Transcript No Slide Title

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
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
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)
Hydrophone groups
#6 #5 #4 #3 #2
Shotpoint # 1
#1
Midpoints
Separation between midpoints is
1/2 separation between hydrophone groups
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
Shotpoint # 2
#1
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 3
#6 #5 #4 #3 #2
#1
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 4
#6 #5 #4 #3 #2
Midpoints
#1
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 5
#6 #5 #4 #3 #2
Midpoints
#1
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 6
#6 #5 #4 #3 #2
Midpoints
#1
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 7
#6 #5 #4 #3 #2
Midpoints
#1
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 8
#6 #5 #4 #3 #2
Midpoints
#1
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 8
#6 #5 #4 #3 #2
Midpoints
#1
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
#1
Midpoints
Shotpoint # 1
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
#1
Shotpoint # 1
Shotpoint # 2
Shotpoint # 1
Shotpoint # 2
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
#1
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
#1
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 4
Shotpoint # 1
Shotpoint # 2
Shotpoint # 3
Shotpoint # 4
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoints # 1-8
1
2
3
Midpoints
45 6 7 8
8
13
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
3
Midpoints
45 6 7 8
8
13
Common Midpoint Method (CMP Method)
Maximum Fold is achieved after the 6th shot
Fold
1
2
3
Midpoints
45 6 7 8
8
13
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
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
Shotpoint Gather
e.g. Shotpoint gather #3
Shotpoint Gather
Shotpoint #3
Hydrophone groups
#6 #5 #4 #3 #2
#1
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
Hydrophone groups
#6 #5 #4 #3 #2
#1
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
Hydrophone groups
#6 #5 #4 #3 #2
#1
Reflecting points
Midpoints
A shotpoint gather samples different reflecting points at a variety of angles
Common Midpoint Method (CMP Method)
Hydrophone group #4
Common source-receiver offset and
common receiver, shotpoints 1-8
Hydrophone group #4
Common source-receiver offset and
common receiver, shotpoints 1-8
COS means equal reflection angle
Midpoints
In the case of a COS gather where are the true
midpoints when the reflecting, geological interface
has a dip?
COS means equal reflection angle
Midpoints
COS NO LONGER implies equal reflection angles
Actual reflecting points
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone group #4
Common mid-points and
shotpoints 1-8
Midpoints
Hydrophone group #4
Common mid-point and
shotpoints 1-8
group
8 76 5 4 3 2 1
Midpoint #6
CMP gathers sample varying angles but a common geological midpoint
What happens to a common midpoint gather when the
reflecting interface has a dip?
group
8 76 5 4 3 2 1
Midpoint #6
CMP gathers sample varying angles but a common geological midpoint
CMP gathers SAMPLE varying angles but with
a relatively smaller spread of
reflecting points than the shotpoint and
common-offset gathers
group
8 76 5 4 3 2 1
Midpoint #6
True Reflecting Points
A common midpoint gather minimizes the effect of dip while it
helps increase the signal-to-noise ratio
Outline-1
CMP METHOD (Harry Mayne)
Seismic sensors
•geophones
•hydrophones
•gimballed geophones and hydrophones
•accelerometers
Sources
•Explosives
• Vibroseis
SEGY data
Geophones
Convert ground motion into electricity
at a rate of about 1 Volt/inch/sec
Natural Resonance Frequency 100 Hz
GS-100 from Geospace
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 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 (3.3/4*10^-7)
Outline-1
CMP 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 ….
Vibroseis images from the Lithoprobe Project, Canada
www.lithoprobe.ca
A vibrator truck
“12 elephants dancing in unison” (LITHOPROBE, CANADA)
Explosives
Noble Explochem Limited
GI Watergun Array
NSF R/VIB NBPalmer- February/March 2003
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-1
CMP METHOD (Harry Mayne)
Seismic sensors
•geophones
•hydrophones
•gimballed geophones and hydrophones
•accelerometers
Sources
•Explosives
• Vibroseis
SEGY data
SEGY data
One line at a time
3200 byte
EBCDIC header
400 byte tape header
240 byte trace header
DATA
240 byte tape header
DATA
240 byte tape header
DATA
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
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!
OTHERWISE ….
Undersampled waveforms
Amplitude
f
Reconstructed frequency
(f -aliased)
f
True frequency (f -true)
Amplitude
Oversampled waveforms
Nyquist frequency
Reconstructed frequency = True frequency (f -true)
frequency is unaliased
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
Amplitude
Oversampled waveforms
Nyquist frequency
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.
Outline-2
Acquisition Parameters
•Sample Rate and Temporal Aliasing
•Offset Range
•Listen Time
• Geophone Spacing and Spatial Aliasing
Offset Range
Maximum shot-receiver
offset
Target depth
One-layer earth of a semi-infinite layer
Maximum shot-receiver offset >= target depth.
Near critical distance
Offset Range
Maximum shot-receiver
offset
Target depth
Multi-layered earth
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
Listen Time
….Twice target time to be sage
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
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 CMP’s per wavelength)
1
Nk 
2(CMPspacing )
IN PRACTICE each wavenumber must be sampled at least four times per minimum
wavelength (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
lambda
CMP interval 
4sin 
Liner, 9.7,p.192
Spatial aliasing
lambda
CMP interval 
4sin 

x

V t
sin  limit
V t

x
For aliasing NOT to occur, delta(t) must be less than T/2
Spatial aliasing
sin  limit
xlim it
VT

2 x
VT

2sin 
Geophone Spacing and Spatial Aliasing
K=0
1/4 wavelength shift per trace
total shift across array=3/4 wavelength
K=+ or -ve?
1/4 wavelength shift per trace
total shift across array=3/4 wavelength
K=?
1/2 wavelength shift per trace
total shift across array=3/2 wavelength
K=0
3/4 wavelength shift per trace
total 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-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
Fundamental Parameters for land 3D shooting
X geophone  ( xg , y g , z g )
X shot  ( xs , ys , zs )
Common Midpoint
X CMP
1
 ( X shot  X geophone )
2
Source-Receiver Offset
X offset  ( X shot  X geophone )
2D
X offset  X shot  X geophone
3D
Azimuth (3D)
 xshot  xgeophone 
  tan 

y y
geophone 
 shot
1
Inline geometry
Matlab code
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
Cross-line geometry
Matlab code
Spatial aliasing
•Degrades (“string of pearls”) stacked sections
•Degrades migration