Electromagnetic and acoustic responses to microstructure characteristics changes and pore water transitions in sandstones

Sandstone serves as a vital porous geologic medium in both reservoir and geotechnical engineering; however, its heterogeneous pore structures and mineralogical variations impede quantitative interpretations of the relationship between water saturation and acoustic responses. This study systematically investigated four types of sandstone using integrated electromagnetic, acoustic, and microscopic observing techniques to elucidate the multiphase coupling relationships among saturation, pore water, and P-wave velocity. An innovative modified Gassmann-Brie model (MG-B), incorporating shear modulus attenuation functions, is proposed. The results indicate that P-wave velocity displays a non-monotonic trend, initially decreasing and then increasing with saturation. Sandstones with low porosity and low clay content exhibit an inflection point at 20%, whereas those with high porosity and high clay content exhibit a delayed inflection at around 60%. The incompressibility of pore water enhances P-wave velocity in saturated sandstones, but interfacial effects of clay minerals attenuate this enhancement efficiency. Furthermore, the MG-B model accurately captures the decreasing trend of P-wave velocity and outperforms classical models in predictive accuracy. Thin water films (bound water) primarily contribute to acoustic attenuation at low saturation stage (< 20%), whereas highly connected pore networks (bulk water) facilitate optimal propagation paths at high saturation (> 60%). Importantly, transition among various interfaces, governed by microstructures and pore water types, control the acoustic response mechanism. This work offers novel insights and theoretical guidance for deep resource exploration and geological hazard assessment.