published_or_final_version / Earth Sciences / Master / Master of Science

A pesquisa teve como objetivo integrar o método GPR (Ground Penetrating Radar) com a sísmica de refração, visando o mapeamento do N.A. e a estimativa do teor de umidade em uma área de estudos hidrogeológicos no campus da USP, assim como analisar a acurácia da medida dos dois métodos. Foram realizados ensaios geofísicos, granulométricos, de teor de umidade, do grau de saturação, do monitoramento do N.A. e acompanhamento dos índices pluviométricos em três períodos com variações sazonais no decorrer de um ano denominados período chuvoso (janeiro/2004); período intermediário (abril/2004) e período seco (setembro/2004). O mapeamento do N.A. com GPR foi feito com geometria multi-offset, com as antenas de freqüências centrais 50 MHz, 100 MHz e 200 MHz, sendo que as antenas de 100 MHz e 200 MHz foram as que caracterizaram o N.A., refletindo sua variação sazonal e mantendo a tendência prof. N.A. janeiro < prof. N.A. abril < prof. N.A. setembro. O emprego da sísmica de refração gerou maiores erros e ambigüidades na inversão dos dados. Neste estudo, a sísmica de refração não foi sensível às variações sazonais e, embora os resultados tenham se aproximado das observações diretas, não permitiram mapear a flutuação do N.A. entre os diferentes períodos. A estimativa do teor de umidade volumétrico pelo GPR foi obtida com duas metodologias diferentes que utilizaram a onda direta no solo. Foram empregadas antenas de 50 MHz, 100 MHz e 200 MHz nos diferentes períodos. Observou-se uma variação da umidade entre os períodos, sendo o teor de umidade de janeiro > abril > setembro. A antena de 200 MHz foi a que possibilitara uma melhor acurácia na estimativa do teor de umidade volumétrico. / The present study had as objective to integrate GPR (Ground Penetrating Radar) method to seismic refraction, focusing on water table mapping and on the estimate of water content in an area of hydrogeological studies at USP\'s campus, as well as analysing the measuring accuracy of both methods. Several assays were accomplished, such as geophysical, granulometric, of water content, saturation degree, water table monitoring assays and also accompaniment of pluviometric indexes in the study periods with seasonal variations, along a year, denominated rainy period (january/2004), intermediaty period (april/2004), and dry period (september/2004). Water table mapping by GPR was made with multi-offset geometry, with 50 MHz, 100 MHz and 200 MHz central frequency antennas, seeing that 100 MHz and 200 MHz antennas were those ones, which characterized the water table, reflecting its seasonal variation and maintaining the depth of the water tablejanuary < depthapril < depthseptember tendency. The use of seismic refraction generated larger mistakes and ambiguities in the data inversion. In this study, seismic refraction was not sensitive to seasonal variations and, although the results have approached to direct observations, they did not allow mapping the water table fluctuation among the different periods. The estimate of volumetric water content by GPR was obtained with two different methodologies that have used the direct ground wave. Fifty MHz, 100 MHz and 200 MHz antennas were used in the different periods. A moisture variation was observed among these periods, being the water content of January > April > September. Two hundred MHz antenna was the one that had facilitated a better accuracy in the estimate of moisture content.

The integrated use of Electromagnetic Ground Conductivity, Ground Electrical Resistivity and Seismic Refraction geophysics methods was made in Alia valley, Eritrea for the aim of developing a methodology best suited for ground water exploration in hard rock areas with limited professional and financial resources. A total area of 25 square kilometer was covered with the geophysical surveys. The effective use of information from wells and geological information from outcrops and existing literature helped to create a preliminary conceptual hydrogeologic model that in turn provided a general picture of the research site in terms of its groundwater potential and related factors. The optimum configuration of the geophysical methods and their interpretation was researched by the use of synthetic mathematical models and experimental surveys in areas of known hydrogeological characteristics like lithology and depth of water table. Two approximately parallel electromagnetic ground conductivity profile lines crossed the survey area in approximately north-south direction to assess the variation in the electrical conductivity of the major geological structures. Vertical electrical sounding (VES) surveys were conducted close to existing wells with lithological information and at some selected sites. Survey sites for the two dimensional electrical resistivity imaging and the seismic refraction surveys were selected based on the results from the electromagnetic ground conductivity surveys. The formal geophysical surveys results were correlated with nearby boreholes and with each other. The responses of the different geophysical methods for each lithological layer were recognized. In addition, the location and characteristics of the major geological structures of the area were studies based on the information from the geophysical surveys and other available information. At the end of the research, parts of the research area with better groundwater potential were identified. As a result of the research, an efficient and cost effective geophysical approach was developed for groundwater exploration in hard rock areas of Eritrea and similar places of the world. Recommendations were made on the wider and effective use of the approach with due consideration for the limited financial, human resource and material capacities available for ground water exploration in most countries of Africa and other countries of the world.

On the lower continental slope off Vancouver Island near scientific ocean drilling IODP Site U1326, traveltime modeling along several ocean bottom seismograph (OBS) profiles shows anomalous high velocities of about 2.0 km/s at 70 - 100 m depth (compared to a no-hydrate reference of about 1.6 km/s). These velocities are consistent with the Site U1326 downhole sonic logs that show velocities up to 2.8 km/s near these depths. The drillhole high velocities are interpreted as caused by nearly massive hydrate with concentrations as large as 60-80% of the pore space. The OBS seismic velocities show that high hydrate concentrations of at least 20-30% are laterally extensive out to distances of at least 6 km on either side of the drillhole. A grid of migrated single-channel data shows a sequence of 15- to 75-m-high seafloor scarps, cutting across the ridge perpendicular to the deformation front. These are interpreted as normal faults. Two of the largest fault scarps bound a prominent ~2.5-km-wide slump feature on the steep seaward slope of the frontal ridge. This provides strong evidence that the slump is fault-controlled, and the base of the slump is near the base of hydrate stability suggesting that the slumping is also related to the presence of gas hydrate. At IODP drill Site U1327 on the mid-continental slope, seismic data were recorded along a 1-km-long profile of 10 OBSs. Traveltimes from wide-angle and vertical-incidence arrivals were inverted simultaneously for velocity structure. Corresponding hydrate concentrations increase with depth with an average of about 15% in the 100-m-thick layer above the base of hydrate stability . The seismic structure shows that this local hydrate distribution extends on the kilometer-scale away from the drillhole, as also suggested by multichannel interval velocities in the region. At Site U1328 (Bullseye Vent), seismic images derived from the very high resolution deep-towed DTAGS reflection data show that the top of a zone of high reflectivity, 10-25 m in thickness, extends from the seafloor to a depth of ~30 m. This zone likely corresponds to the shallow region of massive methane hydrate detected in the upper 40 m in the drillhole, and may represent a system of fractures through which fluids and gas pass from the main vent to the seafloor.

High-resolution electrical-resistivity, seismic-refraction, and seismic-reflection surveys were performed at three locations in the Inner Bluegrass Region of Kentucky along coincident survey lines in order to correlate results and determine which method is most effective at locating karst features in this area. The first two survey locations at Slack’s Cave and the Kentucky Horse Park were chosen in order to investigate known karst features. High and low electrical-resistivity anomalies were correlated to air- and water-filled karst voids, respectively. Seismic velocity anomalies, including parabolic time suppressions, amplitude terminations, and surface-wave backscatters, were also observed and correlated to these karst voids. These findings were applied to a third location along Berea Road in order to investigate undiscovered karst voids. Three seismic targets were selected based on backscatter anomaly locations and were aligned in a northwest trend following the general bedrock dip, joint orientations, and suspected conduit orientation. Overall, the seismic-reflection method provided the highest resolution and least ambiguous results; however, integration of multiple methods was determined to help decrease ambiguities in interpretation created by the inherent non-uniqueness found in the results of each method.

Thesis (Ph.D.(Exploration Geophysics))--University of Pretoria, 2007. / Abstract in English. Includes bibliographical references (leaves 72-76).

Thesis (M.Sc.)--Memorial University of Newfoundland, 1995. / Typescript. Computer disk contains: Appendix A, Fortran programs. Bibliography: leaves 134-137. Also available online.

Cratonic basins cover more than 10% of Earth's continental surface area, yet their origin remains enigmatic. In this thesis a suite of new and legacy geophysical and geological data are integrated to constrain the origin of the Parnaíba basin, a cratonic basin in Northeast Brazil. These data include a 1400 km long, deep (20 s two-way travel time) seismic reflection profile, five +/- 110 km offset wide-angle split-spread receiver gathers, gravity anomaly, and well data. In the centre of the basin, the depth to pre-Paleozoic basement is ~ 3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for ~ 250 km at depths between 17-25 km and Moho depth is ~ 42 +/- 2 km. Gravity and P-wave modelling suggests that the MCR represents the upper surface of a high density (2985 kg m³) and V_p (6.7 - 7.0 km s^-1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that commonly observed in rift-type basins. It is shown, however, that the seismic and gravity data are inconsistent with an extensional origin. It is shown that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly, sediment thickness and basin shape. A buried load model is also consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin centre than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement. Comparative analysis of the Michigan and Congo basins shows gravity data from these basins is also consistent with a lower crustal mass excess, while subsidence analysis shows viscoelastic stress relaxation may also contribute to their early subsidence histories. However, unlike Parnaíba, both of these basins appear to have been subjected to secondary tectonic processes that obscure the primary 'cratonic basin' subsidence signals. Parnaíba basin, therefore, offers an excellent record for the investigation of cratonic basin formation.

A pesquisa teve como objetivo integrar o método GPR (Ground Penetrating Radar) com a sísmica de refração, visando o mapeamento do N.A. e a estimativa do teor de umidade em uma área de estudos hidrogeológicos no campus da USP, assim como analisar a acurácia da medida dos dois métodos. Foram realizados ensaios geofísicos, granulométricos, de teor de umidade, do grau de saturação, do monitoramento do N.A. e acompanhamento dos índices pluviométricos em três períodos com variações sazonais no decorrer de um ano denominados período chuvoso (janeiro/2004); período intermediário (abril/2004) e período seco (setembro/2004). O mapeamento do N.A. com GPR foi feito com geometria multi-offset, com as antenas de freqüências centrais 50 MHz, 100 MHz e 200 MHz, sendo que as antenas de 100 MHz e 200 MHz foram as que caracterizaram o N.A., refletindo sua variação sazonal e mantendo a tendência prof. N.A. janeiro < prof. N.A. abril < prof. N.A. setembro. O emprego da sísmica de refração gerou maiores erros e ambigüidades na inversão dos dados. Neste estudo, a sísmica de refração não foi sensível às variações sazonais e, embora os resultados tenham se aproximado das observações diretas, não permitiram mapear a flutuação do N.A. entre os diferentes períodos. A estimativa do teor de umidade volumétrico pelo GPR foi obtida com duas metodologias diferentes que utilizaram a onda direta no solo. Foram empregadas antenas de 50 MHz, 100 MHz e 200 MHz nos diferentes períodos. Observou-se uma variação da umidade entre os períodos, sendo o teor de umidade de janeiro > abril > setembro. A antena de 200 MHz foi a que possibilitara uma melhor acurácia na estimativa do teor de umidade volumétrico. / The present study had as objective to integrate GPR (Ground Penetrating Radar) method to seismic refraction, focusing on water table mapping and on the estimate of water content in an area of hydrogeological studies at USP\'s campus, as well as analysing the measuring accuracy of both methods. Several assays were accomplished, such as geophysical, granulometric, of water content, saturation degree, water table monitoring assays and also accompaniment of pluviometric indexes in the study periods with seasonal variations, along a year, denominated rainy period (january/2004), intermediaty period (april/2004), and dry period (september/2004). Water table mapping by GPR was made with multi-offset geometry, with 50 MHz, 100 MHz and 200 MHz central frequency antennas, seeing that 100 MHz and 200 MHz antennas were those ones, which characterized the water table, reflecting its seasonal variation and maintaining the depth of the water tablejanuary < depthapril < depthseptember tendency. The use of seismic refraction generated larger mistakes and ambiguities in the data inversion. In this study, seismic refraction was not sensitive to seasonal variations and, although the results have approached to direct observations, they did not allow mapping the water table fluctuation among the different periods. The estimate of volumetric water content by GPR was obtained with two different methodologies that have used the direct ground wave. Fifty MHz, 100 MHz and 200 MHz antennas were used in the different periods. A moisture variation was observed among these periods, being the water content of January > April > September. Two hundred MHz antenna was the one that had facilitated a better accuracy in the estimate of moisture content.

This study sets out to evaluate the use of geophysical methods for delineating buried stream channels, which can act as zones of preferential flow within a less hydraulically conductive aquifer. This information is important for gaining an understanding of flow dynamics of alluvial systems. The most reliable method of delineating the dimensions of aquifers is by drilling, which is an expensive proposition and is best preceded by a preliminary geophysical study to help define target zones for a drilling program. The study area is located adjacent to the Coerney River in the Sundays River Valley. Geologically it consists of approximately 5 metres of alluvial fines, covering 3 metres of coarse cobbles and boulders, all underlain by alternating siltstone and sandstone beds of indeterminate thickness. Throughout the area the water is very shallow at approximately 2 metres depth and the groundwater tends to be very saline. An air photo study revealed an old oxbow channel that had been covered over by subsequent agricultural land use. The geophysical methods available for the study were portable seismic refraction, electrical resistivity and electromagnetics. Preliminary field tests clearly showed that seismics did not produce valid results. The methods of electrical resistivity and electromagnetics produced good data and were subjected to further assessment. A grid was surveyed over the study area and both geophysical methods were applied at regularly spaced stations. Soil samples were taken over the same survey grid and analyzed for electrical conductivity in a soils laboratory. The results were compared to the geophysical data in an attempt to quantify the relationship between geophysical response and soil salinity. The data from the electromagnetic survey showed areas of low electrical conductivity which was a possible indication of zones of preferential groundwater flow. A transect of boreholes was drilled over selected electrical conductivity lows and successfully intersected the buried stream channel. A comparison of the borehole logs with the layered earth models from the Vertical Electrical Soundings indicated that the electrical resistivity method was not responding to the features of the buried stream channel and the cobblestone layer. This proved the electromagnetic method to be more valuable for this particular study