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Definition of 3D terms

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Cross line Direction:The direction that isorthogonal to receiver lines

In-line Direction: The Direction that is parallel to

receiver lines CMP BIN (or BIN)

All mid-points that lie inside this area, or bin, are

assumed to belong to the same common

midpoint. In other words, all traces that lie in thesame bin will be CMP stacked and contribute to

the fold of that bin.

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Fold

The number of midpoints that are stacked

within a CMP bin. Although one usually givesone average fold number for any survey, the fold

varies from bin to bin and for different offsets.

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Swat

The term swath, has been used with different

meanings in the industry. First, and mostcommonly, a swath equals the width of the area

over which source stations are recorded without

any cross-line rolls.

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Bin Size

Bin size can be determined by examining three

factors:target size, maximum unaliasedfrequency due to dip, and lateral resolution, and

then picking the smallest value of bin size

provided by these analyses as the design

parameter

.

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Target SizeNormally two to three traces, positioned so they

pass through a small target, will allow that target to

be seen in a 3-D image, because this means four to

nine traces will be related to the target on a time slice

of the horizon of interest. For example, if the target isa small reef or a narrow channel sand then the bins

should be small enough to get at least two (preferably

three) traces across the target. This imaging requirement

gives a 3-D designer an initial (and generally too

large) estimate for a bin size, which is

Bin Size < Target size/3

.

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Consider the following example where a 3-D survey

in Alberta crossed a 300-m ( mi) wide channel

(Cordsen, 1993b), which had been difficult to define

with 2-D data. Within this narrow channel, a 100-m

( mi) wide sand anomaly surrounded by shale could

be identified on the 3-D data (Figure 2.8). The choice

of 24 m 24 m (78 ft 78 ft) bin size allowed the

sand anomaly to be recognized on four traces crossing

the channel. This 4-trace response is close to the minimumthat is required for target recognition by many

interpreters. Had the bin size been chosen much

larger, the sand anomaly may not have shown up at

all.

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Maximum Unaliased Frequency

An adequate spatial sampling is necessary for

dips and frequencies expected, otherwise spatial

aliasing will occur.

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Referring to Figure

One needs to take account of the fact that t

represents only wavelength since two-waytraveltime is measured and two samples

per wavelength are required to avoid

aliasing.

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Maximum Unaliased Frequency:Each dipping seismic reflection event has maximum possible unaliased

frequency f before migration, that depends to velocity to the target,the value of geological dip and bin size B

B=V/4*f*Sin

Common practice is to use dominant frequency & average velocity

Reference (Planning 3D Seismic Surveys)

Receivers (BIN) should be close (Small) enough to avoidthe possibility of spatial alliasing.

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Calculate the Bin Size for the following

sets of numbers (Vint=3000 m/s)

Dip (in degrees) fmax (Hz)B

(meters)15 100

25 60

40 40