Research And Application Of Microtremor Survey Method In Shallow Exploration

Shallow underground space is not only rich in mineral and geothermal resources,but also closely related to human survival and development.Microtremor is a small daily tremor on the earth's surface,which is equivalent to environmental noise.Microtremor survey method uses surface wave information in microtremor to study the S-wave velocity structure of underground.This method plays an important role in shallow surface exploration because of its advantages of non-intrusion,convenient field observation and non-electromagnetic interference.This paper focuses on the research and application of spatial autocorrelation method(SPAC)and horizontal and vertical spectral ratio method(H/V)in microtremor survey method.By processing the actual data of three areas,the application effect of microtremor survey method and the practical problems solved are illustrated.Firstly,we use SPAC method to process the microtremor data of 8 fixed stations in the capital circle and 8 mobile stations in Baiyun Gold Mine,and obtain the S-wave velocity structure of shallow crust in two regions.The results show that S-wave low-velocity layers are developed at depths of 5-8 km and 12-16 km in the Capital Circle.The upper,middle and lower crusts are clearly separated at depths of 8 km and 20 km.The results obtained from Baiyun Gold Mine in Dandong,Liaoning Province show that there are two low-velocity layers in the upper crust at depths of 1.2-1.8 km and 6.2-7.0 km,which are in good agreement with the results obtained by background noise tomography and magnetotelluric method.We obtained the three-dimensional S-wave velocity structure of the shallow layer of Baiyun Gold Mine by H/V method,and revealed the distribution of sedimentary layers in this area,which is in good agreement with the results of geological map and noise imaging.In this study,we also used the microtremor survey method to detect geothermal resources.To this end,a microtremor array observation profile with seven observation points was set up in Xiji area of Tongzhou,Beijing.Each micro-motion array observation point consists of seven instruments,which constitute the vertices and centers of nested triangles with radius of 200 m and 400 m.The synchronous observation is carried out at least 30 minutes at a sampling rate of 100 Hz.In this paper,the top interface and hidden fracture morphology of thermal reservoirs concerned in geothermal exploration are explored.The results of SPAC and H/Vmethods show that a hidden fault near Xiji-1 geothermal well with a dip of about 75 degrees and a strike to the East and West is developed.The burial depth of the top interface of thermal reservoir is more than 3000m.This result is in good agreement with the results of boreholes and magnetotelluric method.It shows that the microtremor survey exploration method is an effective tool in geothermal exploration.