Still present is the dipping coseismic energy between 0.025 and 0.04 seconds. Because this energy is so strong, and so close to horizontal, it leaks through the PEF, corrupting a portion of the record. This problem is the focus of on-going investigation and is being addressed with waveform separation.
Waveform separation is a standard technique in VSP and cross-well seismic processing, used to remove high-amplitude early arrivals in records by capitalizing on the different move-outs of different arrivals. We employ the method as follows: (1) picks are manually made along the arrival targetted for removal, (2) the gather is moved out such that the arrival (as defined by the picks) is horizontal and then stacked, and (3) the resulting trace is normalized by the number of traces in the gather and then subtracted from each of the moved-out traces. (4) After the subtraction, the gather is moved back to its original alignment. By repeating this process, it is possible to remove more than one coherent arrival from the record. Figure shows a series of images as various arrivals are removed from the record (a). Figure d shows the result after three iterations through the process, and shows that although the process has effectively removed much of the energy of the strong coseismic first arrivals, it has also partially removed the interface response (0.01 to 0.02 seconds). This is a result of the chance line-up of waveforms during the second iteration. We chose to use the data shown in Figure c for the PEF processing sequence.
We apply the second derivative after waveform separation since it has proven successful. Because the coherence of the coseismic noise has been disturbed by the waveform separation technique, we opt to determine the PEF's 332#332 and 338#338 with the data shown in the windows of Figure b and c. The starting datafile (after waveform separation and second derivative) is shown in Figure a, with the final result in Figure b. Here we see that the clarity of the added events is improved, but that some of the coseismic energy remains. In this case, the remaining coseismic energy is closer to horizontal than that in Figure d. Thus, if we were to stack this gather, the resulting trace would definitely include unwanted coseismic energy. We continue to pursue solutions to this problem.