discussion and conclusion

I propose a new imaging condition, with which the wavefield extrapolation is carried out in the depth domain, and the imaging amplitude is extracted from the focused scattered wavefield in the time domain, if the radius of curvature of the wavefront diminishes to zero. I call this imaging condition the depth-focusing imaging condition. I assert that the statement that the arrival time of the downgoing wave equals the take-off time of the upcoming wave is equivalent to saying that the radius of the curvature of the scattering wavefield diminishes to zero. With the imaging condition, the primaries and the multiples can be simultaneously imaged. The source position can be known or unknown; therefore the passive data can be imaged with it. Some shot gathers can be added together according to the receiver positions, and then the new data set is imaged with the above method, thus improving the calculation efficiency. The depth-focusing imaging condition can be used for imaging multicomponent seismic data. However, since the focusing of the scattering wave is detected in the time domain, the data needs to have high S/N ratio. It will be best suited for processing marine data.