Review of progress in passive seismic reflection exploration

Seismic exploration is a key method for the study of underground structures. Active seismic surveys can acquire high signal-to-noise ratio reflection data; however, the operation of active seismic surveys is complicated, and the exploration cost is high. Passive seismic exploration is another type of seismic exploration, which requires no active human build source but utilizes the natural noise recorded by seismograph stations or geophones. As a low-cost and environmentally friendly method, passive seismic reflection exploration can be used to create higher resolution seismic profiles than the surface wave method and plays an increasingly important role in underground mineral exploration, dynamic monitoring of carbon storage, and urban underground structure detection. However, passive source reflection imaging technology faces several challenges. For example, because ambient noise is mainly controlled by surface wave energy, the reflected body wave signal can be weak and difficult to extract. Actual underground sources are limited in number and are unevenly distributed, which leads to artifacts in the virtual shot gathers reconstructed by seismic interference. There are also constraints of massive data computation and storage for long-term observation by a large number of geophones. With the rapid development of portable node geophones and high-performance computing, passive seismic reflection exploration has achieved considerable progress in both method research and practical applications in recent years. This paper briefly reviews the history of seismic interferometry and the construction of virtual shot gathers using various seismic interferometry methods and then introduces in detail how the reflection signals from ambient noise records dominated by surface waves are identified and extracted. We discuss the identification and extraction of weak body wave reflection signals based on various characteristics of surface and body waves, such as differences in signal-to-noise ratio, velocity, and azimuth angle. We then focus on the processing of passive source reflection data, including the beginning of raw data preprocessing, virtual shot gather static correction, coherent noise suppression, multiple suppression, velocity analysis, and direct migration imaging. We also introduce examples of passive seismic reflection applications, including CO₂ storage site monitoring, metal mining, and coal mine underground structure research. Finally, we give an outlook for research prospects in passive seismic reflection exploration.