## 1 Introduction  
Earthquake has accompanied the development of human society all along and has caused huge loss of life and property. A large number of earthquake damage data show that site conditions are very sensitive to earthquake phenomena and earthquake damage level, and the covering soil layer, surface topography, and bedrock surface of local sites become the direct factors for the distribution of earthquake disasters [1]. The shallow shear-wave velocity is one of the most important parameters for the description of engineering sites. Understanding the underground shear-wave velocity structure is the basis for conducting surface seismic impact analysis, evaluating seismic effect, and carrying out engineering earthquake resistance of local sites, which is of great significance for earthquake disaster prediction and prevention [2]. Ground motion intensity is closely related to soil category and nature, and overburden thickness. Understanding the structural characteristics of shallow surface is essential for the study of seismic site effect, especially when the overburden of a site is thick and the site contains a thick weak interlayer, its overall stiffness is small [3] and its stress distribution is not safe for operation under seismic loading [4]. Meanwhile, a weak interlayer can amplify low-frequency signal, which will seriously affect the peak acceleration and characteristic period of the ground motion, causing great harm to engineering projects. Therefore, it is particularly important to carry out a detailed survey on sites with weak interlayer in general, as well as a survey of urban seismic risk. In traditional engineering investigation, the conventional methods to obtain the shallow shear-wave velocity mainly include the cross-(single-)hole method and Rayleigh wave exploration [5]. It is generally considered that more accurate velocity values can be obtained by the cross-(single-)hole method. However, only the shear-wave velocity structure of the strata near boreholes can be obtained, which indicates that it will cost a lot of labor and time to obtain the velocity structure change of the whole site. In comparison, The Rayleigh wave exploration is a nondestructive testing method by which some results have been achieved in detecting low-velocity weak interlayer [6]. However, the currently accepted rule of thumb of the maximum penetration depth is approximately half the longest wavelength [7], and the effective exploration depth should be less than the maximum penetration depth of Rayleigh waves [8]. In recent years, microtremor survey has become one of the research hotspots in geophysical exploration [9] at home and abroad, due to such advantages as signal acquisition from natural sources, wide-frequency band, large detectable depth, and freedom from urban noise interference, and it has been rapidly developed with the development of processing technology and accumulation of observation data. There are two methods for microtremor survey, namely the horizontal-to-vertical spectral ratio (HVSR) [10] method and multichannel analysis [11] method. Research [12; 13] shows that, using the multichannel analysis based on the microtremor array method, the shallow shear-wave velocity can be obtained, and the sediment thickness or bedrock depth can be divided, with very high accuracy especially for hard rock covered with sediment [14]. The microtremor array method has been widely used in densely populated urban areas [15; 16] due to its versatility and convenient operation. [PERSON] [17] applied the microtremor array method to urban rail transit. By conducting a fine exploration of the stratum within the depth of 50 m, they solved geological problems, such as boulders, karst, cracks, fault fracture zones, and soil-rock interface in an urban geological survey. [PERSON] [18] took the geothermal resource area in the south of Jiangsu Province as an example to develop effective stratification within a depth of 2.5 km by adjusting the array radius. Compared with the borehole results, the depth error is small.