Page 109 - Acquisition and Processing of Marine Seismic Data

100 2. MARINE SEISMIC DATA ACQUISITION

an existing NAZ data, typically over a relatively autonomous, controlled by a pair of small ves-
small-scale target, to improve the image of that sels which regulate the position and record the
specific target instead of being applied to a seismic data. Streamers have 4C sensors every
whole 3D survey area. WAZ surveys assure sig- 25 m, each with a hydrophone and geophone
nificant improvements in azimuth distribution, measuring the wave pressure and velocity to
especially for near offsets: While they can pro- cancel out the ghost reflection, which provides
vide almost a full range of azimuths for near off- a broadband high-resolution seismic image of
sets, it is only 30 degrees for far offsets. RAZ the subsurface. Since the streamers are not
surveys use WAZ vessel configuration with a towed, the swell noise from water flow and
MAZ shooting design, and acquires the most mechanical cable noise are completely
regular azimuth distribution for all shots within eliminated.
the survey. A full-azimuth (FAZ) survey was
conducted in 2014 by PGS using five vessels
(Long et al., 2014). During the survey performed 2.4 SPECIFIC ACQUISITION
in the Gulf of Mexico, all five vessels towed TECHNIQUES
source arrays, two vessels tow 10 8100 m
streamers with 120 m separation, and 30-degree, Today, both 2D and 3D towed streamer
90-degree, and 150-degree survey azimuths acquisitions have been employed worldwide
were applied. The result is a uniform full- to map shallow and deep reservoirs, shallow
azimuth sampling for all source-receiver offsets. gas and gas hydrates, faults or small scale frac-
Coil shooting is another approach for full- tures and stratigraphic character of the subsur-
azimuth (FAZ) surveys. It was not realized that face. The towed streamer surveys are
the circular acquisition for rich azimuth data considered to be conventional marine seismic
was practical for 3D surveys until it was success- acquisitions. Apart from narrow- or rich-
fully tested in 2006 by Western Geco. In coil azimuth towed streamer surveys, there are a
shooting surveys, only one single vessel tows couple of unconventional or specific seismic
both source array and streamers and the vessel acquisition techniques which are used based
sails in closely spaced circles in the area to on the requirements of the acquisition environ-
acquire FAZ data for the whole survey. The tech- ment or necessities of the project. In this section,
nique provides narrow azimuths for near off- unconventional acquisition methods, such as
sets, while the far offsets are acquired with four-component (4C) ocean bottom seismic
much larger azimuths. The main advantages of recording by ocean bottom cables or nodes,
the method are that a better illumination of the time-lapse (4D) acquisition for reservoir moni-
target is obtained with higher S/N ratio along toring, transition zone acquisition and P-cable
with a more complete suppression of the multi- acquisition, are briefly introduced.
ples. Furthermore, a proper sorting of coil shoot-
ing data based on the different azimuth ranges 2.4.1 Ocean Bottom Seismic Acquisition
promises anisotropic analysis, such as aniso-
tropic fracture detection or velocity analysis. Although two-component (2C) with a hydro-
A recent acquisition method is known as phone and an accelerometer and four-
FreeSeis which provides full-azimuth seismic component (4C) with a hydrophone and three-
data using several parallel submerged streamer component motion sensor technologies have
cables, while the seismic source is towed by a been successfully utilized for towed streamer
source vessel (Haumont e, 2017). Each streamer, acquisition in recent years, today single sensor
floating in the mid-water, are independent and 2D and 3D towed streamers are still widely used