02 land 2d surveys modified
TRANSCRIPT
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