4.1 Brillouin Distributed Strain Sensing (BDSS)
Several different scattering principles can be used to measure strain (Kishida et al., 2014). Brillouin scattering, the inelastic scattering phenomenon described earlier, is the most common and is based on the principle that longitudinal strain (in the direction of the fiber) will produce a scattered photon whose frequency is shifted from the incident photon in linear proportion to the change in strain at the scattering site and the change in temperature at the site. From Zhang and others (2020), this can be expressed as Equation 4.
| ΔvB = CeΔε + CTΔT | (4) |
where:
| ΔvB | = | change in Brillouin frequency (T−1) |
| Δε | = | change in strain (dimensionless, LL−1) |
| ΔT | = | change in temperature (Θ) |
| Ce | = | calibrated Brillouin frequency shift-strain coefficient of the fiber (T−1) |
| CT | = | calibrated Brillouin frequency shift-temperature coefficient (Θ−1 T−1) |
Because the shift is temperature dependent, it is also necessary to measure the Raman backscatter as in DTS to compensate for any thermal effects, and therefore a Brillouin Distributed Strain Sensing (BDSS) system will have an on-board, low-resolution DTS system as well.
Strain measurements are commonly done using fiber either buried in a trench or road pavement, or a borehole. In both cases, the strain measured in the optical fiber is a function of both the true geologic strain and the mechanical coupling of the fiber to the ground. Zhang and others (2020) demonstrated that when the cable is better mechanically connected to the geologic media (i.e., the stiffer the backfill and less stiff the cable), then more representative measures of true strain in the medium are obtained. Furthermore, increasing the gauge length, i.e., the distance over which the strain is measured in the fiber optic cable, will improve the accuracy of the true strain measurement. This is due to mechanical coupling rather than signal intensity, as is the case in DTS Raman measurements.
BDSS has been widely used in slope stability and landslide monitoring. Schenato (2017) provides an extended review of applications. However, high-end geodetic monitoring of surface displacement outperforms fiber-optic-based methods when it comes to real surface displacement in the cm or greater range as such strains can cause fiber failure. The fiber-optic-based methods are optimal for detailed strain measurements (nm-mm range), which require stiff connections. Additionally, relatively low-cost, fiber-based extensometers have been tested, which change in response to displacement. They use the principle of energy loss due to micro- or macro bending (e.g., Kwon et al., 2006; Higuchi et al., 2007). Displacement of millimeters to a few centimeters can be measured using such designs.
The hydrogeologic applications of BDSS have been limited; however, deep reservoir monitoring and geothermal monitoring continue to benefit from this technology. Its cost is higher due to the need for coincident temperature monitoring. For measurement of the rate of strain, BDSS is now being surpassed by Rayleigh-based methods.