Optimisation of geothermal resources in urban areas

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A numerical investigation of the Somali Current during the Southwest Monsoon

The dynamics of the Somali Current system during the Southwest Monsoon are investigated using a 2½-layer numerical model that includes entrainment of cool water into the upper layer. Entrainment cools the upper layer, provides interfacial drag, and prevents the interface from surfacing in regions of strong coastal upwelling. Solutions are forced by a variety of wind stress fields in ocean basins with western boundaries oriented either meridionally or at a 45° angle. Solutions forced by southern hemisphere easterlies develop a strong coastal current south of the equator. When the western boundary is slanted, this current bends offshore at the equator and meanders back into the ocean interior. No cold wedge forms on the Somali Coast. These solutions suggest that the southern hemisphere trades are not an important forcing mechanism of the Somali Current circulation. Solutions forced by northward alongshore winds differ considerably depending on the orientation of the western boundary and the horizontal structure of the wind. When the boundary is meridional and the wind is uniform (a curl-free wind field), solutions continuously shed eddies which propagate northward along the coast and weaken. When the boundary is meridional and the wind weakens offshore, they reach a completely steady, eddy-free state with no coastal upwelling. If the boundary is slanted and the wind does not vary alongshore, solutions reach a steady state that now contains stationary gyres and cold wedges. If the boundary is slanted and the forcing is a strong wind patch confined north of the equator, the flow field slowly vacillates between single-gyre and double-gyre states. Solutions are also forced by an idealized representation of the observed alongshore wind field, consisting of two components: a moderate background field (∼1 dyn/cm2) turned on in May, and a Findlater jet (∼4 dyn/cm2) turned on gradually in June. A single gyre, the Southern Gyre, initially develops south of 4N due to the background wind, and a second gyre, the Great Whirl, develops later between 4N–9N in response to the Findlater jet. Cold wedges form on the northern flanks of both gyres. In some of the solutions, the Southern Gyre moves northward and coalesces with the Great Whirl in early September, before the monsoon begins to weaken. Thus the collapse of the two-gyre system is part of the adjustment of the model to the peak phase of the Southwest Monsoon, and is not due to a relaxation of the wind

Quantitative characterisation of deltaic and subaqueous clinoforms

AbstractClinoforms are ubiquitous deltaic, shallow-marine and continental-margin depositional morphologies, occurring over a range of spatial scales (1–104m in height). Up to four types of progressively larger-scale clinoforms may prograde synchronously along shoreline-to-abyssal plain transects, albeit at very different rates. Paired subaerial and subaqueous delta clinoforms (or ‘delta-scale compound clinoforms’), in particular, constitute a hitherto overlooked depositional model for ancient shallow-marine sandbodies. The topset-to-foreset rollovers of subaqueous deltas are developed at up to 60m water depths, such that ancient delta-scale clinoforms should not be assumed to record the position of ancient shorelines, even if they are sandstone-rich.This study analyses a large dataset of modern and ancient delta-scale, shelf-prism- and continental-margin-scale clinoforms, in order to characterise diagnostic features of different clinoform systems, and particularly of delta-scale subaqueous clinoforms. Such diagnostic criteria allow different clinoform types and their dominant grain-size characteristics to be interpreted in seismic reflection and/or sedimentological data, and prove that all clinoforms are subject to similar physical laws.The examined dataset demonstrates that progressively larger scale clinoforms are deposited in increasingly deeper waters, over progressively larger time spans. Consequently, depositional flux, sedimentation and progradation rates of continental-margin clinoforms are up to 4–6 orders of magnitude lower than those of deltas. For all clinoform types, due to strong statistical correlations between these parameters, it is now possible to calculate clinoform paleobathymetries once clinoform heights, age spans or progradation rates have been constrained.Muddy and sandy delta-scale subaqueous clinoforms show many different features, but all share four characteristics. (1) They are formed during relative sea-level stillstands (e.g., Late Holocene); (2) their stratigraphic architecture and facies character are dominated by basinal processes, and are quite uniform; (3) their plan-view morphology is shore-parallel and laterally extensive; (4) their sigmoidal cross-sectional geometry contrasts with the oblique profiles of most subaerial deltas. Holocene-age, delta-scale, sand-prone subaqueous clinoforms occur on steep (≥0.26°) and narrow (5–32km) shelves, at typical distances of 0.6–7.2km from the shoreline break. That contrasts with mud-prone subaqueous deltas, which form clinoforms on gently-sloping (0.01–0.38°), wide (23–376km) shelves, at usual distances of 7.5–125km from the shoreline. Delta-scale sand-prone subaqueous clinoforms have diagnostically steep foresets (0.7–23°). Similarly steep gradients were observed in much larger shelf-prism- and continental-margin-scale clinoforms. Gentler foreset gradients are shown by sand-prone subaerial deltas (0.1–2.7°), and mud-prone subaqueous and subaerial deltas (0.03–1.50°). Due to the lack of connections with river mouths, Holocene delta-scale sand-prone subaqueous clinoform deposits have progradation rates (1–5×102km/Myr) and unit-width depositional flux (1–15km2/Myr) that are up to 3–4 and 2–3 orders of magnitude lower, respectively, than age-equivalent input-dominated subaerial deltas and muddy subaqueous deltas. Lower progradation/aggradation ratios are reflected in a larger spread of clinoform trajectory angles (from −0.4° to +3.5°) than the very low values displayed by age-equivalent subaerial and muddy subaqueous deltas.As slowly prograding, steep, sigmoidal clinoforms are strongly suggestive of sand-prone subaqueous deltas, the Sognefjord Formation and Bridport Sand are likely Jurassic examples of this clinoform type, and host hydrocarbon reservoirs. In contrast, the Campanian Blackhawk Formation is an outcrop example of delta-scale compound clinoforms with a muddy subaqueous component

The Joint IOC (of UNESCO) and WMO collaborative effort for met-ocean services

The Joint Committee for Oceanography and Marine Meteorology (JCOMM), a joint technical commission of IOC of UNESCO and WMO, has devised a coordination mechanism for the fit-for-purpose delivery of an end-to-end system, from ocean observations to met-ocean operational services. This paper offers a complete overview of the activities carried out by JCOMM and the status of the achievements up to 2017. The JCOMM stakeholders are the WMO Members and the IOC Member States, their research and operational Institutions, which mandated JCOMM to devise an international strategy to advance toward the achievement of the United Nations Sustainable Development Goals. The three activity areas, namely the Observation Program Area-OPA, the Data Management Program Area-DMPA and the Services and Forecasting Services Program Area-SFSPA have established several expert teams to contribute to the international coordination. OPA is organized in observing networks connected with different observing technologies, DMPA organizes the overall near-real time and delayed mode data assembly and delivery methodology and architecture and the SFSPA coordinates the met-ocean services stemming out of observations and data management. The future developments should strengthen the coordination in the three program areas considering the inclusion of new and emergent observing technologies, the interoperability of met-ocean data assembly centers and the establishment of efficient research to operations protocols, as well as better fit-for-purpose customized services for the public and private sectors

Transfer across the air-sea interface

The efficiency of transfer of gases and particles across the air-sea interface is controlled by several physical, biological and chemical processes in the atmosphere and water which are described here (including waves, large- and small-scale turbulence, bubbles, sea spray, rain and surface films). For a deeper understanding of relevant transport mechanisms, several models have been developed, ranging from conceptual models to numerical models. Most frequently the transfer is described by various functional dependencies of the wind speed, but more detailed descriptions need additional information. The study of gas transfer mechanisms uses a variety of experimental methods ranging from laboratory studies to carbon budgets, mass balance methods, micrometeorological techniques and thermographic techniques. Different methods resolve the transfer at different scales of time and space; this is important to take into account when comparing different results. Air-sea transfer is relevant in a wide range of applications, for example, local and regional fluxes, global models, remote sensing and computations of global inventories. The sensitivity of global models to the description of transfer velocity is limited; it is however likely that the formulations are more important when the resolution increases and other processes in models are improved. For global flux estimates using inventories or remote sensing products the accuracy of the transfer formulation as well as the accuracy of the wind field is crucial

Seeing plants as never before

Authors of the article assert that imaging has long supported our ability to understand the inner life of plants, their development, and response to a dynamic environment. The purpose of this review was to provide the scientific community with an overview of current imaging methods, which rely variously on either nuclear magnetic resonance (NMR), mass spectrometry (MS) or infrared (IR) spectroscopy, and to present some examples of their application in order to illustrate their utility

A review of CO2 storage in view of safety and cost-effectiveness

The emissions of greenhouse gases, especially CO2, have been identified as the main contributor for global warming and climate change. Carbon capture and storage (CCS) is considered to be the most promising strategy to mitigate the anthropogenic CO2 emissions. This review aims to provide the latest developments of CO2 storage from the perspective of improving safety and economics. The mechanisms and strategies of CO2 storage, focusing on their characteristics and current status, are discussed firstly. In the second section, the strategies for assessing and ensuring the security of CO2 storage operations, including the risks assessment approach and monitoring technology associated with CO2 storage, are outlined. In addition, the engineering methods to accelerate CO2 dissolution and mineral carbonation for fixing the mobile CO2 are also compared within the second section. The third part focuses on the strategies for improving economics of CO2 storage operations, namely enhanced industrial production with CO2 storage to generate additional profit, and co-injection of CO2 with impurities to reduce the cost. Moreover, the role of multiple CCS technologies and their distribution on the mitigation of CO2 emissions in the future are summarized. This review demonstrates that CO2 storage in depleted oil and gas reservoirs could play an important role in reducing CO2 emission in the near future and CO2 storage in saline aquifers may make the biggest contribution due to its huge storage capacity. Comparing the various available strategies, CO2-enhanced oil recovery (CO2-EOR) operations are supposed to play the most important role for CO2 mitigation in the next few years, followed by CO2-enhanced gas recovery (CO2-EGR). The direct mineralization of flue gas by coal fly ash and the pH swing mineralization would be the most promising technology for the mineral sequestration of CO2. Furthermore, by accelerating the deployment of CCS projects on large scale, the government can also play its role in reducing the CO2 emissions

The influence of overburden on quantitive time-lapse seismic interpretation

Time-lapse seismic data quality has improved over the past decade, which makes dynamic interpretation of the reservoir changes possible. To push the limits of this technique further, this thesis studies the time-lapse seismic noise generated by overburden heterogeneities, as well as its influence on quantitative seismic interpretation. This is done by testing the accuracy of a multi-attribute pressure and saturation inversion method in this context to gain insight into its performance in the case of seismic acquisitions not being perfectly repeated. Extensive seismic modelling studies are conducted in order to quantify the accumulated error for three different overburden complexities. Channels in the overburden above the Nelson Field, North Sea, are found to cause errors in the time-lapse amplitudes. The magnitude of these amplitude errors decreases with increased repeatability of the monitor survey’s source and receiver positions. On average, saturation change is estimated to an accuracy of less than 6% when affected by amplitude errors only. However, these mean errors significantly increase to more than 20% if the residual time shifts caused by the channels are not removed from the seismic data. Moreover, the maximum saturation change estimation error can exceed the production induced signal locally. In addition, a major finding of this study is that the shape of the channel in conjunction with the acquisition direction has a significant impact on the spatial distribution of the errors at the reservoir level. It is also shown that the commonly used repeatability measures of NRMS or Δsource+ΔReceiver do not correlate well with the spatial distribution of areas with increased saturation change estimation error. Consequently, a layer stripping method is presented which reduces the amplitude errors caused by the overburden channel and the acquisition non-repeatability by a factor of two. Nevertheless, the limits of using post-stack data to invert for timelapse changes become apparent and, as a result, it is strongly advised to do further research into applying this method to pre-stack seismic data. Production-induced amplitude changes inside the stacked reservoirs of a deepwater West of Africa field constitute the second overburden complexity studied. These changes imprint on the lower reservoir channel and reduce the time-lapse amplitude change locally by up to 42%. Furthermore, time-lapse amplitude errors are as large as 38% in case that the velocity change inside the upper reservoir is not included in the monitor migration velocity model. In addition, an important conclusion of this study is that due to the high frequency assumption ray-tracing based seismic modelling does not perform well for cellular models such as this West of Africa example. Finite-difference modelling methods are strongly advised to be used instead. Finally, the effect of overburden changes above the highly compacting Ekofisk chalk reservoir, North Sea, is investigated by combining reservoir simulation, geomechanical and ray-tracing models. The velocity change of the overburden rocks reduces the time-lapse amplitudes at the top reservoir predominantly in the zone of vertical displacements greater than six metres. In this zone, the mean time-lapse amplitude errors in the full and far offset stack data are 9.4% and 4.23%, respectively. These errors decrease below 2.3% in areas of less than six metres vertical displacement. Consequently, the full and far offset stack amplitudes are not suited for quantitative time-lapse interpretation. The time-lapse amplitudes for the near and mid offset stacks are significantly less affected and the mean errors are smaller than 1.5% across the entire reservoir. Therefore, these two partial stacks are recommended for quantitative time-lapse interpretation. Three different overburden complexities in the North Sea and West of Africa are studied and prove to have a measurable impact on the time-lapse amplitudes. It is shown that these errors affect the ability to estimate the saturation change and in a way that is not entirely predictable from inferences using commonly used repeatability measures