No description available.

No description available.

No description available.

No description available.

Ground Penetrating Radar (GPR) identifies subsurface features by distinguishing materials with different dielectric constants and electrical conductivities. Subsurface cavities can, therefore, be detected by the variation in their electrical properties from the electrical properties of the surrounding material. To test the cavity detection ability of GPR, subsurface cavities of varying size, shape and content were modeled. Radar response to the cavity models was found to be affected by the composition of the surrounding soil material, the depth of the groundwater table, and the radar signal frequency. Based on knowledge gained from the cavity modeling study, a natural subsurface cavity was identified during a GPR field investigation. Limestone features such as bedding planes and fractures were mapped, and a detailed lake bottom profile was obtained by the radar system.

The Caribbean and Central America region (CCAR) undergoes the entire spectrum of earthquake types due to its complex tectonic setting comprised of transform zones, young oceanic spreading ridges, and subduction along its eastern and western boundaries. CCAR is, therefore, an ideal setting in which to study the impacts of long-term tectonic deformation on the distribution of present-day seismic activity. In this work, we develop a revised continuous tectonic strain rate model based on interseismic, secular geodetic data. We compare it with its predecessor, the Global Strain Rate Model v2.1 (GSRM). Specifically, we compare predicted fault types with known active faults and evaluate the style of predicted fault types with present-day earthquake focal mechanism data. We first create a 0.25$^{circ}$ x 0.25$^{circ}$ finite element grid that is comprised of block geometries defined from previous studies. Second, we isolate and remove anomalous signals that are inconsistent with rigid block motion from the latest open access community Global Navigation Satellite System (GNSS) velocity solution from UNAVCO and combine it with GNSS data compiled for the GSRM. In a third step, we delineate zones of deformation and rigidity by creating a buffer around the boundary of each block that varies depending on the size of the block and the expected deformation zone, which are based on locations of GNSS data consistent with rigid block motion. Fourth, we assign the regions within the buffer of zero for the deforming areas and a plate index outside the buffer to constrain plate rigidity. Finally, we calculate a tectonic strain rate and continuous velocity model for CCAR using the Haines and Holt finite element approach to fit bicubic Bessel splines to the GNSS data assuming block rotation for zones of rigidity. Our model of the CCAR is consistent with compression along subduction zones, extension across the East Pacific Rise, and a combination of compression and extension across the North America - Caribbean plate boundary with a few exceptions due to limitations with the modeling approach. Modeling results are then used to calculate expected faulting behaviors that we compare with seismic activity, the GSRM, and mapped geologic faults. We find the accumulation of strain rates in areas near the Middle American Trench, Hispaniola, the northeastern Caribbean, and northern South America indicate tectonic deformation that may result in seismic events. We conclude the tectonic deformation plays a critical role in explaining present-day seismicity along land masses adjacent to the subduction zone and the Hispaniola block. / Master of Science / Central America and the Caribbean are areas with high occurrences of earthquakes. This is due to the various types of tectonic plate boundaries that occur in the region. When plates move in relation to each other, they can accumulate strain, which plays a role in the size and type of earthquakes that occur. In this work, we aim to determine the effects on strain on earthquakes. To do this, we utilize an inversion method to calculate strain rates from Global Navigation Satellite System (GNSS) data. In our model, we first create a grid of points and a geometry of the regional tectonic blocks. We then gather data from public and published sources. The model also requires that we define where the plates are allowed to deform (accumulate strain) and where they remain rigid. Using the Haines and Holt method, we invert the GNSS velocities for strain rates and velocities. We find long-term tectonic deformation dominates the present-day seismic activity in three key regions: along the Middle America Trench and across the Hispaniola block.

Após a desativação da disponibilidade seletiva, ocorrida em maio de 2000, a maior fonte de erro presente no sistema GPS passou a ser a refração ionosférica. Os seus efeitos provocam um avanço na fase de batimento da portadora e um atraso no código. Na região equatorial onde o Brasil está inserido, a refração ionosférica apresenta variações que decorrem de diversos fatores tais como: o ciclo solar, o movimento aparente do Sol, as estações do ano, o campo geomagnético, entre outras. Desta forma, a refração ionosférica é um fator limitante pois, degrada a determinação de posições, principalmente, se o usuário estiver utilizando um receptor de apenas uma freqüência. Para avaliar o efeito da refração ionosférica no Brasil, no período compreendido entre janeiro de 1997 a dezembro de 2001, foram utilizados dados coletados com receptores providos de duas freqüências e que integram a Rede Brasileira de Monitoramento Contínuo do Sistema GPS. Todos os dados foram preparados e inseridos em um programa científico de processamento das observáveis do sistema GPS. Devido à grande massa de dados foi necessário o desenvolvimento de um programa para verificar a integridade dos mesmos, converter as efemérides precisas e os dados para o formato de entrada do programa científico e preparar os arquivos de lote. Além das características apresentadas, este programa foi desenvolvido para automatizar o processo de cálculo do conteúdo total de elétrons. Ao final do processamento o programa exibe os parâmetros que representam a refração ionosférica ou seja, os valores calculados para o Conteúdo Total de Elétrons. Tais valores permitiram a criação de uma série histórica do comportamento da refração ionosférica abrangendo um intervalo de tempo que contempla um período em que o ciclo solar apresentava atividade mínima e um outro período em que o ciclo solar alcançou a máxima atividade. Estes dados indicam o comportamento da ionosfera para um período de cinco anos em uma das regiões do planeta que apresenta as maiores variações, comportamento este, que permitirá um melhor conhecimento da ionosfera e a melhoria ou desenvolvimento de modelos mais adequados. Foram gerados ainda, mapas da ionosfera que poderão auxiliar na melhoria da acurácia posicional. / Since the US president decided to turn off the selective availability (SA) one of the biggest error in the GPS system has been the ionosphere refraction. The refraction effect on GPS signals are: delay for the code and an advance for the phase. In the equatorial region, where Brazil is, the refraction presents the biggest variations that are caused by the solar cycle, the time of the day, the season, the geomagnetic field and many others phenomena. In this case, the ionosphere refraction is a limitation because it increases the degradation of the position, specially if the user is using a single frequency receiver. To evaluate the ionospheric effects in Brazil, between January 1997 and December 2001, data from the Brazilian Continuous GPS Monitoring Network were used, provided by double frequency GPS receivers. All data were prepared and used by a scientific software to process the GPS observations. The volume of data was so big that it was necessary to write a software to manage the data integrity, to transform the precise ephemeris and the observations to scientific program format and to create batch files. Also, the program was used to process the data and to compute the total electron content automatically. At the end of the process, the software shows the TEC parameters. These parameters were used to create a historical series of the ionosphere refraction in Brazil. This series represents a period of minimal solar cycle, 1997, and a maximum period of the solar cycle, 2000 / 2001 in one of the most active regions of the planet. This series will improve the knowledge on the ionosphere and will allow to improve models or create new ones. Ionospheric maps that can be used to improve the positional accuracy, were also generated.

The advent of Global Positioning System (GPS) has revolutionized geodesy with high accuracy, fast speed, simple use, and low cost. This dissertation investigates three topics on volcano and earthquake-related deformation using GPS measurements and models to demonstrate the power of the new generation of geodetic methods. The three topics include the 2002-2003 continued episodic inflation at Long Valley Caldera in eastern California, the coseismic and postseismic response of the energetic 2008 MW 6.4 Achaia-Elia Earthquake in northwest Peloponnese, Greece, and the interseismic megathrust coupling and forearc sliver transport near the Nicoya Peninsula in northwest Costa Rica.

Após a desativação da disponibilidade seletiva, ocorrida em maio de 2000, a maior fonte de erro presente no sistema GPS passou a ser a refração ionosférica. Os seus efeitos provocam um avanço na fase de batimento da portadora e um atraso no código. Na região equatorial onde o Brasil está inserido, a refração ionosférica apresenta variações que decorrem de diversos fatores tais como: o ciclo solar, o movimento aparente do Sol, as estações do ano, o campo geomagnético, entre outras. Desta forma, a refração ionosférica é um fator limitante pois, degrada a determinação de posições, principalmente, se o usuário estiver utilizando um receptor de apenas uma freqüência. Para avaliar o efeito da refração ionosférica no Brasil, no período compreendido entre janeiro de 1997 a dezembro de 2001, foram utilizados dados coletados com receptores providos de duas freqüências e que integram a Rede Brasileira de Monitoramento Contínuo do Sistema GPS. Todos os dados foram preparados e inseridos em um programa científico de processamento das observáveis do sistema GPS. Devido à grande massa de dados foi necessário o desenvolvimento de um programa para verificar a integridade dos mesmos, converter as efemérides precisas e os dados para o formato de entrada do programa científico e preparar os arquivos de lote. Além das características apresentadas, este programa foi desenvolvido para automatizar o processo de cálculo do conteúdo total de elétrons. Ao final do processamento o programa exibe os parâmetros que representam a refração ionosférica ou seja, os valores calculados para o Conteúdo Total de Elétrons. Tais valores permitiram a criação de uma série histórica do comportamento da refração ionosférica abrangendo um intervalo de tempo que contempla um período em que o ciclo solar apresentava atividade mínima e um outro período em que o ciclo solar alcançou a máxima atividade. Estes dados indicam o comportamento da ionosfera para um período de cinco anos em uma das regiões do planeta que apresenta as maiores variações, comportamento este, que permitirá um melhor conhecimento da ionosfera e a melhoria ou desenvolvimento de modelos mais adequados. Foram gerados ainda, mapas da ionosfera que poderão auxiliar na melhoria da acurácia posicional. / Since the US president decided to turn off the selective availability (SA) one of the biggest error in the GPS system has been the ionosphere refraction. The refraction effect on GPS signals are: delay for the code and an advance for the phase. In the equatorial region, where Brazil is, the refraction presents the biggest variations that are caused by the solar cycle, the time of the day, the season, the geomagnetic field and many others phenomena. In this case, the ionosphere refraction is a limitation because it increases the degradation of the position, specially if the user is using a single frequency receiver. To evaluate the ionospheric effects in Brazil, between January 1997 and December 2001, data from the Brazilian Continuous GPS Monitoring Network were used, provided by double frequency GPS receivers. All data were prepared and used by a scientific software to process the GPS observations. The volume of data was so big that it was necessary to write a software to manage the data integrity, to transform the precise ephemeris and the observations to scientific program format and to create batch files. Also, the program was used to process the data and to compute the total electron content automatically. At the end of the process, the software shows the TEC parameters. These parameters were used to create a historical series of the ionosphere refraction in Brazil. This series represents a period of minimal solar cycle, 1997, and a maximum period of the solar cycle, 2000 / 2001 in one of the most active regions of the planet. This series will improve the knowledge on the ionosphere and will allow to improve models or create new ones. Ionospheric maps that can be used to improve the positional accuracy, were also generated.

No description available.