Application of the method of passive surface wave to the exploration of urban residential area.

With the rapid development of cities, people are paying increasing attention to urban ground subsidence and underground space. As a means to directly explore the structure of subsurface media, geophysical methods are increasingly being applied to solve the geophysical and geological problems on shallow surface in urban areas. Due to the various electromagnetic interference produced by the human noise and industrial production in cities, the application of traditional geophysical survey methods (gravity, magnetism, electricity and earthquake) is greatly limited in densely populated urban areas, but the human noise provides an effective source for the method of passive surface wave. The method of passive surface wave has developed rapidly due to its strong anti-interference capacity and little influence by site conditions. Specifically, this method has been used in karst collapse detection, prediction of overburden thickness, detection of velocity structure of underground shear wave, site effect evaluation, geothermal resource exploration, active fault detection, urban geological survey and other fields. Since the study area is located in an urban residential area whose ground had subsided, it is urgent to find out the geological structure of the underground medium. The authors adopt the method of passive surface wave for exploration and application research. The core of this method lies in the extraction of the dispersion curve. There are two most commonly used methods at present. One is the method of frequency-wave number domain (F-K method), and the other is the method of spatial autocorrelation (SPAC method). In this study, the authors mainly adopt the latter, while for irregular arrays, the method of extended spatial autocorrelation (ESPAC) is used. The main exposed strata in this study area lies successively as follows: the overlying clay in light yellow and brick red and gravel in Quaternary Holocene Guiping Formation (Qhg), and the underlying bedrock composed of thick limestone in light gray and gray-white in the upper segment of Devonian Upper Rongxian Formation (D3r3). There is no fault structure developed in the area. The overburden is characterized by low velocity. The complete limestone shows characteristics of obviously high velocity. When there are fissures or even water-filled caves in the limestone, the high velocity will be significantly weakened, which is the geophysical premise for the application of this method. Due to the narrow space and the criss-crossing buildings in the area, the method of linear passive surface wave was adopted in actual data collection. The SUMMIT seismograph of the German DMT company was adopted as the instrument to collect data, and a vertical geophone of 4.5 Hz was used to receive data. One-time layout at intervals of 2 m was completed, and the whole survey line was received at the same time. Each survey line collected 20 or more records, and the sampling time of each record was 32,768 ms with the sampling frequency of 500 Hz. Before indoor processing, the original data were extracted in an orderly manner according to the spread length of 11 traces. The specific processing flow included preprocessing, extraction of dispersion curve, calculation of apparent shear wave velocity and drawing of apparent S-wave velocity profiles. The apparent shear wave velocity V_x is a physical parameter of surface wave different from the phase velocity V_r and the shear wave velocity V_s. It has a velocity dimension. Because it avoids the influence of human factors such as setting the initial model and selecting the inversion results during the inversion process, the apparent shear wave velocity was chosen. In this study, the characteristics of apparent shear wave velocity in the backfilling area for the original subsidence were firstly interpreted and analyzed, and then the characteristics of other survey lines were interpreted. The profiles and the slice diagrams at different depths of the apparent shear wave velocity show that: (1) The thickness of overburden layer in this area ranges from several meters to more than ten meters, and the thickness varies greatly at or near the sudden change in velocity. According to the comparison with the backfill area of original subsidence, the obvious abnormal features indicate that the upper soil layer is disturbed and developed in the solution groove, and karst caves are developed in the deep part, resulting in soil leakage and then ground subsidence. (2) Survey lines reveal that the karst in the bedrock is mostly developed within 30 m, mainly composed of Zone I--a strong karst development zone. Zone I is mainly distributed under and around Building 10 in a residential area, with a wide range and a certain extension direction. It is speculated to be a strong runoff zone with the main trend of north-south direction. (3) The statistical results of apparent shear wave velocity of multiple survey lines show that the apparent shear wave velocity range of the overburden in this study area is 220-380 m½s^-1, and the intact limestone is 580-920 m½s^-1. However, due to the development of fissures in broken limestone, its range is 390-510 m½s^-1, and the apparent shear wave velocity of water-filled cave is relatively lower at the range of 340--430 m½s^-1. [ABSTRACT FROM AUTHOR]