5.2.2 Significant Developments
By 1956, Haggerty from Texas Instruments had patented methods for cancelling the ghost reflections and reverberations in the water-layer by deploying hydrophones and geophones, and over/under streamers. His analysis was based on the theory of standing waves. Today, history shows that Haggerty was 50 years ahead of his time. Pavey and Pearson from Sonic Engineering
Company described in 1966 how the frequency components that are cancelled on hydrophone recordings due to the ghost can be replaced by the use of geophones. However, it was found that the combination of P and Z signals tended to degrade the S/N-ratio of the lower frequencies in the seismic band. Te Z-sensor had high noise caused by the specific mounting of the sensor and the rotation of the towed cable. Berni, working for Shell Oil (1982–85), developed
Figure 5.7: Drawing from Pavey and Pearson (1966), who proposed combining hydrophone and geophone measurements to attenuate the effect of ghost reflections from the sea surface.
the geophone principle further. Te first successful P-Z marine streamer, GeoStreamer, developed by PGS, was described by Vaage et al. (2005). In addition to the deghosting advantage, the GeoStreamer reduces weather noise and increases acquisition efficiency by extending the weather acquisition window. In 1976 Parrack from Texaco patented the use of streamers that are spaced
Figure 5.8: Hydrophone and geophone measurements at 13m depth with amplitude spectra from Vaage et al. (2005). The noisy low-frequency part of the geophone signal (53) is recalculated from the recorded pressure signal below ~18 Hz using the Z-P model and merged with the non-noisy geophone signal before deghosting.
apart vertically to cancel the downgoing ghost signals from the sea surface. Te first practical 2D applications of the over/under streamer method started in 1984 in the North Sea, driven by ideas from Sønneland in Geco. Te method was introduced as a means of reducing the weather downtime by deploying two streamers on top of each other at large depths, such as 18m and 25m, to minimise the effect of swell noise. In addition, it allowed deghosting. Te over/ under acquisition, however, had limited applications during that period due to deficiencies in marine acquisition technology related to lack of streamer control in vertical and horizontal planes. With the introduction of new marine acquisition technology that has
accurate positioning and advanced streamer control, the over/under method was applied during the mid-2000s, mainly for 2D applications. Slant streamer marine acquisition was first proposed by Ray and Moore
(1982) from Fairfield Industries. Te novel idea of this method was to have variable receiver depths along streamers and, inherently, variable ghosts from receiver to receiver, and to take advantage of this in the stacking process. However, slant streamer acquisition was not successful at that time due to inadequate data processing algorithms, particularly for the ghost-removal process. Today, variants of the slant streamer idea have been implemented by
CGGVeritas (BroadSeis) and by WesternGeco (ObliQ). Te concept of multicomponent towed streamers was introduced by
Robertsson et al. (2008). Te system, called IsoMetrix, measures pressure with hydrophones, and particle acceleration in y- and z-directions with micro electromechanical systems (MEMS) accelerometers. Based on these measurements, cross-line wavefield reconstruction and deghosting can be performed.
5.2.3 Ghosts
Te effect of the sea or land surface is a well-known obstacle in seismic exploration and has been addressed since the outset of reflection seismology (Leet, 1937). Van Melle and Weatherburn (1953) dubbed the reflections from energy initially reflected above the level of the source, by optical analogy,
Figure 5.10: Drawing from Ray and Moore (1982), who proposed a ‘high resolution, high penetration marine seismic stratigraphic system’ where a slanted cable gathers seismic reflections so that the primary and ghost reflections from a common interface are gradually spaced apart.
199
Figure 5.9: Drawing from Parrack (1976), who proposed using over/under streamers to cancel the downgoing ghost signal. The two upper traces show that the ghost (45) arrives first on the upper cable (35) then on the lower cable (36). By aligning the ghost arrivals and subtracting, the trace (51) is ghost-free.
USPTO
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