Page 62 - Acquisition and Processing of Marine Seismic Data

2.1 COMPONENTS OF MARINE SEISMIC ACQUISITION 53

removing the ghost notches from the amplitude Sparkers are high-resolution seismic sources
spectrum, which ultimately provides much generally used with single-channel surveys, or
higher resolution and imaging of the smaller with short analogue streamers with 24- to 48-
traps in relatively complex stratigraphic envi- channel acquisitions. The seismic signal is pro-
ronments. The streamers can be towed at any duced by a sudden discharge of several thou-
depth and restriction of the available frequency sand volts of electrical voltage into the water.
band depending on the streamer depth is not an Electric discharge is done via specific cables,
issue. Therefore, utilizing the collocated pres- termed electrodes mounted on a catamaran-type
sure and motion sensor technology also allows carrier (Fig. 2.13A). Today, modern sparker sys-
the operators to tow the streamers deeper at a tems use several electrodes known as multitip
quieter and more stable environment to prevent sparker sources, which produce a seismic
the noise from the sea surface, which provides energy typically between 160 and 16,000 J.
longer operating times, even during the rough Sparker electrodes are used for high-resolution
seas, resulting in less downtime. Deeper towing seismic studies, generally for site survey pur-
also provides better recording of low-frequency poses, to map shallow stratigraphy as well as
amplitudes, which ensures a better illumination active tectonism in continental margins, and
of deeper stratigraphy. geo-hazards before drilling. Although the pene-
tration of the signal is relatively low, ranging
from a few tens of meters to approximately
2.1.3 Marine Seismic Energy Sources
500 m depending on the power level of the
The seismic sources used in conventional source, resolution of these systems is consider-
marine seismic surveys produce high- ably high since the multitip arrays produce nar-
amplitude acoustic signals by a positive shock row wavelets with a significantly wide
wave due to the sudden pressure increase. The amplitude spectrum, between 50 and 2000 Hz.
signal generated is of a wide frequency band The standard seismic source used in marine
with a dominant frequency of approximately seismic exploration today is the air gun, which
60 Hz for conventional surveys, and can pene- produces seismic signal by a sudden discharge
trate into the subsurface sediments up to of high pressure air into the water. The pressur-
10 km deep. Even though explosive sources ized air generated by seismic compressors is
were used for seismic signal generation in supplied to the air guns via high pressure hoses,
marine acquisition until the 1950s, they have and released into the water column initiated by a
been replaced by repeatable sources because of time break signal from the gun controller at each
their environmental and safety issues. Different shot point. High-capacity air compressors sup-
marine seismic energy sources are used for var- ply the compressed air, typically at 2000 psi,
ious purposes in offshore surveys today; each and they can recharge the air guns during the
has its own advantages and shortcomings: shooting at every 10–15 s. The most important
disadvantage of the air guns is their bubble noise
• Sparker sources (GeoMarine Survey Systems,
(Sections 2.2.2 and 3.2) and several air guns with
SIG France and Applied Acoustics)
different volumes should be used to suppress
• Air guns
the bubble noise, which is known as array tun-
– Water gun (Sercel) ing. A single air gun can be used for high-
– GI gun (Sercel) resolution shallow surveys, whereas an air gun
– Sleeve gun (Seamap) array consisting of several air guns within the
– G gun II (Sercel) subarrays is utilized at 4–10 m depth during
– LongLife (LL) gun (Teledyne Bolt) the conventional seismic operations by the oil