# Three-Dimensional Processing of Reflections for Passive-Source Seismology Based on Geometric Design
## 2 Extracting 3D Virtual Shot Records Based on Geometry
### Calculating 3D Virtual Shot Records of Passive Source Based on Geometry  
The core of passive seismic reflection exploration is to obtain virtual shot records based on correlations between a channel of ambient noise records and other channels. When the length of the correlation is given, the ambient seismic records received over a long time need to be divided into several windows according to this length, and can then be correlated and stacked. To reconstruct a high-fidelity Green function, the time series of mutual correlation among the seismic data is usually very long. This makes it necessary to increase the number of folds to improve the quality of the Green function, which in turn reduces the computational efficiency of imaging of the passive source. Therefore, the number of virtual shot records, length of the ambient seismic records, length of the correlation window, and number of seismic channels involved in the calculation all influence the time needed to generate the virtual shot records. Passive seismic exploration using reflections of waves from the source is based on correlations between all channels of random signals, and the generated virtual shot records are similar to those of an active source. They should also comply with the corresponding requirements of geometric design in theory. An appropriate geometry can not only improve the quality of imaging, but can also reduce the computational expense. Many seismic channels are involved in correlation and stacking in the calculation of virtual shot records, and the long time needed to record the ambient noise incurs a high computational intensity. The range of seismic channels needed to obtain correlations in 2D calculations is usually selected by setting the spacing and range of offset of points on the source. However, a rule is required in case of 3D calculations to ensure the quality of the generated virtual shot records. The data processing features uniform folds, wide azimuths, and other requirements that are similar to those for active seismic exploration. The geometric design has been extensively studied in the context of conventional seismic exploration, and exploration based on reflections from a passive source can directly learn from the experience accumulated by this research. We designed a method to compute the virtual shot records of reflections from a 3D passive source based on the design of the geometry for active-source seismic data to ensure a sufficient number of folds and quality of imaging while reducing the computational cost. Before acquiring 3D data on the active source for seismic exploration, the geometry required to determine how to position the sources and receivers can be designed according to the known surface and the underground geological conditions of the given area. One can then specify the location of each source and receiver and the corresponding relationships between them. Following this, we must ensure that the collected data contain uniform  
Figure 4: (**a**) Virtual shot records (the source point x = 1500 m). (**b**) Migration profile obtained by conventional processing. folds, and then set the range of the offset distance and the azimuths. The geometric design usually considers the geological target elements and cost of acquisition. The file containing these geometric data is usually called the Shell Processing Support (SPS) Format for 3D Land Survey, and is a standard file for recording information on the source and the receiver as well as their relationship. The SPS file is mainly composed of a source-information file (S file, Table 1), a receiver-information file (R file, Table 2), and a source-receiver relationship file (X file, Table 3). The SPS file determines the unique physical location of the sources and receivers in the working area as well as their relationships. In the context of correlations, the SPS.S file stores information on the sources of all virtual shot records, and the SPS.X file contains the range of the receivers in the SPS.R file that need to be correlated with these sources.