Abstract. Analyses of displacement measurements may provide valuable insight into the characteristics and behaviours of landslides. This paper demonstrates the application of statistical analysis to displacement data collected with Global Positioning System (GPS), total stations and extensometers at theÅknes rockslide site, western Norway. TheÅknes rockslide has particular interest due to the potential for catastrophic consequences if the rockslide accelerates into a rock avalanche and hits the fjord below. This would generate a tsunami in the adjacent fjord system and pose a threat to local settlements and infrastructure as well as to the many tourists visiting nearby areas. The analyses reported in this paper pay special attention to the newly available time series obtained from seven permanent GPS stations. The results from these continuously monitored GPS stations are believed to be an important contribution to the understanding of the complex displacement pattern evident from previous investigations. Results from the statistical analyses show that the displacement rates can be modelled as linear trends superimposed with periodic (sinusoidal) components. This indicates constant average displacement rates with no persistent accelerations. The annual displacement rates estimated from GPS and extensometer measurements range from a few millimetres to about 8 cm, whereas the periodical fluctuations typically have maximum amplitudes of 1-2 mm. Some interpretations of the periodical fluctuations are presented. High correlations between displacements and the groundwater level, measured in a borehole at the upper part of the slope, are evident for extensometers located across the back scarp. For the GPS control points located further down the slope, this correlation is, however, not so clear.
The years ahead will see increased petroleum-related activity in the Barents Sea, with operations far off the coast of Norway.The region is at high geomagnetic latitude in the auroral zone, and therefore, directional drilling by use of magnetic reference will experience enlarged azimuth uncertainty compared with operations in the Norwegian and North Seas. Two main contributors to azimuth uncertainty are magnetic disturbances from electric currents in the ionosphere and axial magnetic interference from the drillstring. The former is more frequent in the Barents Sea than farther south, and the effect of the latter is increased because of diminished value of the magnetic horizontal component. Wellbore directional surveying for operations on the continental shelf in the North Sea and the Norwegian Sea rely on wellestablished procedures for near-real-time magnetic monitoring by use of onshore magnetic-reference stations. The different land and sea configuration, distant offshore oil and gas fields, higher geomagnetic latitude, and different behavior of the magnetic field require the procedures to be reassessed before being applied to the Barents Sea. To reduce drilling delays, procedures must be implemented to enable efficient management of magnetic disturbances. In some areas of the Barents Sea, the management requires new equipment to be developed and tested before drilling, such as seabed magnetometer stations. One simple way to reduce drillstring interference is increasing the amount of nonmagnetic steel in the bottomhole assembly (BHA). To maintain azimuth uncertainty at an acceptable level in northern areas, it is crucial that wellbore-directional-surveying requirements are given high priority and considered early during well planning. During the development phase of an oil and gas field, the planned wells must be assigned adequate positional-uncertainty models and, if possible, be designed in a direction that minimizes the wellbore directional uncertainty.
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