## 6 Conclusions  
In this paper, on the premise of fully understanding regional tectonic background and stratigraphic lithology, a measurement study is conducted on two typical stratigraphic models in the Wuhan area using the long-spread microtremor array method. Through fine processing of the original data and calibration in combination with borehole data, two measured 2D profiles within a length of 800 m of the microtremor apparent-S wave are obtained. The velocity interface showed in the profiles has obvious change trend in the horizontal direction, with very clear fluctuation state; in the longitudinal direction, the depth information is reliable and highly consistent with the borehole information, of which the details are as follows:  
1. In study area I, the velocity of the profile obtained through a measurement along the He'an Line increases gradually from the surface to a depth of 75 m, which, in terms of geology, matches the four layers exposed by the boreholes, namely silty clay, silty fine sand, sand mixed with pebbles, and moderately-weathered conglomerate; the apparent S-wave velocity ranges are less than 200 m/s, 200-300 m/s, 300-500 m/s, and greater than 500 m/s, respectively, which is consistent with the wave velocity results of the in situ test. The measured results of the conventional stratum well verify the correctness of the research method in this paper.
2. In study area II, the velocity of the profile obtained through a measurement along the Shamao Line does not show an increasing trend from the surface to a depth of 120 m. There exists an obvious low-velocity interlayer between 60.0-82.0 m, which has never been seen in previous studies. The velocity of the low-velocity layer is about 200 m/s lower than that of the upper and lower strata. Through borehole verification, both zk10 and zk11 reveal the existence of weak strata within the depth between 50.3-73.6 m. The apparent shear-wave velocity obtained through the microtremor inversion is also consistent with the wave velocity test results, which further verifies the applicability of the research results of this paper to weak strata.
3. Based on the microtremor inversion results, the overburden thickness and the equivalent shear-wave velocity can be estimated, and then the site category can be determined, which provides important seismic parameters for engineering site survey. Compared to the in situ test data, the error of overburden thickness and \(Vs20\) of the two cases in this paper is about 10%, with the site category being both site III. Compared to drilling, with high efficiency, this method can help reduce the drilling workload to a certain extent, and the detection results could provide important basic data for regional seismic safety and seismic risk assessment of major projects in complex urban areas.  
Conceptualization, C.J. and S.L.; methodology, C.J.; software, C.J.; validation, S.L. and J.W.; formal analysis, C.J.; investigation, H.Z.; resources, H.Z.; data curation, C.J. and M.C.; writing--original draft preparation, C.J.; writing--review and editing, S.L.; visualization, C.J.; supervision, S.L.; project administration, C.J.; funding acquisition, S.L. and J.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Scientific Research Fund from the Institute of Seismology, CEA and National Institute of Natural Hazards, Ministry of Emergency Management of China, grant number IS201926301; the Natural Science Foundation of Hebei Province, grant number D2020210009; and the Science and Technology Projects of Hebei Education Department, grant number BJ2021036.  
Not applicable.