Study of the magnitudes of faint galaxies

A method is developed for calculating integrated magnitudes of faint galaxies using COSMOS mapping mode measurements of UKSTU Schmidt photographs. The intensity calibration is provided by the step filters and the zero point of the magnitude scale is taken from the photoelectric sky brightness at the time of the exposure. Tests carried out on both star and galaxy images are described. The galaxy magnitudes are isophotal, the limiting isophote being about 28 mag aresec⁻². The errors range from ±Oᵐ08 at the bright limit of m = 18 to ±Oᵐ43 at the faint limit of m = 22. The magnitudes from several hundred galaxies calculated from COSMOS mapping measurements are calibrated against the logarithm of the image area obtained from COSMOS coarse measurements. Using this calibration the magnitudes of galaxies in three areas near the south galactic pole are calculated. The total area of the survey is about 11 arcdeg² and there are some 83,000 galaxies. The limits of the sample are discussed in detail. The log N(m) relation is compared with standard Friedman cosmo- logical models. Some of these models are modified to take account of selection effects. An excess of observed faint galaxies is found which can be attributed to the effects of evolution or inhomogeneity of the universe. The effect of galaxy evolution is investigated assuming that the luminosity decreases linearly with time. The observations can be explained if, on average, the intrinsic luminosity of a galaxy decreases at the rate of about one magnitude per 10¹⁰ yr

The Effect of Aquifer/Caprock Interface on Geological Storage of CO2

AbstractThe migration of CO2 stored in deep saline aquifers depends on the morphology of the top of the aquifer. Topographical highs, such as anticlines, may trap CO2 and limit the distance migrated, or elevated ridges may provide pathways enabling CO2 to migrate further from the injector. For example, seismic data of the Utsira formation at the Sleipner storage site indicates that a branch of the CO2 plume is moving to the north [1]. It is therefore important to study the interface between the aquifer and the caprock when assessing risk as CO2 storage sites.Undulations in the top surface of an aquifer may either be caused by sedimentary structures [2], or by folding. In addition, irregularities may be generated by faulting [2]. Large-scale features are detected using seismic data (i.e. structures with amplitudes greater than 10 m), and such structures will generally be included in reservoir or aquifer models. However, smaller- scale features could also have an effect on a CO2 plume migration, and this is the topic of our study. We have conducted simulations in models with a range of top-surface morphology, and have examined the distance migrated and the amount of dissolution.The results from this study suggest that the effects of sub-seismic variations in the topography of the aquifer/caprock interface are unlikely to have a significant impact on the migration and dissolution of CO2 in a saline aquifer, compared with tilt or permeability anisotropy. The results were most sensitive to the kv/kh ratio during the injection period

The detectability of free-phase migrating CO₂:A rock physics and seismic modelling feasibility study

AbstractSubsurface monitoring is essential for the successful implementation and public acceptance of CO2 storage. Injected CO2 will need to be monitored to verify the successful containment within its intended formation, and to ensure no loss of containment within the storage complex. The ability for seismic techniques to monitor structurally trapped CO2 has been successfully demonstrated due to the changes in the acoustic properties of the reservoir produced by the displacement of brine by less dense and more compressible CO2. However, the ability for seismic methods to detect free-phase migrating CO2 is still moderately understood. In order to assess the feasibility for seismic monitoring of a migrating front, we estimate the time-lapse signal over a theoretical, clean, homogeneous sandstone reservoir through the application of a three-stage model-driven workflow consisting of fluid-flow, rock physics and seismic forward modelling. To capture the range of responses which could be encountered, two end-member fluid distribution models were used: uniform saturation and the modified patchy saturation model. Analysis of the time- lapse survey highlights the importance of determining and understanding the fluid distribution model impacting the range of velocities prior to generating and interpreting the seismic response. This change in velocity is shown to be directly related to the volume of CO2 occupying the pore-space of a migrating plume front. This highlights the fact that the detectability of a migrating front is a site specific issue which not only depends on the geophysical parameters of the seismic survey but also on the geological variations and spatial distribution in the reservoir

The impact of brine production on aquifer storage of captured CO2

This project aimed to assess the potential for brine production through dedicated wells in target Carbon Dioxide (CO2) storage formations to increase CO2 storage capacity and reduce overall cost of storage - as well as any other potential benefits for CO2 store operators associated with brine production. Brine production is proposed as a method to manage pressure in storage sites, as a corollary to water injection during hydrocarbon extraction. In the case of CO2 storage, the concept is that the production of water creates voidage to increase storage capacity and reduce the extent of pressure increase due to CO2 injection. This in turn reduces the risk of caprock failure, fault reactivation and induced seismicity. Additionally, brine production reduces the energy available to drive fluids through legacy well paths and other potential seep features. Spatial reduction in the extent of the pressure plume cuts down the area of potential drilling interference, the number of impacted legacy wells, and the area of investigation for monitoring where brine movement is a concern. This report presents findings from the entire project, and references other project reports where appropriate.This project aimed to assess the potential for brine production through dedicated wells in target Carbon Dioxide (CO2) storage formations to increase CO2 storage capacity and reduce overall cost of storage - as well as any other potential benefits for CO2 store operators associated with brine production. Brine production is proposed as a method to manage pressure in storage sites, as a corollary to water injection during hydrocarbon extraction. In the case of CO2 storage, the concept is that the production of water creates voidage to increase storage capacity and reduce the extent of pressure increase due to CO2 injection. This in turn reduces the risk of caprock failure, fault reactivation and induced seismicity. Additionally, brine production reduces the energy available to drive fluids through legacy well paths and other potential seep features. Spatial reduction in the extent of the pressure plume cuts down the area of potential drilling interference, the number of impacted legacy wells, and the area of investigation for monitoring where brine movement is a concern. This report presents findings from the entire project, and references other project reports where appropriate