761 research outputs found
Métodos geofísicos de exploração aplicados ao estudo da perigosidade sísmica
Nas últimas décadas os métodos geofísicos ganharam papel de relevo na avaliação da perigosidade do risco sísmico e na sua mitigação. A detecção de falhas
activas em ambientes intra placa onde as taxas de movimentação são relativamente baixas (<0,3 mm/ano) e as roturas superficiais são obliteradas pela
erosão e sedimentação, levaram à utlização daqueles métodos na investigação paleossismológica. A nível da mitigação, em que a avaliação dos efeitos de
sítio é essencial, é necessário um conhecimento das propriedades mecânicas das primeiras dezenas de metros da sub-superfície e da estrutura das bacias
sedimentares, que são obtidos a partir de sondagens geotécnicas e métodos geofísicos.
A utilização dos métodos sísmicos, eléctricos e electromagnéticos na identificação de falhas e na estrutura do subsolo, a delimitação de bacias a partir de
métodos potenciais e a caracterização das propriedades mecânicas do subsolo são analisadas no presente trabalho, com exemplos de aplicação em Portugal
Integration of seismic, well, potential-field and geological data for ore prospecting in the Iberian Pyrite Belt
Ore prospecting using gravimetric and magnetic data has become one of the traditional approaches in
the last decades, often complemented with electric and electromagnetic methods. However, due to the
problem of non-uniqueness inherent to potential-filed modelling, constrains provided by structural
methods such as seismic reflection are often used. During the exploration of massive sulphide
polimetallic minerals in the Iberian Pyrite Belt Figueira de Cavaleiros sector, located in the Sado
Tertiary Basin, several gravimetric and magnetic anomalies were considered as interesting targets. In
order to reduce ambiguity of the gravimetric modelling and to confirm the geological model of the
area, two seismic reflection profiles were acquired. The interpretation of these profiles was assisted by
three mechanical boreholes, two of them located in the research area, in order to make a
seismostratigraphic interpretation. Unfortunately, the gravimetric modelling suggests that the anomaly
has a lithological and structural origin and is not related with massive sulphides. Nevertheless, a good
agreement between the seismic and potential-field data was achieved and new insights into the
geological model for the region were obtained form this work, with accurate data about the Tertiary
cover and Palaeozoic basement
Neotectonic activity on the Carcavai fault zone (Algarve, Portugal)
Studies performed on the Carcavai fault zone, Eastern Algarve, revealed several deformational features in
Plio-Quaternary sediments with potential neotectonic implications. These features include fracturing (faults and
joints) and clastic dikes, located along the Carcavai fault. Most of the fractures appear to be the expression of the
fault activity affecting the Plio-Quaternary sediments. This is suggested by a consistent strike, subparallel to the
major tectonic structure, and by the location pattern. Evidences of the fault activity were also identified in the
Mesozoic and Paleozoic rocks, though the age of that deformation is poorly constrained. It consists in the presence of
zones of intensely brecciated rock and of adjoining coarse sedimentary deposits probably related to the dismantling
of a fault scarp. The acquired data point to a late hercynian structure, reactivated as a left-lateral strike-slip fault
with a thrust component during the Plio-Quaternary
Ground-motion simulation in the Lower Tagus Valley Basin
Throughout history, the Lower Tagus Valley (LTV) region has been shaken by several earthquakes, including some with moderate to large magnitudes and with sources located inside the basin, for example the 1344 (M6.0) and 1909 (M6.0) earthquakes. Previous simulations (Bezzeghoud et al. Natural Hazard 69:
1229–1245, 2011) have revealed strong amplification of the amplitude waves in the region, because of the effect of the low-velocity sediments that fill the basin. The model used in those simulations was updated in this work by including new high-resolution geophysical and geotechnical data available for the area (seismic reflection, aeromagnetic, gravimetric, deep wells, standard penetration tests, and geological data). To contribute to improved assessment of seismic hazard in the LTV, we simulated propagation of seismic waves produced by moderate earthquakes in a 3D heterogeneous medium by using elastic finite-difference wave propagation code. The method, successfully used by Grandin et al. (Geophys J Int 171: 1144–1161, 2007), involves evaluation of the seismic potential of known faults in the area studied and three-dimensional seismic ground motion modelling by use of finite difference methods. On the basis of this methodology, we calculated the ground motion for the April 23, 1909, Benavente (Portugal) earthquake (Mw = 6.0) in dense grid points, then computed the synthetic isoseismic map of the area by use of appropriate relationships between seismic intensity (MMI) and peak ground velocity (PGV). The synthetic results, in contrast with available macroseismic and instrumental data, enable validation of the source models proposed for the area, identification of the
sources of historical earthquakes, and could also indicate which areas are more exposed to seismic ground motion
Excess Thermodynamics of Mixtures Involving Xenon and Light Linear Alkanes by Computer Simulation
Excess molar enthalpies and excess molar volumes as a function of composition for liquid mixtures of xenon + ethane (at 161.40 K), xenon + propane (at 161.40 K) and xenon + n-butane (at 182.34 K) have been obtained by Monte Carlo computer simulations and compared with available experimental data. Simulation conditions were chosen to closely match those of the corresponding experimental results. The TraPPE-UA force field was selected among other force fields to model all the alkanes studied, whereas the one-center Lennard−Jones potential from Bohn et al. was used for xenon. The calculated and for all systems are negative, increasing in magnitude as the alkane chain length increases. The results for these systems were compared with experimental data and with other theoretical calculations using the SAFT approach. An excellent agreement between simulation and experimental results was found for xenon + ethane system, whereas for the remaining two systems, some deviations that become progressively more significant as the alkane chain length increases were observed
The 1909 Benavente (Portugal) earthquake : search for the source
The Lower Tagus River Valley has been affected by severe earthquakes comprising distant events, as in 1755, and
local earthquakes, as in 1344, 1531, and 1909. The 1909 earthquake was located NE of Lisbon, near Benavente,
causing serious damage and many losses. Mw 6.0 has been assessed for this earthquake and a reverse faulting focal
mechanism solution has been calculated. Poor epicenter location, possible directivity and site effects, low fault slip
rates, and the thick Cenozoic sedimentary cover make difficult correlation with regional structures. The focal
mechanism indicates an ENE reverse fault as source, though it does not match any outcropping active structure
suggesting that the event could have been produced by a blind thrust beneath the Cenozoic sedimentary fill. Hidden
sources, inferred from seismic reflection data, are a possible NE structure linking the Vila Franca de Xira and the
Azambuja faults, or the southern extension of the later. Evidence of surface rupturing is inhibited by the thick
Holocene alluvial cover and the high fluvial sedimentation rate, though a slightly depressed area was identified in the
Tagus alluvial plain W of Benavente which was investigated as possible geomorphic evidence of co-seismic surface
deformation. A high-resolution seismic reflection profile was acquired across a 0.5 m high scarp at this site, and two
trenches were opened across the scarp for paleoseismic research. Some deformation of dubious tectonic origin was
found, requiring further studies
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