Expressions of signal saturation in Distributed Dynamic Strain Sensing (DDSS) and how can we address them: An example of seismo-acoustic sources at Mt. Etna, Italy.

Mt. Etna is an iconic volcanoe, not only for being the bigest one in Europe, but also due to its complex behavior, which produces a wide range of frequencies in the seismo-acoustic wavefield (0.05-100 Hz). This characteristic has made it a great example for volcano research and as a unique natural laboratory for testing new seismic instrumentation. During three months in 2019, we deployed a multi-instrument network comprising infrasound sensors and broad-band seismometers (BB). In addition, to understand the capabilities of fibre optic seinsing methods in such a complex environment, we deployed two types of fibre optic cables at 30 cm beneath non-consolidated scoria. Both fibres were simultaneously used for meassurements in Distributed Dynamic Strain Sensing (DDSS), also known as Distributed Acoustic Sensing (DAS). The first fibre was a 1.5 km standard telecom fibre, interrogated by a iDAS unit. The second fibre was a 0.5 km enginnered fibre. The standard and engineered fibres were interrogated simultaneously using an iDAS and a Carina unit, respectively. We recorded numerous seismo-acoustic events, which some of them are shown as saturated signals in the DDSS records. In this work we present methods to detect and indentify signal saturation in DDSS data. In addition, we demonstrate how the true strain-rate signal can be recovered from saturated records, and in the process, overcome the limitations of the dynamic range set by the initial adquisition parameneters during DDSS recordings. We also propose strategies to avoid saturation in future DDSS campaings based on the implementation of proper acquisition parameters.