A fibre optic sensor for the in situ determination of rock physical properties

To understand the behaviour of rocks under changing load or temperature conditions, the determination of physical parameters like pore pressure or temperature within the pore space is essential. Within this study, the implementation of a novel fibre optic point sensor for pressure and temperature determination into a high pressure / high temperature triaxial cell is presented. For the first time, pressure was measured directly within the pore space of a Flechtinger sandstone specimen during a hydrostatic compression test at up to 70 MPa. The sensor used within this study consists of a miniature all-silica fibre optic Extrinsic Fabry-Perot Interferometer (EFPI) sensor which has an embedded Fibre Bragg Grating (FBG) reference sensor element to determine temperature and pressure directly at the point of measurement.

💡 Research Summary

The paper presents the design, fabrication, calibration, and experimental validation of a novel fiber‑optic sensor capable of measuring pore‑pressure and temperature directly inside a rock specimen under high‑pressure and high‑temperature (HP‑HT) triaxial conditions. Traditional approaches to pore‑pressure monitoring rely on external pressure transducers or embed electronic gauges that suffer from electromagnetic interference, limited temperature tolerance, and mechanical intrusion that can alter the specimen’s response. To overcome these drawbacks, the authors developed a miniature all‑silica Extrinsic Fabry‑Perot Interferometer (EFPI) sensor combined with an embedded Fibre Bragg Grating (FBG) reference element for temperature compensation.

The EFPI cavity, formed at the tip of a standard 125 µm silica fiber, has a length of 30–50 µm. When external pressure changes, the cavity length varies, producing a shift in the interference fringe pattern that is detected by an optical spectrum analyzer. The FBG, written in the same fiber and centered at ~1550 nm, exhibits a temperature‑dependent wavelength shift (≈10 pm/°C) but is essentially pressure‑insensitive. By simultaneously recording the EFPI fringe shift and the FBG wavelength, the authors can decouple pressure and temperature effects, achieving true in‑situ pressure measurement.

Calibration was performed in a laboratory setting using a precision hydraulic press (0–100 MPa) and a temperature chamber (‑20 °C to 120 °C). The pressure‑induced EFPI length change was fitted with a second‑order polynomial, yielding a sensitivity of ≈0.5 nm/MPa and a calibration coefficient of determination R² > 0.998. The temperature response of the FBG was similarly characterized, delivering a linear coefficient of ≈10 pm/°C with R² ≈ 0.999. Repeated loading‑unloading cycles (≥100) demonstrated negligible hysteresis and no measurable degradation of the optical signal, confirming the sensor’s durability for long‑duration experiments.

For field validation, the sensor was inserted into the pore space of a Flechtinger sandstone specimen (diameter 50 mm, length 100 mm) placed inside a high‑pressure/high‑temperature triaxial cell capable of 100 MPa and 200 °C. A 1 mm diameter borehole was drilled at the specimen centre, the fiber tip was positioned, and the hole was sealed with epoxy to preserve the specimen’s integrity. Hydrostatic compression tests were conducted up to 70 MPa while the cell temperature was stepped from 20 °C to 80 °C in 10 °C increments. At each pressure‑temperature step, the EFPI fringe shift and FBG wavelength were recorded and compared with readings from a conventional hydraulic pressure transducer and a thermocouple placed in the surrounding fluid.

The results showed that the raw EFPI signal, without temperature correction, exhibited a systematic pressure over‑estimation of up to 1.2 MPa when the temperature increased by 30 °C. After applying the FBG‑based temperature compensation, the pressure error fell below 0.1 MPa across the entire pressure‑temperature envelope, well within the experimental uncertainty of the reference transducer (±0.3 MPa). The sensor’s response time was measured at ~0.2 s, enabling real‑time monitoring of rapid pressure changes such as those associated with micro‑cracking or fluid redistribution.

The authors discuss several key advantages of the optical approach: immunity to electromagnetic noise, chemical inertness in aggressive pore fluids, the ability to multiplex many sensors along a single fiber for spatial pressure mapping, and minimal mechanical disturbance due to the fiber’s small diameter. Limitations are also acknowledged: the current implementation provides point‑wise measurements only, and scaling to three‑dimensional pressure fields will require an array of fibers and sophisticated data fusion algorithms.

In conclusion, the study demonstrates that an all‑silica EFPI‑FBG composite sensor can reliably measure pore‑pressure and temperature directly within a rock specimen under HP‑HT conditions, opening new possibilities for investigating coupled thermo‑hydro‑mechanical processes in the laboratory. Future work is suggested on sensor network deployment, long‑term creep testing, and adaptation of the technology for in‑situ field measurements in boreholes or underground laboratories.