## 2 Experimental Set Up and Test Procedure  
### Experimental Setup  
One Berea and one Tako sandstone core samples, both measuring 100 mm in length and 50 mm in diameter, are used for strain measurement. The Berea cylindrical sample is cored perpendicular to the bedding plane. Unlike the Berea core sample which is relatively homogeneous, the Tako sample is composed of a lower coarse-grained layer and an upper fine-grained layer (see Figure S1 in the Supplement). After drying the sample, an optical fiber (single-mode optical fiber element) is attached to peripheral of the sample (using Araldite and curing for 3 h). In addition, four cross-type strain gauges, each with two 1 cm long arms, are also attached to the sample (using Araldite and curing for 12 h), with one of the arms (channels) aligned along the length of the optical fiber. The instrumented sample is coated with silicone rubber to prevent oil in the high-pressure vessel penetrating into the sample during the experiment (see Figure S1 in the Supplement).  
Figure 2 presents a schematic showing the position of the four strain gauges attached to the two samples, and in relation to the optic fiber. Each strain gauge has two arms (channels). Channels 1, 3, 5 and 7 (Ch1, Ch3, Ch5 and Ch7) are aligned with the direction of optic fiber, while channels 2, 4, 6 and 8 (Ch2, Ch4, Ch6 and Ch8) are orientated in the near-vertical direction. Also shown are the position of the selected points, 11 for the Berea sample and 14 for the Tako sample, where the frequency shifts are monitored at each confining/pore pressure step. Among the 14 monitoring points for the Tako sample, points 1 to 7 are located in the lower coarse grain region, with the rest (points 8 to 14) in the upper fine grain region. Similarly, the four strain gauges are also split into two groups: Ch 1 to Ch4 in the coarse grain portion, and Ch5 to Ch8 are in the fine grain portion.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Brillouin-Based Sensing** & **Rayleigh-Based Sensing** \\ \hline  & BOTDA & BOTDR & TW-COTR \\  & PPP-BOTDA & S-BOTDR & \\ \hline  & Double end & Single & Single \\ \hline Minimum spatial resolution & 2 cm & 10 cm & 2 cm \\ \hline Resolution & 5 \(\mu\epsilon/0.5\)\({}^{\circ}\)C & 5 \(\mu\epsilon/0.5\)\({}^{\circ}\)C & 0.5 \(\mu\epsilon/0.05\)\({}^{\circ}\)C \\ \hline Measurement time & 5 \(\mu\)62000 \(\mu\epsilon\) & 40 s@10,000 \(\mu\epsilon\) & 60 s/2000 \(\mu\epsilon\) \\ \hline Maximum measurement distance & 20 km@1 m & 20 km@1 m & 20 km@20 cm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of Brillouin and Rayleigh.  
Figure 3 shows a schematic diagram of the experimental setup, which features a high-pressure vessel where the instrumented sandstone sample is placed. In addition to being attached to the rock sample (sensor region A), free-standing optic fiber sensors are also employed, inside (sensor region B) as well as outside (sensor region C) the vessel to provide two different baseline readings. All the sensors are enclosed in a container where a constant temperature is maintained. The strain gauges are connected to a data logger, and the strain measurements are recorded on the computer via RS-232C interface. The optical fiber with sub-centimeter bending radius was attached to the measurement sample and then it was connected to the optical interrogator NBX-7020. The frequency shifts measured by NBX-7020 are recorded on another computer.