Coupled multiphysics approach to characterize groundwater flow system around a near-surface fault zone

Fault zones intensively disturb local hydrogeologic structures, and, consequently, can play a critical role in governing small to large-scale groundwater flow. Extensive studies have focused on the permeability variation along faults in the light of the conduit or barrier function for the deep groundwater flow. However, little attempt has been made to characterize the hydrological functions of near-surface fault zone.

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, which functions as a recharge or discharge zone at the near surface, exerts a non-negligible influence on groundwater flow. However, identifying the hydrological function of near-surface fault zone remains challenging when relying solely on conventional, often non-integrated, geophysical or hydrological investigation approaches.

This study proposes a multiphysics coupled strategy to understand 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 proposed multiphysics approach consists of 5 successive steps:

• 2-D Electrical Resistivity Tomography (ERT) Survey: A 2.3 km-long profile crossing the fault zone, consisting of 7 roll-along surveys with a 6-m electrode spacing.
• Self-Potential Survey: Conducted along the 2.3 km ERT profile.
• Rock Property Characterization: A 80 m deep borehole was drilled in the fault zone and physical properties were measured.
• 3-D Groundwater Flow Simulation of the Fault Zone: Utilizing areal hydrogeological data, measured rock properties, and geophysical imaging.
• Model Validation Process: Using the results from the groundwater flow simulation, electrical conductivity and self-potential responses were calculated, and compared with observed field data.

Preliminary results successfully reproduce the overall resistivity signature and the self-potential anomaly (+35 mV) in the fault zone, attributed to local groundwater upwelling. This newly proposed multiphysics approach could be an essential tool to evaluate the groundwater flow in a region including large-size fault zone, which is important for radioactive waste disposal. Furthermore, this approach could also be effective in capturing the local fluid flow circulation for a variety of applications.

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.