Rocks have both mechanical and acoustic properties, and there exist inherent relations among them. The characteristics of ultrasonic waves (UWs) change when passing through rocks, and the UWs carry the structural information of rocks. Thus, the interior properties of rocks can be obtained by analyzing the received UWs. Nowadays, the hydraulic properties of rocks have become a new focus in the engineering field. For example, in oil and gas exploration, the water content affects the density and strength of reservoir rocks. The analysis results of the reservoir structure are directly affected by the varied amplitude and velocity of exploration waves. In rock engineering, such as solution mining, long-distance tunnels, and reservoir bank slopes, the pore water affects the stability of rocks and even threatens the safety of engineering projects. Therefore, it is significant to study the ultrasonic propagation characteristics of rocks during water absorption and softening.

At present, a common method to detect the water content is to employ the piezoelectric transducer (PZT). However, the PZT has some inherent drawbacks, such as large size, narrow bandwidth, and low resistance to electromagnetic disturbance, which decreases the detection resolution and brings large deviations. Optical fiber sensors feature compact size, high sensitivity, broadband response, and sound resistance to electromagnetic interference. The most commonly employed optical fiber sensors in ultrasonic detection are Fabry-Perot interferometer (FPI) and fiber Bragg grating (FBG). The FPI sensors usually suffer from the low-reflection reflectors and the FBG encounter difficulties when utilized with high-frequency UWs. Fortunately, the optical fiber FPI constructed with two FBGs combines the advantages of both FPI and FBG and becomes the preferred solution in ultrasonic rock water content detection.

We propose a new ultrasonic method based on an FBG-FPI optical fiber sensor for water-content detection in rocks. In experiments, red sandstone is employed as the detecting object (cylinder, 80 mm×100 mm). The 1 MHz longitudinal pulsed wave emitted by PZT is adopted as the ultrasonic source. The transmitted UWs are detected by a pair of fiber gratings inscribed into a thin core fiber (TCF). The UWs velocity can be calculated by measuring the transmission distance and flight time inside the rock. The method of fast Fourier transform (FFT) is leveraged to convert time-domain signals into frequency-domain ones. For the frequency-domain results, the main frequency and the normalized amplitude are extracted respectively. By employing the fitted curve between the measured UWs velocity and the rock water variation, the water content is reconstructed, and an average detection deviation is obtained simultaneously. Additionally, the results measured by PZT are also recorded for comparison in identical conditions.

The experiment results show that in the longitudinal wave conditions, the wave velocity of the red sandstone first decreases and then increases with the rising water content, while the main frequency and corresponding amplitude both decrease with the increasing water content. When the water content increases from 0 to 0.16%, the wave velocity measured by the optical fiber sensor (or PZT) decreases from 3440.86 m/s (or 3691.74 m/s) to 3389.83 m/s (or 3681.55 m/s). When the water content rises from 0.16% to 2.33%, the wave velocity measured by the optical fiber sensor (or PZT) increases from 3389.83 m/s (or 3681.55 m/s) to 4020.10 m/s (or 3980.10 m/s) (Fig. 5). When the water content increases from 0 to 2.33%, the main frequency measured by the optical fiber sensor (or PZT) decreases from 1.000 MHz (or 0.987 MHz) to 0.933 MHz (or 0.887 MHz), and the normalized amplitude reduces from 1.000 (or 1.000) to 0.058 (or 0.040) (Fig. 6). The optical fiber sensor and PZT are found to exhibit the similar response tendency with the changing water content. After water content reconstruction, an average absolute deviation between the optical fiber sensor (or PZT) measurement results and the actual values is approximately 0.055 (or 0.069) (Fig. 7). It is shown that the deviation of the FBG-FPI optical fiber sensor is smaller, which proves the optical fiber ultrasonic detection feasibility of rock water.

A new optical fiber method is proposed for the ultrasonic detection of water content in rock mass. The time-domain and frequency-domain results are obtained using an FBG-FPI optical fiber sensor by ultrasonic transmission method. In the comparative experiments, the FBG-FPI optical fiber sensor presents a similar response tendency to PZT with increasing water content. Additionally, the FBG-FPI optical fiber sensor has a smaller detection deviation than that of PZT. Furthermore, laser ultrasound can be employed as a broadband source to replace piezoelectric excitation and helps to improve the detection resolution with the broadband response of optical fiber sensors.