A Preliminary Study On Shear Wave Mutation Based On Atmospheric Energy

Anisotropy is an important feature of the earth’s internal media. Seismicanisotropy research has been a long-sought goal of geophysics over recent year, andshear-wave splitting is a primary method to obtain the properties of anisotropicmedium. In this thesis, seismic anisotropy is comprehensively discussed from theorigin of anisotropy research to its development of theories. We also introducedseveral methods which are commonly used for shear-wave splitting analysis anddescribed their characters.Air-gun is a kind of repeatable artificial source working in reservoir which canproduced low frequency shear-wave effectively. The energy of an air-gun array isequivalent to ML0.5earthquake, which makes it possible to monitor the temporal ofproperties in medium of crustal. In this study, we assumed to process single shotsignal records produced by air-gun to obtain the temporal of properties of crust inexperiment area.We processed single shot records from air-gun experiment in September2007, inShangguanhu reservoir, Hebei province, in rotation correlation and minimum energymethod, and then got fast polarization direction and slow delay of the crust mediumbeneath the stations. We found that there is a correlation between fast polarizationdirection series and delay time with solid earth tide.The preliminary conclusions of our study are as follows. First, the results of fastpolarization direction in ZUH, XIL and JIX station are of good convergence. Thedirections of fast polarization in ZUH and JIX are near EW, which is consistent withprevious research results. There is larger difference between the results obtained viarotation correlation method and minimum energy method in XIL station. We suggestthat it is due to different standard of optimal solution determining in two methods.The consistency of results makes them can be considered with a certain degree ofreliability.The result series of fast polarization direction obtained via two methods exhibit asignificant correlation with theoretical value of north-south line strain caused by solidearth tide, and less correlation with theoretical value of east-west line strain. Thevariation tendencies of fast polarization direction in three stations are consistent.When north-south line strain is compressive, the fast polarization directions changeclockwise, and north-south line strain is tensile, the directions change anticlockwise.The variation trend of fast polarization direction aroused by solid earth tide mayrepresent the property of regional crustal medium.We also analyzed the correlation between the value of barometric pressure,temperature and fast polarization direction series, and the result shows that thecorrelation is unobvious, that may because the propagation path of seismic wave is in deep enough where the cracks do not connect with atmosphere, so that the barometricpressure and temperature can hardly impact the property of the crustal medium in thepropagation path.After increasing the sampling frequency of records in JIX station, we obtainedslow delay result series with higher accuracy via minimum energy method. The resultseries of slow delay in JIX station shows positive correlation with value ofnorth-south line strain, that implies the effect of solid earth tide can also illustrated byslow delay time.The clock bias of instruments does not affect the results of shear wave splitting,which remedies the defect of wave velocity and travel time measurement. The resultsof this study prove that we can get accurate stress variation of crust by shear wavesplitting. It can measure the extent of impact for stress variation caused by solid-earthtide, temperature and barometric pressure.