do traslado da população rural aos serviços de saúde. A orientação teórica se apóia no conceito de distância na Geografia e em concepções sobre a medida da acessibilidade e mobilidade espacial. Faz-se uma ampla observação das abordagens e metodologias voltadas ao estudo da acessibilidade geográfica das populações rurais aos serviços de saúde, sobretudo, as que tratam de fatores espaciais e fazem uso dos SIG. Dados relativos às classes de rodovias rurais, declividade da paisagem, sinuosidade, visibilidade e uso da terra, são considerados na composição de uma síntese de impedâncias. A avaliação toma como parâmetro o tempo de viagem por veículo automotor e as medidas se fazem com base na rede rodoviária ponderada por impedâncias. A população rural e as unidades de atendimento à saúde são os referencias para a avaliação da acessibilidade geográfica. A metodologia foi aplicada à Microrregião de Registro (SP) e os resultados indicam que a velocidade nos percursos muda de acordo com os atributos associados às rodovias, sendo as estradas locais nãopavimentadas as que apresentam maior variabilidade para a geração de condições ótimas de viagem. A distribuição espacial dos serviços de saúde reflete na acessibilidade geográfica e algumas regiões são mais penalizadas no acesso aos serviços hospitalares. O modelo aqui apresentado orienta à montagem de um SIG para a análise da acessibilidade da população rural aos postos de saúde e hospitais e atende a duas principais aplicações: (i) o apoio ao planejamento da viagem da população rural aos postos de atendimento à saúde; (ii) e auxílio em facilitar o acesso da população rural às consultas e atendimento de ocorrências emergenciais, pela aplicação de recursos de análise de rede, tais como: caminho mínimo, matriz de origens-destinos e área de serviço. Ambas as aplicações poderão resultar num melhoramento do planejamento das áreas de influencia dos centros de saúde na região rural. / Distance and travel time are important parameters in the evaluation of access to health services. For the population living in the countryside, this has become a significant issue due to geographical particularities of rural areas. The objective of this work is the development of a GIS-based methodology for the mapping of rural road networks and the determination of travel impedances in order to optimize the travel time of the rural population to health care facilities. The theoretical orientation is based on the concept of geographical distance and on the conception of the extent of accessibility and mobility. A general overview is given of approaches and methodologies which study the geographical accessibility of health services facilities by the rural population with particular consideration of those which deal with spatial factors and make use of GIS (Geographic Information System). Data on rural roads classes, slope, sinuosity, visibility and land use are considered in the composition of the impedances synthesis. The evaluation uses the travel time by car as a parameter, and the quantifications are based on the road network with velocities adjusted by the impedances. This way, network distances, velocity and the travel time are the criteria for the evaluation of the geographical accessibility to health care facilities by the rural population. The methodology was applied to the Region of Registro (SP), and the results indicate that the velocity on the particular road sections changes depending on the attributes associated with the road classes with the observation that unpaved roads represent the worst conditions for establishing optimum travel conditions. The distribution of health service facilities is reflected in the geographical accessibility, and some regions are highly penalized regarding access to hospital services. The model presented here points out methods for the installation of a GIS for the analysis of the accessibility of health care facilities by the rural population with respect to the following applications: (i) support of the rural population when planning journeys to health care facilities; (ii) improved access to medical examination and emergency assistance by analysis of the network, i.e., by determining the minimum traveling distance, the origin-destination matrix and the service areas of the health care facilities. Both applications may improve covering and planning rural health service facilities.

This Ph.D. dissertation focuses on a comprehensive study of seismic waveform similarity aiming at two themes: (1) reliable identification of repeating earthquakes (repeaters) and (2) investigation of the detailed source process of induced seismicity through the three-dimensional spatiotemporal evolution of mainly neighbouring earthquakes. Theme 1: Reliable identification of repeaters. Repeaters, occurring repeatedly on the same fault patch with nearly identical waveforms, are usually identified with the match-filtering (MF) method which essentially measures the degree of waveform similarity between an earthquake pair through the corresponding cross-correlation coefficient (CC). However, the performance of the MF method can be severely affected by the length of the cross‐correlation window, the frequency band of the applied digital filter, and the presence of a large‐amplitude wave train. To optimize the performance of MF, I first examine the effects of different operational parameters and determine generic rules for selecting the window length and the optimal frequency passband. To minimize the impact of a large‐amplitude wave train, I then develop a new method, named the match-filtering with multisegment cross-correlation (MFMC) method. By equally incorporating the contributions from various segments of the waveforms, the new method is much more effective in capturing the minor waveform discrepancy between an event pair due to location difference and hence is more reliable in detecting potential repeaters and discriminating non-repeaters with large inter-event separation. With both synthetic and borehole array waveform data, I further reveal that waveform similarity is controlled by not only the inter-event separation but also many other factors, including station azimuth, epicentral distance, velocity structure, etc. Therefore, in contrast to the traditional view, the results indicate that waveform similarity alone is insufficient to unambiguously identify true repeaters. For reliable repeater identification, we should rely on a physics-based approach considering both the overlapped source area and magnitude difference. Specifically, I define an event pair to be true repeaters if their inter-event separation is smaller than the rupture radius of the larger event and their magnitude difference is no more than 1. For the precise estimation of inter-event distance in cases of limited data, I develop the differential traveltime double-difference (DTDD) method which relies on the relative S-P differential traveltime. The findings of this study imply that previously identified repeaters and their interpretations/hypotheses potentially can be biased and hence may need a systematic reexamination. Theme 2: Investigation of the detailed source process of induced seismicity. Earthquakes induced by hydraulic fracturing (HF), especially those with large magnitudes, are often observed to have occurred near/after well completion. The delayed triggering of induced seismicity with respect to injection commencement poses serious challenges for risk mitigation and hazard assessment. By performing waveform cross-correlation and hierarchical clustering analysis, I reveal a high-resolution three-dimensional source migration process with mainshock delayed triggering that is probably controlled by local hydrogeological conditions. The results suggest that poroelastic effects might contribute to induced seismicity but are likely insufficient to activate a non-critically stressed fault of sufficient size. My analysis shows that the rapid pore-pressure build-up from HF can be very localized and capable of producing large, felt earthquakes on non-critically stressed fault segments. I further infer that the number of critically stressed, large intraplate faults should be very limited, and that reactivation of such faults may require sufficient pore-pressure accumulation. The findings of this study may also explain why so few fluid injections are seismogenic. / Graduate

The South Portuguese Zone (SPZ), which host world-class massive sulphide deposits, forms the southern fold-and-thrust belt of the Iberian Variscan orogeny. This thesis focuses on seismic-reflection and seismic-refraction processing efforts on a subset of the IBERSEIS deep seismic-reflection data set aiming at resolving the SPZ upper crust in high resolution. A comparison of different crooked-line seismic-reflection imaging schemes showed that a processing sequence involving dip-moveout corrections, a common-midpoint projection, and poststack time migration of common-offset gathers provided the most coherent images considering the crooked acquisition geometry. Correlation with surface-geological data allows four units of different reflection character to be identified: the ~0–2 km deep Upper Carboniferous Flysch group, the highly reflective ~2–4 km thick and up to ~5 km deep Volcano-Sedimentary Complex (VSC) group, and two deep Paleozoic metasedimentary units, with the shallower Phyllite-Quartzite group exposed in an antiform. Prominent diffracted energy was enhanced using a modified Kirchhoff imaging routine. High reflectivity and distinct diffractions mark extensive dike bands at 6–12 km depth, possibly related to the intense hydrothermal activity that led to the formation of the ore-bearing VSC group. Source-generated noise obscures potential signals from depths shallower than ~500m depth on the seismic-reflection sections. P- and SV-wave first-arrival traveltimes were inverted for velocity models imaging the shallowest crust. Overall, the velocity models correlate well with surface-geological data marking high (>5.25 km/s) and uniform P-velocities for the Flysch unit in the southern SPZ. A prominent P-wave low-velocity body (~4.5 km/s) is resolved where the Phyllite-Quartzite unit forms the core of an antiform. P-velocities fluctuate the most in the northern SPZ with Flysch group units exhibiting high velocities (>5.25 km/s) and VSC group bodies showing intermediate velocities (~5 km/s). Low VP/VS-ratios (~1.8) computed for the southern profile part are interpreted as less deformed Flysch-group units, whereas high VP/VS-ratios (~1.9) indicate fractured units.