A telescope based on Scintillator technology for assessing massive rock falls – la reunion island

A telescope based on scintillators technology has been installed at the footstep of giant cliffs for assessing massive rocks falls. Such an experience is a first in the “world of muography”. It was made possible thanks to the national geological service, the BRGM, in particular its regional branch based in Reunion Island. Muography was chosen because it allows to access the density variation in time and space, in a passive way, by collecting in the telescope the muons which crossed the rock. The telescope has been installed for up to 6 months at the footwall of the giant cliff.  The rainy season was chosen as the acquisition window to be able to follow the density variations that occur in the massif during rainy events.

The telescope is composed of 3 parallel ~1m² active detection planes recording the positions and the precise time of the particles hits. The detector readout has been developed on the early concept of connected “smart sensors”. It allows an optimized selection of the particles hits to perform their tracking. A post-processing analysis will translate the recorded tracking properties into a detailed image of the target within the acceptance of the detector.

La Reunion Island, located East of Madagascar, is composed of three shield volcanoes among with la Fournaise which is still active. The volcanic cirques are subjected to large-scale rock-falls (>10,000 m³). 

On the top of the wall (or Rempart), the target of our muography experiment, the decompression cracks are concentrated on a strip often equivalent to 10% of the height of the cliff. The cliff can be higher than 1,000 m. These cracks, sometimes more than a meter wide, delimit the rock scales likely to be crumbled. The origin of these cracks, which are almost vertical on the surface, is linked to the natural decompression of the massif by the vacuum. The geometry of these cracks at depth is not well known, but it is likely that they acquire a slightly concave shape, bringing them closer to the wall and cutting out large scales at the crest of the Rempart. The volume of rock falling highly depends on the depth of these cracks.

Our experiment is focused on the Maïdo Rempart overlooking the western part of the Cirque of Mafate where the formations of the ancient volcanic outcrop in 1,000 m high scarps. We have installed a Muons telescope at “l’Ilet de Roche-Plate”, a small village located at the foot of active scree cones at the foot of the Maïdo Rempart. This innovative experiment follows a fire that occurred on the top of the Rempart at the end of 2020, which led to an increase in falling blocks and a potential acceleration in the opening of cracks. One of the issues is to better delimit the volume of "fractured" rocks and if possible, to identify the depth of the decompression cracks that delimit the scales likely to fall.