Fiber Optic Sensors

Overview

Fiber Optic (FO) strain sensors are a promising new technology for advancing the state of the art in in-situ landslide monitoring. General performance advantages include high resolution, fast sampling rate, multiplexing potential, and insensitivity to electrical disturbances such as lightning.

To extend our monitoring strategy at Randa, with the added goal of understanding the seismic response of the rockslide, we installed a Fiber Optic strain monitoring system based on long-gage Fiber Bragg Grating (FBG) sensors.

Monitoring System

Fiber Bragg gratings are short sections of an optical fiber that contain a periodic variation in the index of refraction. When illuminated with broadband light, each FBG reflects a certain wavelength while transmitting all others undisturbed. In this way many sensors can be placed in series on the same optical fiber. The reflected wavelength then changes proportionally with the FBG strain within a linear regime.

System Components

Two types of FO strain sensors have been installed: 1. Borehole sensors encased in grout at depths of 38, 40, and 68 m, and 2. Crackmeters across tension cracks at the upper back surface of the rockslide (Figure). Borehole sensors were installed to span known fractures showing normal mode offset previously identified in borehole surveys. Three borehole sensors make up one chain, and there are two identical chains (for redundancy). The borehole sensors are fully embedded in grout that extends up to 20 m depth.

The two new fiber optic crackmeters at the top of the rockslide have been placed adjacent and parallel to existing vibrating wire crackmeters to allow for direct comparison. The FO crackmeters have both strain and temperature sensing FBG’s so the strain record can be compensated for changes in the sensor temperature.

Figure 1: Fiber optic system layout.

The measurement system consists of the optical sensing interrogator (Micron Optics SM130) and processing module (Micron Optics SP130). Our model operates at a scan rate of 100 Hz. The system is packaged with software from Smartec SA (Switzerland) that allows for customizable data logging and the capability to record both continuous (static) and triggered-dynamic measurements. Our system records one data point every 5 minutes (the average of all 100 Hz data during this time), while waiting for a set trigger threshold to be exceeded. In the event the trigger level is reached, our system records 5 seconds pre-trigger plus 25 seconds post-trigger data at 100 Hz.

The resolution capabilities of the system are defined by two parameters: repeatability and stability. Repeatability refers to the agreement between successive measurements under similar conditions over a short time period, while stability takes into account the effect of changes over the full operating temperature of the device. Table 1 summarizes the sensor-specific resolution parameters based on the different sensor base lengths.

Table 1: Resolution parameters for FO borehole and crack sensors. For the measurement range + indicates elongation while - designates shortening.

Each of the two borehole sensor chains plus each of the crackmeters has a dedicated cable running to the central measurement point about 150 m distant. These stainless steel armored cables consist of an optical fiber protected with stainless steel tubing, stainless steel reinforcing fibers, and a polyamide sheath. They have high mechanical and chemical resistance and are well suited for the harsh environment at the Randa rockslide.

The measurement system is contained within the small hut at the top of the rockslide. Since there is no grid power available, all power is supplied via solar cells. Our solar system was designed to account for the limited sunshine available during the winter months, combined with the chance of prolonged storms lasting a few days. We installed a power system with a total of 1160 W available from 12 photovoltaic panels, plus 1080 Ah of battery storage capacity. Remote communication is possible via wireless modem, in order to download data or change acquisition parameters.

Example Results