Separation of upgoing and downgoing waves in vertical seismic profiling by contour‐slice filtering

The separation of upgoing waves and downgoing waves in vertical seismic profiling is effectively accomplished in the frequency‐wavenumber (f‐k) domain using a new technique of contour‐slice filtering. The response of the filter has no straight edge or boundary and preserves the “natural state” of the spectra. The cutoff contour level can be reduced as low as desirable to remove background noise and other unwanted signals. In this sense it can also be viewed as a noise‐rejection filter. The effectiveness of the process has been clearly demonstrated through synthetic and real examples. Its success with the upgoing waves depends on the degree of coherency of events in the VSP record. To enhance the coherency and to avoid velocity aliasing, we use a novel band‐pass frequency filter which is the result of convolving a boxcar spectral window with a frequency‐shifted Gaussian function.