| As the largest underground gas storage in China,HTB UGS was established from a depleted gas field in 2013.It is of great significance to balance the gas demand and supply through injecting or withdraw natural gas during summer and winter time respectively.It is aslo an effective solution to the natural gas shortage problem in Xinjiang especially the Northern Slope Economic Zone,and has played an important role in protecting the stable gas supply of West-to-East Natural Gas Transmission Project,and realizing the national energy strategic reserve.Located at the eastern end of the second tectonic belt of North Tianshan Piedmont depression in the southern margin of Junggar basin,Hutubi UGS belongs to the Northern Tianshan active tectonic belt,which is one of the major seismic activity zones in china with large-magnitude and high-frequency seismic activity.To assess the earthquake disaster effect and ensure safe operation of the gas storage,it is essential to study the shallow subsurface structure and the medium variations in this area.At present,the available seismic data surrounding HTB UGS is relatively minor,and we thus apply continuous seismic ambient noise into the research on the underground structure and the medium change in this area.Generally,the major work of this dissertation can be summarized as follows:(1)Background noise characteristics around Hutubi UGS in Xinjiang.To evaluate the noise level of the portable station around HTB UGS,we calculate the acceleration power spectral density of several recordings and then acquire the probability density functions within a certain period.The results show that the background noise level is close to or even exceeds the global new high noise model within the high frequency band,and some stations are even more than the global new high noise model within the whole frequency band due to the influence of ultra thick sedimentary layer and significant human activity.High frequency and microseismic noise are the main content,among which the microseismic noise is relatively low since Xinjiang is far from the sea,and the man-made noise band is wide,and even higher than the noise model,indicating that it is available to extract the high frequency Green function between two receiving points for imaging shallow structure.(2)Shallow subsurface structure imaging based on ambient nosie.The background noise around Hutubi UGS is rich in high-frequency,from which we can extract high frequency surface wave reflecting the information of shallow medium.The Empire’s Green functions between different stations are calculated by the method of ambient noise cross correlation,and Rayleigh wave dispersion curves are then obtained using the multiple filter technique.Consequently,we make an average shear wave velocity model,and a two-dimensional inversion Rayleigh group velocity distribution image.The average shear wave velocity model shows that the lithology of the near surface region varies sharply,and that the interaction of sand and mudstone layer confirms that fine velocity stratification is reasonable.An obvious low velocity layer at 420-500 m may be associated with loose mudstone layers.The shallow velocity model has an important reference value for understanding the upper median characteristics,assessing near-surface site effect,removing the shallow effect;group velocity image at different periods showed that Hutubi fault and Hutubi North fault correspond to the high and low group velocity transition zone,while no speed anomaly is observed on both sides of the Hutubi No.001 North Well fault since there is no path coverage.Besides,there are some obvious high velocity anomalies near stations at the southwest of the gas storage reservoir,inferred from the proximity of the orogenic belt.(3)The subsurface temporal variation monitoring using ambient noise.In addition to obtain the regional underground structure,we make a preliminary analysis on the underground medium change using seismic ambient noise.It is found that velocity variation may be related to the charging and discharging of HTB UGS and the variation of the underground water level.However,due to the limited length of time chosen,only one seasonal cycle is observed,so further observation is needed to analyze all possible interference factors.Seasonal changes of environmental noise sources periodically may change waveform and frequency characteristics of cross-correlation function,in fact,this change of underground medium between stations and may not be relevant.Therefore,more techniques have to be applied into wave velocity variations in order to verify the result from the cross correlation of background noise.Nearby the studied area there is an airgun signal transmitting station,which has accumulated a large number of experimental data so far.With such advantages as being highly repeated,the airgun source can detect weak velocity change.Compined passive imaging by ambient noise with this active observation system,the temporal and spatial distribution characteristics of subsurface media velocity can be accurately analyzed.(4)Preliminary study on the role of vertical borehole observations on near-surface media characteristics.The effect of downhole observation on noise reduction is studied in Dazhai well.The borehole seismometer has obvious effect on reducing noise frequency greater than 1 Hz,and especially noise near 5 Hz can be reduced by up to 40 dB.When the frequency is greater than 10 Hz,the noise level gradually decreased along with the increase of frequency.In fact,downhole observation has other unique advantages.We construct the Green function between 375 m depth to the surface,identify the incident wave and surface reflection wave,and thus obtain the average velocity structure of shallow layer by applying the regularization deconvolution method into ten local seismic events with the epicentral distance less than 150 km.Taking one seismic event as an example,the theoretical arrivals of direct wave and surface-reflected wave are proved to be consistent with the observd one by the frequency-wavenumber technique,which confirms the average velocity computed here is reliable.From the initial velocity model estimated by drilling geological data,we can see that there are a number of large wave impedance interfaces near surface.When the seismic wave propagates from the base to the surface in the borehole,several reflections occure at the interface,while some other part of the energy continue to be up to the surface,and then back to the underground seismometer.This leads to the existence of multiple peaks between the incident wave and surface-reflected wave from the deconvolution waveforms,and the amplitude of the surface-reflected wave is relatively weaker than that of the incident wave and can not even be identified.The research has important significance for the analysis and application of vertical borehole station data around the area of Hutubi UGS in the future. |
PDF Full Text Request