No abstract

Regional early warning systems for landslides rely on historic data to forecast future events and to verify and improve alarms. However, databases of landslide events are often spatially biased towards roads or other infrastructure, with few reported in remote areas. In this study, we demonstrate how Google Earth Engine can be used to create multi-temporal change detection image composites with freely available Sentinel-1 and -2 satellite images, in order to improve landslide visibility and facilitate landslide detection. First, multispectral Sentinel-2 images were used to map landslides triggered by a summer rainstorm in Jølster (Norway), based on changes in the normalised difference vegetation index (NDVI) between pre- and post-event images. Pre- and post-event multi-temporal images were then created by reducing across all available images within one month before and after the landslide events, from which final change detection image composites were produced. We used the mean of backscatter intensity in co- (VV) and cross-polarisations (VH) for Sentinel-1 synthetic aperture radar (SAR) data and maximum NDVI for Sentinel-2. The NDVI-based mapping increased the number of registered events from 14 to 120, while spatial bias was decreased, from 100% of events located within 500 m of a road to 30% close to roads in the new inventory. Of the 120 landslides, 43% were also detectable in the multi-temporal SAR image composite in VV polarisation, while only the east-facing landslides were clearly visible in VH. Noise, from clouds and agriculture in Sentinel-2, and speckle in Sentinel-1, was reduced using the multi-temporal composite approaches, improving landslide visibility without compromising spatial resolution. Our results indicate that manual or automated landslide detection could be significantly improved with multi-temporal image composites using freely available earth observation images and Google Earth Engine, with valuable potential for improving spatial bias in landslide inventories. Using the multi-temporal satellite image composites, we observed significant improvements in landslide visibility in Jølster, compared with conventional bi-temporal change detection methods, and applied this for the first time using VV-polarised SAR data. The GEE scripts allow this procedure to be quickly repeated in new areas, which can be helpful for reducing spatial bias in landslide databases.

Nature-based solutions (NBS) are becoming increasingly important in both the EU and individual countries’ political agendas, as a sustainable means to reduce the risk posed by hydrometeorological hazards. However, as the use of NBS is increasing, a number of barriers regarding their practical implementation also become apparent. A number of review studies have summarized and classified barriers, mainly in urban settings. PHUSICOS is a Horizon 2020 Innovation Action to demonstrate the use of NBS in rural and mountain landscapes. Large-scale demonstrator case sites with several sub-projects are established in Italy, Norway and in the French and Spanish Pyrenees. The present paper describes the project’s NBS measures and their experienced barriers, some of which have resulted in full cancellation of the planned interventions. Many of the barriers experienced in rural settings have the same root causes as the ones described from urban areas, and the main barrier-creating mechanisms are institutional factors, resistance among stakeholders and technical and economic issues. The key element, however, is the lack of knowledge about the ability of NBS to deliver a series of co-benefits in addition to their risk-reducing effects and that long-term thinking is required to see the effect of many of these co-benefits.

No abstract

Gas migration into well annuli with resultant high pressures is a common problem in oil and gas production wells. On a North Sea major gas producer high pressure has been observed in the outermost annulus of several wells and anomalous pressures were observed below the gravity platform base. Gas may migrate along the well-path and into the well annulus to create pressure problems and was further suspected to continue to the surface. The source of such gas is often linked to shallow gas pockets or small reservoirs and the mechanism whereby the gas migrates into the annulus and creates excess pressure problems has often been associated with poor cementing or micro-cracking in the cement. The pressure and associated gas volumes are routinely dealt with by bleeding off the gas and/or water, but this action does not necessarily solve the problem. A detailed study was initiated to investigate if the gas migration was a hazard to well integrity and foundation stability. Work to identify the source of the gas and mechanisms involved on this North Sea platform has provided a new insight into the understanding of gas migration in sediments and overburden. A gas source exists even if no gas pockets are present, and the migration into annuli is independent of the presence and quality of cement. The gas migration as a potential hazard for the platform foundation and a possible link with a natural gas flux in the seabed and the formation of pockmarks has been checked. The time scale for gas migration is important to judge the hazard. A model is presented in which water insoluble H2 gas plays an important role. This paper gives a brief overview of the problems associated with gas migration, pressure build up in the wells and the foundation, and its relation to the formation of pockmarks in the soft clayey seabed of the Norwegian Trench. BACKGROUND AND DESCRIPTION OF PROBLEM The Troll A Platform is a huge concrete gravity base structure located in the Norwegian trench at a water depth of 305m, see Figure 1. The platform was installed in 1995 and in the following two years 40 wells were installed, 39 gas production wells and 1 monitoring well. A typical well design is shown in Figure 2. The platform is the North Sea largest gas producer with an average yearly production in the order of 26 GSm3 (923 billion cu.ft). After a few years of production it was realized that pressure bleed-off activity from outer annuli was high, for some wells 10-20 bleed-offs every month. It is standard practice to set safe threshold values for pressure in well annuli and if this value is reached, pressures are reduced by bleeding off gas. The threshold for outer annulus was 6 bar, and this was raised to 15 bar after an evaluation of wellhead seals in 2001. This action reduced the problem of very high bleed-off activity to more normal conditions, typically once a month for an average well.