On the Comparison of Records from Standard and Engineered Fiber Optic Cables at Etna Volcano (Italy)

Abstract

Distributed Dynamic Strain Sensing (DDSS), also known as Distributed Acoustic Sensing (DAS), is becoming a popular tool in array seismology. A new generation of engineered fibers is being developed to improve sensitivity and reduce the noise floor in comparison to standard fibers, which are conventionally used in telecommunication networks. Nevertheless, standard fibers already have extensive coverage around the Earth’s surface, so it motivates the use of the existing infrastructure in DDSS surveys to avoid costs and logistics. In this study, we compare DDSS data from stack instances of standard multi-fiber cable with DDSS data from a co-located single-fiber engineered cable. Both cables were buried in an area located 2.5 km NE from the craters of Mt. Etna. We analyze how stacking can improve signal quality. Our findings indicate that the stack of DDSS records from five standard fiber instances, each 1.5 km long, can reduce optical noise of up to 20%. We also present an algorithm to correct artifacts in the time series that stem from dynamic range saturation. Although stacking is able to reduce optical noise, it is not sufficient for restoring the strain-rate amplitude from saturated signals in standard fiber DDSS. Nevertheless, the algorithm can restore the strain-rate amplitude from saturated DDSS signals of the engineered fiber, allowing us to exceed the dynamic range of the record. We present measurement strategies to increase the dynamic range and avoid saturation.

Fiber optic sensing for volcano monitoring and imaging volcanic processes

“…Artificial discontinuities in the refractive index distribution can be introduced in so called "engineered fibers" to increase the signal-to-noise ratio (Shatalin et al, 2021). The benefits of using engineered fibers over standard telecom fibers were illustrated at Etna volcano (Diaz-Meza et al, 2023).…”

“…On the contrary, the telecom cable in the urban environment on the Eastern flank is likely to be deployed in pre-existing cased conduits and therefore transfer of ground strain to the fiber core is less efficient (Reinsch et al, 2017). Underwater, the recorded strain may be lower due to the cable's coupling conditions with the sea floor, and its increased strength to withstand water column pressure at a depth of 2000 m and submarine currents.This prevents ground strain from being transferred to the sensing fiber (Currenti et al, 2023;Diaz-Meza et al, 2023).…”

“…To address this issue, the deployment of cables in various directions has been proposed and tested, such as the PoroTomo array (Feigl et al, 2021). However, processing data from such multi-directional cable with many short linear sections requires much more complex processing steps and array methods (Johnson & Dudgeon, 1993;Van der Ende & Ampuero, 2021;Muñoz & Soto, 2022) compared to using longer sections of aligned fiber (Currenti et al, 2021;Jousset et al, 2022;Diaz-Meza et al, 2023). Encircling expected epicenters with cables can improve hypocenter determination (e.g., Currenti et al, 2023;Klaasen et al 2023).…”

“…This approach enables stacking of the records (Zwartes & Mateeva, 2015) and enhance accuracy, positioning DDSS as a serious competitor to conventional seismometers. However, signal quality becomes less accurate as the interrogated cable lengthens, creating a trade-off (Diaz-Meza et al, 2023). Fiber technologies have been mainly focused on the ability to measure weak signals.…”

“…Saturation occurs when the strain rate exceeds the interferometric ability of the interrogator, resulting in a skip in the optical phase. In practice, it is possible to correct for the phase skip and recover strain rates exceeding the theoretical limit allowed by the interrogator, as long as the strain/strain rate is not too large, and the interrogator is properly set up (Diaz-Meza et al, 2023).…”