Page 40 - Acquisition and Processing of Marine Seismic Data

1.3 FUNDAMENTALS OF MARINE SEISMICS 31

recording channels and their corresponding shot method termed TopSeis applied by CGG) since
location is termed the offset distance for each the shot must be fired at the front end (the vessel
trace in a shot gather. As a specific case, the side)ofthespread,whichresultsinrecordingonly
distance from the first channel to the shot halfofthereflectionhyperbolas(Fig.1.21B).Noise
point is called the minimum offset. Reflection types involved in land and marine seismics are
events appear as hyperbolas, termed reflection also different. For instance, Rayleigh waves or
hyperbolas on the shot gathers. ground roll is an important type of noise in land
Both for 2D and 3D seismic acquisition, the seismics, but it does not exist in marine seismic
raw seismic data is collected as shot gathers. surveys. In addition, we do not observe air blast
Recording starts at the exact time that the source in the marine surveys, since the streamer is towed
is fired. The recorder receives the seismic ampli- at a certain depthbelow the sea surface. However,
tudes perceived by all channels along the spread there are several different types of noise encoun-
and constitutes a shot gather by sorting them tered in marine surveys and these are discussed
with an increasing order of channel number or in Chapter 3 in detail.
corresponding offset value. This is repeated for
each shot and a shot gather is reproduced and 1.3.4 Reflection Hyperbolas
recorded into the disks for each shot location.
Each shot gather has a unique and regularly Reflections recorded at different receivers con-
increasing (or decreasing, according to the set- stitute hyperbolas on the shot gathers when the
tings of the acquisition parameters) shot number traces are sorted in increasing order regarding
termed the Field File ID Number (FFID) and used the channel number or their offset distance.
to discriminate the successive shots (Fig. 1.20). Fig. 1.22A shows a reflection hyperbola from a
The existence of FFID and recording channel single horizontal reflector model. The shot point
numbers in the seismic trace headers is of vital is located at the center of the spread and the
importance in the processing stage (Section 5.2). recorded hyperbola is symmetrical about the shot
Bothlandandoffshoreseismicreflectiondatais location. As a specific case, the reflected signal
collected asshot gathers. However,there are strict reaches the receiver first at x ¼ 0 m (the shot loca-
distinctions between land and marine shot tion) and the arrival time of the reflected energy
gathersintermsoftheseismiceventstheycontain, at x ¼ 0 m receiver is known as zero-offset time,
types of noise, resolution, etc. These distinctions often designated with t(0) and expressed by
arise due to the differences in acquisition geome-
2h
try, seismic source types, the noise components in t 0ðÞ ¼ (1.11)
land and marine environments, and the presence V
of a water column in marine surveys. In land seis- where h is depth of the reflector and V is velocity
mics, the acquisition geometry is more flexible of the upperlying medium. In such an earth
and can be modified based on the requirements model consisting of a single horizontal interface,
of the survey area: It is possible to collect data the arrival time of the reflected signal at a
along the crooked lines; shots can be located at receiver at offset x can be calculated by
anylocationalongthesurveyline(thesplit-spread x 2
2
2
surveys) or off the survey line (which provides a t xðÞ ¼ t 0ðÞ + 2 (1.12)
better azimuth distribution for 3D surveys). For V
split-spreadsurveys, it is possible to record whole Iftheinterfaceisinclinedwithaninclinationangle
reflection hyperbolas along both sides of the shot of ϕ, the situation is somewhat different: The
point (Fig. 1.21A). This flexibility is not possible recorded reflection hyperbola is asymmetrical
formarinesurveys(exceptaquitenewacquisition about the shot location and the zero-offset time