Characterizing Fault Zone Hydrology Using a Coupled Geophysical and Modeling Approach

Fault zones can play a critical role in controlling small to large scale groundwater flow. Extensive studies have focused on permeability variations along faults in terms of conduit or barrier function for deep groundwater flow. However, little attempt has been made to characterize the hydrologic functions of near-surface fault zones.

When exposed to atmospheric conditions, fault zones are further disturbed by stress relief and chemical weathering, modifying their structure and generally increasing their permeability. Consequently, the fault zone, acting as a near-surface recharge or discharge zone, exerts a non-negligible influence on groundwater flow. However, identifying the hydrological function of such a fault zone remains challenging when relying solely on conventional, often non-integrated, geophysical or hydrological investigation approaches.

This study presents a multiphysics coupled strategy to characterize the groundwater flow regime around near-surface fault zone. The proposed approach is applied to an active reverse fault zone in Kamikita Plain, NE Japan, which extends for 30 km within the recharge zone of the catchment.

The multiphysics approach consists of 5 consecutive steps:

• Electrical Resistivity Tomography (ERT) Survey: A 3.8 km-long profile across the fault zone, with 20 m electrode spacing.
• Self-Potential (SP) survey: Conducted along the ERT profile.
• Rock property characterization: A 160 m deep borehole was drilled in the fault zone and physical properties were measured.
• Groundwater flow simulation of the fault zone: Using hydrogeological data, measured rock properties and a 3D geological model.
• Model evaluation: Post-processing of the groundwater flow simulation to calculate synthetic electrical resistivity and self-potential responses and comparison with observed field data.

The fault zone is identified by a sharp structural change between conductive and resistive geologic units, which also exhibit a small but shifted SP jump (+20 mV) signal. Our model evaluation process reproduces the entire ERT and SP data.

This newly proposed multiphysics approach offers a robust tool for monitoring groundwater flow in geologically complex regions, with applications in radioactive waste disposal safety, groundwater contamination management, and understanding hydrogeologic processes in tectonically active areas.

Acknowledgements: Main part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of the Nuclear Regulation Authority, Japan.