Karteller i Sverige : En analys av fällda karteller

Validerat; 20131124 (global_studentproject_submitter

Two-dimensional audiomagnetotelluric and magnetotelluric modelling of ore deposits : Improvements in model constraints by inclusion of borehole measurements

A combination of magnetotelluric (MT) measurements on the surface and in boreholes (without metal casing) can be expected to enhance resolution and reduce the ambiguity in models of electrical resistivity derived from MT surface measurements alone. In order to quantify potential improvement in inversion models and to aid design of electromagnetic (EM) borehole sensors, we considered two synthetic 2D models containing ore bodies down to 3000 m depth (the first with two dipping conductors in resistive crystalline host rock and the second with three mineralisation zones in a sedimentary succession exhibiting only moderate resistivity contrasts). We computed 2D inversion models from the forward responses based on combinations of surface impedance measurements and borehole measurements such as (1) skin-effect transfer functions relating horizontal magnetic fields at depth to those on the surface, (2) vertical magnetic transfer functions relating vertical magnetic fields at depth to horizontal magnetic fields on the surface and (3) vertical electric transfer functions relating vertical electric fields at depth to horizontal magnetic fields on the surface. Whereas skin-effect transfer functions are sensitive to the resistivity of the background medium and 2D anomalies, the vertical magnetic and electric field transfer functions have the disadvantage that they are comparatively insensitive to the resistivity of the layered background medium. This insensitivity introduces convergence problems in the inversion of data from structures with strong 2D resistivity contrasts. Hence, we adjusted the inversion approach to a three-step procedure, where (1) an initial inversion model is computed from surface impedance measurements, (2) this inversion model from surface impedances is used as the initial model for a joint inversion of surface impedances and skin-effect transfer functions and (3) the joint inversion model derived from the surface impedances and skin-effect transfer functions is used as the initial model for the inversion of the surface impedances, skin-effect transfer functions and vertical magnetic and electric transfer functions. For both synthetic examples, the inversion models resulting from surface and borehole measurements have higher similarity to the true models than models computed exclusively from surface measurements. However, the most prominent improvements were obtained for the first example, in which a deep small-sized ore body is more easily distinguished from a shallow main ore body penetrated by a borehole and the extent of the shadow zone (a conductive artefact) underneath the main conductor is strongly reduced. Formal model error and resolution analysis demonstrated that predominantly the skin-effect transfer functions improve model resolution at depth below the sensors and at distance of similar to 300-1000 m laterally off a borehole, whereas the vertical electric and magnetic transfer functions improve resolution along the borehole and in its immediate vicinity. Furthermore, we studied the signal levels at depth and provided specifications of borehole magnetic and electric field sensors to be developed in a future project. Our results suggest that three-component SQUID and fluxgate magnetometers should be developed to facilitate borehole MT measurements at signal frequencies above and below 1 Hz, respectively.Innovative Deep Exploration (INDEX

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

3D seismic survey at the Millennium uranium deposit, Saskatchewan, Canada: Mapping depth to basement and imaging post-Athabasca structure near the orebody

Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the ~6.5 km 2 survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wide-angle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial. The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low.Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration

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

Comparison of surface seismic sources at the CO<inf>2</inf> SINK site, Ketzin, Germany

In 2004 three seismic surface sources (VIBSIST, accelerated weight drop and MiniVib) were tested in a pilot study at the Ketzin test site, Germany, a study site for geological storage of CO2 (EU project CO2 SINK). The main objectives of this pilot study were to 1) evaluate the response of the Ketzin site to reflection seismics, especially at the planned injection depth, 2) test different acquisition parameters and 3) use the results to guide the planning of the 3D survey. As part of these objectives, we emphasize the source performance comparison in this study. The sources were tested along two perpendicular lines of 2.4 km length each. Data were acquired by shooting at all stations (source and receiver spacing of 20 m) on both lines, allowing common-midpoint stacked sections to be produced. The sources' signal characteristics based on signal-to-noise ratio, signal penetration and frequency content of raw shot records were analysed and stacked sections were compared. The results show that all three surface sources are suitable for reflection seismic studies down to a depth of about 1 km and provide enough bandwidth for resolving the geological targets at the site, i.e., the Weser and Stuttgart Formations. Near surface conditions, especially a thick weathering layer present in this particular area, strongly influence the data quality, as indicated by the difference in reflectivity and signal-to-noise ratio of the two common-midpoint lines. The stacked sections of the MiniVib source show the highest frequency signals down to about 500 ms traveltime (approximately 500 m depth) but also the shallowest signal penetration depth. The VIBSIST source generates signals with the highest signal-to-noise ratio and greatest signal penetration depth of the tested sources. In particular, reflections below 900 ms (approximately 1 km depth) are best imaged by the VIBSIST source. The weight drop performance lies in between these two sources and might be recommended as an appropriate source for a 3D survey at this site because of the shorter production time compared to the VIBSIST and MiniVib sources. © 2009 European Association of Geoscientists & Engineers

Experimental estimation of velocities and anisotropy of a series of Swedish crystalline rocks and ores

To provide a guide for future deep (<1.5 km) seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of Sweden, more than 50 rock and ore samples were collected and measured for their seismic velocities. The samples are geographically from the northern and central parts of Sweden, ranging from metallic ore deposits, meta-volcanic and meta-intrusive rocks to deformed and metamorphosed rocks. First, ultrasonic measurements of P- and S-wave velocities at both atmospheric and elevated pressures, using 0.5 MHz P- and S-wave transducers were conducted. The ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low P- and S-wave velocities. The low P- and S-wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from Norway, which shows significantly higher P- and S-wave velocities.Later, an iron ore sample from the central part of Sweden was measured using a low-frequency (0.1–50 Hz) apparatus to provide comparison with the ultrasonic velocity measurements. The low-frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation. The iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods. This is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of Sweden. Finally, a laser-interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone. Up to 10% velocity-anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser-interferometery measurements. However, the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone

Quantitative assessment of seismic source performance: Feasibility of small and affordable seismic sources for long term monitoring at the Ketzin CO2 storage site, Germany

We apply a range of quantitative pre-stack analysis techniques to assess the feasibility of using smaller and cheaper seismic sources, than those currently used at the Ketzin CO2 storage site. Results from two smaller land sources are presented alongside those from a larger, more powerful source, typically utilized for seismic acquisition at the Ketzin. The geological target for the study is the Triassic Stuttgart Formation which contains a saline aquifer currently used for CO2 storage. The reservoir lies at a depth of approximately 630m, equivalent to a travel time of 500ms along the study profile. The three sources discussed in the study are the Vibsist 3000, Vibsist 500 (using industrial hydraulic driven concrete breaking hammers) and a drop hammer source.Data were collected for the comparison using the three sources in 2011, 2012 and 2013 along a 984m long line with 24m receiver spacing and 12m shot spacing. Initially a quantitative analysis is performed of the noise levels between the 3 surveys. The raw shot gathers are then analyzed quantitatively to investigate the relative energy output, signal to noise ratio, penetration depth, repeatability and frequency content for the different sources. The performance of the sources is also assessed based on stacked seismic sections. Based on the results from this study it appears that both of the smaller sources are capable of producing good images of the target reservoir and can both be considered suitable as lower cost, less invasive sources for use at the Ketzin site or other shallow CO2 storage projects. Finally, the results from the various pre-stack analysis techniques are discussed in terms of how representative they are of the final stacked sections. © 2014