Abstract: The multichannel analysis of surface wave (MASW) method has been widely used in near surface explorations. Accurate extraction of dispersion curves is the key to surface wave imaging. In this study, a new seismic surface wave dispersionimaging method, named the extended range phase shift, is introduced. Based on the traditional phase shift method, the newly proposed method uses the internal arrays to extract dispersions in the medium-to-high frequency range, while it uses the external array to extract dispersions in the low-to-medium frequency range. The two dispersion curves in different frequency ranges are then merged to obtain a new dispersion curve with a wider frequency band, which can be used to invert for the subsurface velocity structure. Not only can this method ensure high-resolution imaging of near surface structures, but also greatly increases the depth which can be effectively constrained by the ambient noise data. From September to October in 2019, we deployed 8 dense linear arrays in Woxi, Hunan Province to collect passive seismic ambient noise data for about one month. 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 using the background noise data from three dense linear arrays, the S-wave velocity structure down to 2.5 km in depth is obtained. Compared with existing geological evidence, the S-wave velocity inversion result for Line 160 shows a good correspondence with known faults, lithological interfaces and the veins. Also, many new geological features can be interpreted based on the tomographic results showing that the new seismic shear-wave structure for part of the Woxi ore field reflects the ore controlling structures, lithological interfaces and ore body distribution. This provides an important foundation for deep ore exploration in this area. In this study, actual data are used to test the effectiveness of the proposed extended range phase shift method, showing that it is an effective and high-precision imaging method for exploring deep ore fields in the future.

Key words: dense linear arrays, ambient noise tomography, extended range phase shift, deep ore exploration