Method and system for wavefield separation of sonic data using cross-correlation

1. A method for wavefield separation of sonic data, the method comprising:

estimating direct phases of waveforms of sonic data observed with two or more sensors by using cross-correlation of waveform traces at adjacent sensor locations;

removing the direct phases from the observed waveforms; and

extracting event signals from the waveforms after removing the direct phases;

wherein the direct phases of waveforms are approximated by polynomial fitting.

2. The method according to claim 1 , further comprising:

estimating time differences of phase arrivals to a reference time using cross-correlation of the waveform traces; and

estimating the direct phases of waveforms based on the time differences of phase arrivals.

3. The method according to claim 2 , wherein the time differences of phase arrivals are estimated by successively applying time shift to the waveform traces and computing cross-correlation for the time shifted waveform traces.

4. The method according to claim 1 , further comprising applying a k-f filter to remove unwanted signals from the waveforms.

5. The method according to claim 1 , further comprising applying a k-f filter to select up-going or down-going waves from the waveforms.

6. The method according to claim 1 , further comprising analyzing a subterranean structure near a borehole by processing the event signals.

7. A system for wavefield separation of sonic data, the system comprising:

a memory to record sonic data and a processor used to:

estimate direct phases of waveforms of sonic data observed with two or more sensors by using cross-correlation of waveform traces at adjacent sensor locations;

remove the direct phases from the observed waveforms; and

extract event signals from the waveforms after removing the direct phases;

wherein the direct phases of waveforms are approximated by polynomial fitting.

8. The system according to claim 7 , wherein the processor is used further to:

estimate time differences of phase arrivals to a reference time using cross-correlation of the waveform traces; and

estimate the direct phases of waveforms based on the time differences of phase arrivals.

9. The system according to claim 8 , wherein the time differences of phase arrivals are estimated by successively applying time shift to the waveform traces and computing cross-correlation for the time shifted waveform traces.

10. The system according to claim 7 , wherein the processor is further used to apply a k-f filter to remove unwanted signals from the waveforms.

11. The system according to claim 7 , wherein the processor is further used to apply a k-f filter to select up-going or down-going waves from the waveforms.

12. The system according to claim 9 , wherein the processor is further used to analyze a subterranean structure near a borehole by processing the event signals.