Dynamic displacements and stresses in the vicinity of a cylindrical cavity embedded in a half-space

The two-dimensional response of a viscoelastic half-space containing a buried, unlined, infinitely long cylindrical cavity of circular cross-section subjected to harmonic plane SH, P, SV and Rayleigh waves is obtained by an indirect boundary integral method based on the two-dimensional Green's functions for a viscoelastic half-space. An extensive critical review of the existing numerical results obtained by other techniques is presented together with some new numerical results describing the motion on the ground surface and the motion and stresses on the wall of the cavity for P, SV, SH and Rayleigh waves.     

Seismic response of a cylindrical shell embedded in a layered viscoelastic half-space. I: Formulation

A method to obtain the three-dimensional harmonic response of a infinitely long cylindrical shell of circular cross-section embedded in a layered viscoelastic half-space and subjected to harmonic plane waves impinging at an oblique angle with respect to the axis of the shell is presented. The procedure combines an indirect integral representation for the field in the exterior half-space with a model of the pipeline or tunnel based on Donnell shell theory. The integral representation for the soil is based on the use of moving Green's functions for the layered viscoelastic half-space. The accuracy of the formulation is tested by comparison of results obtained by using different discretizations. Extensive comparisons with previous two- and three-dimensional results for the case of a shell embedded in a uniform half-space and some new numerical results for a shell embedded in a multilayered half-space are presented in a companion paper.     

Seismic response of a cylindrical shell embedded in a layered viscoelastic half-space. II: Validation and numerical results

A procedure to calculate the three-dimensional harmonic response of a infinitely long cylindrical shell of circular cross-section embedded in a layered viscoelastic half-space and subjected to harmonic plane waves impinging at an oblique angle with respect to the axis of the shell is validated by extensive comparisons with previous two- and three-dimensional results for the particular case of a shell embedded in a uniform half-space. New numerical results describing the motion and stresses within a shell embedded in a multilayered half-space and subjected to obliquely incident P-, SV- and SH-waves with different horizontal angles of incidence are presented and discussed.     

The Amazon river breeze and the local boundary layer: I. Observations

Observations of the diurnal evolution of the planetary boundary layer over the Amazon rain forest, made at sites close to the confluence of the Solimões and Negro rivers (approximately at 3°S, 60°W) near Manaus, Amazonas, Brazil, show the existence of a diurnal rotation of the wind near the surface and the frequent presence of low-level nocturnal wind maxima. These circulations are shown to be plausibly explained as elements of a river and land breeze circulation induced by the thermal contrast between the rivers and the adjacent forest.     

Identification and occurrence of novel C36–C54 3,4-dialkylthiophenes with an unusual carbon skeleton in immature sediments

A series of novel long-chain 3,4-dialkylthiophenes (C₃₆–C₅₄) was identified in a number of sediments ranging from Pleistocene to Cretaceous. The identifications were based on mass spectral characterisation, desulphurisation and mass spectral data of synthesised model compounds. These organic sulphur compounds are probably formed by sulphur incorporation into mid-chain dimethylalkadienes with two methylenic double bonds. These putative precursor lipids are unprecedented and may be considered rather unusual. The distribution of 3,4-dialkylthiophenes in sediments varies considerably with the depositional palaeoenvironment, indicating that these compounds have a potential as molecular markers reflecting changes in palaeoenvironment.     

Geophysical investigations of the Platanares geothermal site, Honduras, Central America

Geophysical studies were carried out in Platanares in Western Honduras, as part of an investigation of the geothermal resource potential of Honduras by Los Alamos National Laboratory in cooperation with the Honduran government and the U.S. Geological Survey. Geological and geochemical studies later followed by exploratory bore-holes indicate this area of rugged topography has the best geothermal potential in Honduras. In order to better understand the geology of the area, gravity, audiomagnetotelluric and telluric surveys were carried out. The site is in a graben of Tertiary volcanics and Cretaceous red beds, with numerous hot springs along faults in the center of the graben. Gravity mapped the thickness of low-density tuffs and the position of associated normal faults. It also indicated the presence of persuasive, shallow, highly silicified, high-density tuffs and gravels. The electrical surveys mapped shallow, high-resistivity silicified zones and faults as low-resistivity zones. The red beds below the tuffs are considered to contain the geothermal reservoir, but their maximum thickness could not be determined by these surveys due to the lack of density contrast and the high surface electrical conductivities. Three geologic cross-sections constructed by others have been re-evaluated using gravity modeling and analysis of eletrical anomalies, showing general agreement but in detail significant differences.     

Exploration drilling and reservoir model of the Platanares geothermal system, Honduras, Central America

Results of drilling, logging, and testing of three exploration core holes, combined with results of geologic and hydrogeochemical investigations, have been used to present a reservoir model of the Platanares geothermal system, Honduras. Geothermal fluids circulate at depths ≥ 1.5 km in a region of active tectonism devoid of Quaternary volcanism. Large, artesian water entries of 160 to 165°C geothermal fluid in two core holes at 625 to 644 m and 460 to 635 m depth have maximum flow rates of roughly 355 and 560 l/min, respectively, which are equivalent to power outputs of about 3.1 and 5.1 MW(thermal). Dilute, alkali-chloride reservoir fluids (TDS ≤ 1200 mg/kg) are produced from fractured Miocene andesite and Cretaceous to Eocene redbeds that are hydrothermally altered. Fracture permeabillity in producing horizons is locally greater than 1500 and bulk porosity is ≤ 6%. A simple, fracture-dominated, volume-impedance model assuming turbulent flow indicates that the calculated reservoir storage capacity of each flowing hole is approximately 9.7 × 10⁶ l/(kg cm⁻²), Tritium data indicate a mean residence time of 450 yr for water in the reservoir. Multiplying the natural fluid discharge rate by the mean residence time gives an estimated water volume of the Platanares system of ≥ 0.78 km³. Downward continuation of a 139°C/km “conductive” gradient at a depth of 400 m in a third core hole implies that the depth to a 225°C source reservoir (predicted from chemical geothermometers) is at least 1.5 km. Uranium-thorium disequilibrium ages on calcite veins at the surface and in the core holes indicate that the present Platanares hydrothermal system has been active for the last 0.25 m.y.     

Influence of basement structures on the evolution of the major sedimentary basins of Brazil: A case of tectonic heritage

The present study deals with the correlation between the geotectonic features of the basement and the internal structure, shape, geologic evolution, etc. of the major sedimentary basins of the South American platform. The Paraná, Parnaíba and Amazonas basins occupy an area of the order of 3.6 × 10⁶ km², and their sedimentary cratonic sequences were deposited from Silurian to Triassic times. Subsidence rates are estimated around 15 m/Ma in the main depocenters.A geologic study was carried out along the basement features in the surroundings of such basins in order to identify the major structural, geotectonic and geochronological discontinuities. The extension of these basement characteristics towards the interior of the basins are examined. Basement core samples from deep wells were investigated through petrological and geochronological analyses, and pre-existing geophysical and structural maps of those basins were taken again into consideration.It can be concluded that many of the identified basement discontinuities display a direct influence on the depositional history of the basins as well as on their internal subdivisions and external outlines. Basement structures generated during the late Precambrian Brasiliano Cycle turned out to be particularly important.The Paraná and Parnaíba basins are considered to represent cratonic basins, located on rigid lithosphere, tectonically stabilized in the latest Precambrian/early Palaeozoic, and their subsidence is attributed to the establishement of some initial rifted grabens. The Amazonas basin is more complex and includes three large sub-basins with distinct evolutions, each located on a different tectonic segment of the basement.     

Volatiles H2O,CO2, and Cl in a subduction related basalt

The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H₂O and CO₂ in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K₂O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO₂ from 51 to 54 wt.% SiO₂. The concentration of H₂O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H₂O in these inclusions. The gas saturation pressures for pure H₂O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H₂O)(Pa)=(SiO₂−48.5 wt.%) × 1.45 × 10⁷. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.