# Fiber-Optic System for Monitoring Pit Collapse Prevention
## 1 Introduction
It should be noted that the main advantage of OF is the ability to measure not only one parameter but also two interrelated parameters [12; 13], for example, displacement and mechanical stress or two parameters with a low correlation, such as temperature and mechanical stress [14; 15], and in some cases, three parameters [16]. However, this application requires large computing power or more complex designs. In the case of point-by-point measurement application, the presence of corrosion on steel reinforcement can also be monitored [17]. On the other hand, it is possible to measure the force exerted by rock displacement using a point-by-point fiber-optic sensor [18]. The advantages of this solution are high measurement accuracy, as well as compensation for signal interference caused by temperature changes. Despite this, the quality of the welding of the sensing element can greatly affect the measurement, while the welding itself complicates the mounting and dismantling processes. Another possible solution to this problem in measuring stress can be a device made in the form of a metal ring, on which Bragg gratings, a polling device and a controller based on a Field-Programmable Gate Array (FPGA) are mounted [19]. The special design of the device provides high accuracy and linear dependence of measurements; the controller provides temperature compensation. However, the above options have a high cost and a complex design. Another case of using FOS is to determine the increase of the displacements of rocks, stone slabs, for example, [12]. This solution is made in the form of a special design mounted in a metal box. The principle of operation is to measure the angle of inclination along the wavelength. The advantages are the accuracy of crack measurement, detuning from the temperature changes. The disadvantages are the design complexity and high cost. On the other hand, a temperature-compensated sensor can be considered, which consists of a cable made using a multi-mode fiber, an input and output single-mode fiber and is made in the form of a spiral [20]. The advantages of the device are an increased measurement accuracy and temperature compensation. The disadvantages are the absence of the consideration of the vibrations effect on readings, as well as high requirements for equipment to correct the error caused by temperature changes. It should be noted that the use of FOS makes it possible to monitor the integrity of mines. For example, there is information about developing a system for monitoring rock displacement and preventing the collapse of mine workings in coal mines in China [21]. Fiber-optic vertical and horizontal seismic receivers based on a Mach-Zehnder interferometer have been considered. Authors have proposed an original design of a sensitive element that provides a high-threshold sensitivity of the measuring device to seismic accelerations. Stability of the Mach-Zehnder interferometer operation is ensured using an active stabilization system for the operating point [22]. Developing their area, the authors proposed their own construction that has certain common features with those considered earlier in [23; 24]. On the other hand, increasing the sensitivity of fiber-optic interferometric sensors is achieved using a multi-turn sensitive functional element (acting element), in which the FOS is based on a two-arm Mach-Zehnder interferometer [25]. The theoretical basis for the development of long-baseline strainmeters was work [26] that presented the results of studying long-baseline strainmeters designed to monitor the mechanical states of large-scale objects. In this work, there was partially solved the important problem of reducing the fading of the strainmeter output signal with change of the external temperature. Although the use of a multi-turn sensing element in the design of FOS can significantly improve the performance of FOS in strain measurements, it was not possible to completely eliminate the temperature negative effect (the effects of thermal elasticity) on the measurement process.