As the detection of urban active faults becomes increasingly important, high-resolution detection of urban blind active faults is very important for urban planning, land use, and disaster risk reduction. However, it is difficult to determine the corresponding surface positions in the city environment for noise and building restrictions. The active source reflection seismic technique is considered the best technique to image faults with a high resolution and deep penetration. However, urban geophysical exploration must often consider the complex urban environment, which includes moving vehicles, dense power grids, and irregular buildings. These features make active source reflection seismic exploration difficult for wide application due to its drawbacks of high cost and the necessary use of explosives. In contrast, ambient noise seismic surveys have the advantages of continuous ambient noise sources, low cost, and fast deployment. These advantages are good for urban exploration. Although ambient noise seismic surveys have a lower resolution than active seismic surveys, their ultrahigh-density layout can improve the resolution. We have conducted two active source seismic lines and two ambient noise seismic lines near the Huangzhuang-Gaoliying fault (HGF) in a northern suburb of Beijing. The autocorrelation and crosscorrelation results are consistent with the active source reflection seismic results. They revealed the location of the HGF, which is composed of a set of steep dip faults. The study of the combination of the two techniques demonstrates that ambient noise seismic surveys are effective for urban active fault detection, especially for larger scale area surveys, and active source reflection seismic surveys can be used for detailed surveys. The combination of the two techniques has a higher efficiency and lower costs and can be widely used in blind urban active fault surveys.
Extensional shear zones as imaged by reflection seismic lines: the Larderello geothermal field (central Italy)