02 land 2d surveys modified

Land 2D Geometry & Surveys

Aim of this Course

The aim of this course is to understand the geophysical &

operation concerns of acquiring 2D seismic surveys

Objectives of this Course

At the end of this course you will be able to:

• Define the various 2D acquisition geometries

• Understand 2D spread roll-on & roll-off

• Understand 2D fold & coverage

• Understand the effect of skips & offsets on CMPs

• Understand different source geometries

What is 2D Seismic Acquisition?

• 2 Dimensions – Distance along line & time

• Build up a cross-section of data along line

• Single receiver line – may be straight or crooked

• Sources between receiver stations & offset from line

• Multiple intersecting lines in a prospect

2D Prospect Geometry

Why 2D?

• Exploration Surveys

• Fast Results

• Large scale, regional picture, low resolution

– Noisy due to poor attenuation of cross-line noise

– Distorted view – only looking in plane of 2D line

• Identify areas of interest & target in 3D

2D Line Geometry

• Which direction do you choose for your 2D line?

DIP

2D Line Direction

2D Geometry – Off-End

• Source at one end of live receiver spread

• Occurs at start or end of spread

• Is half the live spread operationally (channel count) &

geophysically (CMPs)

• Only geometry for marine acquisition

Coverage

2D Geometry – Asymmetric Split-Spread

• Unequal numbers of live receivers on each side of shot

• Occurs during roll-on & roll-off

Coverage

2D Geometry – Symmetrical Split-Spread

• Occurs within parts of the line where its number of live channels is less

than the total line length

• Also known as full spread, the channel count is greater than off-end but

greater than/equal to asymmetric split spread

• Occurs when equal number of channels and CMP’s exists on either side

of the shot point

Coverage

2D Geometry – Spread Roll

• In Marine we pull the receivers behind the vessel – End-on shooting

– The “live spread” = active length of streamer

• In Land we have the advantage of being able to lay the receivers along

our entire line

– Only limitation is amount of equipment available

• However we only shoot into some of these receivers for any one shot –

the live spread i.e. the channels on which we will record for that shot

• The live spread rolls-along the length of our line from start to end

• At the start of the line we shoot end-on, then roll-on asymmetrically until

we have achieved full symmetrical split-spread & do full-spread roll until

we roll-off the end of the line

2D Geometry – Spread Roll

Start of

Line

End of

Line

Far Trace Offset

Live Spread e.g. 200 Channels @ 50m int. 50m

Live Spread Template (as described in client parameter letter)

1st Shot: End-on

Roll-On

Asymmetrical Symmetrical Asymmetrical Last Shot: End-on

Roll-Off Full Spread

2D Technique – Coverage Build-up

1 2 3 4 4 4 4 4 3 3 2 2 1 1 CMP Fold

CMPs

Stacking Diagram

1st Full Fold CMP

2D Technique - Coverage Build-up

Area of FULL FOLD

As required by Client

0

Full Fold Full Fold

2D Line

Start of

Line End of

Line

Fo

ld F

old

Live Spread

¼ Live

Spread

¼ Live

Spread

2D Coverage – Coverage Build-up

• Client only cares about full-fold extent

• To achieve clients full-fold we need to extend the line in each direction –

extra source & receiver points

• Client won’t pay for more shots than necessary – therefore fold must

build-up as quickly as possible

• For SP=RP, first shot starts ¼ live spread from full-fold edge

• Why End-on spread for 1st & last shot? – no point shooting split-spread

where CMP’s don’t contribute to full-fold

Area of FULL FOLD

As required by Client

2D Fold

• 2D Fold is given by the length of the coverage area divided by

the moveup

2D Fold = RPint

SPint x

N

2

Where: RPint = Receiver station interval (m)

SPint = Source station interval (m)

N = Number of live receivers per shot

• Example: RPint = SPint = 50m, N = 200, Fold = 100

2D Fold

Relationships between moveup & fold where:

• RP = SP

– then F = Chn/2 i.e.. 2 CMPs between shots

• RP = 2*SP

– then F = Chn i.e.. 1 CMP between shots

• RP = SP/2

– then F = Chn /4 i.e.. 4 CMPs between shots

Offset – Far Trace Offset

Far Trace Offset (FTO)

• Approximately target depth

• Shows data from the last group of receivers for each source point

• Has to be sufficient for NMO discrimination of primaries & multiples

• Can be extended (e.g. 120% target depth) to identify multiples

• If extended too far, reflections no longer hyperbolic, NMO stretch too

great FTO = Target Depth

Target Depth

Z

Offsets – Near Trace Offset

Near Trace Offset (NTO)

• Shows shallowest reflector of interest

• Normally < 1 station interval maximum, typically 0.5 station interval

• Shows data from the first group of receivers from each source point

• Important for static corrections

• Important for NMO velocities (small offset = zero moveout)

Offsets – Source & Receiver Skips

• Source and/or receiver skips result in loss of near-offset & shallow

information

Near Offset/Shallow Data Loss

First Breaks

• Far-offsets provide coverage at depth, however fold is reduced

• This can pose problems if near-surface information required – e.g. for

statics

• For skips greater than ½ spread length we loose continuity completely

Is this anomaly real?

or

is it a static problem?

X

T

Anomaly or Statics?

• Even after compensations and In-fills you cannot recover the

information

• Need both sources & receivers in the area for near-offset information

2D Source Offsets

Offset CMPs

Offset CMPs

SP Lateral Offset

SP

SP Lateral Offset

SP SP RP RP RP RP RP RP RP CMPs S/L & R/L

• The shifted CMPs are grouped (binned) with the CMPs on the main

line resulting in SMEARING of the subsurface reflections

2D Receiver Offsets

CMPs Offset

CMPs Offset

RP Lateral Offset

RP Lateral Offset

S/L & R/L SP SP SP RP RP RP RP RP

CMP’s

• Offset receiver stations – the effect is identical to a source offset

• Typically offsets < 200m allowed

2D Offsets

• 200m surface offsets give 100m CMP offsets

• No significant increase in smear

• However, crossline noise becomes significant

• Need some crossline protection in arrays

Inline

CrossLine

Multiple Coverage - Binning

• In the perfect world all our reflection points would come from the same

place – the CMP

• In reality sources & receivers may be shifted from theoretical positions

due to obstacles, positioning errors etc.

• Reflection points no longer coincide

• A scheme must be devised to group traces as if they came from the

same location so they can be stacked - BINNING

2D Acquisition – Crooked Lines

• Lines are rarely straight, they deviate around obstacles

• Have to bin CMPs along a line of best fit & process

CMPs

Binning ( Start of 3D?)

2D Acquisition – Crooked Lines

• The greater q, the greater the smear

• Therefore limit q (client/contract dependant)

q

CMP smear

CMP smear CMP smear Obstacle

2D Acquisition – Crooked Lines

• Reduced smearing by separating sources & receivers

• Vibrators can go around the dune, geophones can be laid

across the top

q

Dune - Obstacle

CMP’s

RP

SP

R/L & S/L

2D Acquisition – Crooked Lines

2D Acquisition

2D Vibrator Moveup

Inline Array – Moveup within the VP

• 4 sweeps/VP, 6.25m moveup/sweep, 6.25m moveup between VPs

25.00m

25.00m

SWEEP 1

SWEEP 2

SWEEP 3

SWEEP 4

6.25m 14.50m

Not to Scale

2D Vibrator Moveup

Parallelogram Array – Standing Sweeps

• Standing Sweeps x 4 & 25m moveup between VPs

15.00m

10.75m

6.50m 10.75m

21.50m

25.00m

25.00m

25.00m

Not to Scale

2D Vibrator Moveup

Parallelogram Array – Move up within the VP

• 4 sweeps/VP, 3.38m moveup/sweep, 14.88m moveup between VPs

25.00m

25.00m

25.31m

3.38m

14.88m

Not to Scale