3D Acquisition

8/4/2019 3D Acquisition

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Box (sometimes called Unit Cell) In orthogonal

3-D surveys, this term applies to the area boundedby two adjacent source lines and two adjacentreceiver lines . The box usually represents thesmallest area of a 3-D survey that contains the entiresurvey statistics (within the full fold area). In an

orthogonal survey, the midpointbin located at the exact center of the box hascontributions from many source-receiver pairs; theshortest offset trace belonging to that bin has the

largest minimum offset of the entire survey.


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Cross-line Direction The direction that isorthogonal to receiver lines

Fold The number of midpoints that are stackedwithin a CMP bin. Although one usually givesone average fold number for any survey, the foldvaries from bin to bin and for different offsets.

Fold TaperThe width of the additional fringe areathat needs to be added to the 3-D surface area to buildup full fold . Often there is some overlap between thefold taper and the migration apron because one cantolerate reduced fold on the outer edges of themigration apron.


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In-line DirectionThe direction that is parallel toreceiver lines.

MidpointThe point located exactly halfwaybetween a source and a receiver location. If a 480-channel receiver patch is laid out, each source

point will create 480 midpoints. Midpoints willoften be scattered and may not necessarily form aregular grid.


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Migration ApronThe width of the fringe area that

needs to be added to the 3-D survey to allow propermigration of any dipping event. This width does notneed to be the same on all sides of the survey.Although this parameter is a distance rather than anangle, it has been commonly referred to as themigration aperture. The quality of images achieved by3-D migration is the single most important advantageof 3-D versus 2-D imaging.


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PatchA patch refers to all live receiver stationsthat record data from a given source point in the 3-

D survey. The patch usually forms a rectangle ofseveral parallel receiver lines. The patch movesaround the survey and occupies different templatepositions as the survey moves to different source

stations.

Receiver LineA line (perhaps a road or a cut-line

through bush) along which receivers are laid outat regular intervals.


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Scattering AngleAssuming the presence of a pointscatterer (diffraction point) at depth, the scattering

angle is the angle between the vertical downgoingsource-scatterer raypath and the upgoing scatterer-receiver raypath.

Signal-to-Noise RatioThe ratio of the energy of thesignal over the energy of the noise. Usuallyabbreviated as S/N.

Source LineA line (perhaps a road) along whichsource points (e.g., dynamite or vibrator points)are taken at regular intervals.


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Source Point Density (sometimes called shotdensity), SD

The number of source points/km2 or sourcepoints/mi2. Together with the number of channels,NC, and the size of the CMP bin, SD determinesthe fold.

Super BinThis term (and others like macro bin ormaxi bin) applies to a group of neighboring CMPbins


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SwathThe term swath, has been used with differentmeanings in the industry. First, and most commonly, a

swath equals the width of the area over which sourcestations are recorded without any cross-line rolls.Second, the term describes a parallel acquisitiongeometry, rather than an orthogonal geometry, in which

there are some stacked lines that have no surface linesassociated with them.

TemplateA particular receiver patch into which anumber of source points are recorded


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Survey Design Decision Table.Parameter Definitions and RequirementsFoldShould be fold (if the S/N is good) up to 2-D fold (if high frequencies areexpected).

In-line fold number of receivers RI (2 SLI).Cross-line fold NRL 2.Binsize Use 3 to 4 traces across target. Should be Vint (4 fmax sin ); for aliasingfrequency.Should provide N ( 2 to 4) points per wavelength of dominant frequency. LateralResolution available: N orVint (N fdom ).XminShould be less than 1.0 to 1.2 times depth of shallowest horizon to be mapped.

XmaxShould be approximately the same as target depth. Should not be large enoughto cause direct wave interference, refracted wave interference (first breaks), or deephorizon critical reflection offset, particularly in the cross-line direction, or intolerableNMO stretch. Should exceed offset required to see deepest LVL (refractor), offsetrequired to cause NMO t> one wavelength of fdom, offset required to get multiplediscrimination>3 wavelengths, and offset necessary for AVO analysis. Should be large

enough to measure Xmax as a function of dip.MigrationapronMust exceed radius of first Fresnel zone, diffraction width (apex to tail)for an upward scattering(full-fold) angle of 30, i.e., Z tan 30 0.58Z, and dip lateralmovement after migration, which isZ tan. Can overlap with fold taper.Fold taper Is approximately patch dimension 4.Record length Must be sufficient to capture target horizons, migration apron, anddiffraction tails.


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Orthogonal survey design.

Orthogonal designzoomed.


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FOLD

Stacking fold (or fold-of-coverage) is the number offield traces that contribute to one stack trace, i.e.,the number of midpoints per CMP bin. It is also thenumber of overlapping midpoint areas

TOTAL FOLD

The total 3-D nominal fold is the product of in-linefold and cross-line fold:total nominal fold = (in-line fold) (cross-line fold).


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number of receivers x station interval..2 x source interval along the receiver line

IN-LINE FOLD=

CROSS-LINE FOLD =source line length..2 x receiver line interval


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The above equations assume that thebin size remains constant and is equalto half of the receiver interval, which in

turn is equal to half the source interval.They also assume an orthogonal layoutwith all the source points

within the patch.


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XMIN


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XMAX

The required maximum offset depends on the depth to thedeeper targets that must be imaged. One also needs to take intoaccount normal moveout (NMO) assumptions and dip.

3D Acquisition

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