Induced seismicity in the Castarthao reservoir, NE Brazil - Preliminary results

The Castanhao reservoir was built in the state of Ceara, a dry region in Northeastern Brazil, to regulate the flow of the Jaguaribe River, for irrigation, and for power generation. It is an earth-filled dam, 60 m high, with a water capacity of 4.5 x 10(9) m(3). The seismicity in the area has been monitored since 1998, with a few interruptions, using one analog or one digital station and, during a few periods, a three-station network. The first earthquakes likely to be induced events were detected in 2003, when the water level was about 20 in high. In early 2004 a very heavy rainfall season quickly filled the reservoir. Shortly after, an increase in the seismic activity occurred and many micro-earthquakes were recorded. We suggest that this activity resulted from an increase in pore pressure due to undrained response. Therefore, we may classify this cluster of microearthquakes as ""initial seismicity."" We deployed a network with four analog stations in the area, following this activity, to determine the epicentral zone. At least three epicentral areas under the reservoir were detected. The spatio-temporal analysis of the available data revealed that the seismicity occurs in clusters and that these were activated at different periods. We identified four sets of faults (N-S-, E-W-, NW-SE-, and NE-SW-oriented), some of which moved in shallow crustal levels and as recently as the Quaternary (1.8 Ma). Under the present-day stress regime, the last two sets moved as strike-slip structures. We suggest a possible correlation between dormant faults and the observed induced seismicity. (c) 2008 Elsevier B.V. All rights reserved.

TDEM survey in an area of seismicity induced by water wells in Parana sedimentary basin, Northern Sao Paulo State, Brazil

This article presents TDEM results from an area with recent induced shallow seismicity. The purpose was to do a geoelectrical mapping of sedimentary and fractured basaltic aquifers for better understanding of the hydrogeologic setting. The study area is in the Parana basin where flood basalts are overlain by sedimentary units near the city of Bebedouro, northern Sao Paulo State, Brazil. 86 TDEM soundings were acquired in an area of 90 km(2) in the Andes and Botafogo study areas. The soundings were chosen next to wells for calibration, and also along profiles crossing the seismically active areas. 1-D interpretation results showed the general geoelectrical stratigraphy of this part of the Parana basin. The upper geoelectrical layer is the shallow sedimentary aquifer (Adamantina formation) with less than 80 m thickness. The second geoelectrical layer contains the upper basalts of the Serra Geral formation at about 60-80 m depths. A saturated fractured basalt zone between 100 and 300 m depths was identifiable on various TDEM soundings. This depth range corresponds to the range of hypocentral depths for more than 3000 micro-earthquakes in this area. The lower basalt layer was estimated to lie between 400 and 650 m depth. The deepest geoelectrical layer detected by various TDEM soundings corresponds to the Botucatu sandstone (Guarani aquifer). Results suggest that the high-discharge wells are located in the fractured zone in the middle basalt of the Serra Geral formation. There is a good correlation between seismically active areas, high discharge wells (>190 m(3)/h), and fracture zones in the middle basalt. The results reinforce the hypothesis that the shallow seismic activity in the Bebedouro region is being triggered by high rates of groundwater withdrawal. (C) 2012 Elsevier B.V. All rights reserved.

The double Caldeira-Leggett model: Derivation and solutions of the master equations, reservoir-induced interactions and decoherence

In this paper we analyze the double Caldeira-Leggett model: the path integral approach to two interacting dissipative harmonic oscillators. Assuming a general form of the interaction between the oscillators, we consider two different situations: (i) when each oscillator is coupled to its own reservoir, and (ii) when both oscillators are coupled to a common reservoir. After deriving and solving the master equation for each case, we analyze the decoherence process of particular entanglements in the positional space of both oscillators. To analyze the decoherence mechanism we have derived a general decay function, for the off-diagonal peaks of the density matrix, which applies both to common and separate reservoirs. We have also identified the expected interaction between the two dissipative oscillators induced by their common reservoir. Such a reservoir-induced interaction, which gives rise to interesting collective damping effects, such as the emergence of relaxation- and decoherence-free subspaces, is shown to be blurred by the high-temperature regime considered in this study. However, we find that different interactions between the dissipative oscillators, described by rotating or counter-rotating terms, result in different decay rates for the interference terms of the density matrix. (C) 2010 Elsevier B.V. All rights reserved.

Seismic activity triggered by water wells in the Parana Basin, Brazil

Triggered seismicity is commonly associated with deep water reservoirs or injection wells where water is injected at high pressure into the reservoir rock. However, earth tremors related solely to the opening of groundwater wells are extremely rare. Here we present a clear case of seismicity induced by pore-pressure changes following the drilling of water wells that exploit a confined aquifer in the intracratonic Parana Basin of southeastern Brazil. Since 2004, shallow seismic activity, with magnitudes up to 2.9 and intensities V MM, has been observed near deep wells (120-200 m) that were drilled in early 2003 near the town of Bebedouro. The wells were drilled for irrigation purposes, cross a sandstone layer about 60-80 m thick and extract water from a confined aquifer in fractured zones between basalt flow layers. Seismic activity, mainly event swarms, has occurred yearly since 2004, mostly during the rainy season when the wells are not pumped. During the dry season when the wells are pumped almost continuously, the activity is very low. A seismographic network, installed in March 2005, has located more than 2000 microearthquakes. The events are less than 1 km deep (mostly within the 0.5 km thick basalt layer) and cover an area roughly 1.5 km x 5 km across. The seismicity generally starts in a small area and expands to larger distances with an equivalent hydraulic diffusivity ranging from 0.06 to 0.6 m(2)/s. Geophysical and geothermal logging of several wells in the area showed that water from the shallow sandstone aquifer enters the well at the top and usually forms waterfalls. The waterfalls flow down the sides of the wells and feed the confined, fractured aquifer in the basalt layer at the bottom. Two seismic areas are observed: the main area surrounds several wells that are pumped continuously during the dry season, and a second area near another well (about 10 km from the first area) that is not used for irrigation and not pumped regularly. The main area displays cyclic annual activity, but the second area does not. We explain the earthquake swarms as being triggered by pore pressure diffusion in the fractured basalt layer due to additional pressure from the newly connected surface aquifer. This reaches critically prestressed areas up to a few kilometers away from the wells. During periods of continuous pumping, the reduction of pore pressure in the confined aquifer stops the seismic activity. Our study suggests that this kind of activity may be more common than previously thought and implies that many other cases of small tremors associated with the drilling of water wells may have gone unnoticed.

Estudo da sismicidade na barragem do Açu no período de 2012 a 2013

The seismic activity in the Northeastern of Brazil has been a constant target of study, since it is the most active region of the country. However, some areas have their earthquakes related to human action, what means they are induced. The Açu dam is a classic example of reservoir-induced seismicity and it has been the subject of several studies. Recently, after a considerable period of inactivity, the LabSis / UFRN recorded events related to the dam, which led to the installation of a network around the reservoir. From the data provided by this network, it was observed that the seismic activity is related to a new epicental area inside the lake. The epicentral parameters and the focal mechanism were determined. It was found that the events were related to the reactivation of a basement structure in a new seismogenic subvertical fault with NE-SW-striking subparallel to the São Rafael Fault. These results were used in the preparation of a scientific paper, which discussed the relationship between this seismicity with the geology of the region and with the reservoir water level. The paper showed that the diffusion of pore pressure was the main mechanism that controlled the triggering of the induced seismicity at the reservoir.

Estudo da sismicidade na barragem do Açu no período de 2012 a 2013

The seismic activity in the Northeastern of Brazil has been a constant target of study, since it is the most active region of the country. However, some areas have their earthquakes related to human action, what means they are induced. The Açu dam is a classic example of reservoir-induced seismicity and it has been the subject of several studies. Recently, after a considerable period of inactivity, the LabSis / UFRN recorded events related to the dam, which led to the installation of a network around the reservoir. From the data provided by this network, it was observed that the seismic activity is related to a new epicental area inside the lake. The epicentral parameters and the focal mechanism were determined. It was found that the events were related to the reactivation of a basement structure in a new seismogenic subvertical fault with NE-SW-striking subparallel to the São Rafael Fault. These results were used in the preparation of a scientific paper, which discussed the relationship between this seismicity with the geology of the region and with the reservoir water level. The paper showed that the diffusion of pore pressure was the main mechanism that controlled the triggering of the induced seismicity at the reservoir.

Sviluppo di una metodologia per la modellazione geologica-geomeccanica ed il monitoraggio di reservoir adibiti a siti di stoccaggio di gas naturale

La valutazione dei rischi associati all’operatività dei sistemi di stoccaggio, quali la sismicità indotta e la subsidenza, è requisito basilare per una loro corretta gestione e progettazione, e passa attraverso la definizione dell’influenza sullo stato tensionale delle variazioni di pressione di poro nel sottosuolo. Principale scopo di questo progetto è lo sviluppo di una metodologia in grado di quantificare le deformazioni dei reservoir in funzione della pressione di poro, di tarare i modelli utilizzati con casi studio che presentino dati di monitoraggio reali, tali da consentire un confronto con le previsioni di modello. In questa tesi, la teoria delle inomogeneità è stata utilizzata, tramite un approccio semianalitico, per definire le variazioni dei campi elastici derivanti dalle operazioni di prelievo e immissione di fluidi in serbatoi geologici. Estensione, forma e magnitudo delle variazioni di stress indotte sono state valutate tramite il concetto di variazione dello sforzo critico secondo il criterio di rottura di Coulomb, tramite un’analisi numerica agli elementi finiti. La metodologia sviluppata è stata applicata e tarata su due reservoir sfruttati e riconvertiti a sistemi di stoccaggio che presentano dataset, geologia, petrofisica, e condizioni operative differenti. Sono state calcolate le variazioni dei campi elastici e la subsidenza; è stata mappata la variazione di sforzo critico di Coulomb per entrambi i casi. I risultati ottenuti mostrano buon accordo con le osservazioni dei monitoraggi, suggerendo la bontà della metodologia e indicando la scarsa probabilità di sismicità indotta. Questo progetto ha consentito la creazione di una piattaforma metodologica di rapido ed efficace utilizzo, per stimare l’influenza dei sistemi di stoccaggio di gas sullo stato tensionale della crosta terrestre; in fase di stoccaggio, permette di monitorare le deformazioni e gli sforzi indotti; in fase di progettazione, consente di valutare le strategie operative per monitorare e mitigare i rischi geologici associati a questi sistemi.

Tectônica rúptil e sismicidade na área de inundação do açude do Castamhão (CE): implicações para o risco sísmico

This study describes brittle deformation and seismicity in the Castanhão Dam region, Ceará State, Brazil. This reservoir will include a hidroeletric power plant and will store about 6,7 billions m3 of water. Five main litostratigraphic unit were identified in the region: gneissic-migmatitic basement, metavolcanosedimentary sequence, granitoid plutons of Brasiliano age, Mesozoic basaltic dike swarm, and Cenozoic fluvial terraces of the Jaguaribe river. The region has experienced several faulting events that occurred at different crustal levels. Faults formed at depths less than about 12 km present left-lateral movement and are associated with epidote and quartz infillings. Faults formed at depths less than 7 km are mainly strike-slip present cataclastic rocks, fault breccia and gouge. Both fault groups form mainly NE-trendind lineaments and represent reactivation of ductile shear zones or new formed faults that cut across existing structures. Seismically-induced liquefaction fractures take place in Cenozoic terraces and indicate paleoearthquakes that may have reached at leat 6,8 MS. In short, this work indicate that the level of paleoseismicity is much greater than one observed in the instrumental record. Several faults are favourably oriented for reactivation and induced seismicity should be expected after the Castanhão Dam impoudment

Estudo do potencial da técnica de nanossísmica para o monitoramento de hidrofraturamento em reservatórios de hidrocarbonetos

Hydraulic fracturing is an operation in which pressurised fluid is injected in the geological formation surrounding the producing well to create new permeable paths for hydrocarbons. The injection of such fluids in the reservoir induces seismic events. The measurement of this reservoir stimulation can be made by location these induced microseismic events. However, microseismic monitoring is an expensive operation because the acquisition and data interpretation system using in this monitoring rely on high signal-to-noise ratios (SNR). In general, the sensors are deployed in a monitoring well near the treated well and can make a microseismic monitoring quite an expensive operation. In this dissertation we propose the application of a new method for recording and location of microseismic events called nanoseismic monitoring (Joswig, 2006). In this new method, a continuous recording is performed and the interpreter can separate events from noise using sonograms. This new method also allows the location of seismic sources even when P and S phases onsets are not clear like in situations of 0 dB SNR. The clear technical advantage of this new method is also economically advantageous since the sensors can potentially be installed on the surface rather than in observation well. In this dissertation field tests with controlled sources were made. In the first test small explosives using fire works at 28 m (slant distances) were detected yealding magnitudes between -2.4 ≤ ML ≤ -1.6.. In a second test, we monitored perforation shots in a producing oil field. In this second test, one perforation shot was located with slant distances of 861 m and magnitude 2.4 ML. Data from the tests allow us to say that the method has potential to be used in the oil industry to monitor hydrofracture

Geothermal investigations in an area of induced seismic activity, northern São Paulo State, Brazil

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Sismicidad inducida por la manipulación del subsuelo

Los terremotos inducidos por la inyección de gas realizadas en la plataforma continental del golfo de Valencia, proyecto El Castor, en septiembre-octubre 2013, han creado cierta alarma social en las poblaciones costeras próximas. Esto, que es novedoso en España, ha ocurrido con cierta frecuencia en otras áreas pobladas en distintos países y por causa de varios tipos de manipulación del subsuelo. Este artículo intenta ilustrar el problema de la sismicidad inducida por la actividad del hombre. Según se describe en el texto, la sismicidad inducida tiene unos límites, máximo terremoto inducido, que pueden ser investigados en cada caso. De esa manera, se pueden establecer protocolos de actuación que impidan que dichas actividades causen daños materiales en las localidades próximas. Earthquakes induced by the gas injection in the continental platform, of the golf of Valencia, Castor Project, last September-October 2013, has motivated some social alarm within the nearby coastal population. That, which is new for Spain, has happened with some frequency at some other populated areas, in different countries and due to various types of subsoil manipulations. This article tries to illustrate the induced seismicity problem due to human activities. As it is described in the text, the induced seismicity has some limits (maximum induced earthquake) that can be investigated for each particular case. By this way, protocols of operation can be established to avoid that theses activities might cause material damage to local population.

Forest structure of artificial islands in the Tucuruí dam reservoir in northern Brazil: a test core-area model

Construction of hydroelectric dams in tropical regions has been contributing significantly to forest fragmentation. Alterations at edges of forest fragments impact plant communities that suffer increases in tree damage and dead, and decreases in seedling recruitment. This study aimed to test the core-area model in a fragmented landscape caused by construction of a hydroelectric power plant in the Brazilian Amazon. We studied variations in forest structure between the margin and interiors of 17 islands of 8-100 hectares in the Tucuruí dam reservoir, in two plots (30 and >100m from the margin) per island. Mean tree density, basal area, seedling density and forest cover did not significantly differ between marginal and interior island plots. Also, no significant differences were found in liana density, dead tree or damage for margin and interior plots. The peculiar topographic conditions associated with the matrix habitat and shapes of the island seem to extend edge effects to the islands' centers independently of the island size, giving the interior similar physical microclimatic conditions as at the edges. We propose a protocol for assessing the ecological impacts of edge effects in fragments of natural habitat surrounded by induced (artificial) edges. The protocol involves three steps: (1) identification of focal taxa of particular conservation or management interest, (2) measurement of an "edge function" that describes the response of these taxa to induced edges, and (3) use of a "Core-Area Model" to extrapolate edge function parameters to existing or novel situations.

Trypanosoma cruzi strains isolated from human, vector, and animal reservoir in the same endemic region in Mexico and typed as T. cruzi I, discrete typing unit 1 exhibit considerable biological diversity

In this study, three strains of Trypanosoma cruzi were isolated at the same time and in the same endemic region in Mexico from a human patient with chronic chagasic cardiomyopathy (RyC-H); vector (Triatoma barberi) (RyC-V); and rodent reservoir (Peromyscus peromyscus) (RyC-R). The three strains were characterized by multilocus enzyme electrophoresis, random amplified polymorphic DNA, and by pathological profiles in experimental animals (biodemes). Based on the analysis of genetic markers the three parasite strains were typed as belonging to T. cruzi I major group, discrete typing unit 1. The pathological profile of RyC-H and RyC-V strains indicated medium virulence and low mortality and, accordingly, the strains should be considered as belonging to biodeme Type III. On the other hand, the parasites from RyC-R strain induced more severe inflammatory processes and high mortality (> 40%) and were considered as belonging to biodeme Type II. The relationship between genotypes and biological characteristics in T. cruzi strains is still debated and not clearly understood. An expert committee recommended in 1999 that Biodeme Type III would correspond to T. cruzi I group, whereas Biodeme Type II, to T. cruzi II group. Our findings suggest that, at least for Mexican isolates, this correlation does not stand and that biological characteristics such as pathogenicity and virulence could be determined by factors different from those identified in the genotypic characterization

Numerical simulation of ambient seismic wavefield modification caused by pore-fluid effects in an oil reservoir

We have modeled numerically the seismic response of a poroelastic inclusion with properties applicable to an oil reservoir that interacts with an ambient wavefield. The model includes wave-induced fluid flow caused by pressure differences between mesoscopic-scale (i.e., in the order of centimeters to meters) heterogeneities. We used a viscoelastic approximation on the macroscopic scale to implement the attenuation and dispersion resulting from this mesoscopic-scale theory in numerical simulations of wave propagation on the kilometer scale. This upscaling method includes finite-element modeling of wave-induced fluid flow to determine effective seismic properties of the poroelastic media, such as attenuation of P- and S-waves. The fitted, equivalent, viscoelastic behavior is implemented in finite-difference wave propagation simulations. With this two-stage process, we model numerically the quasi-poroelastic wave-propagation on the kilometer scale and study the impact of fluid properties and fluid saturation on the modeled seismic amplitudes. In particular, we addressed the question of whether poroelastic effects within an oil reservoir may be a plausible explanation for low-frequency ambient wavefield modifications observed at oil fields in recent years. Our results indicate that ambient wavefield modification is expected to occur for oil reservoirs exhibiting high attenuation. Whether or not such modifications can be detected in surface recordings, however, will depend on acquisition design and noise mitigation processing as well as site-specific conditions, such as the geologic complexity of the subsurface, the nature of the ambient wavefield, and the amount of surface noise.

Numerical analysis of wave-induced fluid flow effects on seismic data: Application to monitoring of CO2 storage at the Sleipner Field

In this work we analyze how patchy distributions of CO2 and brine within sand reservoirs may lead to significant attenuation and velocity dispersion effects, which in turn may have a profound impact on surface seismic data. The ultimate goal of this paper is to contribute to the understanding of these processes within the framework of the seismic monitoring of CO2 sequestration, a key strategy to mitigate global warming. We first carry out a Monte Carlo analysis to study the statistical behavior of attenuation and velocity dispersion of compressional waves traveling through rocks with properties similar to those at the Utsira Sand, Sleipner field, containing quasi-fractal patchy distributions of CO2 and brine. These results show that the mean patch size and CO2 saturation play key roles in the observed wave-induced fluid flow effects. The latter can be remarkably important when CO2 concentrations are low and mean patch sizes are relatively large. To analyze these effects on the corresponding surface seismic data, we perform numerical simulations of wave propagation considering reservoir models and CO2 accumulation patterns similar to the CO2 injection site in the Sleipner field. These numerical experiments suggest that wave-induced fluid flow effects may produce changes in the reservoir's seismic response, modifying significantly the main seismic attributes usually employed in the characterization of these environments. Consequently, the determination of the nature of the fluid distributions as well as the proper modeling of the seismic data constitute important aspects that should not be ignored in the seismic monitoring of CO2 sequestration problems.