# Fiber-Optic System for Monitoring Pit Collapse Prevention
## 1 Introduction  
In the last 10-15 years, control and measuring instruments, devices (sensors, hardware and software components for computers, sensors, etc.) and process monitoring systems have used automated high-speed functional elements and circuits based on fiber-optic materials (FOM). They have caused significant applied scientific and technical interest for a number of branches of modern industry: laser technology, microelectronics and radio electronics, mining technologies and metallurgy, non-destructive testing technologies and resistance of materials, mechanical engineering, aircraft building and rocket science, etc., first of all, because of organizing high-quality electronic measurements with a high degree of accuracy of results (often in real time and with remote access) when monitoring the technical conditions (deformation and mechanical stress parameters) of extended large-scale objects ( dams, bridges, embankments, mining pits, etc.) [1]. A possible solution is to build systems using fiber-optic sensors. Moreover, the advantages of fiber-optic control and measuring systems have been known for a long time. For example, they are notaffected by electromagnetic interference (noise); they have high electric physical (nonlinear relaxation polarization and conductivity), optical (nonlinear electric optical phenomena) and metrological characteristics; they have distributed and quasi-distributed sensitivity; they are able to work in an explosive environment. At the same time, the measuring channels have a significant length, and the cost of fiber-optic conductors is lower than that of copper ones; every year, the cost of optical fiber (OF) is reduced, and its properties are improved. Another feature of using fiber-optic sensors (FOS) is their low power consumption and the ability to work at a considerable distance from the data processing unit, since the light signal is weakly attenuated, unlike the electric one. In addition, OFs do not oxidate [2]. OFs can be used to build distributed and quasi-distributed measuring systems for monitoring geotechnical parameters and the safety of mining operations in pits and coal mines [3], as well as the other extended objects. Security systems for perimeters and borders of various types and purposes have been developed based on the FOS [4]. FOS are actively used in the aerospace and oil and gas industries [5], since they have significant advantages over electrical sensors based on the clarity of the physical principle of operation and their positive properties, which ensure their large-scale implementation. Sufficiently serious scientific work has been carried out in the field of improving the safety of open-cast marble extraction [6]. There are image surveillance systems, Global Positioning System (GPS) technologies, ultrasonic measurements, specialized sensors and IoT-applications [7], according to the analyzed literature, that are used in order to monitor the technical condition of concrete, metal structures. Moreover, FOS are widely used [8]. The reasons for the development were the decrease of the materials cost and improvements in measurement tools [9]. The main methods of measuring external impacts include Tunable Wavelength Coherent Optical Time Domain Reflectometry (TW-COTDR), Phase Shift Pulse Brillouin Optical Time Domain Reflectometry (PSP-BOTDR), Pulse-Prepump Brillouin Optical Time Domain Analysis (PPP-BOTDA), Brillouin Optical Time Domain Analysis (BOTDA), Brillouin Optical Time Domain Reflectometry (BOTDR), Optical Frequency Domain Reflectometry (OFDR) or methods based on measuring losses in an OF caused by Brillouin scattering [10]. Moreover, the most widely used among FOS sensors are those built on Bragg gratings, with the measurement of losses by scattering of Brillouin, Raman, Rayleigh [11].