# FBG-Based Monitoring of Geohazards: Current Status and Trends
## 1 Introduction  
Natural disasters occur from time to time all over the world. Earthquakes, tsunamis and volcanic eruptions have caused abundant loss of life and properties in the past few decades. Due to these natural processes, landslides, rockfalls, debris flows, and surface collapses have been triggered. More importantly, under the impact of accelerated industrialization and urbanization, anthropogenic geohazards such as land subsidence and earth fissures in urban areas are growing more serious for communities and infrastructures. The mitigation of these geohazards is a pressing concern for engineering geologists and geotechnical engineers, especially in developing zones and countries. Geomaterials are natural products of geological processes. Their properties vary continuously in response to geoenvironmental changes and human activities. In order to effectively reduce or prevent the loss of property and human life, real-time monitoring and early warning of potential geohazards are of crucial importance. Since 1930s, geotechnical instrumentation has contributed much to investigate the triggering mechanism of geohazards and to ensure the safety of geotechnical projects. Based on this approach, analytical and numerical geomechanical models were validated, and the effectiveness of stabilizing measures were evaluated. However, as the conventional geotechnical monitoring techniques are normally based on optical, mechanical, hydraulic or electrical sensing elements, there are some inherent limitations [1]. The total station, interferometric synthetic aperture radar (InSAR), and global positioning system (GPS) are widely used ground surface movement detection technologies. They canmeasure local and regional displacements, but the measuring accuracy is highly dependent on weather and vegetation conditions. In engineering practice, these remote sensing systems are usually used in conjunction with contact monitoring methods. Geotechnical instruments for subsurface movement monitoring, such as inclinometers and extensometers, are incapable of large-scale and long-distance monitoring. For instance, due to the difficulty of arranging data communication cables in the borehole, an in-place inclinometer can hardly measure the slope displacement at a number of depths [2]. Most sensors for measuring earth pressure, pore water pressure, ground temperature, and vibration are point (discrete) sensors. The readings taken in the field are often sensitive to electromagnetic interference (EMI) [3]. The quality of waterproof measures also significantly affects the durability (long-term performance) of these sensors [4]. In order to capture dynamic deformations and evaluate stability conditions of geological structures in real time, it is of great importance to develop an improved smart monitoring system which can overcome the above shortcomings. Only in this way can we forecast geohazard risks and perform early warning with confidence. In recent years, several fiber optic sensing (FOS) technologies have been developed for monitoring civil infrastructures, such as fiber Bragg grating (FBG), optical time domain reflectometry (OTDR), Raman optical time domain reflectometry (ROTDR), Brillouin optical time domain reflectometry (BOTDR), Brillouin optical time domain analysis (BOTDA), and Brillouin optical frequency domain analysis (BOFDA). Compared with conventional sensors, fiber optic sensors offer a variety of benefits including immunity to EMI, resistance to corrosion, high precision, miniature size and the capability of multiplexing [5]. Among these technologies, the quasi-distributed FBG is one of the most popular and widely used techniques [6]. Using FBG sensing systems, elaborate instrumentation of geohazards can be performed and the detailed information concerning local and overall stability conditions of earth structures can be collected with high reliability.