*2.3. Model Test*

We designed a forward model to verify the theory of 3D passive seismic exploration. The size of the model was 3000 m × 3000 m × 2100 m, and the grid spacing dx = dy = dz = 30 m. The model consisted of four geological horizons with velocities of 1000, 1600, 2500, and 4000 m/s from top to bottom, as shown in Figure 6a. As we sought to study reflections from the source in deep layers, we used the acoustic finite-difference algorithm to generate data on ambient noise, and randomly set 160,000 sources in the range of 200–1800 m from the model, as shown in Figure 6b. A survey line was arranged every 30 m on the surface, and 100 receivers were arranged for each survey line. A total of 10,000 receivers were thus set, that is, the distance between the receivers in both the x and the y directions was 30 m. The seismic waves were stimulated at randomly distributed sources, and a total of 200 s of seismic records were collected by the receivers on the surface (shown in Figure 7). During acquisition, the source was made to continuously shake at random amplitudes and phases. This was done to simulate the collection of noise records of reflections from a passive source by the surface receivers.

**Figure 6.** (**a**) A 3D model of velocity. (**b**) Distribution of sources of random noise in the model (blue dots).

**Figure 7.** Random noise records (200 s).
