Design and Building of a Cost-Effective Six-Component Optical Borehole Strainmeter

Due to their high resolution and near real-time capabilities, borehole strainmeters are a critical complement of satellite-based geodetic systems [GPS/global navigation satellite system (GNSS)] and satellite interferometry. However, commercial strainmeters remain expensive and only provide the three horizontal components of the strain tensor. We propose a novel design able to detect the six components of the 3-D strain tensor at relatively low cost. Here, we embed six compliant elastic gauges in a sphere in order to evenly sample space directions. Each gauge exhibits an amplification ratio of ~30. Interrogated by Fabry-Perot interferometers illuminated by a single laser diode (LD) through a multichannel fiber cable, these opto-mechanical systems exhibit a resolution of ~89 pm/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\surd $ </tex-math></inline-formula>Hz over a dc – 500-Hz bandwidth, ultimately providing a strainmeter resolution <1 nanostrain. A supplementary interferometric device is also implemented to detect and correct nongeometrical optical phase changes due to pressure and temperature variations. An original building method allowed us to mold fibered cement around a thin sphere equipped by the optical strain gauges. Moreover, an adjustable pressure device is integrated to allow in situ calibration in order to correct the 3-D strain tensor from borehole and cement heterogeneities. This cost-effective strainmeter was successfully installed in November 2023 in a 30-m deep borehole at the Larzac Observatory in the French Massif Central.