Study of the near-surface velocity structure of the Hutubi gas storage area in Xinjiang from ambient noise tomography

Recently, ambient noise tomography has been widely used in retrieving high frequency surface waves and getting near-surface velocity models. Here we applied this method to the Hutubi gas storage area, which is located at the Hutubi anticline in the southern margin of Junggar basin in Xinjiang. We calculated the cross-correlation functions of the vertical component of continuous ambient noise data of 22 stations in the Hutubi gas storage and its neighboring regions, and measured the fundamental-mode Rayleigh wave group velocity dispersion curves in the period band 0.5 similar to 1.5 s. Firstly, using the average dispersion curve in this area, we obtained the one-dimensional average shear-velocity model, and then we used a direct three-dimensional surface-wave tomographic method based on period-dependent raytracing to invert for the 3-D near-surface shear-velocity structure to the maximum depth about 500 meters underground. The results show that the sedimentary layers in this area are thick and have low shear-wave velocity (about 0.4 similar to 0.9 km . s(-1)) in general. The low velocity structure in the gas storage region is probably caused by the fractured sedimentary rocks due to gas and water extraction and injection. In the southeast and northeast of the gas storage, there are two zones with relatively high velocities, which are probably due to the integrated effects of the local groundwater level and topographic relief. The near-surface velocity model can provide an important basis for understanding the overlying formation characteristics, precise location of local microearthquakes, assessing near-surface site effect, and removing the shallow structural effect in imaging the deep structures.