Study Of Surface-wave Exploration Method Base On Ambient Noise

Huangtuling landslide is located between Erdaogou and Sidaogou in the old town of Badong County, which is one of the most serious landslides in the Three Gorges Reservoir Area. It is consisting of Riverside collapse deposits, substation landslide, market gardening landslide and some recent small landslides. The state has put a lot of manpower and material resources for the relocation of the landslide prevention and research. The institute of geophysics and geomatics of CUG participated in the test site of Huangtuling landslide for a week to record seismic noise. We deployed four kinds of observations, line a,b,d and c array. Line a,b and c array are consisting of8broadband seismic stations. Line d is a dense array consisting of forty-seven Texans125A seismic recorders with4.5-Hz vertical receivers across the base of a tunnel structure, which is excavated through the No.1riverside collapse deposits of Huangtuling landslide. In order to detect the landslide internal shear wave velocity structure, the author carried out the study of surface-wave exploration method base on ambient noise.In recent years, seismic interferometry can be used to retrieval Green’s function of seismic ambient noise recorded at each receiver. And conducting surface wave tomography or inversion of shear wave velocity structure of the Earth’s interior has become a rapidly emerging field of seismological research and a mature technology. Ambient noise tomography has been proven particularly useful in imaging Earth’s crust and uppermost mantle on both regional and global scales, but there are few applicability examples of the seismic interferometry technique to near surface geological studies. The application of the seismic noise interferometry to near surface geological study is not a merely change of scale, since it involves important problems still under discussion. To explore these, the first phase of data processing consists of preparing waveform data from each station individually and doing cross-correlation of seismic interferometry to retrieval surface wave Green’s function from the seismic ambient noise recordings from line d. The record length was consistently24hours with a500Hz sampling interval. Phase of single station data preparation has several steps, such as stored in2880time widows of30s duration, de-meaning, de-trending and bandpass filtering the seismogram, time-domain normalization and spectral whitening. Further more, we used the standard seismic interferometry approach on the shot gather generated by virtual source.Seventeen shot gathers were constructed from the entire data set. The virtual sources are No.l to17detectors, and the near offset trace and track pitch is8m with a232m trace gather length. Using the high-resolution linear Radon transform we generate an image of the dispersive energy. The dominant Rayleigh waves observed in the shot gather have a frequency range from2.5-9Hz. The dispersion curve shows the4Hz phase velocity of the study area is about2000m/s,5Hz about at1500m/s,7-8Hz about at1000m/s. We use the ESPAC method compared with the result of MASW, and find that the two results have better consistency. We used the total dispersion curve generated for each of the seventeen shot gathers when inverting for each1D S-wave velocity profile used to generate the2-D velocity cross section. Two low velocity layers (LVLs) were revealed by the S-wave velocity cross section. The top LVL is continuously distributed from the midpoint of fifteen and sixteen receivers with60meters deep to the right up to about20meters, and potentially linked to the slide plane exposed at the tunnel base of twenty-five receiver position, the average inclination of the top LVL is about35°. And the second LVL can be found at the middle of the cross section approaching100meters deep to the right endpoint up to50meters deep, the average inclination of the second LVL is about35°too. These LVLs of S-wave are deduced the potential slide planes of Huangtuling landslide.Beamforming of seismic noise recorded on the c array was used to analyze these source locations, with a consistently22hours record length and a500Hz sampling interval. Azimuth-slowness maps (dB) of the2Hz,4Hz,6Hz,8Hz,10Hz vertical component surface wave show that the sources region during the daytime is significantly more than the number of night. Azimuth-slowness map (dB) of the2Hz shows that during the daytime there are energy group from Pier and during the night energy group distributed more evenly along the Yangtze River. Therefore the2Hz noise sources are mainly from the Yangtze river flow, waves and ship activities and so on. Azimuth-slowness map (dB) of the4Hz shows that the energy group during the daytime not only in the direction of Yangtze river, but also the direction of the city. But the energy group during the night is mainly in the direction of Yangtze river. This indicates that in this band it contains human activities noise source during the daytime. Azimuth-slowness map (dB) of the6Hz,8Hz,10Hz show that regardless of day or night the noise sources are dispersed distributed. So these bands noise sources are not only Yangtze river flowing, but also the human activities such as traffic noise and engineering construction and so on. Using the estimated Green function, the group speeds as a function of period can be measured by using traditional frequency time analysis. A total of24group velocity dispersion curves are for further research use.The passive surface wave exploration has enormous advantages. We don’t have to worry about noise and disturbing behavior in conventional seismic data. The method requires less about topographic conditions of the site and reduces the seismic exploration costs and the destruction of the natural environment. Combined use of active source and passive surface waves exploration can effectively increase detection depth. The human activities such as traffic noise and engineering construction and so on in the city are thestable source, so we don’t have to worry about "source". In the future, passive surface wave exploration can be widely used in engineering geophysics and geotechnical engineering.