*3.3. Testing the Model of Direct Migration*

The model of velocity (Figure 6a) and the random noise records (Figure 7) described in the previous section were used to conduct an experiment on the direct migration of random reflection-induced noise. The process of calculation is shown in Figure 18. A geometry with a wide azimuth was used, as shown in Figure 14. The maximum offset along the inline direction and the crossline direction was 1500 m, that is, the signal from each source was received by 51 × 51 receivers at most, and the aspect ratio was one. One line was extracted from every four for the calculations. After reading the SPS.X file, the source points of the data on the offset noise were placed in the channels described in the file, and the corresponding seismic channels of the receiver point in the SPS.X file were extracted to calculate the one-way wave migration with a time window of 20 s. The final migration profile obtained by stacking all of the results of migration is shown in Figure 19b. The velocities required for the migration and nmo were taken from the model of velocity. The results obtained by the conventional method of passive seismic exploration were consistent with those based on calculating the direct migration of reflections. The direct migration method required less computational and storage-related resources, and was accurate.

**Figure 19.** (**a**) Migration profile of 3D virtual shot records obtained by using a geometry with a wide azimuth. (**b**) Direct migration profile of reflection from a 3D passive source using a geometry with a wide azimuth.

When the forward migration-based imaging is carried out by using a geometry with a wide azimuth, there is no need for all sources or receivers to participate in the calculation; this significantly reduces the time needed for computations and yields uniform folds. By applying this geometry, we used only 1/4 of the source lines, and each source used only 1/4 of the near-offset receivers (the computational time was only about 1/16 of that incurred when using the entire dataset). As shown in Figure 18, the 3D seismic records obtained by the wide-azimuth geometry were processed by the direct migration method and yielded results of imaging similar to those of the virtual shot records of reflections from a passive source by using the migration method. This indicates that the proposed method can obtain accurate migration profiles by omitting the correlation in the virtual shot records, and thus requires less time and computational resources.
