Page 34 - Acquisition and Processing of Marine Seismic Data

1.3 FUNDAMENTALS OF MARINE SEISMICS 25

marine seismic reflection exploration can be • New processing methods for 3D, 4D, and
listed as follows: 4C data to increase the resolution and better
suppress the noise
• 3D acquisition up to 24 streamers, each • Widespread incorporation of anisotropy
12 km long
• 50 m streamer separation
• Two synchronized seismic source arrays at
different depths 1.3 FUNDAMENTALS OF MARINE
• Two synchronized streamers at different SEISMICS
depths
• Fan mode acquisition 1.3.1 Seismic Waves
• Slanted streamer acquisition
• Shooting over the seismic spread (TopSeis) Seismic waves are often termed elastic waves,
which can be described as the energy caused by
The petroleum industry will lead in the devel-
opment of new sophisticated instruments and a sudden breaking of rock within the earth, such
as from earthquakes, or explosions, such as from
acquisition as well as data-processing tech-
the sources in seismic surveys. They can travel in
niques in seismic exploration in forthcoming
air and in solid or liquid materials, e.g., through
years. The most important advancements will
the earth’s interior or within the oceans. The par-
possibly be in the production and monitoring
ticles during the wave energy passage are suc-
of existing reservoirs, utilizing time-lapse 4D
cessively compressed (termed compressions)
seismics. Incorporating shear waves with multi-
and pulled apart (termed rarefactions), or they
component acquisition using ocean-bottom
perform a lateral movement (termed shear),
cables or permanently positioned nodes will
provide a better imaging of structures with dif- depending on the elastic wave type they are
ferent fluid types, such as gas chimneys or reser- affected by.
voir evaluations, to reduce uncertainties. In Seismic waves propagate in an expanding
forthcoming years, marine seismic exploration spherical shape in a homogeneous medium after
will possibly involve the following new they are generated by an explosive source in
developments: seismic surveys, such as the expanding spherical
waves formed by a pebble thrown into calm
• Full azimuth acquisition with water. This frontal energy is termed the wave
permanent nodes front. The direction perpendicular to any point
• Increasing number of time-lapse or 4D on the wave front is named the ray path. There
acquisition are two main seismic waves generated by seis-
• Widespread multicomponent mic sources as well as earthquakes, termed body
towed streamer technology for waves and surface waves. Body waves are of
ghost-free data higher frequency and propagation velocity than
• Commercial use of converted wave surface waves.
acquisition and processing There are two types of body waves, termed
• Increasing number of multiazimuth data primary (P) waves and secondary (S) waves;
acquisition both can penetrate into the earth through the
• Longer streamers (12 km), deeper deeper layers. P waves are faster than S waves
penetration and arrive at the seismometers first, followed
• Much smaller streamer separation by S waves, during an earthquake. That is why
• Use of synchronized multisources we call them primary and secondary waves,