Land and Marine Seismic Acquisition from 2D to 3D

1
Land and Marine Seismic Acquisition from 2D to 3D
From chapters 7-12 Elements of 3D Seismology
by Chris Liner
2
Outline-1

• CMP METHOD (Harry Mayne)
• Seismic sensors
• geophones
• hydrophones
• gimballed geophones and hydrophones
• accelerometers
• Sources
• Explosives
• Vibroseis
• SEGY data

3
Outline-2

• Acquisition Parameters
• Time Sample Rate
• Offset Range
• Listen Time
• Sample Rate and Temporal Aliasing
• Geophone Spacing and Spatial Aliasing
• Shooting geometry
• inline
• cross-line

4
Common Midpoint Method (CMP Method)
Please take a look at the powerpoint presentation
for the radio-telemetry field trip at the
following link http//www.geol.lsu.edu/Faculty/J
uan/ReflectSeismol05/labs/ppt/Radio-telemetry.ppt
This link has information to complement the
explanation on the CMP method.
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Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 1
6
4
5
2
3
1
Midpoints
Separation between midpoints is 1/2 separation
between hydrophone groups
6
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 2
6
4
5
2
3
1
Midpoints
7
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 3
Midpoints
8
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 4
Midpoints
9
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 5
Midpoints
10
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 6
Midpoints
11
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 7
Midpoints
12
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 8
Midpoints
13
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint 8
Midpoints
14
Common Midpoint Method (CMP Method)
Hydrophone groups
6
4
5
2
3
1
Shotpoint 1
Midpoints
15
Common Midpoint Method (CMP Method)
Hydrophone groups
6
4
5
2
3
1
Shotpoint 1
Shotpoint 2
Shotpoint 1
Shotpoint 2
Midpoints
16
Common Midpoint Method (CMP Method)
Hydrophone groups
6
4
5
2
3
1
Shotpoint 1
Shotpoint 2
Shotpoint 3
Shotpoint 1
Shotpoint 2
Shotpoint 3
Midpoints
17
Common Midpoint Method (CMP Method)
Hydrophone groups
6
4
5
2
3
1
Shotpoint 1
Shotpoint 2
Shotpoint 3
Shotpoint 4
Shotpoint 1
Shotpoint 2
Shotpoint 3
Shotpoint 4
Midpoints
18
Common Midpoint Method (CMP Method)
Hydrophone groups
6
4
5
2
3
1
Shotpoints 1-8
1
2
Midpoints
3
4
6
7
8
13
8
5
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Common Midpoint Method (CMP Method)
Fold or Multiplicity is the number of times that
the same midpoint is sampled by different shots
and different receivers Signal-to-Noise increases
as the square root of the fold
Fold
1
2
Midpoints
3
4
6
7
8
13
8
5
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Common Midpoint Method (CMP Method)
Maximum Fold is achieved after the 6th shot
Fold
1
2
Midpoints
3
4
6
7
8
13
8
5
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Common Midpoint Method (CMP Method)
When shotpoint spacing and group spacing are
equal then Maximum fold number of geophones or
hydrophones Midpoint separation 1/2 distance
between geophones In a more general case
Maximum Fold recording groups distance
between groups 2 distance between
shots Midpoint separation 1/2 smaller of the
two receiver group spacing or shot spacing
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Gather Types
A gather i.e. a subset of the traces from the
entire data set can be of different types

• Shotpoint gather
• Common source-receiver offset gather (COS)
• Common midpoint gather

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Shotpoint Gather
e.g. Shotpoint gather 3
6
4
5
3
2
1
24
Shotpoint Gather
Shotpoint 3
6
4
5
3
2
1
Hydrophone groups
A shotpoint gather samples various midpoints and
a variety of angles
25
What happens to the reflecting points in a
shotpoint gather when the reflecting interrface
dips?
Shotpoint 3
6
4
5
3
2
1
Hydrophone groups
A shotpoint gather samples various midpoints and
a variety of angles
26
What happens to the reflecting points in a
shotpoint gather when the reflecting interface
dips?
Shotpoint 3
6
4
5
3
2
1
Hydrophone groups
Reflecting points
Midpoints
A shotpoint gather samples different reflecting
points at a variety of angles
27
Common Midpoint Method (CMP Method)
Hydrophone group 4 Common source-receiver offset
and common receiver, shotpoints 1-8
6
4
5
3
2
1
28
Hydrophone group 4 Common source-receiver offset
and common receiver, shotpoints 1-8
6
4
5
3
2
1
COS means equal reflection angle
Midpoints
29
In the case of a COS gather where are the true
midpoints when the reflecting, geological
interface has a dip?
6
4
5
3
2
1
COS means equal reflection angle
Midpoints
30
COS NO LONGER implies equal reflection angles
6
4
5
3
2
1
Actual reflecting points
Midpoints
31
Common Midpoint Method (CMP Method)
Hydrophone group 4 Common mid-points
and shotpoints 1-8
6
4
5
3
2
1
Midpoints
32
Hydrophone group 4 Common mid-point
and shotpoints 1-8
6
4
5
3
2
1
group
1
2
3
4
5
6
7
8
Midpoint 6
CMP gathers sample varying angles but a common
geological midpoint
33
What happens to a common midpoint gather when the
reflecting interface has a dip?
6
4
5
3
2
1
group
1
2
3
4
5
6
7
8
Midpoint 6
CMP gathers sample varying angles but a common
geological midpoint
34
CMP gathers SAMPLE varying angles but with a
relatively smaller spread of reflecting points
than the shotpoint and common-offset gathers
6
4
5
3
2
1
group
1
2
3
4
5
6
7
8
Midpoint 6
True Reflecting Points
35
A common midpoint gather minimizes the effect of
dip while it helps increase the signal-to-noise
ratio
36
Outline-1

• CMP METHOD (Harry Mayne)
• Seismic sensors
• geophones
• hydrophones
• gimballed geophones and hydrophones
• accelerometers
• Sources
• Explosives
• Vibroseis
• SEGY data

37
Geophones
Convert ground motion into electricity at a rate
of about 1 Volt/inch/sec
Natural Resonance Frequency 100 Hz
GS-100 from Geospace
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Geophone layout
39
Geophone layout
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Seismic Sensors

• Hydrophones convert changing pressure into Volts
• (Volts/bar)

e.g. Preseis 2517 from I/O 1V/microPascal
41

• Gimballed Geophone-hydrophone combinations for
  sea-bottom work

Sea-Array 4 from Geospace
42
Streamer layout
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• Accelerometers

Convert ground acceleration into
Volts d(dx/dt) dt
E.g. VectorSeis from I/O 3-component digital
accelerometer (requires battery) full-scale at
3.3 m/s2 noise level 0.44 microm/s2 140db 20
log (3.3/410-7)
44
Outline-1

• CMP METHOD (Harry Mayne)
• Seismic sensors
• geophones
• hydrophones
• gimballed geophones and hydrophones
• accelerometers
• Sources
• Explosives
• Vibroseis
• SEGY data

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Vibroseis Method (Liner, 2004 p.157, para. 4, )
An output sweep
(e.g., 10-80 Hz)
enters the earth
..and undergoes various reflections
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...something too complicated to draw
Field correlation unravels the raw data into .
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Vibroseis images from the Lithoprobe Project,
Canada www.lithoprobe.ca
A vibrator truck
12 elephants dancing in unison (LITHOPROBE,
CANADA)
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(No Transcript)
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Explosives
Noble Explochem Limited
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GI Watergun Array
NSF R/VIB NBPalmer- February/March 2003
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Sercel G. GUN 150 cu. In. firing at 2,000 p.s.i.

• Link to movie of this G. Gun working in a pool

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Outline-1

• CMP METHOD (Harry Mayne)
• Seismic sensors
• geophones
• hydrophones
• gimballed geophones and hydrophones
• accelerometers
• Sources
• Explosives
• Vibroseis
• SEGY data

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SEGY data
3200 byte EBCDIC header
One line at a time
400 byte tape header
240 byte trace header
DATA
240 byte tape header
DATA
240 byte tape header
DATA
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Outline-2

• Acquisition Parameters
• Time Sample Rate
• Offset Range
• Listen Time
• Sample Rate and Temporal Aliasing
• Geophone Spacing and Spatial Aliasing
• Shooting geometry
• inline
• cross-line

55
Sample Rates
What is the fewest number of times I need to
sample this waveform per second?
?
?
?
56
Sample Rates
57
Sample Rates
58
Sample Rates
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Sample Rates
What is the fewest number of times I need to
sample this waveform per second?
At least twice per wavelength or period!
OTHERWISE .
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Undersampled waveforms
Amplitude
True frequency (f -true)
Reconstructed frequency (f -aliased)
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Oversampled waveforms
Nyquist frequency
Amplitude
Reconstructed frequency frequency is unaliased
True frequency (f -true)
Nyquist frequency 1 / twice the sampling rate
Minimum sampling rate must be at least twice the
desired frequency
E.g., 1000 samples per second for 500Hz, 2000
samples per second for 1000 Hz
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Oversampled waveforms
Nyquist frequency
Amplitude
In practice we are best oversampling by double
the required minimum i.e. 1000 samples per second
for a maximum of 500 Hz i.e., 2000 samples per
second for a maximum of 1000 Hz Oversampling is
relatively cheap.
63
Outline-2

• Acquisition Parameters
• Sample Rate and Temporal Aliasing
• Offset Range
• Listen Time
• Geophone Spacing and Spatial Aliasing

64
Offset Range
Maximum shot-receiver offset
Target depth
One-layer earth of a semi-infinite layer
Maximum shot-receiver offset gt target
depth. Near critical distance
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Offset Range
Maximum shot-receiver offset
Target depth
Multi-layered earth
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Outline-2

• Acquisition Parameters
• Time Sample Rate
• Offset Range
• Listen Time
• Sample Rate and Temporal Aliasing
• Geophone Spacing and Spatial Aliasing
• Shooting geometry
• inline
• cross-line

67
Listen Time
.Twice target time to be sage
68
Outline-2

• Acquisition Parameters
• Time Sample Rate
• Offset Range
• Listen Time
• Sample Rate and Temporal Aliasing
• Geophone Spacing and Spatial Aliasing
• Shooting geometry
• inline
• cross-line

69
Spatial aliasing
Spatial frequency, or wavenumber (k) is the
number of cycles per unit distance. One
spatial cycle or wavenumber frequency/velocity.

Each wavenumber must be sampled at least twice
per wavelength (two CMPs per wavelength)

IN PRACTICE each wavenumber must be sampled at
least four times per minimum wavelength (two
CMPs per wavelength)
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Spatial aliasing
However, dip (theta) as well as frequency and
velocity event changes the number of cycles per
distance, so
Liner, 9.7,p.192

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Spatial aliasing

For aliasing NOT to occur, delta(t) must be less
than T/2
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Spatial aliasing

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Geophone Spacing and Spatial Aliasing
K0
74
1/4 wavelength shift per trace total shift across
array3/4 wavelength
K or -ve?
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1/4 wavelength shift per trace total shift across
array3/4 wavelength
K?
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1/2 wavelength shift per trace total shift across
array3/2 wavelength
K0
77
3/4 wavelength shift per trace total shift across
array2 1/4 wavelength
78
Spatial aliasing

• Degrades (string of pearls) stacked sections
• Degrades migration

79
Signal-to-Noise

• Improves with stacking
• greater fold
• greater repetition of shots

80
Outline-2

• Acquisition Parameters
• Time Sample Rate
• Offset Range
• Listen Time
• Sample Rate and Temporal Aliasing
• Geophone Spacing and Spatial Aliasing
• Shooting geometry
• inline
• cross-line

81
Fundamental Parameters for land 3D shooting

82
Common Midpoint

83
Source-Receiver Offset
2D
3D

84
Azimuth (3D)

85
Inline geometry

Matlab code
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Outline-2

• Acquisition Parameters
• Time Sample Rate
• Offset Range
• Listen Time
• Sample Rate and Temporal Aliasing
• Geophone Spacing and Spatial Aliasing
• Shooting geometry
• inline
• cross-line

87
Cross-line geometry

Matlab code
88
Spatial aliasing

• Degrades (string of pearls) stacked sections
• Degrades migration