76336191 gathering and sorting

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GATHERING

A gather is a collection of seismic traces which havean acquisition attribute (parameter) in common.

Sorting

Gathers are sorted from field records in order to

examine the dependence of amplitude, signal to noise

ratio, move-out, frequency, phase, incidence angle,

azimuth and other geometric attributes that areimportant for data processing and imaging.


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1) COMMON-RECEIVER GATHER

The complete set of data that was received

at one geophone. Each trace in a common-

receiver gather represents the subsurface image

beneath the receiver contributed by a particularshot.


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The common-receiver gather represents the

electromagnetic signal at the receiver whichis induced by the source at all different

locations and different times during the

survey. This gather is useful for

studying problems associated with one

particular receiver. For example, a bad

receiver that generates one noisytrace per

shot.


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2) COMMON-OFFSET GATHEROR

COMMON

RANGE GATHERDisplays the succession of data traces

which have the same offset

distance (which is the distance betweenthe shot point and the receiver). It uses

one trace per shot section.


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In common-offset shooting, if the target is flat

and no changes occur, then all traces recordedwill be identical. Common offset is used in areaswhere the targets are well known and easilyidentified. It aims to detect changes in the targets

depthandcharacter. Since all the traces are at thesame offset, it is sometimes used in AVO analysisas well; one can quickly take a quick look at thevariations in layering along a line. It requires very

little processing and is considered cheap methodto track reflections.


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3) COMMON-SHOTPOINT GATHER

The traces of the gather come from a single shotand many receivers.

A shotpoint gather samples

various midpoints and a

variety of angles.


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EXAMPLEONSHOTGATHER

It is easy to inspect traces in these displays for bad

receivers or bad shots.


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4) COMMON-MIDPOINT GATHER

OR COMMON-DEPTHPOINT GATHER

Acquiring a series of traces which are reflected

from the same commonsubsurfacemid-point.

The number of traces in a CMP gather definesthefoldof the data. Where the fold is the

number of times that the same midpoint is

sampled by different shots and different

receivers.


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USING CMP TRACES IN NMO CORRECTION

AND STACKING:

Velocity functions are calculated for selected CMPgathers based on arrival time variations as a function

of the offset distance. CMP velocity functions arethen interpolated to construct a velocity model ofthe subsurface. This velocity model is used toperform normal move-out (NMO) corrections.

Where NMO is the effect of the delayin reflection arrival times when geophones andshotpoints are offset from each other.


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NMO correction is applied to each trace in the

gather so that the reflection travel times

approximate that of a trace with zero offset

distance (a coincident source and receiver). After

NMO correction, all the traces in a CMP gathercan be summed (stacked). By repeating this

process for all the CMP gathers in the survey, the

pre-stack data set is replaced by a much smaller

post-stack data set of much higher signal quality.


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REFLECTION POINT DISPERSAL

The phenomenon where the CMP gather no

longer relates to a single reflection point. This

is noticed in the presence of subsurface dip.

Since midpoints are surface co-ordinates,they are independent of subsurface conditions

unlike the depth points (vertical) and reflection

points (lateral). It is for this reason that the

preferred term is common mid point (CMP)

shooting rather than common depth point

(CDP) or common reflection point (CRP)


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Data from several shotsand receivers are combinedinto a single gather.

The CMP gather will appear similar to acommon-shotpoint gather, but should have lessvariability as all the traces refer to a commonmidpoint

CMP gathers samplevarying angles but a

common geological

midpoint.


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CMP acquisition is accomplished by firing the source into many

receivers simultaneously as shown in the figure which depicts

a shot gatherwhere a single shot is fired into 12 receivers.


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The figure shows the original shot and second shot. In this case, theshot has moved a distance equal to the receiver spacing andCMP spacing is equal to halfthe receiver spacing.


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The figure shows how the fold of the CMP gathersis starting to build up after six shots have been

fired.


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76336191 gathering and sorting

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