Page 77 - Acquisition and Processing of Marine Seismic Data

68 2. MARINE SEISMIC DATA ACQUISITION

shrink (Fig. 2.25B). This increases the internal receiver sides (Fig. 2.27B), which are respectively
pressure of the bubble again and prevents it from termed the source and receiver ghosts. Interfer-
collapse (Fig. 2.25C). The time span between fir- ence of the ghost signal significantly restricts the
ing of the generator and injector chambers is on frequency band of the seismic data: It results in
the order of a few milliseconds and is known as periodic notches in the spectrum (Fig. 2.27C),
injector time delay, which depends on the depth andtheavailablefrequencybandbecomeslimited
of the GI gun and application pressure. It is between 0 and the first ghost notch frequency,
important to determine this parameter correctly which decreases as the gun depth increases. For
for a complete suppression of the bubble effect. instance, the first notch frequency is 250 Hz for a
3 m source depth, whereas it is 125 and 82.5 Hz
2.2.3 Ghost Reflection for 6 and 9 m source depth, respectively.
Elimination of ghost interference from the
The reflection coefficient of the sea surface is
source signature widens the amplitude spec-
approximately 1.0, and the upgoing signal emit-
trum of the data since it removes the ghost
tedfromanair gunbelow the seasurfaceisalmost
notches and hence we can obtain a much higher
completelyreflectedbackintothewatercolumnat
resolution seismic data. There are a number of
all reflection angles. This signal is termed the
developments which aim to deghost the data
ghostreflectionandisadelayedversionofthepri-
during the acquisition stage:
mary seismic signal with an opposite polarity.
Since we cannot separate the ghost signal from • Variable-depth or slant streamers: The method
the primary signal, it is considered to be a part was introduced by CGG Veritas and known
ofthesourcesignature.Therefore,thefar-fieldsig- as BroadSeis acquisition technology
nature of the air guns consists of the combination (Soubaras, 2010). The streamer is configured
of both signals (Fig. 2.26). The time separation to have variable receiver depths so that the
between the primary and source ghost is receiver ghost notches occur at different
Δt ¼ 2h/V W ,where V W is the water velocity and frequencies along the offset, which is then
histhe gundepth (Fig. 2.27A). This time separa- exploited to deghost the data (Section 2.3.6).
tion is normally quite small: the ghost signal • Over-under streamers: The method was first
arrives at the receivers only 8 ms later than the introduced by Western Geco in the early
primary signal for a source fired at 6 m depth. 1980s. It comprises two streamers on top of
The ghost signal occurs both at the source and each other at depths of approximately 20 and

FIG. 2.26 (A) Primary signal produced by an air gun, (B) source ghost, which is the polarity reversed version of the primary
signal, and (C) the signature of an air gun is the combination of the primary signal and the ghost interference.