Seismic Ambient Noise Tomography With Dense Linear Arrays And Its Application In The Exploration Of Deposits In Hunan

The development and utilization of shallow surface is of great significance for human survival and development.How to accurately obtain the shear wave velocity of shallow surface is an important direction for geophysicists.In this paper,the extended range phase shift method is applied to study the shallow underground structure of Woxi and Pingbao mining areas in Hunan Province,which provides an effective high-precision imaging method for medium and deep ore prospecting in the future.In recent years,the background noise imaging method has been widely used in shallow surface imaging.Its main principle is to calculate the seismic ambient noise cross-correlation function between stations to obtain empirical Green's function,and to study the underground structure through tomography.The multichannel analysis of surface wave(MASW)method is the most commonly used method in surface wave exploration.Its main idea is to normalize the amplitude of the signal in time domain after Fourier transform.At the same time,the integration method is used in space domain to obtain the phase velocity dispersion curves.By combining the traditional phase shift method with the background noise cross-correlation,the problem that the traditional phase shift method is poor in extracting the low-frequency dispersion signal is greatly improved,and the available frequency band for extracting the dispersion signal is effectively broadened.On the basis of the traditional phase shift method,the extended range phase shift method uses the internal arrays to extract the medium and high frequency signal of the surface wave with small aperture range,and uses the external arrays to extract the medium and low frequency signal of the surface wave with large aperture range,and then merges the two parts of the dispersion curve to obtain a wider frequency band surface wave dispersion curve for the inversion of underground velocity structure.This method can not only ensure the high resolution imaging of near surface structure,but also greatly increase the effective constraints of deep structure.In this paper,extended range phase shift method is applied to Woxi and Pingbao mining areas in Hunan Province.After the seismic ambient noise method is used to process the continuous waveform data,we extract the Rayleigh surface wave dispersion curves in the frequency band of 0.1-2 s using the proposed method.Through the preliminary inversion,the seismic shear wave velocity structure with a depth of less than 2.5 km is obtained.The inversion results of Woxi mining area in Hunan Province show that there is a velocity interface at the depth of 1.5-2.5 km,and the top of the interface is uplifted,which reflects the existence of dome structure in this area and the existence of high-speed interlayer structure on both sides of the section,which well reflects the interlayer folds and faults.Compared with the known geological data,the S-wave velocity inversion results of line 160 have a good correspondence with faults,lithologic interfaces and veins.The inversion results of Pingbao mining area in Hunan Province show that there are high and low velocity interlayers in the middle of the profile,which reflects the existence of a large number of fractures and fissures in the region.The transverse profile of the shear wave velocity structure of the shallow surface has a good corresponding relationship with the surface lithologic interface.Through the surface wave dispersion extraction and S-wave velocity structure inversion in Hunan mining area,the effectiveness of extension distance phase shift method is verified.S-wave velocity inversion results can better reflect the fault structure and lithologic interface,and some velocity anomaly structures also provide ideas for further exploration and prospecting.