Quantitative Analysis And Comparison Of Drilling Strata Across Buried Active Faults

Buried active faults refer to active faults that are not exposed on the surface and are buried below the surface(Geoscience Dictionary).The possible reasons for its concealment are:? the fault that cuts through the bedrock is covered by new sediments;? it is formed deep underground and the fault plane is not cut to the surface(blind fault).Buried active faults under the city,due to the high density of people and buildings,once the activity of buried fault will cause a great disaster.Therefore,it is of great practical significance to determine the accurate location and activity habits of buried active faults for earthquake prevention and disaster mitigation and land use planningShallow seismic exploration and borehole detection are the basic detection techniques for detecting buried active faults.Shallow seismic exploration mainly uses longitudinal/transverse wave reflection survey to obtain seismic time profile and reflect the underground structure,and then analyze the geometry,dislocation mode,accumulated dislocation amount,buried depth and plane position of buried faults.Drilling exploration work is generally based on geophysical exploration results,and boreholes are layoutted across buried faults.Through the drilling stratum division and comparison,a composite drilling section is formed to determine the fault spatial location,dislocation mode,and dislocation period,coseismic dislocation amount and burial depth of the upper breakpoint more accurate.Combining with the dating of core samples to determine quantitative parameters such as the fault slip rate and the latest active age.In this thesis,aiming at the problem of division and comparison of borehole strata during the detection of buried active faults,the buried section of the Lijiang-Xiaojinhe fault in the Lijiang Basin is selected as the research object.Drilling detection is carried out based on the shallow seismic time profile.Through the core cataloging,magnetic susceptibility curve,and resistivity curve,the borehole strata are divided and compared finely,and a composite drilling section across buried faults is established.This experimental study has achieved the following points(1)It reveals that there is a significant positive correlation between the sediment magnetic susceptibility and the median particle size in the adjacent layer.The sediment magnetic susceptibility and median particle size in different horizons have great dispersion,which provides a theoretical basis for the quantitative division of borehole formations and quantitative comparison of strata across boreholes using the magnetic susceptibility curve.The first phenomenon helps to use the magnetic susceptibility to carry out the lithological division of borehole formations;The second phenomenon indicates that the formations of the same age and the same particle size in adjacent boreholes have similar magnetic susceptibility,the susceptibility of formations with the same particle size in different eras may have large differences,which helps quantitative comparison of formations between boreholes(2)The magnetic susceptibility values of silt and fine sand with different horizons show large fluctuations as a whole,which is conducive to the identification of the same set of sedimentary strata in different boreholes;Clay and silty clay show the characteristics of low value and small change range,while the medium sand and gravel layer show the characteristics of high value and small change range.This feature is conducive to stratification in the same borehole.(3)The resistivity value of the borehole deposits in a shallow depth(above 15m)responds well to sedimentary lithology changes,while the resistivity value below 15m basically has no change.Therefore,resistivity can be used to help correct the results of quantitative analysis of shallow magnetic susceptibility.(4)Compared with the composite drilling section obtained based on core cataloging only,the section based on core cataloging and physical property testing is more finely divided,and the lateral comparison is more reliable and more conducive to identification fault period.(5)Combining the detection results of shallow seismic exploration profile and composite drilling section,it is considered that the concealed section of the Lijiang-Xiaojinhe fault in the Lijiang Basin has an asymmetric positive flower-like structure as a whole,but the top part in the middle of the fault zone formed a negative flower structure due to local tension effect,which faulted the Late Quaternary strata.