综合物探方法在碎屑岩地区找水中的应用——以云南宣威大路边村为例

Application of comprehensive geophysical prospecting method to water detection in the clastic rock area of Dalubian village,Xuanwei,Yunnan Province

为探索地球物理方法在碎屑地区找水应用的有效性,文章以云南宣威大路边村为研究对象,选取高密度电阻率法、联合剖面法、音频大地电磁法对碎屑岩层进行找水研究。结果表明:三叠系上统飞仙关组(T1f)碎屑岩富水性中等,含水性较均匀;受地面场地狭窄限制,高密度电法及联合剖面法测线长度有限,探测深度较浅,受碎屑岩电阻率小的影响,其找水应用效果不佳,在可探测的深度范围内较难识别基岩裂隙等找水有效异常,需结合其他物探方法甄别异常;音频大地电磁法找水应用效果相对较好,具有较高的横向分辨率,可有效识别碎屑岩中的断裂破碎带及构造裂隙带,表现为相对低阻异常特征;视电阻率单支测深曲线能有效分辨不同深度的岩土层结构,具有较高的纵向分辨率,对裂隙破碎带有较好指示意义。在地面场地较为狭窄的碎屑岩地区开展找水工作,音频大地电磁法结合视电阻单支测深曲线能取得较好效果,以音频大地电磁法确定碎屑岩地区中的裂隙发育带位置,以视电阻单支测深曲线确定裂隙发育带的深度。

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 greyishgreen 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 mediumlevel water abundance,with uniform water content.Constrained by the narrow ground,the lengths of survey lines in highdensity 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.