Global Characteristics Of Ambient Seismic Noise

Ambient seismic noise is an important part of seismic data,which plays an important part in velocity model inversion of velocity model inversion,seismic imaging and geological structure monitoring.Ambient seismic noise can not only study the source area of the signal in a certain frequency band,but also deeply understand the evolution of wave climate.Therefore,people pay more and more attention to how to extract noise signal efficiently and better.First,we analyzing the three-component data of two broadband seismic stations OBN and M54A by two different data processing methods that frequency polarization analysis method and the probability spectral density function method.Concluding the analysis results of two different methods.We found that the variation characteristics of noise power with frequency fluctuation obtained by frequency polarization analysis method and probability spectral density function method are roughly the same,and the peak periods of double-frequency microseismic(3-8s)and single-frequency microseismic(10-16s)obtained by the two methods almost coincide.However,the probability spectral density function method is more suitable for daily monitoring of data observation quality because of its smoothing variation with frequency and short operation time.It may be caused by the probability spectral density function method is smoothed twice in the calculation process.And The frequency polarization analysis method can obtain more information about particle motion.For example,the degree of polarization(?~2),the back azimuth of the signal(?_H),the incident Angle(?_V),the phase difference between the vertical component and the horizontal component(?_VH),the phase difference between the two horizontal components(?_HH)changes with frequency.This Angle information can help us to analyze the type of signal(body wave or surface wave)and the direction of source,so as to track and study the activity of cyclone over the sea.Second,by analyzing continuous seismic data form 40 stations in the II network for3 years and ocean significant wave height and 10m wind speed,we found that DF microseismic energy level and its frequency extent mainly controlled by the ocean wave height.For example,the DF microseismic showing high energy level and wider frequency extent in autumn-winter-spring and low energy level and narrower frequency extent in summer consistent with the ocean wave height time in the north Pacific,north Atlantic,and the Antarctic ocean,respectively.The station near the Arabian sea has DF low energy level and narrow frequency extent in summer which is reversed compared with other stations,because the time of the high ocean wave height in the Arabian sea is reversed from the other ocean areas.Due to the high wave height through the whole year in Antarctic ocean,the station closes to this ocean area does not have the variation of DF microseismic energy and frequency extent.Through the analysis of the polarization characteristics of the azimuth Angle,we find that the DF microseismic signal is mainly generated by the nearby sea area.Since DF microseism is radiated as Rayleigh wave,its energy attenuates very quick when it travels through shoreline to the continent crust.High frequency ambient noise is called microtremor,which is generated by the breaking waves for the island station and anthropogenic activities with diurnal,weekly,and annually variations.