Study on multi-scale geophysical detection methods for vertical dissolution fissures in limestone areas

XI Chunfei; NIU Guangtian; LI Fan; CHEN Di; CHEN Huapeng; LEI Qiguo

doi:10.11932/karst2025y025

Abstract：To address the challenges of strong concealment and high detection accuracy requirements for vertical dissolution fissures in limestone areas,this study proposes a multi-scale integrated detection system that combines high-density Electrical Resistivity Tomography(ERT)for macro-scale anomaly delineation with cross-hole electromagnetic wave CT imaging for refined characterization.Through numerical simulations comparing the vertical fissure identification capabilities of the Wenner array,three-pole array,and dipole-dipole array,the dipole-dipole array-with superior lateral resolution and noise resistance-was selected for field experiments.The ERT method rapidly delineated low-resistivity anomaly zones,while cross-hole electromagnetic wave CT imaging precisely resolved fissure development characteristics.The results demonstrate that the dipole-dipole array effectively reconstructs the spatial distribution of vertical fissures.Strong absorption anomaly zones identified by electromagnetic wave CT spatially align with ERT-derived low-resistivity anomalies.Borehole coring revealed significantly higher rock fragmentation in geophysically delineated fissure zones compared to adjacent layers,validating the reliability of the geophysical results.This integrated technical framework provides scientific support for concealed fissure identification,engineering stability evaluation,and geological hazard prevention in karst regions. This study area is located in Yichang City,Hubei Province.Geological structure and weathering/erosion processes have resulted in steep terrain with significant slopes.Well-developed solution grooves,varying in width from centimeters to meters and in depth from shallow surface etching to several meters,are evident on the rock surfaces.Ground fissures are often filled with argillaceous material.Bedrock(limestone)is extensively exposed in the area.A specific ground fissure was observed,approximately 30 meters in length,with an average width of 0.5 meters and a visible depth ranging between 0.5 to 2 meters.This fissure trends approximately N10°E and extends towards a vertical dissolution slot to the north.This slot feature is about 100 meters in height and exhibits dense vegetation growth within.Given its characteristics and proximity to the slot,it is inferred that this surface fissure may extend downward into the subsurface rock mass,prompting the implementation of our geophysical investigation here. Building upon previous research and site survey,we first constructed a digital resistivity model for forward modeling.Comparative analysis of various electrode array configurations and noise levels led to the selection of a dipole-dipole array with a minimum electrode spacing of 5 meters for the initial,broad-scale delineation of the fissure zone.Results successfully located the fissure,and a confirmation borehole drilled within the anomalous resistivity zone revealed highly fractured core material,providing strong validation for the effectiveness of the ERT method in identifying fractured regions.However,inherent limitations of the ERT method concerning site conditions and resolution meant the delineated fissure boundary remained relatively broad.To achieve a more precise characterization of the dissolution fissure's morphology and extent,cross-hole electromagnetic wave CT was subsequently conducted between the boreholes,which provided detailed imaging of the subsurface dissolution features.