Determining the soil-bedrock interface and fracture-zone scope in the central urban area of the Jinan city, China, by using microtremor signals

Abstract

Abstract To explore and utilize the urban underground space, it is important for us to accurately obtain information on the near surface structures. Microtremor signals play a vital role in that they carry abundant information about the subsurface structure and can be easily observed under disturbance of human activities in urban areas. We collected microtremor signals at 24 sites along a survey line in central Jinan city, obtained the dispersion curves by using the Extended Spatial Autocorrelation method and the resonance frequencies by using the horizontal-to-vertical spectral ratio method, and inverted the near surface structures by using the forking genetic algorithm. The lateral variation of the soil–rock interface was delineated at a maximum depth of 33 m and a large fracture zone was determined at a width of approximately 300 m. The frequency peaks changed from 2.5 to 7.3 Hz for the fracture and stable zones. A frequency-dependent model of the sediment thickness was obtained as $h = 101{f_r}^{ - 1.13}$, where h is the depth with a unit of m and fr is the resonance frequency. These results may be useful for understanding near surface structures and then benefit exploration of urban underground space.