L19-3D Survey Design

8/11/2019 L19-3D Survey Design

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Day 1

Introduction

Data Acquisition

Analysis in the frequency domain

Basic Processing

Day 5

3-D acquisition

3-D imaging

3-D Survey design

Day 2Statics

Velocity & rock properties

Synthetic seismogram generationFiltering and sampling

Day 3

Stratigraphic AnalysisAmplitudes

Spatial filtering & analysis

Multiple suppression

Day 4

Modeling IIImaging and migration

Migration application

Prestack / depth migration

Seismic Imaging of Subsurface GeologySeismic Imaging of Subsurface Geology -- OutlineOutline


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The dilemma in modern seismic acquisitionThe dilemma in modern seismic acquisition

New acquisition techniquessave time and money...

But

Data Quality Suffers

Due to Poorer Subsurface Sampling


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The Challenge: Obtain the proper balanceThe Challenge: Obtain the proper balance

New acquisition techniques save money but degrade data quality.

High Data quality requires high acquisition and processing costs.


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Poor communication: the road to failurePoor communication: the road to failure


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Good acquisition begins with a good designGood acquisition begins with a good design

Survey

Design

Target

information

Sampling

theory

Time

Business

Needs Experience

Model

studies

Interpretation

considerationsField

techniques

Processing

considerations

Costs$$$

Information integration requires a multi-disciplined approach


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Seismic InterpreterSeismic Interpreters role ins role in

survey designsurvey design

Starts the ball rolling Provides input on

Business needs, target information Interpretation experience / requirements

Operational constraints

Helps determine minimum acceptable dataquality

Contributes to cost / benefit analysis


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Determining Acquisition SpecificationsDetermining Acquisition Specifications


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Design Initiation

Request from Business unit

Initial communicationCost and timing estimate

Basic Input

Business/ imaging objectivesExperience (interpretation,

processing)

Multidisciplinary work session


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Asia case history 3Asia case history 3--D design: InputD design: Input

Business Objectives Determine reservoir geometry & continuity in the section tested by

wells

Use seismic attributes to predict & map hydrocarbon distributionfor accurate volumetrics and well placement

Determine the structural framework for the deeper, untestedsection and generate additional prospects

Products Structure maps Hydrocarbon distribution maps

Seismic facies maps AVO analyses

Reservoir thickness maps Detailed Velocity Model

Constraints / concerns Limited operational widow due to weather (130 -150 days)

Availability of equipment


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Asia case history 3Asia case history 3--D design: InputD design: Input

Target-specific information

Primary target Secondary target

Depth 1700 2400 m 2700 4200 m

Minimum thickness 20 m 40 m

Minimum fault throw 10 m 15 m

Minimum fracture width 150 m 250 m

Maximum structural dip 15o 35o

Initial structural

interpretation requires only

moderate quality, butlonger-term needs will

require high data quality

Structural and stratigraphic

interpretation require high

data quality

Data requirements

Experience for primary target

interval velocities range up to 3000 m / smaximum frequency is 60 Hz


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Geophysical requirements

& Survey Parameters

ResolutionBin Size

Signal-to-noise ratio

FoldAperture


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Asia case history 3Asia case history 3--D design: ResolutionD design: Resolution

primary targetprimary target

RV= vertical resolution

VINT_MAX= maximum interval velocityBW = bandwidth BW

V

R

MAXINT

V 2

=

Fmax = 60 Hz BW = 50 Hz RV = 3000 / 2* 50 = 30m

RLM= lateral resolution

Min width = width of smallest feature = 150 m

# traces = no. traces to be confident that a feature is present(assume a value of 3 is reasonable)

m

m

traces

width

RLM 503

150

#

.min

===


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Asia case history 3Asia case history 3--D design: bin sizeD design: bin size

= required migration aperture angle

=

sinV

LMRR 6.0sin ==

LM

V

RRRV= 30 m

RLM= 50m = 37o

For bin size determination, we use the larger of the migrationaperture angle or the maximum structural dip. In this case, 37o

is larger than the maximum structural dip.

mfVx

MAX

MAXINT 216.*60*4

3000sin4

==


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Asia case history: 3Asia case history: 3--D fold predictionD fold prediction

based on 2based on 2--D dataD data

Obtain acceptable 2-D data from area

Reduce data fold in data processing by discarding shotrecords and/or receiver stations until data are no longeracceptable

42 is the minimum acceptable fold on Asia data with 30 mtrace spacing

2-D Effort level = 2-D fold / trace spacing

Minimum acceptable 2-D effort level = 1.4 Determine 3-D fold with equivalent S/N, using empirical

relation between 2-D and 3-D effort levels


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Design formulation

Geometry & methodology

Design evaluationFold / offset / azimuth analysis

Geometry / artifact modeling

Cost benefit analysis


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Asia case history 3Asia case history 3--D designD designPossible Acquisition configurationsPossible Acquisition configurations

Scenario A

1 vessel

2 source arrays

6 streamers

Scenario B

1 vessel

2 source arrays

8 streamers

Scenario C

2 vessels

2 source arrays

9 streamers


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Geometry artifact modelingGeometry artifact modeling

Determine whether the magnitude of geometry-

induced artifacts will exceed an acceptable level

A tif tA tif t C li lit d i tiC li lit d i ti


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ArtifactsArtifacts -- Crossline amplitude variationsCrossline amplitude variationsAsia c. h. 3Asia c. h. 3--D designD design-- Acquisition window = 130Acquisition window = 130 -- 150 days150 days

3%4%

Am

plitude

Scenario A

(6 streamers) 92%

3%

5%

92%

Amplitude

Scenario B

(8 streamers)

92%

4%

16%

A

mplitude

Inline position

Scenario C

(9 streamers)


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CostCost--benefit analysis: decimation studiesbenefit analysis: decimation studies

Simulate data that would have been acquired at lower cost

Processing

Assess impact of lower effort on meeting business needs S/N analysis, seismic sections, maps, volumetrics


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Typical cost distributionTypical cost distribution -- 33--D surveysD surveys

The principal cost drivers in

acquisition are

Fold Bin size

Aperture

These factors control Acquisition cost and time

Data quality and interpretability

Acquisition

Processinginte

rpretati

on

The challenge:

obtain the survey with the best cost-benefit ratio.

Impact of fold on acq isition costImpact of fold on acquisition cost


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Impact of fold on acquisition costImpact of fold on acquisition cost

Single source Dual sourceMarine surveysArea = 1 x 1km

Bin = 12.5 x 25m

40 inlines

80 stations20 fold

4 lines per boat pass

10 boat passes

40 fold2 lines per boat pass

20 boat passes

Lower fold Fewer boat passes Less time & money

Land surveys Shoot every station Shoot every second stationArea = 1 x 1km

Bin = 12.5 x 25m

40 inlines80 stations

4 lines per swath40 fold

10 swaths x 80 shots/swath

800 shot locations

20 fold


400 shot locations

Lower fold Fewer shot locations Less time & money


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Impact of fold on data qualityImpact of fold on data quality

As fold is decreased, S/N decreasesnonlinearly

Reflections are disrupted by noise

Weak reflections are impacted morethan strong reflections

Interpretation is more difficult

Auto-picking is less reliable

Maps have more non-geologicirregularities

Seismic attributes are less sensitive

to changes in physical properties

3-D fold

S/N

ratio

We need to determine the best tradeoff between

data quality (Fold) and cost Decimation studies are useful

Impact of bin size on acquisition costImpact of bin size on acquisition cost


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Impact of bin size on acquisition costImpact of bin size on acquisition cost

Streamers at 200mStreamers at 100mMarine surveysArea = 1 x 1km

Inline spacing = 12.5m

80 stations

4 lines / boat pass 12.5 x 50 m bins200m swath per boat pass

5 boat passes

12.5 x 25 m bins100m swath per boat pass

10 boat passes

Larger bins Fewer boat passes Less time & money

Land surveys Cables at 50m spacing

Area = 1 x 1km

Inline spacing = 12.5m80 stations

4 lines per swath

Cables at 100m spacing

12.5 x 50 m bins


400 shot locations

12.5 x 25 m bins


800 shot locations

Larger bins Fewer shot locations Less time & money


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Impact of bin size on data qualityImpact of bin size on data quality

As Bin Size is increased, spatial density decreases Effort level decreases, leading to S/N ratio decrease

Lateral resolution decreases small features can be missed

More diffractions are improperly imaged (more migrationnoise)

Interpretation and auto-picking are less reliable, due toaliasing effects

Steeply dipping events are improperly imaged

Attributes are smoothed and their values can be clipped

We need to determine the best tradeoff betweendata quality (bin size) and cost

Resolution equations / model studies are useful

Tradeoff must consider the impact on business needs

I t f t i iti tImpact of aperture on acquisition costs


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Impact of aperture on acquisition costsImpact of aperture on acquisition costs

Image area

8km

6km

15o surface coverage case

30o surface coverage case

1000m0m 475m aperture

Image area = 48 sq. km

Surface coverage

15o case = 62 sq. km

30o case = 80 sq. km

Smaller aperture Fewer/ shorter lines Less time & money

l iB l i d lid li


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Balancing costBalancing cost vsvs data quality: aperturedata quality: aperture

30o case: 80 sq. km

20o case: 68 sq. km10o case: 57 sq. km

Imagability considerations Location of steeply dipping reflectors

How far beyond the image area do raypaths extend?

Lateral resolution needed near edge of image area Impact of diffractions near edge of image area

A i hi & i i i iA i hi & i i i i


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Asia case history: costs & acquisition timeAsia case history: costs & acquisition time

Scenario A 6 streamers

Scenario B 8 streamers

Scenario C 9 streamers

Asia c h : benefitAsia c h : benefit -- cost comparisoncost comparison


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Asia c. h.: benefitAsia c. h.: benefit - cost comparisoncost comparisonRequirement

Primarytarget

Scenario

A

Scenario

B

Scenario

C

Vessels 1 1 2

Sources 2 2 2Streamers 6 8 9

Bin size 12.5x20 12.5x20 12.5x25

Fold 54 54 54

Vert. resolution 20m 18m 18m 18m

Lat. Resolution 100m 86m 86m 108m

Capture angle 31o 31o 31o 14o

Predicted S/N Adequate adequate adequate adequate

Artifacts 8% max 4% 5% 16%

Cost ($M) 30.2 28.8 22.8

Time (days) 130-150 167 125 81

A i h I t f l t l l tiA i h I t f l t l l ti


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Asia c. h.: Impact of lateral resolutionAsia c. h.: Impact of lateral resolution

Requirement R LM < 100m R LM < 120m

Bin size 12.5x20m 12.5x25m

Fold 54 54

Lat. Resolution 92m 115m

S/N Adequate adequate

Cost ($M) 29 24

Time 126 98

Based on scenario B

A slight change in required resolution (from 100m to 120m) can

result in a significant savings in time (28 days) and money ($5M),with only a small degradation in predicted data quality.


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Survey design summarySurvey design summary

Key survey design to imaging target(s)

Assess tradeoffs between acquisition cost and data

quality

Choose field equipment and parameters to Get energy into subsurface with good characteristics

Magnify reflection energy while suppressing noise

Be safe with low environmental impact

Design a 3-D survey over the Theta prospect ExerciseTime: 0 75 hour


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Time: 0.75 hour

Objective: To illustrate key steps in planning a 3-D seismic survey:

Required input, useful analysis, balancing quality & costResources:1: A plot of a Wiener spectrum obtained from a 2-D survey from the area

2: A map showing the target dimensions and maximum dips

3: A diagram showing raypath modeling from the target zone4: A sketch map to use in determining surface coverage

5 :A plot of the relationship between 2-D effort level and 3-D effort level

6: A diagram of a simple geologic model and some seismic models

Prospect / equipment Information:

The target is at a depth of 3390 m.

The reservoir has beds on the order of 25 m in thickness.

The reservoir is dissected by shale-filled channels that are typically 150 m wide.Interval velocities for the reservoir unit range between 2100 and 2300 m/s.

The area to be imaged is 10 km by 10 km

The seismic vessel can accommodate two source arrays and up to 8 streamers Each

streamer can be up to 4200 m in length

Design a 3Design a 3--D survey over the ThetaD survey over the Theta


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gg yy

prospect Exerciseprospect Exercise

Figure 1Wiener spectrum from 2-D data

near Theta Prospect

S/N =1

Design a 3Design a 3--D survey over the Theta prospectD survey over the Theta prospect


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g y p p

ExerciseExercise

Theta

Closure

Image Area

6 x 6 km

15o

15o

15o

35o

Figure 2Structure Map of Theta Prospect Figure 3

Ray Tracing: Theta Prospect

0 m

3390 m

2210 m

1840 m

260 m

730 m

1320 m

1500 m/s

1800 m/s

2000 m/s

2400 m/s

2570 m/s

2650 m/s

2700 m/s

2875 m/s

22.30

Sea Level

Sea Floor

Top of Target

Top of K

Top of Mio

2830 m

2480 m

2010m1210m

350

Image area

10 x 10 km

Design a 3Design a 3--D survey over the Theta prospect ExerciseD survey over the Theta prospect Exercise


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es g 3g su vey ove e e p ospec e c sey p p

Figure 4

Surface Coverage Theta Prospect Figure 5Predict 3-D fold

from2-D Fold

ImageArea

6x6 km

Fringe to Capture Rays

based on dip/diffraction requirements

Fold Build-up

Fold Build-up

BoatSailDirection

Image area10 x 10 km

Fold build-up

Fold build-up

Fold build-up

Fringe to capture rays

Based on diffraction requirements

Boat

saildirection

0

40

80

120

160

1 1.5 43.52.5 32

2-D Effort Level (2-D Fold / Trace spacing in m)

3-D

EffortLevel

(3-D

Fold*1000/(in-line*cross-linespacinginm)

?

North?

Design a 3Design a 3--D surveyD survey


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gg yy

over the Theta prospectover the Theta prospect

ExerciseExercise

Figure 6

Gas-filled modeland its seismic

responses



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gg y p py p p

Part 1. Vertical ResolutionCalculate the vertical resolution that should be anticipated using equation 1 andFigure 1 (for the bandwidth).

RV = Vint-max / ( 2 BW) (1 )

RV =Is the anticipated vertical resolution less than or equal to the bed

thickness in the target?

_____YES ____NO

2. Lateral ResolutionThe smallest feature to be interpreted is 100 m wide. Let us assume that we

need 3 traces to make a confident interpretation. Use equation 2 to determinethe required lateral resolution.

RLM = (min width) / (# of traces) (2)

RLM =

SOLUTION: Theta prospect Survey DesignSOLUTION: Theta prospect Survey Design


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p p y gp p y g

art 1. Vertical Resolution

Calculate the vertical resolution that should be anticipated using equation 1 and Figure 1 (forthe bandwidth).

RV = Vint-max / ( 2 BW)

RV = 2300 / (2 [65 5]) = 19.2 m

Is the anticipated vertical resolution less than or equal to the bed thickness in the target?

__X___YES ____NO2. Lateral Resolution

The smallest feature to be interpreted is 100 m wide. Let us assume that we

need 3 traces to make a confident interpretation. Use equation 2 to determinethe required lateral resolution.

RLM = (min width) / (# of traces) (2)

RLM = 150 / 3 = 50 m

(Between 5 Hz and 65 Hz, the Wiener spectrum exceeds

0.5 and therefore S/N > 1.)

The max velocity in the reservoir zone is 2300 m /s.



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Part 3. Estimate bin size (

x) from

x < Vtar/ (4 * Fmax * sin )

= The larger of maximum structural dip or from the formula sin > RV/RLM

Vtar= interval velocity above the target .

x west = ______________

x north = ______________

For a survey that is close to a square in shape, we would shoot in the direction requiring the

smaller bin dimension. Based on the above considerations, we would shoot the Thetasurvey with the in-lines oriented:

______ East - West ______ North - South

We recommend an in-line trace spacing of ________ meters

We recommend a cross-line trace spacing of ________ meters

3 Bin Size



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3. Bin Size

the smaller of that needeed for lateral resolution and maximum dip

a) For lateral resolution (diffraction requirements)Sin > RV / Rlm = 19.2 / 50 = 0.38 ,res = 22.3

o

b) For structural dip requirements:

for the west flank, maximum angle is 35, which exceedsres(22.3o)

(sin 35o=.57) x < 2650 / (4 * 65 * 0.57) =17.9 m

for the north, east, and south flanks, maximum angle is 15, which is less than

res(22.3o) . Therefore, we use the resolution value. (Sin 22.3o = .38)

x < 2650 / (4 * 65 * 0.38) =26.8 mFor a survey that is close to a square in shape, we would shoot in the direction

requiring the smaller bin dimension. Based on the above considerations, we wouldshoot the Theta survey with the in-lines oriented:

_X___EAST-WEST ____NORTH-SOUTH

We would recommend an in-line trace spacing of: __15__meters

and we would recommend a cross-line spacing of: __25__meters

Survey Surface CoverageSurvey Surface Coverage


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y g

ProspectFull-foldSeismic line

Shots occupy a distance = prospect length + ApertureLeft + ApertureRight + 1/2 spread

Line length = positions occupied by either sources of receivers = prospect length +ApertureLeft + ApertureRight + 1 1/2 spread

Aperture Aperture

Shot point 1

Shooting direction

(off-end shooting)

streamer

Last shot

point

Full-fold buildup(1/2 spread)

Full-fold buildup(1/2 spread)



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4. Surface Coverage

To determine the surface coverage, we need to consider the image area plus the fringing

area on each side needed to capture the necessary seismic energy. In the cross-line

direction, we need the target width plus aperture needed to capture rays reflected from

the steepest dips. In the inline direction, we need the target length plus aperture to

capture rays reflected from the steepest dips, plus half the cable length (to allow for the

build up of fold).Use Figure 3 to complete the following chart:

Max dip angle (or

diffraction need)

Required

aperture A

Half cable

length B

North flank

East flank

South flank

West flank

Use Figure 4 and equations 5 and 6 to determine the surface coverage

In-line dimension = Target + East Flank (A+B) + West Flank (A+B) (5)=

X-line dimension = Target + North Flank (A) + South Flank (A) (6)

Solution: Theta prospect Survey DesignSolution: Theta prospect Survey Design


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4. Surface Coverage

Max dip angle (or

diffraction need)

Required

aperture A

Half cable

length B

North flank 22.3o 1210 0

East flank 22.3o 1210 2100South flank 22.3o 1210 0West flank 35o 2010 2100

Note half cable length added at both ends, which simplifies data acquisition.

Use Figure 4 and equations 5 and 6 to determine the surface coverage

Inline dimension = Target + East Flank (A+B) + West Flank (A+B) (5)

= 10 000 + 1210+ 2100 + 2010 +2100 = 17 420 m

X-line dimension = Target + North Flank (A) + South Flank (A) (6)

= 10 000 + 1210 + 1210 = 12 240 m


t E it E i


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5. Number of In-Lines & Traces (or Cross-Lines)

The number of in-lines equals the cross-line dimension divided by the

cross-line spacing plus one extra line

# in-lines =(N-S surface dimension / x-line bin size) + 1 (7)

=

The number of traces per in-line equals the in-line dimension divided by

the in-line trace spacing plus one extra trace

# traces = (E-W surface dimension / in-line bin size) + 1 (8)

=

Solution: Theta prospect Survey DesignSolution: Theta prospect Survey Design


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p p y gp p y g

5. Number of In-Lines & Traces (or Cross-Lines)

The number of in-lines equals the cross-line dimension divided by the

cross-line spacing plus one extra line# in-lines =(N-S surface dimension / x-line bin size) + 1 (7)

= (17 420 / 25) + 1 = 491 lines

The number of traces per in-line equals the in-line dimension divided bythe in-line trace spacing plus one extra trace

# traces = (E-W surface dimension / in-line bin size) + 1 (8)

= ( 12 240 / 15) +1 = 1162 traces

Design a 3Design a 3--D survey over the Theta prospect Exercise: 6. desired foldD survey over the Theta prospect Exercise: 6. desired fold


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For many surveys, the only existing seismic data are 2-D surveys. First, we perform a fold

decimation study to simulate lower fold data with existing surveys. Next, we estimate the lowest

fold we can have and still meet business needs. Then, we use an empirical relationship toestimate the 3-D fold that will give us a signal-to-noise (S/N) ratio that is comparable to that of

the minimum-fold 2-D data. For the Theta prospect, assume

1) a 2-D line exists with 15-m trace spacing.

2) a fold decimation study concludes that 40-fold 2-D data has the minimally acceptable S/N

Calculate the 2-D effort level for 40 fold data with 15-m trace spacing, using equation 9

2-D Effort = 2-D fold * 1000/trace spacing (9)

=

Using the empirical relationship in Figure 5, estimate the equivalent 3-D effort level

3-D Effort =

Calculate the 3-D fold needed to obtain an acceptable S/N level, using equation 10

3-D Effort = 3-D Fold * 1000/in-line bin * 1000/x-line bin (10)

Rearranging terms:

3-D Fold = 3-D Effort / (1000 /in-line bin * 1000/x-line bin) (11)

3-D Fold =

Solution: Theta prospect Survey DesignSolution: Theta prospect Survey Design : 6. desired fold: 6. desired fold


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Calculate the 2-D effort level for 40 fold data with 15-m trace spacing,using equation 9

2-D Effort = 2-D fold * 1000/trace spacing (9)

= 40 * 1000 / 15 =2667

Using the empirical relationship in Figure 5, estimate the equivalent 3-D

effort level

3-D Effort = 62 000

Calculate the 3-D fold needed to obtain an acceptable S/N level, using

equation 10

3-D Effort = 3-D Fold * 1000/in-line bin * 1000/x-line bin (10)

Rearranging terms:

3-D Fold = 3-D Effort / (1000 /in-line bin * 1000/x-line bin) (11)

3-D Fold = 62 000 / (1000/15 * 1000/25) = 23



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0

40

80

120

160

1 1.5 43.52.5 32

2-D Effort Level (2-D Fold / Trace spacing in m) x1000

3-DEffort

Levelx1000

(3-D

F

old*1000/(in-line

*cross-linespacinginm)

2-D Effort Level = 1000 * 2-D Fold / Trace Spacing

= 40 000 / 15 = 2667

3-D Effort Level = 62 000

p p y g


7 Realizable Fold (permitted by equipment)


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7. Realizable Fold (permitted by equipment)

We have estimated the fold that we would like the survey to have. Nowwe have to consider whether the available equipment will permit that

fold level. The realizable fold is a function of the shot point interval, the

group interval in the streamers, and the number of traces that the

streamer length will allow.

a) If there is only one source array, the shot point interval is simply the

interval at which the guns are fired. With multiple source arrays, the fact

that the arrays are fired alternately must be taken into account. Let us

assume two source arrays in which alternate arrays fire at an interval

equal to the in-line bin size. For this situation, we can use equation 12 to

determine the effective shot point interval.

eSPI = # sources * in-line bin size (12)

=



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Design a 3Design a 3 D survey over the ThetaD survey over the Theta

prospect Exercise:prospect Exercise: 7.7. Realizable FoldRealizable Fold (cont.)(cont.)

b) The group interval (GI) is the distance hydrophone array

centers are separated in the cable. Normally, the inline binsize = GI/2. Therefore,

GI = 2* inline bin size

=c) The number of traces (NT) can be obtained using equation 13.

NT = (cable length / group interval) (13)

=d) We can now calculate the realizable fold using equation 14

Fold = NT * GI / (2 * eSPi) (14)

Solution: Theta prospect Survey DesignSolution: Theta prospect Survey Design : 7.: 7.

realizable foldrealizable fold


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realizable foldrealizable fold

Let us assume two source arrays in which alternate arrays fire at an interval equal to the

in-line bin size. For this situation, we can use equation 12 to determine the effective

shot point interval.

eSPI = # sources * in-line bin size (12)= 2 * 15m = 30m

b) The group interval (GI) is the distance hydrophone array centers are separated in the

cable. Normally, the inline bin size = GI/2. Therefore, GI = 2* inline binsize

= 2 * 15m = 30m

For the Theta survey, we assume that a streamer with group interval of 30 m is available.

c) The number of traces (NT) can be obtained using equation 13.

NT = (cable length / group interval) (13)

= (4200 / 30) = 140

d) We can now calculate the realizable fold using equation 14Fold = NT * GI / (2 * eSPi) (14)

Design a 3Design a 3--D survey over the Theta prospect Exercise: 8. 3 scenariosD survey over the Theta prospect Exercise: 8. 3 scenarios

We have determined the set of geophysical parameters that are required for the Theta survey. We


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We ve de e ed e se o geop ys c p e e s e equ ed o e e su vey. We

have also started to consider the type of available equipment. There is more than one way to

collect the data while maintaining these requirements. For the Theta survey, let us considerthe following three scenarios:

Scenario A: Dual Source with Four Streamers

Scenario B: Dual Source with Six Streamers

Scenario C: Dual Source with Eight Streamers

If the boat is able to accommodate a maximum of 1000 channels, could each streamer be 4200 m

long? Complete the following chart to find out:

# streamers # traces # channelsScenario A

Scenario B

Scenario C

Scenario C would need more than 1000 channels to accommodate 4200 m cables. How many

traces could we have for Scenario C given the limitation of 1000 channels?

NT = # Channels / # Streamers (15)

What would be the impact on realizable fold (use equation 14)?Fold =

Design a 3Design a 3--D survey over the Theta prospect Solution: 8. 3 scenariosD survey over the Theta prospect Solution: 8. 3 scenariosWe have determined the set of geophysical parameters that are required for the Theta survey. We have also

started to consider the type of available equipment There is more than one way to collect the data while


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started to consider the type of available equipment. There is more than one way to collect the data while

maintaining these requirements. For the Theta survey, let us consider the following three scenarios:

Scenario A: Dual Source with Four Streamers, Scenario B: Dual Source with Six Streamers

Scenario C: Dual Source with Eight Streamers

If the boat is able to accommodate a maximum of 1000 channels, could each streamer be 4200 m long?

Complete the following chart to find out:

# streamers # traces # channels

Scenario A 4 140 560

Scenario B 6 140 840

Scenario C 8 140 1120

Scenario C would need more than 1000 channels to accommodate 4200 m cables. How many traces could we

have for Scenario C given the limitation of 1000 channels?

NT = # Channels / # Streamers (15)= 1000 / 8 = 125

What would be the impact on realizable fold (use equation 14)?

Fold = 125 *30 / (2 * 30) =63 (which is acceptable). However, decreasing

the number of channels by decreasing the maximum offset would make

velocity analysis less precise.

b) Now we want to consider the magnitude of acquisition artifacts relative to the geologic



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response. Let us assume that seismic modeling indicates a gas sand will have a response

of 6,000 amplitude units and a water sand will have a response of 18,000 amplitude units.Modeling of geometry-induced artifacts has determined the minimum and maximum

amplitudes for each scenario in the presence of constant geology. Calculate the amplitude

range and the ratio of the amplitude artifact range to the expected geologic range.

Acquisition footprint Amp Range as

Min Amp. Max Amp Amp. Range % of Geologic Range

Scenario A -800 900

Scenario B -1100 1200

Scenario C -1800 1900

Figure 6 shows a simple model of two gas traps. We assume that amplitude is directlyproportional to the thickness of the gas sand at a trace location. The edges of the reservoir

and the boundaries where the sand is 100% gas-filled can be interpreted readily on the

ideal response. We have superimposed on the ideal response the amplitude variations

assumed for each scenario. How does your confidence in picking the edges of the reservoirand the 100% gas-filled boundaries vary with the number of streamers?

b) Now we want to consider the magnitude of acquisition artifacts relative to the geologic



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response. Let us assume that seismic modeling indicates a gas sand will have a response

of 6,000 amplitude units and a water sand will have a response of 18,000 amplitude units.Modeling of geometry-induced artifacts has determined the minimum and maximum

amplitudes for each scenario in the presence of constant geology. Calculate the amplitude

range and the ratio of the amplitude artifact range to the expected geologic range.

Acquisition footprint Amp Range asMin Amp. Max Amp Amp. Range % of Geologic Range

Scenario A -800 900 1700 14

Scenario B -1100 1200 2300 19

Scenario C -1800 1900 3700 31

Figure 6 shows a simple model of two gas traps. We assume that amplitude is directly

proportional to the thickness of the gas sand at a trace location. The edges of the reservoir

and the boundaries where the sand is 100% gas-filled can be interpreted readily on the

ideal response. We have superimposed on the ideal response the amplitude variations

assumed for each scenario. How does your confidence in picking the edges of the reservoir

and the 100% gas-filled boundaries vary with the number of streamers?

Confidence decreases as the number of streamers increases from A to C and the

artifact amplitudes become a larger fraction of the geologic amplitudes .




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p pp p

c) Now we want to consider some cost factors. Assume that

mobilization costs are $150k (fixed for each scenario) and that each

boat pass costs $30k for A, $32k for B and $34k for C.Complete the following table:

In-lines per

Boat Pass

# In-Lines

(from page 3)

# Boat

Passes

Cost

Scenario A

Scenario B

Scenario C

Design a 3Design a 3 D survey over the ThetaD survey over the Theta


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prospectprospect -- solutionsolution

c) Now we want to consider some cost factors. Assume that

mobilization costs are $150k (fixed for each scenario) and that eachboat pass costs $30k for A, $32k for B and $34k for C.

Complete the following table:

In-lines per

Boat Pass

# In-Lines

(from page 3)

# Boat

Passes

Cost

Scenario A

Scenario B

Scenario C

8

12

16

491

491

491

62

41

31

$2010k

$1462k

$1204k

Design a 3Design a 3--D survey over the Theta prospect ExerciseD survey over the Theta prospect Exercise9. Recommendations

Consider the advantages and disadvantages of each scenario


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Consider the advantages and disadvantages of each scenario

Advantages Disadvantages

Scenario A

Scenario B

Scenario C

Keeping in mind the need to balance data quality and cost, which of the three

scenarios would you recommend?

Why?


i


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prospectprospect -- solutionsolution

9. Recommendations

Advantages Disadvantages

Scenario A 70 fold is possible

smallest acquisition footprint

Highest data quality

Highest cost

Longest acquisition time

More traces than C

Scenario B 70 fold data

only minor increases in

acquisition footprint

Costs more than C

More ship time than C

More traces than CScenario C Costs the least

Shortest acquisition time

Fewest traces to process

Only 63 fold

Less accurate velocities

Largest acquisition footprint

Lowest data quality

Design a 3Design a 3--D survey over the Theta prospectD survey over the Theta prospect --solutionsolution


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Keeping in mind the need to balance data quality and cost, which of the three scenarios wouldyou recommend?

A matter of opinion, but B may be the best of the three options.

Why?

Scenario B has the same fold as A and the acquisition footprint is

only slightly worse than A. Based on Figure 6, the edges of a gas

reservoir should be interpretable with a fair degree of confidence.

The cost of B is about 73% the cost of A ( and 66% of the acquisitiontime). Based on the available information, the small degradation in

data quality expected in going from Scenario A to Scenario B should

be worth the savings of 27% ($548 000).One would need a better understanding of the business need and the

impact that the predicted changes in data quality would have on

meeting that need before making a final decision.

33--D Land Survey Design ExerciseD Land Survey Design Exercise


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y gy g

Specifications:

Depth to Objective = 4500 m

Bin Size = 25 m squareCMP Fold = 30

CMP Recording with a Symmetrical Split Spread

Step 1. Find the Station Spacing or group interval.

Step 2. Determine the Maximum Offset.

For a first estimate, we can use 85-95 % of the depth of the objective. We usuallychoose a round number in this range.

Step 3. Determine the number of stations on each side of the symmetrical split spread.

Step 4.Determine total number of stations recorded per line for each shot.

33--D Land Survey Design ExerciseD Land Survey Design Exercise --page 2page 2


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Step 5. Determine the number of active receiver lines per shot.

Depends upon the amount of equipment available, the objective and the terrain. Assume

no problem with objective or terrain and we have a 1000-channel recording system.

Step 6. Determine Receiver line spacing

200 m is very narrow, 400 m is normal, 600 m is wide. Lets use a normal spacing.

Step 7. Determine Shot Spacing Crossline to Receivers

The crossline shot spacing normally sets the bin size in this direction.

Step 8. Determine Crossline Fold

Let us shoot in one lane and roll one line.

Step 9. Determine Inline Fold

The desired fold is one of the specifications, and crossline fold was computed in step 8.

Step 10. Determine Shot Spacing Inline with Receivers

Since the group interval and the number of inline stations has been determined, the inline

shot spacing determines the inline Fold.

33--D Land Survey Design ExerciseD Land Survey Design Exercise SolutionSolution


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Specifications:

Depth to Objective = 4500 m

Bin Size = 25 m square

CMP Fold = 30

CMP Recording with a Symmetrical Split Spread

Step 1. Find the Station Spacing or group interval.

The Station Spacing is normally two times the Bin Size.

25 m Bin x 2 = 50 m Station SpacingStep 2. Determine the Maximum Offset.

For a first estimate, we can use 85-95 % of the depth of the objective. We usually

choose a round number in this range.

Maximum Offset = 4500 m x 90% = 4050 m

Let us round the distance to 4000 m. 1

33--D Land Survey Design Exercise Solution (cont)D Land Survey Design Exercise Solution (cont)


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Step 3. Determine the number of stations on each side of the symmetrical split spread.

4000 m Offset Range divided by 50 m Stations = 80 Stations

Step 4. Determine total number of stations recorded per line for each shot.

Split spread 80 stations x 2 = 160 stations per line

Step 5. Determine the number of active receiver lines per shot. Depends upon the amount of equipment available, the objective and the terrain. Assume

no problem with objective or terrain and we have a 1000-channel recording system.

With 160 stations per line and 1000 channels available, we can have

Six active lines using a total of 960 stations live per shot.

Step 6. Determine Receiver line spacing

200 m is very narrow, 400 m is normal, 600 m is wide. Lets use a normal spacing.

400 m 2

33--D Land Survey Design Exercise Solution (cont)D Land Survey Design Exercise Solution (cont)

Step 7 Determine Shot Spacing Crossline to Receivers


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intint*

2 shotline

reclineSwathlinesNC =

Step 7. Determine Shot Spacing Crossline to Receivers

The crossline shot spacing normally sets the bin size in this direction.

25 m bin size x 2 = 50 m cross-line shot spacing

Step 8. Determine Crossline Fold

Let us shoot in one lane and roll one line.

Crossline Fold = 6 / 2 * 1 = 3

Step 9. Determine Inline Fold

The desired fold is one of the specifications, and crossline fold was computed in step 8.

Crossline Fold = 3, desired fold = 30 Inline fold = 10.

Step 10. Determine Shot Spacing Inline with Receivers

SI

GITRNI *

2=

NI

GITRSI *

2=

The InThe In--Line Shot Spacing =Line Shot Spacing = SI = (160 / 2) * (50 m / 10) = 400 m.400 m.3

The Active SpreadThe Active Spread

Six Lines with 160 Stations per line.Six Lines with 160 Stations per line.

ExerciseExerciseSolutionSolution


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Eight Shots areEight Shots are recorded before the spread is rolled.recorded before the spread is rolled.

Inline GI = 50 m Inline Bin Size = 25 m

Crossline SI = 50 m

Crossline Bin Size = 25 m

Receiver Line Spacing = 400 m

Wide line spacing poor

sampling of shallow data.

33--D Land Survey Design ExerciseD Land Survey Design Exercise --page 3page 3


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Step 11. Determine shot pattern (orthogonal, brick, triplebrick, or slant).

The objective is a good Offset Distribution. Using

acquisition planning software, small models should be

laid out; shots and live patches simulated; and offsets and

azimuths displayed. The results of the modeling can be

studied and different geometries can be compared

The final choice depends on the terrain and the type of

source that can be used. The orthogonal pattern is usually

avoided, because of its poor offset distribution.


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Five steps in survey designFive steps in survey design1. Determine imaging objective

Depth, resolution, data quality2. Gather information

Seismic lines, velocity information, topographic maps

3. Analyze variables Past experience, fold decimation, max frequency

4. Calculate parameters

Stack bin, vertical resolution, migration aperture5. Create implementation plan

Surface coverage, source, acquisition geometry

Survey Design: SummarySurvey Design: Summary


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3-D design procedure is explicitly tied to surveyobjectives

Communication and active participation byinterpreters, processors, and acquisition specialists isessential Business unit specifications in terms of structural and

stratigraphic objectives must be translated intogeophysical specifications

Vertical resolution RV depends upon bandwidth and int. velocity

Lateral resolution depends upon migration capture angle and RV S/N based upon experience with previous surveys and empirical

relationships

Survey Design: SummarySurvey Design: Summary -- IIII


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Resolution and S/N requirements determine binsize, fold and aperture

Bin size and aperture take into account both diffractioncapture angle and maximum structural dip

S/N and fold are determined empirically

Survey design requires a tradeoff between qualityand cost Least expensive approach (in time and money) often

produces artifacts that degrade seismic attributes

Objective is to obtain acceptable resolution, S/N, andacquisition footprint at the lowest cost and shortestacquisition time.

Economics of various acquisition geometries arecalculated based on available acquisition equipment

L19-3D Survey Design

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