In 2017, a string of eight three-component geophones was deployed in one of the vertical boreholes (62.17954 N, 6.99089 E, elevation of 673 m). The geophone spacing is 5 m; the first geophone (1) is approximately at 15 m below ground level while the deepest geophone (8) is at 50 m, i.e., just above the sliding plane which depth is estimated between 52 and 55 m at this place. Data are recorded continuously with a 1000 Hz sampling rate.
The histogram in the figure below shows the distribution of high frequency (HF) seismic events detected on the borehole over the past 30 days. Those events may be related to microcracks or shearing on the sliding plane occurring in the vicinity of the borehole string. The events are classified in eight categories ("HF_1" to "HF_8") depending on which geophone recorded the events' first onset (1 for the first level, 8 for the deepest level), providing information on the depth at which they occur. The black line represents the mean seismic energy and is indicative of the background seismic noise level. High levels of noise may affect the detectability, but large and sudden increases can also be caused by an increase of the seismic activity. The number of detections per day displayed in the figure is limited to a maximum of 100, but if this number is exceeded, this will be highlighted.
The figures below show the vertical component records of each geophone for a chosen day. Automatic detections are indicated by red stars. Events detected by the surface network are also highlighted by coloured patches.