Seepage–acoustic coupling numerical simulation on heterogeneous thick sandstone aquifer formation

Accurate characterization of aquifer heterogeneity is critical for preventing water inrush hazards, yet traditional methods often lack the required resolution. To address this problem, a coupled seepage–acoustic numerical simulation framework was developed to investigate the physical mechanisms of flow-induced noise within boreholes. The influences of aquifer lithology, seepage rate, seepage location, and borehole diameter on the flow and acoustic fields were systematically analyzed. The results demonstrate that hydrodynamic noise can be decomposed into dynamic pressure (pseudo-sound) and sound pressure (radiated noise), and these two components exhibit complementary spatial characteristics. Dynamic pressure is governed by local flow velocity and reaches its maximum at the seepage location, while it decays monotonically with increasing frequency. In contrast, sound pressure reflects global pressure fluctuations and shows its minimum at the seepage location, with distinct resonant peaks appearing in the spectrum (typically 0–400 Hz). A regime transition is observed in that dynamic pressure predominates under low seepage conditions (<1.0 m3/h), whereas sound pressure becomes significantly enhanced and eventually dominant under high seepage conditions (>1.45 m3/h), although the fundamental frequency characteristics remain unchanged. Lithological variations and seepage position primarily affect the signal amplitude rather than the frequency consistent with Strouhal number scaling, where finer grains together with proximity to the seepage point generate higher amplitudes. Furthermore, borehole diameter exerts opposite effects on the two components. A smaller diameter increases the radiated sound pressure because of higher average velocity, while a larger diameter enhances the local dynamic pressure due to the stabilization of vortices.