2-D SURVEY DESIGN_Final.ppt

14-03-05 2D Survey Design 1
2-D SURVEY DESIGN

2-D SURVEY DESIGN
To optimize acquisition survey
parameters to acquire optimum
quality of seismic data for a given
objective with optimum cost

PARAMETERS TO BE DECIDED
• Type of spread
• Direction of shooting
• Fold
• Group Interval, Shot interval
• Near offset, Far offset
• Migration Aperture
• Recording parameters
– Record length
– Sampling interval
– Filters-High, low & Notch

Pre - Survey Studies
A. Formation of data base
B. Reprocessing/processing
C. Analysis of data and results of processing
D. Optimization of parameters

 Geological & seismo-geological objectives of the
survey
 Surface & subsurface geological information
 Satellite images
 Time structure maps at target zones to study
major structural aspects, fault patterns and
other subsurface features
 Details of wells drilled in survey area
 Well log data (sonic, density & dip meter )

 Gravity and Magnetic survey data
 Parameters of earlier acquired seismic data
 Key seismic sections in dip & strike direction
 VSP data of wells in survey area
 Information about drilling difficulties
encountered, poor data quality & logistic
 Near surface information, uphole survey data
and near surface models
 Location & results of experimental surveys

 Extensive reconnaissance survey in the area
 Topo sheets and other topographic survey data
 Meteorological and tidal data for transition zones
and backwaters
 PEL / ML details pertaining to the area
 Preparation of detailed project map with
information about logistics, earlier seismic
data, drilled wells, surface & sub surface
geological features, major obstacles in the
area, PEL/ ML information etc.

B. Reprocessing/Processing
 Results of reprocessing of raw and stack data of
earlier vintages for fold study
 Amplitude frequency analysis of earlier seismic
data including VSP data in the zone of
interest
 Generation of synthetic seismogram from log
data
 Modeling studies in geologically complex areas
areas ( after completing desk computations)

C. Analysis of Data
&
Results of Processing
 Depth of shallowest target
 Depth of deepest target
 Foldage required in the zone of interest
 Frequency at target level
 Narrowest dimension of geological target
 Dip & thickness of the target zone
 Velocity function of the area

Foldage may be decided based on :
 Results of fold optimization of earlier data
 S/N ratio in the area
 Shot interval
 Number of channels
D. OPTIMIZATION- FOLD

Fold of coverage/ Fold: Number of times a subsurface point
is sampled
Nominal fold/Total fold: Provided by a geometry if
implemented without skip. However, the staking fold
( actual number of traces staked). These is less than due to:
1. Skip of shot points in field
2. Dead Channels in field
3. Editing of noisy/sluggish Channels at processing
4. Muting during processing
5. Pre-stack migration
D. OPTIMIZATION- FOLD

Split Spread:
• Minimizes shadow zones
• Ensure updip shooting for half of ray paths
• Cost effective
• Easy to correct positioning of shot location in processing
centre
• In-between the pickets provides better offsets.
End On:
• Limited number of channels and required far offset is
large.
ASS:
• Interest is shallow as well as deep
D. TYPE OF SPREAD

Comparison of spreads
assuming limited number of channels is available
Sl.
No.
End-on Spread Split-spread
1. longer spread which enables us to
look deeper.
suitable for shallow targets
2. suitable for better multiple
suppression
area is free of multiples
3. better velocity analysis. It reduces the NMO stretch.
4. It is convenient for field
operations.
any activity at the corresponding
and successive shot points
hinder the active spread.
5. It may ensure up-dip shooting if
dip direction is monocline and
it is known.
it is better suited for conflicting dip
or unknown dip areas.
It is also suitable in horst / graben
set up.
In mapping of geologically
complex subsurface, it
minimizes the shadow zones.

• If, End on spread
• Receiver updip in
relative to shot
• Wave travelling updip
suffers less scattering
• Intra – array travel
time difference is low
• Total surface coverage
to map a steeply
dipping reflector is
less
Source Midpoint Receiver
Receiver Midpoint Source
UPDIP
DOWNDIP
D. DIRECTION OF SHOOTING

Spatial aliasing criteria
• No aliasing of maximum dipping events of
highest frequency
• Proper sampling of Diffraction events
Horizontal resolution
• Fresnel Zone criteria:Three-four traces in
Fresnel’s zone
Inputs:
• Group interval=(Xfar-Xmin)/(n-1)
D. GROUP INTERVAL

Sampling theorem
• There should be two samples in a time period of
highest frequency signal.
Nyquist Frequency=1/(2*Sample rate)
• In seismic surveys two types of sampling
– Sampling of a trace from a channels- Temporal
– Sampling of continuous wavefield at different
Geophones - Spatial

Temporal Aliasing-Nyquist Rule
A
T= 2 ms
t
Frequency = 500 Hz
t = 1 ms
IF ANALOG SIGNAL OF:
Frequency (Fs) = 500 Hz
Time period (T) = 2 ms
Then it is to be digitized by:
Nyquist Rate(τ) = 1 ms
Nyquist Freq(Fn=1/2*τ) =500 HZ

t (ms)
A
0
Real Sinusoïd
Aliased sinusoïd
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Temporal Aliasing-Example
Grey Wave (Actual Signal)
Frequency (Fs) :125 HZ
Time period (T) :8 ms
So required Sample Rate : 4 ms
But if it sampled by
Sample rate (τ) : 5 ms
Nyquist frequency (Fn=1/2*5) : 100 Hz
Result Green (aliased signal)={Fn-(Fs-Fn)}=(2Fn-Fs)=75 Hz

Temporal aliasing
• Sampling interval just depends upon:
– Time period of maximum frequency of interest.
• Since frequency above nyquist frequency creates
noise (aliased frequency) when reconstructing the
trace from sampled values.
• Hence, it is necessary to filter out the frequency
above nyquist frequency- high cut filter( nyquist
filter).

Spatial Sampling
• There should be at least two sample locations within a
wavelength of a wave of highest wavenumber.
Nyquist Wavenumber=1/(2*trace interval)
• Proper spatial sampling of continuous wavefield is
important for multi-channels processing otherwise noise
(aliased wavenumber) gets created in applying multi-
channels processing. trace interpolation/spatial anti-alias
filter for:
FILTERING OUT WAVE NUMBER HIGHER THAN
NYQUIST WAVE NUMBER
SO DEICIDE SPATIAL SAMPLING (TRACE
INTERVAL) CAREFULLY

Anti-alias filter
without with

TIME AND SPACE
• t~f
Frequency
Period T=1/f
Sampling criteria: at least
two sample per period
Δt≤1/2fmax
Nyquist frequency:
Fn=1/2Δt
• x~k
Wave number/spatial frequency
Wavelength λ=1/k
Sampling criteria: at
least two sample per wavelength
Δx≤1/2kmax
Nyquist wavenumber
Kn=1/2Δx
Max unaliased freq. Fmax=V * Kn
t
x
x=Vt
f
k
k=f/V

Spatial Aliasing
00
10
20
30
40
50
Frequency=1000/20=50 Hz AND GI=1  NO Aliasing
Time in
ms
# 1 2 3 4 5
Trace

Spatial Aliasing
00
10
20
30
40
50
Time in
ms
# 1 2 3
Trace
Frequency=1000/20=50 Hz: GI=2 NO Aliasing

Spatial Aliasing
# 1 5
Trace
00
10
20
30
40
50
Time in
ms
Frequency=1000/20=50 Hz: GI=4 Aliasing
?
?

?
?
0
0
1
0
2
0
3
0
4
0
0
0
1
0
2
0
3
0
4
0
T
i
m
e
Trace
Spatial Aliasing
GI=1 GI=2 GI=4
1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
00
10
20
30
40
50

Spatial Aliasing
# 1 2 3 4 5
Trace
00
10
20
30
40
50
Time in
ms
Frequency=1000/10=100 Hz: GI=1 NO Aliasing

Spatial Aliasing
# 1 2 3 4 5
Trace
00
10
20
30
40
50
Time in
ms
?
?

Spatial Sampling
In seismic, spatial sampling is more complex due to dips
and velocity
• Apparent wavelength depends upon:
– apparent velocity and frequency
• Apparent velocity depends upon:
– velocity and dips
• Hence , apparent wavelength (so trace interval)
ultimately depends upon:
– Velocity
– Frequency
– dip

Apparent Velocity
Apparent Velocity: The
velocity at which a events
seems to reach at different
channels.
dt=(dx*Sinθ)/V
dx/dt= V/(Sinθ)
Vapp=V/ Sinθ
If θ=0 ; Vapp= infinite
If θ=90; Vapp=V
x x+dx
t
t+dt
dx
θ

V * dt/2 V * dt
Sin θ = ------------- = -----------
dx 2*dx
(Spatial aliasing occur when the dt is one half period (of max freq signal)
= (V * T)/(4 * dx)
= V/(4 * fmax * dx)
or,
dx = V/(4 * fmax * Sin θ)
Spatial Aliasing
Where, V is interval velocity at Target

Far offset should be
Sufficient enough for :
Velocity analysis
 Multiple attenuation
Limited because of:
 NMO stretch criteria
 To avoid wide angle reflection
 Depth of zone of interest
 Number of channels & group interval
Rule of Thumb
Far offset equal to target depth of deepest interest
D. FAR OFFSET

CDP-NMO
S M R
V
D
tx =SDG
to=2*SC =2*MD
tnmo = tx-to
= [(x2+V2
*to
2)1/2/V]-to
= to[(x/V*to)2+1]1/2-to
=to[1+(x2/V2*to
2)1/2-1]
=x2/2*V2*to
i.e. tnmo α Square of offset
α Inverse of travel time at
zero offset
α Inverse of square of
stacking velocity
C
V
x
Offset
Time

NMO Stretch
Time
Offset

NMO Stretch
Time
Part
of
trace
NMO
corrected
Part
of
trace
NMO
corrected
Part
of
trace
NMO
corrected
Offset

Noise at near trace (esp. Vibrators)
Stretch at shallow target
Sufficient fold at shallow target
As minimum as possible
D. NEAR OFFSET

D. MIGRATION APERTURE
The maximum value of :
• The lateral migration movement of dipping
events ( Z*tanθ)
• To capture diffraction energy for migration:
– Take θ equal to 30 degrees.
• Radius of first fresnel zone.
• Migration+zone of influence

Migration Aperture
• It is the distance by which the
survey line/area to be extended
to get full-migrated coverage
and to capture diffracted energy.
– Migration aperture equal to
Z*tanθ
– If the requirement of
PSTM/PSDM, by analysis of unit
impulse response
Z*tanθ
θ
Z
θ

Fresnel Zone
• Energy from all the points of
reflection disk with radius
OA` will arrive
constructively if time period
equals half of dominant time
period. This is called first
Fresnel zone and its r is
given by:
R=(Vav/2)√(to/fmax)
Bx or By=(2/3)R
A A`
O
Z
S
Zo
Z+λ/4

D. Migration aperture
• MA=Migration distance + zone of influence
Z*tanθ

D. RECORD LENGTH
Record Length:
• Sufficient enough to capture target horizons at
farthest offset, migration and diffraction tails.
• Add twice of the length of the longest filter in
time
• Does not affect much on cost.

D. SAMPLING INTERVAL
• To avoid temporal aliasing of highest
frequency.
• At least 4 sample in the time period of highest
frequency
• Does not affect much on cost.

D. RECORDING FILTERS
High Cut Filter:
• To attenuate frequencies above the Nyquist frequency
which depends upon sampling interval
Low Cut Filter:
• Generally out
• May be applied if D/A converter is becoming
saturated.
Notch Filter:
• If necessary like in the area of HT Powerline

It is amazing what you can
accomplish if you do not care who
gets its credit
-Harry S. Truman

2-D SURVEY DESIGN_Final.ppt

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