Ability of decreasing noise and the characteristics of nearsurface wave field around Dazhai borehole in Pu'er

In 2011, the Dazhai borebole station was established in Pu'er, Yunnan Province, to carry out some researches on noise suppression and the wavefield characteristics. We compute the power spectral density of continuous noise recordings within ten months to quantify the reduction in seismic noise with the station depth. The results demonstrate that the downhole station has obvious effect on decreasing noise up to 40 dB within the frequency band more than 1 Hz, which might be associated with the local site conditions. The Green's functions are then obtained between the two receivers by deconvoluting the down-hole recordings with the surface ones. From the deconvolution wavefield, the arrivals of up-going incident and down-going surface-reflection waves are directly identified that cannot be distinguished from the original seismic recordings. The two phases are utilized to set up a shallow seismic wave velocity model, which is consistent with the theoretical results. A comparison with the surface observation suggests that the borehole stations have great advantages in reducing noise and studying the propagation characteristics of near-surface seismic waves. The research here is of great significance for future down-hole observations in other sites.