Abstract
In this paper, we present the results of eight years of continuous monitoring with a ground-based, interferometric, real-aperture radar of two unstable mountain blocks at Tafjord on the western coast of Norway. A real-time, interferometric, ground-based radar has the capability to provide high accuracy range measurements by using the phase of the transmitted signal, thus achieving sub-millimeter accuracy when a sufficient signal-to-noise level is present. The main challenge with long term monitoring is the variations in radio refractivity caused by changes in the atmosphere. The range variations caused by refractive changes in the atmosphere are corrected using meteorological data. We use triangular corner reflectors as references to improve the signal-to-clutter ratio and improve the accuracy of the measurements. We have also shown that by using differential interferometry, a significant part of the variation caused by radio refractivity variations is removed. The overall reduction in path length variation when using differential interferometry varies from 27 to 164 times depending on the radar-to-reflector path length. The measurements reveal cyclic seasonal variations, which are coherent with air temperature. The results show that radar measurements are as accurate as data from in situ instruments like extensometers and crack meters, making it possible to monitor inaccessible areas. The total measured displacement is between 1.2 mm and 4.7 mm for the two monitored mountain blocks.
Highlights
High precipitation, erosion, and temperature variation or extreme stresses from earthquakes can trigger rockslides [1]
We have presented eight years of mountain monitoring with a real-time differential interferometric ground-based radar
We have shown that the radar can produce reliable, long-time, weather-independent output and that results from the radar measurements are as accurate as in situ measurements with geotechnical instruments, making it possible to remotely monitor inaccessible areas with high accuracy
Summary
Erosion, and temperature variation or extreme stresses from earthquakes can trigger rockslides [1]. Geologists from the NVE have identified two potentially unstable blocks at the Hegguraksla Mountain above the fjord Tafjorden on the west coast of Norway, see Figure 1. Each of these blocks could potentially create a flood wave if one of them should fall into the fjord [2]. The block has a counter clockwise motion moving outwards at the base and inwards at the top. There is some uncertainty regarding the movements of the upper block, but it is likely to be moving outward at the base (away from the mountain), while the top has a backwards rotation that leads to vertical motion with a horizontal component. For a detailed analysis of the motion of these two instabilities, see [3] (pp. 127–131)
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