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Figure 4 – Vertical wave test spectra comparing the merits of vibrator wavelets generated
with different sweep parameters. The wavelets that were analyzed were recorded
at a depth of 5,000 feet (1,500 meters), immediately above a targeted reservoir
interval. The comment sweep rate = 3 dB/octave means that when the vibrator
sweep advanced into a new octave of frequencies, the vibrator dwelled two times
longer on each frequency component than it did in the preceding octave band.
The intent of this test was to determine if this vibrator operation caused more
high-frequency energy to remain in the wavelet when it reached target depth. In
this instance, the optimal sweep parameters were judged to be a sweep of 10 to
120 Hz at a sweep rate of 3 dB/octave (lower right spectrum).