Infiltration Monitoring Of Grottoes Overburden Based On Time-lapse ERT

The cultural relics in the grottoes are the artistic treasures of human civilization,carrying the artistic attainments of religion,culture and politics of ancient civilization,with cultural value that cannot be copied.As time goes by,it suffers from structural instability,weathering,water seepage,microorganisms and other diseases.Taking Yungang Grottoes as an example,the damage of water-rock erosion after rainfall infiltration is obvious,and the preservation of the grottoes is worrying.Rainfall infiltration is one of the main sources of water in caves.In order to protect the grottoes from water seepage diseases,it is urgent to carry out long-term monitoring research on rainfall infiltration of grottoes overburden.In view of the above problems,this paper developed a long-term automatic monitoring system by means of Time-lapse ERT,and carried out a one-year monitoring study on rainfall infiltration regularity of the overcover layer in Yungang Grottoes.The main research contents and achievements are as follows:(1)Based on the Linux operating system,the electrical instrument was realized to realize the automatic collection and transmission of resistivity data.A long-term rainfall infiltration monitoring system suiTab for field working conditions was designed,which included integrated measuring device,network transmission module,power supply device,temperature and humidity monitoring sensor and data visualization cloud platform,and was applied to field monitoring work.(2)The numerical simulation experiments based on rainfall infiltration and artificial local infiltration process proved that Time-lapse ERT could monitor the infiltration experiment process and identify the infiltration area,and determine the electrode spacing of the infiltration area under field conditions.The simulation results showed that the drastic change of shallow resistivity would lead to the appearance of artifacts under the layer.(3)A geophysical survey experiment was carried out on the overburden layer of caves 14～21 in the study area.Seven Electrical Resistivity Tomography lines were arranged and geological structure of the study area was divided by combining with field drilling data.It was found that the stratum distribution was mainly composed of clay layer,sand layer,mudstone layer and sandstone layer,and the middle-sand layer of shallow stratum with high permeability was mainly located in the middle and east of the study area.A one-year long-term monitoring experiment was carried out on the roof of the cave,and 1052 groups of resistivity data were collected.The sensor of shallow formation temperature and water content and the collection device of atmospheric rainfall are arranged.The monitoring results show that the design system can capture rainfall weather and describe the overall trend of rainfall infiltration process.(4)Based on long-term monitoring results,artificial infiltration experiments,downfall head experiments,and grain size analysis of soil samples were further carried out to obtain the rainfall infiltration rule of the overburden of Yungang Grottoes,proving that the special topography during rainfall leads to the accumulation of surface runoff in the study area,and rainwater can quickly penetrate below the surface of the overburden.However,due to the obstruction of mudstone layer and sandstone layer,When the rainwater penetrates to 4m or so,it will be discharged from the drainage channel in the east of the study area by the highly permeable sand layer,which indicates that rainfall infiltration is difficult to reach the sandstone layer of the grottoes and cannot be the main source of water in the grottoes in this area.