Abstract: Urban geophysical exploration often requires innovative thinking or sometimes seemingly off-the-wall approaches to meet data challenges of urban environment associated with human sprawl, such as moving vehicles, dense power grids, jagged buildings and so on. Therefore, the China Deep Exploration Center (CDEC, formerly SinoProbe Plan Center) officially launched the government-lead geological survey of urban underground space resources (also named “urban underground space-exploration project”) in pilot cities. One of the principal aims of the project is to develop geophysical characterization techniques that are accurate and noninvasive, and can be adapted to noisy and culturally complex urban settings. We selected Jinan city in Shandong Province as the first pilot city for both its great demand of underground infrastructure including a metro-subway system and its unique subsurface geology. In this pilot study, we seek a geophysical methodology that can meet urban challenges, such as noisy environment, large urban areas, restriction for equipment deployment, and logistics dealing with paved surfaces and roads. We applied ANT (ambient noise tomography) to a dense short-period array and obtained good results: (1)Using high-frequency waves generated by ambient noise tomography, we obtained surface waves with good signal-to-noise ratio after processing continuous ambient noise data collected from 49 broadband seismic stations for 32 days, demonstrating that ANT method is suitable for exploring background rock structures in urban underground space. (2)Using direct surface-wave tomographic method with period-dependent ray-tracing, all surface-wave dispersion data were inverted in the 0.21.5 s period band simultaneously for 3D variations of shear-velocity structure. The shear-velocity structure correlates well with the geological features and general lithological distribution of igneous and limestone rocks, as well as the spatial distribution of faults at depth. (3)Shear-velocity obtained from the inversion showed that the lateral and vertical velocity variation is much bigger. Specifically, in shallow regions at above 300 m depth, the lithological characters are mainly of limestone (higher shear-velocity), with much wider distribution, and of magma intrusion (highest shear-velocity relative to limestone); an obvious boundary between low and high shear-velocity indicates a concealed fault at the study area; and extensive magma intrusion occurs at greater depth whereas limestone exists only in the middle part of the north region. Overall, the study results proved that our method can be effective in helping us to better understanding local geologic structures, evaluating lithological distributions and assessing hazardous concealed active faults and their effects on springs in the future.

Key words: ambient noise, shear wave velocity, urban geophysics, urban underground space, dense array