Application of high-density electrical method in karst exploration in Liannan county,Guangdong

ZHAO Libo; JIANG Shoujun; NI Zehua; TU Shiliang; HUANG Wenlong; ZHUANG Zhuohan

doi:10.11932/karst20250610

Abstract：In covered karst areas,the Quaternary overburden is connected to underlying karst cavities and fissure networks,making these regions susceptible to karst ground collapses due to natural or anthropogenic factors.The northwestern Guangdong region is widely distributed with extensive carbonate rock formations where karst processes are prominent.In recent years,the increasing frequency of extreme climate events has intensified the occurrence of shallow covered karst collapse disasters in this area.These disasters are widespread,highly concealed,and sudden,posing a significant threat to the safety of lives and property.Therefore,studying the geological structural characteristics of potential karst ground collapse zones is of considerable practical significance and profound social value. The high-density resistivity method comprises a set of direct current(DC)electrical exploration techniques that exploit the differences in electrical properties between subsurface targets and surrounding media.This method involves artificially establishing a stable DC electric field underground and performing scanning observations using a predetermined electrode array configuration.By analyzing extensive spatial variations in resistivity within a specific subsurface area,it enables the investigation and elucidation of relevant geological issues.The high-density electrical method has proven valuable in various scenarios involving karst ground collapses.For example,in geological surveys of covered karst areas,the Wenner quadrupole array has been used to detect underground cavities,yielding results that align well with borehole data.Therefore,this study selects a typical shallow covered karst collapse-prone area in Liannan county,Qingyuan City,to investigate the distribution and development characteristics of underground karst using the high-density electrical method combined with drilling. First,the profile of a high-density electrical test was established in Meicun,Liannan county,where data were collected using both the Wenner and Schlumberger array.Inversion was performed on the respective datasets as well as on the combined array data.The inversion profiles clearly reveal significant high-resistivity zones interspersed with low-resistivity anomalies.The Wenner array inversion profile displays a relatively wide low-resistivity anomaly zone located between 185 m and 225 m along the survey line,with a lateral extent of about 40 m and a depth of about 30 m,covering an area of approximately 1,200 m2.However,the resistivity contour anomaly zone is not distinctly delineated.In contrast,the Schlumberger array inversion profile reveals a narrower anomaly zone located between 180 m and 205 m along the survey line,with a lateral extent of about 25 m and a depth of about 25 m,covering an area of approximately 625 m2.The resistivity contour anomaly zone is more clearly defined,exhibiting sharper lateral boundaries.The combined array provides a more refined depiction of the anomaly zone,better constraining the subsurface anomaly.By integrating borehole data,it was concluded that both arrays accurately reflect the low-resistivity anomaly zone.In terms of detection objectives,the Wenner array is better suited for identifying layered structures,while the Schlumberger array excels at delineating anomalous bodies.Simultaneously,joint inversion of the combined arrays leverages the strengths of each array to improve resolution.Second,analysis of the single-point apparent resistivity sounding curve extracted from borehole locations shows that the resistivity value is about 100 Ω·m at the minimum electrode spacing of 7.5 m.As the spacing increases,the resistivity gradually decreases,stabilizing at 22.5 m,followed by a turning point of increasing resistivity at 27.5 m,which is inferred to be the rock-soil interface.As the resistivity continues to rise,a V-shaped anomaly appears in the 47.5 to 57.5 m segment,likely caused by karst fissures or partially to fully filled cavities.The anomaly location on the sounding curve aligns well with borehole findings.Finally,the high-density electrical method was applied to analyze the study area. The results indicate the followings,(1)Different acquisition arrays exhibit distinct characteristics in the resistivity contours of the same target during inversion.Therefore,the optimal array should be selected based on the detection objective prior to fieldwork,and combining multiple arrays can enhance resolution.(2)Integrating high-density electrical inversion results with single-point apparent resistivity sounding curves can improve the accuracy of borehole placement.(3)The high-density electrical method can precisely delineate the rock-soil interface,measure overburden thickness,and successfully identify eight karst anomaly zones and two underground karst development belts.These geophysical inferences show strong consistency with verification borehole data.(4)This study successfully delineated eight potential karst collapse hazard zones.Overall,the research demonstrates that the high-density electrical method is highly effective in identifying cavities and karst fissure zones,providing critical and reliable support for subsequent prevention and mitigation efforts.