<p>Online monitoring of water migration processes in two typical low-permeability (granite) and high-permeability (sandstone) rocks was carried out using the low-field nuclear magnetic resonance (NMR) testing device. Many online NMR measurement techniques, including total <i>T</i>₂, layered <i>T</i>₂, and MRI imaging, were carried out to characterize the water migration process under confining pressure and water pressure. Distinct patterns and mechanisms of water migration were observed in granite and sandstone during the seepage process. For the low-permeability Beishan granite under high seepage pressure, the water migration process lasted for a long time. The measured <i>T</i>₂, layered <i>T</i>₂, and NMR imaging showed water was gradually sucked into the granite then gradually filled the pore/micro-fractures. While for high-permeability sandstone, water was quickly passed through the sample even under low seepage pressure. The online total <i>T</i>₂ and NMR imaging results showed that water was firstly pressured into large pores, then gradually immigrated into small pores with the water seepage process in sandstone. The water migration process is controlled by capillary forces in granite dominated by elongated micro-fractures, while it is driven by the seepage pressure for sandstone with large pores. This study provides visualized results of water migration processes in typical rocks, which could support the further development of rock seepage theory.</p>