Application of the opposing coils transient electromagnetic method in a shallow groundwater-rich area:A case study of Xiacun Town,Xinyu City
Unfavorable geobodies such as Karsts,weak soil,and water-rich areas are extensively distributed in China.Under heavy rainfall,they are prone to geologic hazards like collapse.A severe geological collapse occurred in Xiacun Town,Yushui District,Xinyu City,near the Shanghai-Kunming high-speed railway.The space around the collapsed foundation pit was limited,with many interfer-ence sources like underground pipelines.With early signals subjected to the mutual inductance effects of receiver and transmitter coils,the conventional transient electromagnetic method exhibited low detection accuracy and anti-interference ability,encountering signifi-cant shallow blind zones.To locate unfavorable geobodies in the study area and provide suggestions for the prevention and control of ge-ologic hazards,this study innovatively applied the opposing-coils transient electromagnetic method(OCTEM),supplemented by bore-hole-based verification.The results show that:(1)The OCTEM exhibited high accuracy,as demonstrated by the high consistency be-tween the geophysical exploration results and the drilling results of the study area;(2)The low-resistivity zone spread across the study area,and the low-resistivity anomalies revealed by geophysical exploration were caused by groundwater according to borehole-based ver-ification;(3)The strata from top to bottom were composed of soft plastic silty clay,hard plastic silty clay,soft plastic silty clay,and moderately weathered limestones;(4)The subsurface micro-confined water in the collapse area surged upward,gradually eroding the soft plastic silty clay layer around the area.The static water level in the collapsed foundation pit manifested an elevation of 55.60 m,lo-cated approximately 1.4 m below the surface;(5)A groundwater channel existed under the collapse area,with soil caves formed in the limestone layer under the prolonged erosion effect of water flow;(6)Long-term groundwater extraction may expand the underground seepage zone;(7)The administrative department in charge must promptly contain groundwater in the collapse area to prevent it from further eroding the surrounding unconsolidated soil layer.
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