Application of comprehensive geophysical prospecting method to water detection in the clastic rock area of Dalubian village,Xuanwei,Yunnan Province
The development of fractured and fissured zones characterized by strong water abundance is limited in the clastic rock aquifer with generally poor water-rich properties.Additionally,the small contrast in electrical properties between the water storage structures and the clastic rock makes water detection difficult.In order to explore the effectiveness of geophysical methods in water detection in clastic rock areas,the high-density resistivity method,combined with profile method and audio frequency magnetotelluric method(EH4)are used to study the water detection in clastic rock strata in Dalubian village,Xuanwei,Yunnan Province.The study findings may provide a theoretical basis for the selection of geophysical methods under the same site condition and the same geological background so as to avoid blind method selection and to save time and cost.The study area is located in Xuanwei county,Yunnan Province,belonging to the Beipan river basin.The terrain is low in the north and high in the south,with large undulations and a height difference of 150-250 m.The terrain falls into the type of the eroded middle-mountain trough valley.The exposed strata in the study area are purplish red or greyish-green sandy mudstone,mudstone,sandstone interbedded with limestone,mudstone,etc.of the lower Triassic Feixianguan Formation(T1f).The study area is located in the northwest wing of Tianba syncline,about 1 km away from the core.The core is composed of the Feixianguan Formation(T1f)of the lower Triassic,and the two wings are composed of the Carboniferous(C)to Permian(P)strata.The main aquifer in the study area is composed of purplish red or greyish green sandy mudstone,mudstone,and sandstone layers of the Feixianguan Formation(T1f)in the lower Triassic,with fractures as the main aquifer medium.The fractured aquifer is mainly composed of insoluble hard and brittle rock layers in clastic rocks,igneous rocks,and metamorphic rocks.Interbedded fractured aquifers,fractured aquifers,and weathered fractured aquifers are commonly present.Atmospheric precipitation infiltrates along cracks and joints to recharge groundwater,with runoff direction from south to north,and is ultimately enriched in valleys.The study area is located near the core of the Tianba syncline,with well-developed joints and fractures,and a large storage space for groundwater.The results show that the clastic rocks of Feixianguan Formation(T1f)in the upper Triassic system are at medium-level water abundance,with uniform water content.Constrained by the narrow ground,the lengths of survey lines in high-density electrical method and the combined profiling method are limited,and hence the small detecting depths.Affected by the small resistivity of clastic rocks,the effect of using high-density electrical method and combined profile method in water detection is not good,because it is difficult to identify effective anomalies such as bedrock fractures within the detectable depth range by these methods.Therefore,it is necessary for us to combine other geophysical methods to effectively distinguish the anomalies.The audio frequency magnetotelluric method with high lateral resolution can effectively identify the development sections of structural fault fracture zones and fracture zones in clastic rocks.The anomaly is characterized by relatively low resistance.The apparent resistivity sounding curve,with a high vertical resolution and good indication significance for fracture,can effectively distinguish the structure of rock and soil layers at different depths.To carry out water detection in the clastic rock area with relatively narrow ground site,the audio frequency magnetotelluric method combined with apparent resistivity sounding curve can achieve good results.The audio frequency magnetotelluric method can determine the location of fracture development zone in clastic rock areas,and apparent resistivity sounding curve can determine the depth of fracture development zone.
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