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Land and Marine Seismic Acquisition from 2D to 3D. From chapters 7-12 “Elements of 3D Seismology” by Chris Liner. Outline-1. CMP METHOD (Harry Mayne) Seismic sensors geophones hydrophones gimballed geophones and hydrophones accelerometers Sources Explosives Vibroseis SEGY data. - PowerPoint PPT Presentation
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Land and Marine Seismic Acquisition from 2D to 3D
From chapters 7-12 “Elements of 3D Seismology” by Chris Liner
Outline-1CMP METHOD (Harry Mayne)Seismic sensors
•geophones•hydrophones•gimballed geophones and hydrophones•accelerometers
Sources •Explosives• Vibroseis
SEGY data
Outline-2Acquisition Parameters
•Time Sample Rate•Offset Range•Listen Time•Sample Rate and Temporal Aliasing• Geophone Spacing and Spatial Aliasing•Shooting geometry
•inline•cross-line
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/Juan/ReflectSeismol05/labs/ppt/Radio-telemetry.ppt
This link has information to complement the explanation on the CMP method.
Common Midpoint Method (CMP Method)
Shotpoint # 1Hydrophone groups#1#2#3#4#5#6
Separation between midpoints is 1/2 separation between hydrophone groups
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 2Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 3Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 4Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 5Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 6Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 7Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 8Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 8Hydrophone groups#1#2#3#4#5#6
Midpoints
Common Midpoint Method (CMP Method)Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1
Common Midpoint Method (CMP Method)Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1Shotpoint # 2
Shotpoint # 1Shotpoint # 2
Common Midpoint Method (CMP Method)Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1Shotpoint # 2
Shotpoint # 1Shotpoint # 2
Shotpoint # 3
Shotpoint # 3
Common Midpoint Method (CMP Method)Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoint # 1Shotpoint # 2
Shotpoint # 1Shotpoint # 2
Shotpoint # 3
Shotpoint # 3
Shotpoint # 4
Shotpoint # 4
Common Midpoint Method (CMP Method)Hydrophone groups
#1#2#3#4#5#6
Midpoints
Shotpoints # 1-8
12 3
4 5 6 7 8 138
Common Midpoint Method (CMP Method)
Midpoints1
2 34 5 6 7 8 138
FoldFold or Multiplicity is the number of times that the same midpoint is sampled by different shots and different
receiversSignal-to-Noise increases as the square root of the fold
Fold
Common Midpoint Method (CMP Method)
Midpoints1
2 34 5 6 7 8 138
Maximum Fold is achieved after the 6th shotMaximum Fold is achieved after the 6th shot
Fold
Common Midpoint Method (CMP Method)
When shotpoint spacing and group spacing are equal thenMaximum fold = number of geophones or hydrophonesMidpoint separation = 1/2 distance between geophones
In a more general case: Maximum Fold = #recording groups * distance between
groups2 * distance between shots
Midpoint separation = 1/2 smaller of the two: receiver group spacing or shot spacing
A gathergather 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
Gather Types
Shotpoint Gather
e.g. Shotpoint gather #3
#1#2#3#4#5#6
Shotpoint GatherShotpoint #3
#1#2#3#4#5#6
Hydrophone groups#1#2#3#4#5#6
A shotpoint gather samples various midpoints and a variety of angles
What happens to the reflecting points in a shotpoint gather when the reflecting interrface dips?
Shotpoint #3
#1#2#3#4#5#6
Hydrophone groups#1#2#3#4#5#6
A shotpoint gather samples various midpoints and a variety of angles
What happens to the reflecting points in a shotpoint gather when the reflecting interface dips?
Shotpoint #3
#1#2#3#4#5#6
Hydrophone groups#1#2#3#4#5#6
A shotpoint gather samples different reflecting points at a variety of anglesMidpoints
Reflecting points
Common Midpoint Method (CMP Method)
Hydrophone group #4Common source-receiver offset and
common receiver, shotpoints 1-8
#1#2#3#4#5#6
Hydrophone group #4Common source-receiver offset and
common receiver, shotpoints 1-8
#1#2#3#4#5#6
Midpoints
COS means equal reflection angle
In the case of a COS gather where are the true midpoints when the reflecting, geological
interface has a dip?
#1#2#3#4#5#6
Midpoints
COS means equal reflection angle
#1#2#3#4#5#6
Midpoints
COS NO LONGER implies equal reflection angles
Actual reflecting points
Common Midpoint Method (CMP Method)
Hydrophone group #4Common mid-points and
shotpoints 1-8
#1#2#3#4#5#6
Midpoints
Hydrophone group #4Common mid-point and
shotpoints 1-8
#1#2#3#4#5#6
Midpoint #6
group12345678
CMP gathers sample varying angles but a common geological midpoint
What happens to a common midpoint gather when the reflecting interface has a dip?
#1#2#3#4#5#6
Midpoint #6
group12345678
CMP gathers sample varying angles but a common geological midpoint
#1#2#3#4#5#6
Midpoint #6
group12345678
CMP gathers SAMPLE varying angles but with a relatively smaller spreadrelatively smaller spread of
reflecting points than the shotpoint and common-offset gathers
True Reflecting Points
A common midpoint gather minimizes the effect of dip while it helps increase the signal-to-noise ratio
Outline-1CMP METHOD (Harry Mayne)Seismic sensors
•geophones•hydrophones•gimballed geophones and hydrophones•accelerometers
Sources •Explosives• Vibroseis
SEGY data
Geophones
Convert ground motion into electricityat a rate of about 1 Volt/inch/sec
GS-100 from Geospace
Natural Resonance Frequency 100 Hz
Geophone layout
Geophone layout
Seismic Sensors
•Hydrophones convert changing pressure into Volts(Volts/bar)
e.g. Preseis 2517 from I/O 1V/microPascal
•Gimballed Geophone-hydrophone combinations for sea-bottom work
Sea-Array 4 from Geospace
Streamer layout
•Accelerometers
Convert ground acceleration into Voltsd(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 (3.3/4*10^-7)
Outline-1CMP METHOD (Harry Mayne)Seismic sensors
•geophones•hydrophones•gimballed geophones and hydrophones•accelerometers
Sources •Explosives• Vibroseis
SEGY data
Vibroseis Method (Liner, 2004; p.157, para. 4, )
An output sweep (e.g., 10-80 Hz)
enters the earth
…..and undergoes various reflections
++ =
...something too complicated to draw
Field correlation “unravels” the raw data into ….
“12 elephants dancing in unison” (LITHOPROBE, CANADA)
A vibrator truck
Vibroseis images from the Lithoprobe Project, Canada www.lithoprobe.ca
Explosives
Noble Explochem Limited
NSF R/VIB NBPalmer- February/March 2003
GI Watergun Array
Sercel G. GUN 150 cu. In. firing at 2,000 p.s.i.
• Link to movie of this G. Gun working in a pool
Outline-1CMP METHOD (Harry Mayne)Seismic sensors
•geophones•hydrophones•gimballed geophones and hydrophones•accelerometers
Sources •Explosives• Vibroseis
SEGY data
SEGY dataOne line at a time
400 byte tape header
3200 byteEBCDIC header
240 byte trace header
240 byte tape header
240 byte tape header
DATA
DATA
DATA
Outline-2Acquisition Parameters
•Time Sample Rate•Offset Range•Listen Time•Sample Rate and Temporal Aliasing• Geophone Spacing and Spatial Aliasing•Shooting geometry
•inline•cross-line
Sample Rates
What is the fewest number of times I need to sample this waveform per second?
?
?
?
Sample Rates
Sample Rates
Sample Rates
Sample Rates
What is the fewest number of times I need to sample this waveform per second?
At least twice per wavelength or period!At least twice per wavelength or period!
OTHERWISE ….
Undersampled waveforms
True frequency (f -true)
Ampl
itude
Reconstructed frequency(f -aliased)
ff
Oversampled waveforms
= True frequency (f -true)
Ampl
itude
Reconstructed frequencyfrequency is unaliased
Nyquist frequency
Nyquist frequency = 1 / twice the sampling rate
Minimum sampling rate must be at least twice the desired frequencyE.g., 1000 samples per second for 500Hz,
2000 samples per second for 1000 Hz
Oversampled waveformsAm
plitu
deNyquist frequency
In practice we are best oversampling by double the required minimumi.e. 1000 samples per second for a maximum of 500 Hz
i.e., 2000 samples per second for a maximum of 1000 HzOversampling is relatively cheap.
Outline-2Acquisition Parameters
•Sample Rate and Temporal Aliasing•Offset Range•Listen Time• Geophone Spacing and Spatial Aliasing
Offset Range
One-layer earth of a semi-infinite layer
Target depth
Maximum shot-receiver offset
Maximum shot-receiver offset >= target depth.Near critical distance
Offset Range
Multi-layered earth
Target depth
Maximum shot-receiver offset
Outline-2Acquisition Parameters
•Time Sample Rate•Offset Range•Listen Time•Sample Rate and Temporal Aliasing• Geophone Spacing and Spatial Aliasing•Shooting geometry
•inline•cross-line
Listen Time
….Twice target time to be sage
Outline-2Acquisition Parameters
•Time Sample Rate•Offset Range•Listen Time•Sample Rate and Temporal Aliasing• Geophone Spacing and Spatial Aliasing•Shooting geometry
•inline•cross-line
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 CMP’s per wavelength)
Spatial aliasing
12( )kN CMPspacing
IN PRACTICE each wavenumber must be sampled at least four times per minimumwavelength (two CMP’s per wavelength)
Spatial aliasing
However, dip (theta) as well as frequency and velocity event changes the number of cycles per distance, so
4sinlambdaCMPinterval
Liner, 9.7,p.192
Spatial aliasing
4sinlambdaCMPinterval
x
V t
limitsin V tx
For aliasing NOT to occur, delta(t) must be less than T/2
Spatial aliasing
limitsin2VTx
lim 2sinitVTx
Geophone Spacing and Spatial Aliasing
K=0
1/4 wavelength shift per tracetotal shift across array=3/4 wavelength
K=+ or -ve?
1/4 wavelength shift per tracetotal shift across array=3/4 wavelength
K=?
1/2 wavelength shift per tracetotal shift across array=3/2 wavelength
K=0
3/4 wavelength shift per tracetotal shift across array=2 1/4 wavelength
Spatial aliasing
•Degrades (“string of pearls”) stacked sections•Degrades migration
Signal-to-Noise
Improves with stacking:•greater fold•greater repetition of shots
/S N CMP fold vertical stack
Outline-2Acquisition Parameters
•Time Sample Rate•Offset Range•Listen Time•Sample Rate and Temporal Aliasing• Geophone Spacing and Spatial Aliasing•Shooting geometry
•inline•cross-line
Fundamental Parameters for land 3D shooting
( , , )
( , , )
geophone g g g
shot s s s
X x y z
X x y z
Common Midpoint
1 ( )2
CMP shot geophoneX X X
Source-Receiver Offset
( )offset shot geophoneX X X
2D
offset shot geophoneX X X
3D
Azimuth (3D)
1tan shot geophone
shot geophone
x xy y
Inline geometry
Matlab code
Outline-2Acquisition Parameters
•Time Sample Rate•Offset Range•Listen Time•Sample Rate and Temporal Aliasing• Geophone Spacing and Spatial Aliasing•Shooting geometry
•inline•cross-line
Cross-line geometry
Matlab code
Spatial aliasing
•Degrades (“string of pearls”) stacked sections•Degrades migration
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