T

2D Survey Design Basics

5.1 Basic Concepts in 2D Surveys

he guiding principle should be to design a seismic survey that will image the

selected target in the most economical way for costs and time. Resolution parameters, such

as the frequency required to image the target, are starting design factors. Shallow horizon of

interest and deeper horizons may be interpretational needs; thus, the definition of the

representative horizons is the beginning of the design.

5.1.1 Near Surface layer: The velocity of the surface layer is used as a factor in

computing offsets and determining the effect of ground roll. Usually the weathered layer is

very low velocity because of exposure and erosion, but it may be quite complex and have

several layers of variant velocity. The velocity and maximum dip of each layer are initial

parameters. This information can be obtained in approximate form from existing well logs or

seismic data in the area. If the area is frontier area, then noise tests, experience, or geologic

theory can be the source of this information.

5.1.2 Shallow Layer: While the target layer is most important for imaging, a shallow

layer may be necessary for processing or interpretation. Good data in the shallow part is

needed to use the velocity analysis with confidence. The velocity Vs and the approximate

arrival time are the needed parameters. These parameters allow computation of depth of the

layer Zs by the familiar time-distance formula

where: t = two-way travel time to the shallow horizon

Vs = average velocity to the layer, and

Zs = depth to the shallow layer

This formula provides the information needed for the near offset, i.e.,

5.1.3 Target Layer: The layer is the horizon of primary interest for the survey. When

parameters conflicts arise during the design, the requirements for the target layer should

prevail. For imaging the target horizon, geologic knowledge of the expected thickness and

reflectivity is needed to estimate the frequency range

5.1.4 Group Interval: Group Interval is the basic sampling on the earth’s surface by the

survey. It is the distance on the ground between receiver stations. Group interval represents

which largest spatial sampling shall prevent aliasing during migration:

where, Vmin = minimum velocity,

θ = maximum dip of the target horizon in degrees, and

Fmax = maximum frequency expected

5.1.5 Fresnel Zone: Fresnel zone (figure 7 (b)) is the smallest part of the reflector

making an unambiguous image of the individual event and is circular at zero offset but

elliptical with offset. Fresnel zone is given by:

where, tz = two-way record time of the target horizon

5.1.6 Far Offset: Far offset is a function of the depth modified by the velocity field. The

far offset required should be computed first for the target horizon and then for the deep

horizon. The velocity of the surface layer is involved because of the initial angular influence

on the down going seismic waveform at the depth of the horizon. Once the maximum offset

Maximum group interval =

is computed in combination with the near / offset, the group interval, the ideal parameters can

be evaluated within the framework of the available equipment.

where, Z is the depth of horizon, Vs is the velocity of the surface layer, and V the average

velocity to the target.

If the horizon is dipping, then the distance, Hmax should be extended by:

where, Z = depth of horizon and

θ = dip

This extension is quite important in 3D exploration (Migration aperture). Neglect of this

factor can result in underestimating field costs. The well-founded rule of thumb says that the

spread length should be equal to or a little greater than the depth of the reflection being

imaged. The far-trace distance should preserve full fold on the target horizon. Data

processing often requires considerable muting of the shallow data on the greater offsets

because of NMO stretch, noise trains, and other factors. The target horizon should be

protected by the survey for the mute. The custom is to automatically mute below the “20 to

30 percent stretch factor” the formula most used for this step is:

where H = offset distance, V= velocity at time Tz and Tz = arrival time of the event at H = 0.

When Tm exceeds 0.3, then data processing will probably form an automatic mute.

Too large on offset range for a given number of receiver stations may result in

inadequate fold for the shallow layer or even the target layer. On the other hand, if the offset

range is not long enough, accurate velocity analysis and the suppression of multiples during

the processing can be endangered.

5.1.7 Record Length: Part of the survey design is to determine the required sampling

rate in time and the record length is a function of depth and velocity of the deepest horizon.

Where Td = two-way arrival time of the deepest horizon of interest at the maximum offset

Tr = required record length in time, and

L = length in time of the longest processing filter.

Normally 200ms is adequate for the filter length. The extra time in recording is balanced

against the possible benefits from data from very deep horizons. Signal length becomes more

important when the source is vibratory in nature. Some allowance should also be made for

migration.

5.1.8 Sample Rate: The sampling rate in time is more or less standard, ranging from 2

to 4 ms depending on the resolution needed. The rule is

A sample rate of 2ms is used for most seismic surveys.

5.1.9 Group Interval and Field Equipment: The group interval possible with a

particular recording equipment given by

where, Hmax = far offset,

Hmin = near offset, and

NC = the number of channels available for recording.

5.1.10 Fold Coverage: Each source position yields a certain amount of subsurface

coverage. For flat layers, the sampled point is half the distance from source to receiver. The

subsurface sampling is half the interval of the surface coverage. Foldage is defined as the

number of times a particular sub-surface sampling point (CMP) is covered by different

sources receiver locations. The maximum fold of coverage is given by

Where, S is the number of units of group interval in the source spacing and NC the number of

channels available.

5.1.11 Source Interval: The source interval in the distance between source positions.

The source interval is function of the desired fold coverage and the number of channels

available.

Where, NC is the number of channels and F is the fold.

5.1.12 Source Power: There is a decision to be made in some cases on the source power.

For dynamite, the charge size in kilograms if the unit. For vibratory sources, the available

power (in pounds per square inch) is specified by equipment model. For instance, a large

vibrator can generate 50,000 psi. Marine sources such as air guns and water guns are defined

in terms of their volume and peak-to-peak strength. The power needed is function of target

depth and the environmental noise. As the earth has a natural attenuation, target depth is the

primary consideration. Noise is also involved since more power has the potential to generate

more noise.

The amount of energy generated in a shot hole is proportional to the quality of

dynamite. It is well known that an explosive source in a cylindrical enclosure generates

pressure waves and shear waves of both polarizations.

5.1.13 Line Location and Orientation: The geometry of the survey is not

independent of the target. The location, direction, and length of the lines are important

considerations in the survey design. Dip lines for instance, are favored over strike lines.

Some of basic concepts generally accepted for lines locations/orientations are:

• The lines should, when possible, be perpendicular to fault planes. Since

definition of the fault plane is best on the seismogram when the lines are

perpendicular to the plane.

• Line ties are important to interpretation. When there is existing seismic data

nearby, new lines are planned in such a manner that they can be ties with the

existing data. One very helpful ties is to a well. The closer the line can

approach the well, the more useful the tie of the seismic data to the well log.

• When there is no conflict with other needs, lines should be planned to

minimize elevation and terrain problems. Sometimes a small shift in the line

location can avoid a troublesome obstacle.

Figure 7(a) shows the Surface Geometry and Sub surface Nature and Behavior of 2D

Layout.

S

3D Survey Design Basics

6.1 Why 3D Seismic Survey?

ub-surface geological features of interest in hydrocarbon exploration are 3-dimensional in

nature. A 2D seismic section is a cross-section of 3D seismic response. Despite the fact that

2D seismic section contains signal from all directions, including out-of-plane of the profile,

2D migration normally assumes that all the signal comes from the plane of the profile itself.

Although out of plane reflections (side-sweeps) are often recognizable by the experienced

seismic interpreter, the out of plane signal sometimes causes 2D migrated section to mistie.

These misties are due to inadequate imaging of the subsurface resulting from the use of 2D

rather than 3D migration. On the other hand, 3D migration of 3D data provides adequate and

detailed 3D image of the subsurface, leading to a more reliable interpretation. When

integrated with well logs, core and other petrophysical and production data, 3D data permits

reservoir characterization. The integrity of any 3 D data set leans heavily upon the suitability

of acquisition geometry.

6.2 Basic Concepts in 3D Surveys

The 2D surveys are as linear as the terrain allows. Source and receiver are normally

in-line with each other. Arrays may be multi-dimensional, but most often are also in the line

of survey. For 3D surveys, this is seldom the case.

The source interval of a 2D survey must be extended to include a definition of the

source line. For 3D surveys, source line must be defined, since for most common designs, the

source line is orthogonal to the receiver lines. The receiver line becomes the receiver lines.

As many receiver lines are laid out as the equipment for acquisition allows. Also, the

receiver, layout may not be lines but circles, checkerboards and other patterns developed for

3D surveys. Thus the simple parameters that defined the traditional 2D line now must be

extended to include more geometry.

The analysis of 2D designs centers on the subsurface coverage in the form of

common-depth points (CDPs). For 3D surveys, the CDP becomes two-dimensional and is

termed a “bin”. These bins may be square or rectangular and define the spatial resolution of

the data sampling. Indeed, deciding the bin size will be the first step in designing a 3D

template. Subsurface sampling will be, as with the CDP, half the surface size. The accent of

2D lines is on the fold of coverage and the offset range. For 3D survey the fold may less, but

the azimuth range is added to the offset range as a parameter. If structure is complex, then

good azimuths range becomes more important. Where structure is complex the velocity

analysis must include an azimuthal property. The range of azimuths in the bin is also a

consideration.

Another new factor is the use of computers to do the design. Moreover, interpretation

is usually conducted on workstations. The multiple source and receiver lines, the difficulty of

computing, fold coverage, azimuthal distribution, and offset ranges in the bins make the use

of a computer program to aid in 3D design almost a necessity. The Fresnel zone assumes

some new characteristics in three dimensions. Essentially, the theoretical point source

expands as it propagates in depth, “illuminating” a circular area at vertical incidence. In a

seismic context, this is the reflecting surface constructively contributing to the reflection. A

good approximation to the radius of the zone is which shows that the zone increases in radius

with depth but decreases with higher frequency wave fronts. Migration serves to reduce the

zone to some minimal size when accurately done and the data fits the assumption.

It should be noted that when the reflecting point is offset, the circle becomes

elliptical. This angular effect actually reduces the size of the zone along the minor axis of the

elliptical response. Dip and structure also are factors in the actual response..

where, V = average velocity to the event,

T = arrival time and

t = peak-to-zero crossing of the wavelet.

An important aspect of 3D data and Fresnel zones is the extra dimension of focusing possible

with migration.

6.2.1 Preliminary Parameters: There are some parameters that need to be estimated

as input when designing the 3D survey. The physics and concepts are somewhat independent

of whether the survey is to have two or three dimensions.

6.2.1.1 Offset: The imaging of shallow target and deep horizons still requires certain offset

of source and receiver. An approximation to the required offset for a given horizon is very

simple and used often when surveys are designed in the field:

Offset = depth of the horizon.

New factors include the fact that the offset may now be measured at an angle and the depth is

now that of a plane rather than a line.

6.2.1.2 Fold: The fold required for noise suppression is a function of the S/N conditions.

This translates in 3D to the number of traces in a bin. Because of the extra focusing by

migration and the flexibility of binning, fold can be less than required in 2D surveys. Field

tests or existing 2D seismic data can yield an estimate of the needed fold for the 3D survey.

6.2.1.3 Frequency: The temporal frequency required is not much different from that of

2D surveys. The rules for the resolution of layer of given thickness are best determined by

modeling. The general rule is that the resolution of a thin bed requires it to be sampled twice

within a quarter wavelength of the highest frequency. As a field approximation, the

maximum frequency expected:

T = Two way time of the horizon

6.2.1.4 Objectives of the Survey: The most important information is defining the

objectives of the survey. Although this seems a rather obvious comment, many times the

objectives of the survey except for the aerial extent and approximate spatial sampling are not

part of the input to design. Requirements of good fold on a shallow reference layer or a deep

reflection for survey are clearly stated.

6.2.1.5 Migration Aperture: When the beds are dipping, the extent of the survey must

be increased by:

D = Z tan θ

where, Z = depth,

θ = dip

6.2.1.6 Seismic Data Input: The most useful direct input is existing seismic data. The

seismic sections give information abut many of the design parameters such as noise, source

power, weathering problems, and general structure.

Field records and final stack should be checked for environmental and source

generated noise conditions. The array design should be studied for possible use in the 3D

survey. There are areas where neither source nor environmental noise is a problem, which

greatly simplifies the survey design and makes the whole project less expensive. Type of

source, the power used in either surveys, quality of reflections at depth, frequency content of

shallow data are some of the key factors in deciding the source power to be used. If extensive

static corrections made during processing indicate problems in the near surface, this should

be noted on the survey design. The design of the survey can reduce processing problems in

many cases.

6.3 3-D Survey Design Sequence

There are many ways to begin and complete a survey design. The specific sequence

of steps that follow are general guide lines. Some design templates will dictate a different

sequence of other parameters.

A summary of the proposed sequence for developing a design sequence is:

• Determine the subsurface bin size. Twice the chosen bin size is the source

and receiver station spacings.

• Compute the number of source stations per kilometer required to achieve

fold with available equipment. The number of stations per square kilometer

allows computation of the source line spacing.

• Compute the receiver line spacing.

• Find the number of receiver lines allowed by the field equipment, constrained

by the required offset ranges. The result is the template.

• Decide the in-line and cross-line roll alongs.

• Allow for obstacles and run analyses of the offset distribution ranges of

offsets and azimuthal properties of the bins.

• Estimate time and costs of the script and iterate until attributes, costs, and

time are satisfied.

6.3.1 Bin size: For 3-D data the bin is the basic building block for the rest of the survey.

Bin size depends on target size, spatial resolution needed, and economics. The traces when

their subsurface reflection point falls with in the bin, are treated as a CDP, and corrected and

summed to represent that bin position by a point. A bin can be any size but rectangles and

squares are the popular. The basic sampling theorem applies to the bin.

where, b is the bin size,

Fm = Maximum frequency expected,

θ = maximum dip in degrees and

Vmin = minimum velocity.

6.3.2 Source line spacing: The bin size will, however, allow more design calculations

if the fold and number of channels on the equipment are known .

where, NS = shots per square kilometer,

F = desired fold,

R = number of channels

B = subsurface bin size.

Determining NS allows for the computation of the next important parameter source line

spacing.

6.3.3 Receiver line spacing: The new information required is the minimum offset and

the offset ranges needed. The controlling parameter will be the largest minimum offset within

a bin. The minimum offset has been previously established with preliminary calculations and

modeling. The approximation is that the maximum offset needs to be at least as long as the

depth of the most reflection to be imaged.

Thus, a2 = (c2-b2)1/2 where ‘a’ is the receiver line spacing, c is the largest minimum

offset and b is the source line spacing. A smaller near offset would require ‘a’ smaller

receiver line spacing and be more expensive. As with the source line spacing, the receiver

line spacing from calculation may be reduced.

6.3.4 Number and length of the receiver lines in the template: The problem

is to be determine the number of receiver lines possible with the template. The number of

lines is constrained by the required maximum offset which sets the length of the lines. The

maximum offset found in the preliminary 2-D calculations or 3-D modeling is a function of

deepest horizon to be imaged. The field estimate is that the maximum offset should be a little

greater than the depth of the deep horizon, but exact formula include dip. The target

parameters are the number and length of the receiver lines. The source line shift is an

adjustable variable. The second constraint is the number of channels available with the

equipment.

6.3.5 Determining the template movement: Usually the field people prefer to

roll along the direction of receiver lines. The increment is at the source line spacing. At the

end of the coverage in in-line direction the next swath would be done in same manner

incremented in the source direction and continued until the coverage was completed.

6.3.6 Estimation of nominal fold: Stacking fold is the number of field traces that

contribute one stack trace. Fold controls the signal to noise ratio. Fold should be decided by

looking at previous 2-D and 3-D surveys in the area.

6.3.6.1 In-line fold: For an orthogonal straight-line survey, in-line fold is defined

similarly to the fold on 2-D data. The formula is as follows:

In line fold = ___( no. of receivers x station interval )__ .

2 x Source interval along the receiver line

(or)

In line fold = ( number of receivers x receiver interval )

2 x Shot line interval

6.3.6.2 Cross-line fold: Similar to the calculation of in-line fold, the cross-line fold is:

Cross line fold = source line length _.

2 x receiver line interval

6.3.6.3 Total Fold: The total 3D nominal fold is the produce of in-line fold and cross-line

fold:

Total nominal fold = ( in-line fold ) x ( cross-line fold).

6.4 Land 3D Layouts: Numerous layout strategies have been developed for land 3D

surveys. One has to establish which features are important in the area of the survey in order

to select the best design option.

6.4.1 Full fold 3-D: A full fold 3D survey is one where source points and receiver

stations are distributed on an even two-dimensional grid with station spacings equal to the

line spacings. The grids are offset by one bin size. A full fold 3D survey has outstanding

offset and azimuth distributions as long as one can afford to record with a large number of

channels. All other 3D designs are basically subsets of such full-fold surveys, and the

designer has to decide which aspects of a 3D design are absolutely necessary and which can

be compromised.

6.4.2 Swath: The swath acquisition method was used in the earliest 3D designs. In this

geometry 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 swath lines halfway between pairs of source and

receiver lines. The offset distribution in all occupied bin lines is excellent. However

inadequate sampling in the cross-line direction makes this design a “poor man’s 3-D”,

because many bins are empty. The azimuth mix is very narrow and depends on the number of

live receiver lines in the recording patch and the line spacing. Most companies prefer to have

the source points at the half-integer positions. Parallel swaths are sometimes considered on

land when severe surface restrictions exist, or when costs have to be minimized. The

operational advantages are attractive, but are achieved at the cost of a poor azimuth mix and

poor cross-line sampling.

6.4.3 Orthogonal: Generally, 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. In an orthogonal design, the active

receiver lines form a rectangular patch surrounding each source point location creating a

series of cross spreads that overlap each other. This technique allows more surface area to be

acquired prior to receiver stations moves.

This method is easy to lay out in the field and can accommodate the extra equipment

and roll along operation. Usually all the source points between adjacent receiver lines are

recorded. Then the receiver patch is rolled over one and the process is repeated. The azimuth

distribution for the orthogonal method is uniform as long as wide recording patch is used.

Figure 8(a) shows a 3D layout and the subsurface nature while figure 8(b) shows the

3D cable layout used by GP “Y”.