Ghost

Key Takeaways

• The physics of the ghost reflection are governed by the sea surface reflection coefficient and the travel time geometry: the sea surface acts as a pressure-release boundary (acoustic impedance of air is effectively zero compared to water at 1.5 MPa acoustic impedance), reflecting upward-traveling compressional waves with a polarity reversal (reflection coefficient approximately -1.0); this means the ghost arrival has opposite polarity to the primary and arrives with a delay of 2d/v (where d is the depth of the source or receiver below the sea surface and v is the water velocity); in the frequency domain, the interference of the primary and ghost creates the characteristic ghost notch at frequency f_notch = v/(2d) (where the two arrive exactly out of phase) and a constructive peak at 2*f_notch (where they are in phase); for a hydrophone streamer at 8 m depth in 1,500 m/s water, the receiver ghost notch is at 1,500/(2*8) = 93.75 Hz, which is above the exploration bandwidth and relatively benign; for a streamer at 25 m depth, the notch falls at 30 Hz, directly within the target bandwidth for most exploration applications; the trend toward deeper streamers (for noise attenuation) therefore traded ghost notch frequency against noise, until broadband deghosting techniques made it possible to operate at depths that suppress noise while recovering the attenuated frequencies computationally.
• Conventional towed-streamer acquisition operated with streamers at 6 to 8 m depth (shallow ghost notch at 90 to 125 Hz, well above the exploration bandwidth) and sources at 5 to 7 m depth (source ghost notch at 107 to 150 Hz), accepting the ghost notch at high frequencies as a cost of low-noise shallow acquisition; the shift to dual-sensor streamer technology (co-located hydrophones and geophones in the same cable) in the mid-2000s enabled the first practical broadband ghost attenuation at depth, because the hydrophone records pressure (ghost polarity inverted) while the geophone records vertical particle velocity (ghost polarity same as primary), and the sum of the two after scaling removes the ghost notch from both source and receiver ghosts simultaneously; WesternGeco's Dual Sensor Solid Streamer (Sercel GeoMarine), CGG's BroadSeis, PGS's GeoStreamer (a dual-sensor hydrophone-geophone cable), and ION's Calera are all dual-sensor systems that implement variations of the same deghosting principle; dual-sensor streamers can operate at 20 to 30 m depth (for excellent noise attenuation) while recovering the full low-frequency content that would be notched out at conventional shallow-tow depth, extending the usable bandwidth from the conventional 6 to 80 Hz to 2 to 200 Hz and dramatically improving imaging of thin beds, steep dips, and complex velocity structures.
• Variable-depth streamer (VDS) technology, developed by CGG (BroadSeis VDS) and Sercel, uses a streamer that intentionally varies in depth along its length (from shallow at the near end to deep at the far end, following a curved profile), creating a different ghost notch frequency at each point along the streamer; when the data from all receiver positions along the VDS are summed in the deghosting process, the different notch frequencies at different offsets cancel in the composite, providing a notch-free response across the full bandwidth; WesternGeco's OverUnder (two parallel streamers at different depths, one above the other) uses a similar principle where the different notch frequencies of the shallow and deep streamer sum to provide a notch-free response; these multi-depth acquisition geometries achieve broadband ghost attenuation without requiring per-receiver dual-sensor measurement, at the cost of more complex tow geometry and processing workflows; the OverUnder system (developed in the late 1990s, commercialized in the 2000s) was one of the first broadband marine acquisition methods to reach commercial deployment at scale, particularly for deep-water West Africa and North Sea surveys where the low frequencies (below 5 Hz) are critical for full-waveform inversion velocity model building.
• Source ghost deghosting is a complementary problem to receiver ghost deghosting: the conventional air gun array (fired at 5 to 7 m depth) generates both a downward-traveling primary wavefield and an upward-traveling ghost that reflects from the sea surface with polarity reversal; the composite source wavelet received at any reflector is the sum of the downward primary and the downward-reflected ghost, creating the same notch-and-peak structure as the receiver ghost but driven by the source geometry; near-surface source ghost acquisition (using very shallow arrays, at 1 to 3 m depth, to push the notch to 250 to 750 Hz above the exploration bandwidth) has been used in shallow-water surveys where the notch must be avoided; the more general solution for deep-source ghost removal is computational (minimum-phase or zero-phase source deghosting filters applied in processing), or the use of over/under source arrays (two air gun arrays at different depths that can be simultaneously deghosted using the same sum-of-different-notch principle as the OverUnder streamer); PGS's Tuned Source Array and other optimized source designs also seek to reshape the source spectrum to minimize the ghost notch impact within the target bandwidth by optimizing the array geometry and timing.
• Ocean-bottom cable (OBC) and ocean-bottom node (OBN) acquisition inherently eliminates the receiver ghost in one component: geophones resting on the seabed record particle velocity, which is not subject to the sea surface ghost reflection because the geophone is in contact with the solid seafloor rather than the water column; hydrophones on the ocean bottom do record a ghost (energy that traveled upward through the water column, reflected off the sea surface, and returned down), but the geophone-hydrophone summation (the same dual-sensor principle used in GeoStreamer) removes this ghost in processing; this is why OBC/OBN acquisition provides inherently broader bandwidth than towed-streamer acquisition even before broadband processing, particularly at low frequencies where the receiver ghost of deep streamers suppresses energy -- the OBN hydrophone record has the sea-surface ghost but at a delay of 2*water_depth/v, which for 500 m water depth is 667 ms, far outside the primary reflection window for most targets; in shallow water (less than 100 m), the ghost delay is shorter and OBN deghosting must be applied, but OBN still avoids the noise penalties of shallow tow while providing full-azimuth and four-component (4C) data.