Papers by João G Carvalho
The Iberian Pyrite Belt (IPB) hosts one of the largest concentrations of massive sulfides in the ... more The Iberian Pyrite Belt (IPB) hosts one of the largest concentrations of massive sulfides in the Earth's crust. This highly productive VMS belt contains more than 85 massive sulfide deposits, totalling an estimate of 1600 Mt of massive ore and about 250 Mt of stockwork ore (Leistel et al., 1998; Oliveira et al., 2005; Tornos, 2006). Included in the South Portuguese Zone the IPB is represented by the Phyllite-Quartzite Group (PQG) composed of shales and quartzites of late Devonian age followed by the Volcanic-Sedimentary Complex (VSC) a submarine succession of sediments and felsic and basic volcanic rocks (late Famennian-late Viséan age). Above the IPB a turbidite sedimentary unit occurs being represented by the Baixo Alentejo Flysch Group (BAFG). The ore deposits are hosted by felsic volcanic rocks and sediments that are dominant in the lower part of the VSC succession. The Neves Corvo (ProMine, EU FP7) project area is focused on the Neves Corvo deposit, an active copper mine. T...
Geophysical Prospecting, 2009
The study area is located in the Sinai Peninsula, which is considered one of the most promising r... more The study area is located in the Sinai Peninsula, which is considered one of the most promising regions for oil resources. Three different tectonic forces affect the area in the triple junction structures associated with the opening of the Gulf of Suez and the strike slip movement along the Gulf of Aqaba. The main goal of this work is to model the structure of the basement rocks in the study area using magnetic methods. To achieve this, a high-resolution land magnetic survey was acquired and the results were combined with existing seismic reflection data. The magnetic interpretation was carried out using the analytical signal, horizontal gradient, Euler and Werner deconvolution and 3D magnetic modelling methods. We concluded that most of the deduced structures are trending in N-S, N35 •-N45 • west and E-W directions. The Aqaba trend (N15 •-N25 • east) is barely noticeable. The depth to the basement rocks ranges from 1 km to more than 2 km below sea level and these results are in agreement with the available well log data. In addition, interpretation of seismic reflection sections was carried out and compared with overlapping magnetic profiles interpreted using Euler deconvolution. They show that the sedimentary section was affected by the basement tectonics, with faults extending from the basement upwards through the sedimentary cover. These faults constitute good potential structural traps for oil accumulation.
Journal of Applied Geophysics, 2009
The Algarve province of Portugal is located near the E-W Eurasia-Africa plate boundary. It is cha... more The Algarve province of Portugal is located near the E-W Eurasia-Africa plate boundary. It is characterized by a moderate seismicity with some important historical and instrumental earthquakes causing important loss of lives, serious damage and economical problems. It has therefore been a target of several risk assessment projects. This paper focuses the evaluation of the most interesting and useful geotechnical near-surface parameters, through the acquisition, processing and interpretation of P and S-waves refraction profiles and the use of SPT parameters. V P /V S ratios and the Poisson's ratio were estimated and a subsoil classification based on geophysical and geotechnical parameters is presented. The classification based upon the European Code 8 for civil engineering and SPT bedrock data, was carried out for land use planning and design of critical facilities. Other parameters were computed to provide information for future site effect studies. The quality and volume of the data gathered here using established approaches can be quite useful to estimate soil microzoning in the absence of local earthquake records. The results also show that the lithology is the most influent parameter on the values of seismic velocities and SPT data. Therefore, surficial geology is inappropriate for this kind of studies and lithological maps should be used instead.
Journal of Applied Geophysics, 2009
A major plan for Portugal Mainland is being envisaged to use old open pits from abandoned uranium... more A major plan for Portugal Mainland is being envisaged to use old open pits from abandoned uranium mining sites as "Waste Containment Deposits". These areas will store mining waste from other adjacent mines. The old mining sites classification to this kind of usage is carried out accordingly to its location, accessibility, geological and hydrogeological conditions. Mining waste deposition in the open pits may however cause environmental problems related with geological and hydrogeological features that must be predicted and prevented before a particular site is chosen. Therefore, the identification of faults and conductive zones that may promote groundwater circulation and the spread of contaminated waters is of great importance, since the surrounding area is highly populated. The possible negative environmental impacts of the presence of such potential waste disposal sites are therefore being assessed using geophysical methods and geological outcrop studies in several geological and hydrogeological critical areas. The abandoned Quinta do Bispo uranium mine is one of such places. This old open pit, chosen as one of the sites to be used in the near future as a "Waste Containment Deposit" (accordingly to the above mentioned criteria), needs to be characterized at depth to prevent any possible negative environmental impacts. Thus, the acquisition, processing and interpretation of electromagnetic, electrical and both seismic refraction and reflection have been carried out. 2D schematic models have been constructed, showing alteration and faults zones at depth. These fault zones control groundwater circulation and therefore, future water circulation problems with negative environmental impact may be predicted and prevented.
Geophysical Research Letters, 2013
Small-throw seismogenic fault segments hidden in the Holocene sediments are crucial but difficult... more Small-throw seismogenic fault segments hidden in the Holocene sediments are crucial but difficult targets in seismic exploration. We report here the detection of the deformation pattern and a concealed fault segment in the unconsolidated sediments at Vila Franca Xira, Portugal, through identification in shear wave reflection data of multiple signatures of ductile deformation associated with faulting. We find step-like changes in the stacking velocity along a shallow subsoil layer boundary, indicating synsedimentary faulting. We also recognize a consistent distortion in the moveout of the reflection events in the raw shear wave data. Synthetic modeling of seismic data helps in interpreting these observations and identifying backscattered energy from a steeply dipping shallow fault zone. Prior to this finding, there was no evidence for Holocene activity of this fault, although the fault is considered to be the most probable source for the disastrous 1531 earthquake.
SHARE (Seismic Hazard Harmonization in Europe) is an EC-funded project (FP7) that aims to evaluat... more SHARE (Seismic Hazard Harmonization in Europe) is an EC-funded project (FP7) that aims to evaluate European seismic hazards using an integrated, standardized approach. In the context of SHARE, we are compiling a fully-parameterized active fault database for Iberia and the nearby offshore region. The principal goal of this initiative is for fault sources in the Iberian region to be represented in SHARE and incorporated into the source model that will be used to produce seismic hazard maps at the European scale. The SHARE project relies heavily on input from many regional experts throughout the Euro-Mediterranean region. At the SHARE regional meeting for Iberia, the 2010 Working Group on Iberian Seismogenic Sources (WGISS) was established; these researchers are contributing to this large effort by providing their data to the Iberian regional integrators in a standardized format. The development of the SHARE Iberian active fault database is occurring in parallel with IBERFAULT, another ongoing effort to compile a database of active faults in the Iberian region. The SHARE Iberian active fault database synthesizes a wide range of geological and geophysical observations on active seismogenic sources, and incorporates existing compilations (e.g., Cabral, 1995; Silva et al., 2008), original data contributed directly from researchers, data compiled from the literature, parameters estimated using empirical and analytical relationships, and, where necessary, parameters derived using expert judgment. The Iberian seismogenic source model derived for SHARE will be the first regional-scale source model for Iberia that includes fault data and follows an internationally standardized approach (Basili et al., 2008; 2009). This model can be used in both seismic hazard and risk analyses and will be appropriate for use in Iberian-and European-scale assessments.
VIII CNG …, 2010
Page 1. e-Terra http://e-terra.geopor.pt ISSN 1645-0388 Volume 10 nº 5 2010 Revista Electrónica... more Page 1. e-Terra http://e-terra.geopor.pt ISSN 1645-0388 Volume 10 nº 5 2010 Revista Electrónica de Ciências da Terra Geosciences On-line Journal GEOTIC Sociedade Geológica de Portugal VIII Congresso Nacional de Geologia ...
Pure and Applied Geophysics, 2011
... Elevation differences along each profile are minimal except at Escanxinhas and Trafal 4, wher... more ... Elevation differences along each profile are minimal except at Escanxinhas and Trafal 4, where they do not exceed 3 m. A fixed spread with 48 channels, 50 Hz vertical receivers and a 250kg accelerated weight-drop seismic source were used in the acquisition of the seismic ...
Nas últimas décadas os métodos geofísicos ganharam papel de relevo na avaliação da perigosidade, ... more 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 utilização 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 subsuperfície 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.
Journal of Geophysics and Engineering, 2011
This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep se... more This paper focuses on the interpretation of seismic reflection, gravimetric, topographic, deep seismic refraction and seismicity data to study the recently proposed Ota-Vila Franca de Xira-Lisbon-Sesimbra (OVLS) fault zone and the lower Tagus Cenozoic basin (LTCB). The studied structure is located in the lower Tagus valley (LTV), an area with over 2 million inhabitants that has experienced historical earthquakes which caused significant damage and economical losses (1344, 1531 and 1909 earthquakes) and whose tectonic sources are thought to be local but mostly remain unknown. This study, which is intended as a contribution to improve the seismic hazard of the area and the neotectonics of the region, shows that the above-proposed fault zone is probably a large crustal thrust fault that constitutes the western limit of the LTCB. Gravimetric, deep refraction and seismic reflection data suggest that the LTCB is a foreland basin, as suggested previously by some authors, and that the OVLS northern and central sectors act as the major thrusts. The southern sector fault has been dominated by strike-slip kinematics due to a different orientation to the stress field. Indeed, geological outcrop and seismic reflection data interpretation suggests that, based on fault geometry and type of deformation at depth, the structure is composed of three major segments. These data suggest that these segments have different kinematics in agreement with their orientation to the regional stress field. The OVLS apparently controls the distribution of the seismicity in the area. Geological and geophysical information previously gathered also points that the central segment is active into the Quaternary. The segment lengths vary between 20 and 45 km. Since faults usually rupture only by segments, maximum expectable earthquake magnitudes and other parameters have been calculated for the three sectors of the OVLS fault zone using empirical relationships between earthquake statistics and geological parameters available from the literature. Calculated slip rates are compatible with previous estimates for the area (0.33 mm yr-1
Geophysical Journal International, 2008
This paper focuses on the reprocessing of seismic reflection profiles, aeromagnetic and seismicit... more This paper focuses on the reprocessing of seismic reflection profiles, aeromagnetic and seismicity data, to locate and characterize the Ota-Vila Franca de Xira-Lisbon-Sesimbra fault zone. The studied structure is sited in the Lower Tagus Valley, an area with over 2 million inhabitants, that has experienced historical earthquakes causing many casualties, serious damage and economical losses (e.g. 1531 January 26 and 1909 April 23 earthquakes), whose tectonic sources are mostly unknown. The fault zone trends NNE-SSW to N-S, is located near the eastern border of the Mesozoic Lusitanian Basin and partially delimits the Lower Tagus Cenozoic Basin at the west, mostly hidden under the Cenozoic sedimentary fill. According to the data presented here, the normal structures that compose the fault zone were reactivated in Cenozoic times, with positive inversion and the development of splays towards the east. The fault zone shows three distinct segments with different behaviour, in conformity with their various orientations relative to the NW-SE maximum compressive stress. The northern segment splays into a series of NNE-SSW oriented, east verging, imbricate thrusts, which merge to the west into a major reverse fault that resulted from the tectonic inversion of the former normal fault bordering the Mesozoic Lusitanian Basin in this area-the well known Ota (or Pragança) fault. The central segment corresponds to the approximately 20 km long outcropping Vila Franca de Xira fault, which suffered a maximum degree of inversion. The southern segment extends for ∼45 km, crossing Lisbon and the Setúbal Península at depth until approximately Sesimbra (probably continuing offshore), with an N-S trend and distinct geometry. South of Vila Franca de Xira, there is evidence for a WSW-ENE fault located at depth, producing a righ-lateral stepover on the major structure and splitting the central from the southern segment. We hypothesize that this obliquely trending fault is a possible source of the 1909 Benavente earthquake.
VIII CNG …, 2010
RESUMO: Estudos levados a cabo na zona da falha de Carcavai revelaram a ocorrência de deformações... more RESUMO: Estudos levados a cabo na zona da falha de Carcavai revelaram a ocorrência de deformações (fracturação e filões detríticos) em sedimentos plio-quaternários, indicando actividade neotectónica. A generalidade das fracturas aparenta corresponder à expressão superficial secundária da actividade naquela zona de falha. Os dados adquiridos apontam para uma zona de falha complexa de desligamento esquerdo, com componente vertical inversa, desde o final do Mesozóico ou início do Cenozóico. Os filões detríticos foram interpretados como estruturas resultantes de liquefacção induzida sismicamente.
Comunicações Geológicas
Studies performed on the Carcavai fault zone, Eastern Algarve, revealed several deformational fea... more 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.
The development of seismogenic source models is one of the first steps in seismic hazard assessme... more The development of seismogenic source models is one of the first steps in seismic hazard assessment. In seismic hazard terminology, seismic source zones (SSZ) are polygons that delineate areas with homogeneous characteristics of seismicity. The importance of using knowledge on geology, seismicity and tectonics in the definition of source zones has been recognized for a long time. However, the definition of SSZ tends to be subjective and controversial. In the SHARE (Seismic Hazard Harmonization in Europe) FP7 project, seismic hazard will be assessed with a logic tree approach that allows for three types of branches for seismicity models: a) smoothed seismicity, b) SSZ, c) SSZ and faults. In this context, a large-scale zonation model for use in the smoothed seismicity branch, and a new consensus SSZ model for Portugal and Azores have been developed. The new models are being achieved with the participation of regional experts and external experts, according the SSHAC (1997) level-3 met...
The Algarve province of Portugal is located near the E-W Eurasia-Africa plate boundary. It is cha... more The Algarve province of Portugal is located near the E-W Eurasia-Africa plate boundary. It is characterized by a moderate seismicity, with some important historical earthquakes causing important damage and economical losses. Not only has it suffered the effects of large plate boundary events but also the impact of local onshore moderate-sized earthquake sources. The seismic hazard evaluation and mitigation of the area is therefore of great importance to the local populations and the large number of tourists that frequent the region. This paper focuses the evaluation of the most interesting and useful geotechnical near-surface parameters and a soil classification. The classification based upon the European Code 8 for civil engineering and SPT bedrock data, was carried out for land use planning and design of critical facilities. P-wave and S-wave seismic velocities were obtained through the acquisition, processing and interpretation refraction profiles. Hundreds of SPT parameters from...
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Papers by João G Carvalho