La estructura sísmica de la corteza de la Zona de Ossa Morena y su interpretación geológica

El experimento de sísmica de reflexión profunda IBERSEIS ha proporcionado una imagen de la corteza del Orógeno Varisco en el sudoeste de Iberia. Este artículo se centra en la descripción de la corteza de la Zona de Ossa Morena (OMZ), que está claramente dividida en una corteza superior, con reflectividad de buzamiento al NE, y una corteza inferior de pobre reflectividad. Las estructuras geológicas cartografiadas en superficie se correlacionan bien con la reflectividad de la corteza superior, y en la imagen sísmica se ven enraizar en la corteza media. Ésta está constituida por un cuerpo muy reflectivo, interpretado como una gran intrusión de rocas básicas. La imagen de las suturas que limitan la OMZ muestra el carácter fuertemente transpresivo de la colisión orogénica varisca registrada en el sudoeste de Iberia. La Moho actual es plana y, en consecuencia, no se observa la raíz del orógeno

ГЛУБИННОЕ СТРОЕНИЕ ЗЕМНОЙ КОРЫ СЕВЕРО-ВОСТОЧНОЙ ЕВРАЗИИ И ЕЕ КОНТИНЕНТАЛЬНЫХ ОКРАИН

The paper reports on the deep geophysical studies performed by the Geological Survey of Russia (VSEGEI) under the international project – Deep Processes and Metallogeny of Northern, Central and Eastern Asia. A model of the deep crustal structure is represented by a set of crustal thickness maps and a 5400-km long geotransect across the major tectonic areas of Northeastern Eurasia. An area of 50000000 km2 is digitally mapped in the uniform projection. The maps show the Moho depths, thicknesses of the main crustal units (i.e. the sedimentary cover and the consolidated crust), anomalous gravity and magnetic fields (in a schematic zoning map of the study area), and types of the crust. The geotransect gives the vertical section of the crust and upper mantle at the passive margin of the Eurasian continent (including submarine uplifts and shelf areas of the Arctic Ocean) and the active eastern continental margin, as well as an area of the Pacific plate.В работе представлены результаты обобщения и интерпретации глубинных геофизических исследований, выполненных Геологической службой России (ВСЕГЕИ) в рамках международного проекта «Глубинные процессы и металлогения Северной, Центральной и Восточной Азии». Модель глубинного строения земной коры представлена комплектом карт, отражающих мощностные параметры земной коры, и геотрансектом протяженностью 5400 км, пересекающим основные тектонические области Северо-Восточной Евразии. Комплект цифровых карт, охватывающих область в 50 млн км2, создан в единой проекции и включает карты глубины залегания поверхности Мохоровичича, мощности основных подразделений земной коры (осадочный чехол и консолидированная земная кора), аномального поля силы тяжести и аномального магнитного поля, использованных для районирования территории, а также карту типов земной коры. Геотрансект пересекает северо-восточную часть Евразии и характеризует вертикальный срез земной коры и верхней мантии пассивной окраины Евразийского континента (включая глубоководные поднятия Северного Ледовитого океана и его шельфовую часть), активную восточную континентальную окраину и выходит в область Тихоокеанской плиты

Multidisciplinary study of the hanging wall of the Kiirunavaara iron ore deposit, northern Sweden

Potential weakness zones due to mining-related fracture development under the town of Kiruna, Sweden, have been investigated by integration of seismic, gravity, and petrophysical data. Reflection seismic data were acquired along two subparallel 2D profiles within the residential area of the town. The profiles of ~3.5 km, each oriented approximately east-west, nearly perpendicular to the general strike of the local geology, crossed several contact zones between quartz-bearing porphyries, a sequence of interchanging sedimentary rocks (siltstone, sandstone, conglomerate, and agglomerate), and metabasalt. The resulting reflection seismic sections revealed a strong east-dipping reflectivity that is imaged down to approximately 1.5 km. The location and orientation of major features agree well between the profiles and with the surface geology and known contact zones between the different rock types. Our imaging results, supported by traveltime modelling, indicate that the contact zones dip 40°-50° to the east. The deepest and the weakest reflections are associated with a ~60° dipping structure that is presumably related to the Kiirunavaara iron mineralization. Tomographic inversion of refracted arrivals revealed a more detailed image of the velocity distribution in the upper 100-200 m along the profiles, enabling us to identify near-surface low velocity zones. These could be possible weakness zones developed along the lithological contacts and within the geologic units.The structural image obtained from the seismic data was used to constrain data inversion along a 28 km long east-northeast to west-southwest-oriented gravity profile. The resulting density model indicates that the quartz-bearing porphyry in the hanging wall of the Kiirunavaara mineralization can be separated into two blocks oriented parallel to the ore body. One block has an unexpected low density, which could be an indication of extensive fracturing and deformation

Geophysical monitoring of CO2 at the Ketzin storage site - The Results of the second 3D repeat seismic survey

Various geophysical methods applied at the Ketzin storage site have successfully imaged migration of the injected CO2 within the target reservoir zone of the ~ 650-680 m deep saline aquifer. Results from the first 3D repeat seismic survey conducted in 2009, after about 15 months of injection (~22,000 t), showed that the CO2 plume was concentrated around the injection well with a lateral extent of approximately 300-400 m and a thickness of about 5–20 m. The plume, however, was not radially symmetric, but had a rather westerly trending tendency, revealing the heterogeneous nature of the reservoir. A second 3D repeat seismic survey was acquired in the Summer/Fall of 2012, when ~ 61,000 tons of CO2 had been injected. Preliminary results show further growth and migration of the anomaly which has been interpreted to be induced by the CO2 injection. It is similar in shape to the one observed at the time of the first repeat survey, but larger by approximately 100-200 m and much stronger with the highest amplitudes nearly centered at the injection well. There is still a pronounced westward propagating tendency. The new seismic data show no indication of upward migration into the caprock

4D result enhancement with crosscorrelation-based time-lapse static correction at Ketzin, Germany

A method for correction of time-lapse differences (TLD) in the statics of seismic data from repeated surveys is presented. Such static differences are typically caused by changes in the near-surface velocities between the acquisition repeats and have a deteriorating impact on the time-lapse image. Trace-to-trace time shifts are determined from the pre-stack data sets using cross-correlations. These time shifts are decomposed in a surface-consistent manner, which is providing a static correction that is capable of aligning the repeat data to the baseline data. The approach is demonstrated on a 4D seismic data set from the Ketzin CO2 pilot storage site, Germany, and is compared with results of an initial processing that was based on individual refraction static corrections. It is shown that the proposed TLD static correction reduces 4D noise more effectively than refraction static corrections while being significantly less labor intensive

A footprint of rainfall on land seismic data repeatability at the CO2 storage pilot site, Ketzin, Germany

Two vintages of land time-lapse seismic data were acquired in 2005 and in 2009 at the Ketzin CO2 storage site in Germany. The datasets showed some differences in frequency content, signal-to-noise ratio and refraction statics despite that they were acquired with the same equipment and during the same seasons. The spatial variations in the data appeared to show good correlation with the difference in precipitation during the campaigns. These observations provide a ground for estimation of spatially varying operators that may be used in pre-stack or post-stack cross-equalization corrections of the time-lapse datasets

The relationship of soil-moisture saturation and time-lapse static shifts-An example from the Ketzin pilot site

Changes in the near surface are a major problem for land time-lapse seismic projects. Three seismic surveys at the Ketzin pilot site for CO2 storage in Germany demonstrated the importance of removing the variations in the shallow subsurface by applying spatially variable, relative time shifts to the different vintages prior to 4D interpretation. The main reason for these time shifts is a change in seismic velocities in the ground layer above the water table due to different soil-moisture saturation at the times of acquisition. We compared the variation in precipitation, groundwater level and trace-to-trace time shifts between the baseline and two monitor surveys and revealed that delays in reflected energy are in a qualitative sense, proportional to the moisture content in the soil

Monitoring and volumetric estimation of injected CO 2 using 4D seismic, petrophysical data, core measurements and well logging: A case study at Ketzin, Germany

More than 50 000 tons of CO 2 have been injected at Ketzin into the Stuttgart Formation, a saline aquifer, at approximately 620 m depth, as of summer 2011. We present here results from the 1 st repeat 3D seismic survey that was performed at the site in autumn 2009, after about 22 000 tons of CO 2 had been injected. We show here that rather complex time-lapse signatures of this CO 2 can be clearly observed within a radius of about 300 m from the injection well. The highly irregular amplitude response within this radius is attributed to the heterogeneity of the injection reservoir. Time delays to a reflection below the injection level are also observed. Petrophysical measurements on core samples and geophysical logging of CO 2 saturation levels allow an estimate of the total amount of CO 2 visible in the seismic data to be made. These estimates are somewhat lower than the actual amount of CO 2 injected at the time of the survey and they are dependent upon the choice of a number of parameters. In spite of some uncertainty, the close agreement between the amount injected and the amount observed is encouraging for quantitative monitoring of a CO 2 storage site using seismic methods. © 2012 European Association of Geoscientists & Engineers

4D seismic monitoring of small CO 2 injection: Results from the Ketzin pilot site (Germany)

As a response to global warming, mainly caused by increasing atmospheric concentration of carbon dioxide, and growing energy consumption world-wide, Carbon Capture and Storage (CCS) is regarded as one option to ensure safe energy provision and mitigation of climate change in the near future. Although the technology of geological storage of carbon dioxide has been applied in the framework of oil and gas exploration for over a decade now, it has not yet reached a mature state for, e.g., coal fired power plants or energy intensive industries. One of the first pilot sites for onshore storage of CO2 in a deep saline aquifer has been deployed at the town of Ketzin (Germany). The characteristics of the site are not typical for future industrial demonstration projects which will be operated in much larger dimensions and in deeper reservoirs. However, the storage operations at the Ketzin site are combined with a wide range of geophysical, geochemical and microbial monitoring methods which are tested for their applicability on larger storage sites. Seismic monitoring plays a crucial role in this context as it has proven to deliver the most comprehensive information on the spatial distribution of the injected CO2 in the reservoir. The first 3D seismic repeat survey was acquired after approximately 14 months of injection and delivered high resolution images of the lateral distribution of the injected CO2. A mass estimation of the CO2 imaged by the seismic measurements, using petrophysical and borehole logging results showed that the seismic surveys were able to image approximately 93-95% of the injected CO2. The remaining 5-7% are assumed to be undetected within the reservoir, or dissolved in the reservoir brine and thus undetectable for seismic measurements

Cross-correlation time-lapse static corrections versus refraction static corrections on 4D land seismic CO2 monitoring

Difficulties encountered during the processing of the timelapse 3D land-seismic data at the CO2 geological storage site at Ketzin, Germany, were to a large extent attributed to changes in near-surface velocities. Two workflows for processing of the 4D data were tested. The first one included re-calculation of the refraction static corrections based on new information about the near-surface from first breaks. This workflow showed that the near-surface changes could only imperfectly be resolved by new refraction static corrections. The second workflow included cross-correlation of the traces acquired at the same locations but during different campaigns and calculation of the prestack time shifts between the surveys. Both workflows demonstrated their capability to minimize the time-lapse noise and enhance the time-lapse reservoir signature. They provide similar time-lapse results, except that the cross-correlation workflow is quicker, more accurate and displays less time-lapse noise