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.

Microearthquake activity near the Idukki Reservoir, south India: A rare example of renewed triggered seismicity

Earthquakes triggered by artificial reservoirs have been documented for more than seven decades and the processes leading to this phenomenon are fairly well understood. Larger among such earthquakes are known to occur within a few years of reservoir impoundment and usually the activity decreases with time. A documented example of Reservoir Triggered Seismicity (RTS), the Idukki Reservoir in Kerala, south India, impounded in 1975, is an exception wherein the triggered activity has been revived in 2011, nearly 35 years after the initial burst of activity in 1977, two years after the dam was filled. The magnitude of the largest shock in the 2011 sequence exceeded that of the previously documented largest microearthquake. Presence of faults that are close to failure and vulnerable to increase in pore pressure due to reservoir loading or increased rainfall, or a combination of both seems to trigger shocks in this area. The renewed burst of earthquakes after a prolonged period of reduced activity at the Idukki Reservoir is a rare example of RTS. (C) 2012 Elsevier B.V. All rights reserved.

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)

水库滑坡复活机理及其发展趋势预测研究

Various numbers of ancient landslides of various scales are frequently distributed on both banks of reservoirs, especially large reservoirs, both in China and abroad. During inundation and operation of theses reservoirs, some of the landslides are reactivated, which caused losses of people's lives and properties to various extents, some even disasters. Systematic studies are, however, very few on the reservoir-induced reactivation mechanism and development tendency prediction. Based on investigation of reservoir-induced reactivation phenomena of ancient landslides and relevant existing research problems, a systematic study is carried out on the field identification, induced reactivation mechanism, development tendency prediction, risk decision-making and treatment of reservoir-related ancient landslides, through analysis of large numbers of engineering geological investigation results, scientific experimental and research results, in combination with prevention and treatment practices of reservoir-related landslides both in China and abroad, and a series of research results have been obtained. 1. On the basis of study of the distribution features, genesis mechanism of ancient landslides on river banks, a set of scientific methods are summarized on field identification of ancient landslides, and a significant method named "lithologic sequence method" or "indicator layer method", is proposed, which is proved to be very useful. 2. A detail study is made on the reservoir-induced hydraulic effects and material mechanic effects (or softening effects) on the ancient landslide through model and case studies, which concludes that the magnitude and properties of reservoir-induced hydraulic effects are related to the shapes of sliding planes, water content and permeability of landslide materials and variation rate and magnitude of reservoir levels; the magnitude of material mechanic effects are related to the material composition (including mineral composition and grain size), natural water content and saturation state of sliding zones. Also a sensitive analysis is made on the factors that are related to the stabilities of the landslides, which indicate that the stability of a landslide is more sensitive to the groundwater head h_w in the slides and the inner friction angleψof sliding zones than others. 3. The joint inducing mechanism of rainfall and reservoir is also discussed in the paper through model analysis and case study, which proves that reservoir inundation increases firstly the sensitivity of a landslides to rainfall through reduction of its stability or cracking deformation which will increase the rainfall infiltration to the slide body, and then rainfall triggers reactivation or intensifies the reservoir-induced deformation of a landslide. 4. Based on rheologic test results of sliding zones of several reservoir-related ancient landslides, the rheologic characteristics of sliding zones have been discussed in detail and several typical rheologic models have been set up, which well explains the dynamic process of slide deformation. The response types to reservoir inundation and development tendency of reservoir -related ancient landslide are discussed in the paper based on field investigation results. And prediction methods for reservoir-related landslides have been studied based on the Mate-Synthetic principle of quantitative and qualitative analysis, as well as combination of computation and internal mechanism analysis, and a rheologic analytical method is proposed which is proved very useful for prediction of the landslide development tendency. 6. In disaster-prevention and treatment of reservoir-related landslides, risk decision-making has been proved very significant both in engineering and economics. Based on the practices in disaster-prevention and treatment of reservoir-related landslides both in China and abroad, the disaster-prevention risk decision-making for reservoir-related landslides has been proposed in terms of philosophy, methods and procedures, and well put into practice. A summary is also made through case study of the experiences of treatment of reservoir-related landslides both in China and abroad in terms of principle, methods and technical lines. 7 A detail study is made as a case study of the reactivated Maoping ancient landslide on the left bank of the Geheyan Reservoir on Qingjiang river in Hubei province, China, including its field identification features, reservoir-induced reactivation characteristics and mechanism, development tendency prediction and proposed counter measures based on risk analysis.

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.

Calcite precipitation induced by clay-bleach cation exchange in andesitic reservoir rocks

Experiments were carried out on the sodium hypochlorite bleach sensitivity of a deep subsurface andesitic reservoir in order to predict possible deleterious mineral transformations during a downhole clean-up job. Experiments involved examination of core samples from the reservoir using an Environmental Scanning Electron Microscope (ESEM) with an attached Energy Dispersive Spectrometer (EDS) before and after the samples were immersed in bleach. Bleach immersion of whole-rock samples resulted in rapid (less than 1 min) precipitation of abundant 3.0-10.0-μm-wide calcite rhombs within clay-associated micropores and on clay and feldspar grain surfaces. Abundant microporefilling calcite rhombs also formed in pure separates of constituent chlorite/corrensite, whereas no calcite formed in a pure separate of constituent zeolite. These experiments indicate that corrensite is the likely calcium source in this experimental fluid-rock system. Formation of calcite occurs via a cation exchange reaction in which calcium in the smectitic interlayers of corrensite exchanges for sodium in the bleach. Serious formation damage due to calcite precipitation would have occurred in the andesite reservoir had it been exposed to bleach. This finding gives credence to earlier suggestions that cation exchange reactions have the potential to cause calcite precipitation in some sandstone reservoirs when exposed to drilling, completion or stimulation fluids. © 1993.

Mortalities induced by the copepod Sinergasilus polycolpus in farmed silver and bighead carp in a reservoir

The frequency distributions of the parasitic copepod Sinergasilus polycolpus were examined in silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis during a disease outbreak of the 2 species of fish in a reservoir in China. The mean abundance of the copepod was positively related with host length and age, and the overdispersion of the copepod in both silver and bighead carp was fitted well with negative binomial distribution. Although parasite-induced host mortality was observed, a peaked age-parasite abundance curve was not detected in the present parasite-host system. It is also proposed that this peaked age-abundance curve is unlikely to be observed in its natural host populations.