3-D Field methods

IntroductionWhat is 3-D seismic?

•It is a group of closely spaced 2-D source and receiver lines forming a grid that covers an area.

•Receiver and source lines may be perpendicular or at an angle.

Why 3-D seismic?•3-D migration eliminates misties over dipping reflectors.

•presents a more detailed image of the subsurface

3-D TerminologyInline: direction parallel receiver lines.

Crossline: direction orthogonal receiver lines.

CMP bin: a small rectangle (1/2 RI x 1/2 SI) that contains all the traces which belong to the same CMP.

Box (unit cell): area bounded by two adjacent receiver lines and two adjacent source lines.

Patch (template) : area of all live receivers recording from the same source.

Swath: length over which sources are recorded without crossline rollover.

3-D Swath Shooting Method1.Receiver lines are laid in parallel lines.

2.Source lines are laid in parallel lines in a direction orthogonal to receiver lines.

3.An area of receivers (patch) is activated.4.Source at patch center is shot and recorded.5.Patch is moved crossline one source interval.

6.Source at new patch center is shot and recorded.7.Repeat this until source line is finished.

8.This is one swath.9.Roll over one source-line interval and begin recording the next swath.

10.Keep doing this until the survey is finished.

Factors Controlling 3-D Survey Design1.Shallowest reflector of interest

2.Deepest reflector of interest3.Target Size

4.Target dip5.Multiples

6.CostParameters of 3-D Survey Design

-RI and SI-RLI and SLI

-CMP Fold-Maximum Offset-Minimum Offset

-Record length-Frequency

-Migration Aperture

-

Box (sometimes called “Unit Cell”) In orthogonal3-D surveys, this term applies to the area bounded

by two adjacent source lines and two adjacent receiver lines . The box usually represents the smallest area of a 3-D survey that contains the entire survey statistics (within the full fold area). In an orthogonal survey, the midpointbin located at the exact center of the box has contributions from many source-receiver pairs; theshortest offset trace belonging to that bin has thelargest minimum offset of the entire survey.

Cross-line Direction The direction that is orthogonal to receiver linesFold The number of midpoints that are stackedwithin a CMP bin. Although one usually gives one average fold number for any survey, the fold varies from bin to bin and for different offsets.Fold Taper The 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 the fold taper and the migration apron because one can tolerate reduced fold on the outer edges of the migration apron.

In-line Direction The direction that is parallel to receiver lines.Midpoint The point located exactly halfway between a source and a receiver location. If a 480-channel receiver patch is laid out, each source point will create 480 midpoints. Midpoints will often be scattered and may not necessarily form a regular grid.

Migration Apron The width of the fringe area thatneeds to be added to the 3-D survey to allow propermigration of any dipping event. This width does not need to be the same on all sides of the survey. Although this parameter is a distance rather than an angle, it has been commonly referred to as themigration aperture. The quality of images achieved by

3-D migration is the single most important advantageof 3-D versus 2-D imaging.

Patch A patch refers to all live receiver stations that record data from a given source point in the 3-D survey. The patch usually forms a rectangle of several parallel receiver lines. The patch moves around the survey and occupies different template positions as the survey moves to different source stations.

Receiver Line A line (perhaps a road or a cut-linethrough bush) along which receivers are laid out at regular intervals.

Scattering Angle Assuming 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.

Source Line A line (perhaps a road) along whichsource points (e.g., dynamite or vibrator points) are taken at regular intervals.Source Point Density (sometimes called shot density), SDThe number of source points/km2 or source points/mi2. Together with the number of channels, NC, and the size of the CMP bin, SD determinesthe fold.Super Bin This term (and others like macro bin ormaxi bin) applies to a group of neighboring CMPbins

Swath The term swath, has been used with differentmeanings in the industry. First, and most commonly, aswath equals the width of the area over which sourcestations are recorded without any cross-line rolls. Second, the term describes a parallel acquisition geometry, rather than an orthogonal geometry, in whichthere are some stacked lines that have no surface linesassociated with them.

Template A particular receiver patch into which anumber of source points are recorded

Survey Design Decision Table.Parameter Definitions and RequirementsFold Should be fold (if the S/N is good) up to 2-D fold (if high frequencies are expected).In-line fold number of receivers RI (2 SLI).Cross-line fold NRL 2.Bin size Use 3 to 4 traces across target. Should be Vint (4 fmax sin ); for aliasing frequency.Should provide N ( 2 to 4) points per wavelength of dominant frequency. Lateral Resolution available: N or Vint (N fdom ).Xmin Should be less than 1.0 to 1.2 times depth of shallowest horizon to be mapped.Xmax Should be approximately the same as target depth. Should not be large enough to cause direct wave interference, refracted wave interference (first breaks), or deep horizon critical reflection offset, particularly in the cross-line direction, or intolerable NMO stretch. Should exceed offset required to see deepest LVL (refractor), offset

required to cause NMO t> one wavelength of fdom, offset required to get multiple discrimination >3 wavelengths, and offset necessary for AVO analysis. Should be large enough to measure Xmax as a function of dip.Migration apron Must 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° 0.58 Z, and dip lateral movement after migration, which is Z tan . Can overlap with fold taper.Fold taper Is approximately patch dimension 4.Record length Must be sufficient to capture target horizons, migration apron, and diffraction tails.

Orthogonal survey design. Orthogonal design—zoomed.

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

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

IN-LINE FOLD=number of receivers x station interval ..……

2 x source interval along the receiver line

CROSS-LINE FOLD= source line length ..……

2 x receiver line interval

The above equations assume that the bin size remains constant and is equal to half of the receiver interval, which in turn is equal to half the source interval. They also assume an orthogonal layout with all the source pointswithin the patch.

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

Xmin

XMAX

Field Layouts

FULL-FOLD 3-DSAMPLING THE 5-D

PRESTACK WAVEFIELDSWATH

ORTHOGONAL

BRICK

NONORTHOGONAL

FLEXI-BIN® OR BIN FRACTIONATION

BUTTON PATCH

ZIG-ZAG

MEGA-BIN

HEXAGONAL BINNING

STAR

RADIAL

RANDOM

CIRCULAR PATCH

Field layouts—pros and cons of various layout strategies.

FULL-FOLD 3-D

A full-fold 3-D survey is one where source points and receiver stations are distributed on an even two dimensional grid with station spacing equal to line spacing. The grids are offset by one bin size. A full-fold 3-D survey has outstanding offset and azimuth distributions as long as one can afford to record with a large number of channels

SAMPLING THE 5-D PRESTACK WAVEFIELDThe ideal survey samples the 5-D pre-stack wave field W(t, xs, ys, xr, yr) that is dependent upon travel time t and the source and receiver locations, i.e., xs, ys, xr, yr (Vermeer, 1998a.) Three dimensional surveys generally record only a portion of this 5-D wave field with linespacing that are greater than the station spacing.

In thisgeometry, source and receiver lines are parallel and usually coincident.While source points are taken on one line, receivers are recording not only along the

source line but also along neighboring parallel receiver lines, creating

Cross-spread. Cross-spread with midpoint coverage.

SWATH

swath lines halfway between pairs of source and receiver lines. The Xmin distribution has almost zero values on the source lines and values equal to the line

spacing on the swath lines (Figure 5.5c). The offset distribution in all occupied bin lines is excellent

ORTHOGONALGenerally, source and receiver lines are laid out orthogonal to each other. Because the receivers cover a large area, this method is sometimes referred to as the patch method . This geometry is particularly easy for the survey crew and recording crew, and keeping track of station numbering is straightforward.

BRICKThe brick pattern was developed in an attempt to improve the offset distribution of the orthogonal method. By moving the groups of source points that lie between

alternate receiver lines to a half-line position, the pattern of offset distribution becomes more random in nature

NONORTHOGONALNon-orthogonal (or slanted) arrangements of source and receiver lines are used to get the benefits of the offset distributions of the brick design without some of the disadvantages, such as 90° turns and non-continuous source lines (and thereforecommon receiver-gathers(

FLEXI-BIN® OR BIN FRACTIONATIONIn this method, source points and receivers can be laid out in many different ways. Basically, one must ensure that source and receiver line spacing are non-integer with respect to the group interval.

BUTTON PATCHThe button patch method was developed and patented by ARCO (Bremner et al., 1990) and has routinely been used in many ARCO 3-D surveys. Eachbutton contains a tight pattern of receivers typically 6 6, 6 8, or 8 8. The final button geometry is largely determined by equipment considerations and cable restrictions. There is no requirement to keep the receiver patterns square.

ZIG-ZAGThe zig-zag pattern is popular in desert areas, or other locations where one has good access between receiver lines. Single source lines are located between adjacent pairs of receiver lines for a single zig-zag The source point positions should be located on a grid to create central midpoints.

MEGA-BINThe geometry is based on several concepts that are brought together in a unique way. Any asymmetry between the station and line dimensions of conventional designs aliases nonrandom, source-generated surface noise. The redistribution of the source and receiver locations in the mega-bin design reduces this asymmetry and samples such noise better. In addition, the acquisition footprint, which is typical for wide line spacings, is significantly reduced.

STARStar shooting involves laying out receiver lines in an arrangement resembling the spokes of a wheel Source points are taken along those same lines. In such surveys one can often have all receivers live while acquiring the source points. Fold is very high near the center and drops off quickly towards the edges of the survey

RADIALAn improvement over the star design is the radial design. This concept involves laying out receiver lines in an arrangement resembling the spokes of a wheel similar to the star design. However, source points are placed along concentric circles around the center of the survey

RANDOMThe main advantages of a true randomization of sources and receivers are improvements in the offset and azimuth distribution. Complete static coupling is also achieved.