Spectral analysis of a ghost

Abstract

A seismic line was shot such that the source‐ghost effect from two different dynamite source patterns could be compared. Two 15‐shot seismic datasets were created that were identical in all respects except that one used 4 kg of explosives in a single 18‐m hole, while the other used 2 kg of explosives in each of two 9‐m holes. After identical processing, the final stacked sections of the 18‐m and 9‐m datasets are dramatically different in character and temporal resolution. An f‐x spectral analysis of the stacked sections reveals that the 18‐m data shows a loss in power and phase coherence from 45 to 58 Hz, while the 9‐m data shows a similar effect from 65 to 78 Hz. A spectral notch, centered near 55 Hz, due to a source ghost is suggested as the reason for the lower power in the 18‐m dataset. The 9‐m data is consistent with a spectral notch at a higher central frequency near 72 Hz. Above its spectral notch, 18‐m data shows a reemergence of weak signal that persists to near 80 Hz; the 9 m dataset shows little signal above 65 Hz. Examination of raw shot records shows that these effects are very difficult to observe in field records. Without specialized deghosting, the 9‐m dataset shows greater temporal resolution; however, the 18‐m dataset has a broader signal bandwidth and is potentially superior.