Seismic resolution of towed marine streamer data is affected by the ghost reflections at the low end and at the high end of the spectrum. The ghost reflections occur at the source side and the receiver side and propagate from the sea surface to the hydrophones in the streamer, being part of the downgoing seismic wavefield. The Over/under towedstreamer acquisition method allows the separation of the upgoing seismic wavefield from the downgoing seismic wavefield using a two-hydrophone vertical receiver array. In this paper, we present an over/under towed-streamer experiment performed in the Gulf of Mexico at the beginning of 2004. The results of this experiment demonstrate that the seismic bandwidth is extended and consistent with theoretical design.
In marine towed-streamer acquisition, seismic resolution is limited by source and receiver ghosts. Figure 1 shows the amplitude spectra of seismic data recorded with streamers deployed at 8 m and 30 m. The streamer deployed at 30 m preserves the low frequencies but the ghost notch repeats every 25 Hz and destroys the higher frequencies. The shallow streamer better preserves the higher frequencies but the notch at zero Hz attenuates the lower frequencies. The source ghost has a similar effect on seismic resolution. The key geophysical issue that must be resolved in order to improve the seismic bandwidth of towed-streamer data is to remove the ghost notches, a process known as deghosting. Separation of the upgoing seismic wavefield from the downgoing seismic wavefield, which contains the ghost reflections, is a well-known deghosting method. This is routinely done with seabed acquisition, OBC or node systems, by recording pressure with hydrophones and vertical particle velocity with geophones. Over-under streamer acquisition allows performing wavefield separation for marine towed-streamer data. The over/under method was introduced in the early eighties in the North Sea as a mean to reduce the weather downtime (Brink and Svendsen, 1987). By deploying two streamers at large depths, 20 to 30 m, and separated by 5 to 10 m, the weather noise can be significantly reduced and the upgoing wavefield can be separated from the downgoing wavefield (Sonneland et al., 1986). The success of the wavefield separation method for ghost removal depends on the accurately maintaining the over/under streamers in the same vertical plane. This requirement was difficult to fulfill with the marine streamer acquisition technology of the mideighties and a very good geophysical idea was “shelved” for several years. New marine towed-streamer technology, developed in recent years, has streamer steering, accurate positioning, fine receiver sampling and point receiver acquisition and these features enable accurate wavefield separation using the over/under streamer method. In the over-under towed streamer experiment, the seismic signal is generated by a seismic source and the pressure is then measured at two receivers placed at different depths. The reciprocity principle allows replacing the over and under receivers with over and under sources and measuring the pressure with a single receiver (Moldoveanu, 2000). The wavefield separation could be applied for over-under sources as well .
