Fig. 1

Instrumentation setup of the DAS-sand-fiber system. (a) Dry-sand setup, consisting of a DAS interrogator (red rectangle A), a sand-filled cylinder embedded with 2.28 m of optical fiber, and an inverted plastic bottle (yellow rectangle B) used to drip water onto the top of the cylinder. (b) Wet-sand setup, where the sealed plastic plate from the dry-sand test is replaced with a permeable plastic membrane, allowing water to enter through the cylinder base. The cylinder is mounted on a 4-cm-thick metal ring bracket, enabling upward water flow from the surrounding tank. A water pipe connected to the laboratory tap and sink forms the water input/output (I/O) system, which regulates water rise and drainage in the tank. (c) Sand–fiber system showing the start and end points (O and P) of the optical fiber (top) and the orientations of the six vertical fiber segments (bottom). Odd-numbered segments (#1, #3, and #5) descend, while even-numbered segments (#2, #4, and #6) ascend. Support points (A–E) prevent sharp bends in the fiber that could otherwise cause breakage.