Direct monitoring of active geohazards: emerging geophysical tools for deep-water assessments

Seafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing communication links, information, and energy supplies. Despite their global importance, these networks are vulnerable to damage by a number of natural seafloor hazards, including landslides, turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high-resolution reflection seismic and side-scan sonar, are commonly employed in geohazard assessments. These conventional tools provide essential information for route planning and design; however, such surveys provide only indirect evidence of past processes and do not observe or measure the geohazard itself. As such, many numerical-based impact models lack field-scale calibration, and much uncertainty exists about the triggers, nature, and frequency of deep-water geohazards. Recent advances in technology now enable a step change in their understanding through direct monitoring. We outline some emerging monitoring tools and how they can quantify key parameters for deepwater geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on evidence from past deposits can lead to an under-representation of the geohazard events. Acoustic Doppler current profiling provides new insights into the structure of turbidity currents, whereas instrumented mobile sensors record the nature of movement at the base of those flows for the first time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed measurements of parameters such as strain across large areas of seafloor. These techniques provide valuable new measurements that will improve geohazard assessments and should be deployed in a complementary manner alongside conventional geophysical tools

Distribution and fate of methane released from submarine sources - Results of measurements using an improved in situ mass spectrometer

Methane (CH4) is the most frequent organic compound in the atmosphere and its influence on the global climate is subject of currently conducted scientific discussion. Despite its limited content in the atmosphere (1787 ppbv in 2003), it contributes to ~15 % of the global warming as a result of its 20 to 40 times higher global warming potential compared to carbon dioxide (CO2) on a 100 year timescale. One source of atmospheric methane is the release of biogenic and/or thermogenic CH4 from the oceans seafloor, which is currently one of the research priorities of the marine geosciences. These submarine sources are characterized by rising gas bubbles or diffusive methane flux into the water column. It is estimated that these point sources release a total of ~30 Tg CH4 per year into the ocean, and after its biological oxidation or dissolving in the water, ~10 Tg CH4 are released into the atmosphere per year. Additionally, due to the warming of the oceans, an increasing release of methane can be expected as a result of the melting of permafrost and gas hydrates. Steep gradients over very short distances (< 20 m) and high time-based variability (few hours) are known from dissolved methane concentrations in the water column above these submarine CH4 sources. Due to the limited number of samples taken by conventional ex situ methods, an accurate quantification of the methane distribution could hardly be estimated. Nevertheless, one objective of the present thesis was the detailed spatial representation of the dissolved CH4 in the water column originates from submarine seeps as well as the study of relevant pathways such as vertical or horizontal transport, dilution and its microbial oxidation. Therefore, the first part of the dissertation deals with the optimization and establishment of a novel underwater mass spectrometer (UWMS, Inspectr200-200, Applied Microsystems Limited ) designed for inline, real time and in situ sampling in high frequency. Analysis and evaluation of several thousand samples per day take place in one step, so that one obtains the measurement result in situ and, unlike using conventional methods, without delay, and thus the sampling strategies can be adapted to the existing environment. Additionally, through the use of this novel analytical tool, potential sources of errors that occur during sampling or transport to the laboratories are eliminated. In order to be able to use the potential of this mass spectrometer for scientific research questions, it was necessary to optimize the detection limit for the trace gases that were to be determined. For this purpose, a Stirling cooler was applied, which serves as a trapping system for water vapour and thus leads to optimized conditions for the analysis. Within the framework of this thesis two gas ebullition areas were studied in detail. While one, which is located in the continental shelf northwest of Spitsbergen, is in the center of scientific attention, the gas ebullition area that was studied in the North Sea has not yet been examined until now with regard to the methane release into the water column and its subsequent pathways. With the help of the optimized mass spectrometer it became possible for the first time to obtain distribution patterns of dissolved CH4 in the water column in high resolution. With respect to the geochemical functionality of these increasingly important methane sources, the research conducted in this dissertation contribute to improve our knowledge of the entry of CH4 into the water column as well as its fate. Therefore, the applied novel technique can contribute to revolutionize our understanding of the behavior of seep plumes as suggested by Judd and Hovland (2007)

Anthropogenic emissions of greenhouse gases from the seafloor into the North Sea

The present thesis provides the first methane (CH4) emissions measurements from offshore abandoned wells in the North Sea and found that boreholes constitute unrecognized, but important conduits for the release of biogenic CH4 originating from shallow gas accumulations (<1,000 m sediment depth) in the overburden of deep reservoirs. Such kind of leakages is not currently considered in any regulatory framework or greenhouse gas emission inventory. In the North Sea and in other hydrocarbon-prolific areas of the world shallow gas pockets are frequently observed in the sedimentary overburden above the deep hydrocarbon reservoirs and aggregate emissions along numerous wells may be significant. This conclusion also has important implications for Carbon Capture and Storage (CCS) since it implies that leakage from a carbon dioxide (CO2) storage site can potentially occur along any type of well (production, exploration, or abandoned), as long as it penetrates the subsurface CO2 plume. The second part of this thesis focuses on the Sleipner CO2 storage site in the Central North Sea and investigates hypothetic, but probably realistic leakage of CO2 along a well that penetrates the subsurface CO2 plume and leaks into the ~80 m deep water column, using a combination of experimental field data and numerical modelling. Small footprints of CO2 leakage in seawater imply that the environmental consequences of a single well leaking CO2 are insignificant and finding those leaks may pose challenging. Considering the millions of oil and gas wells drilled world-wide and the prospective implementation of CCS at a scale that would have significant impact on global CO2 emissions, this thesis stresses that pressure-based testing of well integrity is not sufficient for identifying and quantifying gas emissions (CH4 or CO2) along hydrocarbon wells. Improving our surveying and monitoring efforts and adapting the respective regulatory frameworks (national and international) is important