Improved electrical resistivity tomography reveals near-surface structures beneath fumaroles at Tatun Volcano Group, Taiwan

The Tatun Volcano Group (TVG) in northern Taiwan is a potentially active volcanic system situated in close proximity to the Taipei metropolitan area and critical global infrastructure, including major semiconductor manufacturing facilities. While previous geophysical investigations have successfully delineated the TVG’s hydrothermal and magmatic reservoirs at the kilometer scale, a significant resolution gap remains regarding the near-surface (meters to hundreds of meters) structures that govern fluid migration and fumarolic activity. This study implements an improved Electrical Resistivity Tomography (ERT) approach to characterize the fine-scale subsurface architecture beneath three prominent hydrothermal sites: Dayoukeng, Matsao, and Xiaoyoukeng.

To overcome the limitations of conventional ERT in rugged volcanic terrains, we utilized a Remote Resistivity Monitoring System (R2MS) equipped with a nontraditional hybrid electrode array and dense electrode spacings of less than 10 meters. A robust bootstrapping resampling workflow was developed to process massive datasets (approximately 180,000 points per line), allowing for the generation of median resistivity profiles and the quantification of model uncertainty through the Quartile Coefficient of Variation (QCV). This statistical framework ensures that identified anomalies are data-constrained rather than inversion artifacts.

Our results reveal distinct electrical signatures associated with varying degrees of hydrothermal maturation. Beneath Dayoukeng, we identified a prominent arch-shaped low-resistivity structure (1 to 10 Ohm-m), featuring a vertical active fluid conduit that facilitates the ascent of magmatic gases. In contrast, the Matsao area exhibits more scattered and diffuse low-resistivity anomalies (2 to 10 Ohm-m), suggesting a less advanced or currently less vigorous hydrothermal pathway compared to Dayoukeng. The Xiaoyoukeng profiles demonstrate a stratified resistivity structure, where high-resistivity shallow layers (300 to 2000 Ohm-m) overlie deeper low-resistivity zones (10 to 30 Ohm-m), showing strong correlation with lithological data from Borehole W1.

A critical scientific finding is the recurring "low-over-high" resistivity pattern observed in the vicinity of active vent holes. This signature, characterized by extremely conductive altered zones overlying more competent andesitic bedrock, provides a diagnostic geoelectrical indicator for identifying subsurface gas migration pathways. Furthermore, the study identifies "immature" conduits beneath certain profiles where fluids appear trapped under impermeable rock layers, potentially increasing internal pressure.

In conclusion, this research provides a high-resolution visualization of the TVG’s shallow plumbing system, offering new insights into the spatial heterogeneity of volcanic degassing. The integration of automated R2MS acquisition with statistical uncertainty quantification establishes a reliable framework for long-term volcanic monitoring. These findings are essential for refining risk assessments and enhancing disaster preparedness for future phreatic eruptions in northern Taiwan.