Land and Marine Seismic Acquisition from 2D to 3D

Land and Marine Seismic Acquisition from 2D to 3D

From chapters 7-12 “Elements of 3D Seismology” by Chris Liner

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CMP METHOD (Harry Mayne)

Seismic sensors

•geophones

•hydrophones

•gimballed geophones and hydrophones

•accelerometers

Sources

•Explosives

• Vibroseis

SEGY data

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Acquisition Parameters

•Time Sample Rate

•Offset Range

•Listen Time

•Sample Rate and Temporal Aliasing

• Geophone Spacing and Spatial Aliasing

•Shooting geometry

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•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.


Shotpoint # 1Hydrophone groups

#1#2#3#4#5#6

Separation between midpoints is

1/2 separation between hydrophone groups

Midpoints



#1#2#3#4#5#6

Midpoints


Hydrophone groups

#1#2#3#4#5#6

Midpoints

Shotpoint # 1


Hydrophone groups

#1#2#3#4#5#6

Midpoints

Shotpoint # 1Shotpoint # 2



Hydrophone groups

#1#2#3#4#5#6

Midpoints



Shotpoint # 3

Shotpoint # 3


Hydrophone groups

#1#2#3#4#5#6

Midpoints



Shotpoint # 3

Shotpoint # 3

Shotpoint # 4

Shotpoint # 4


Hydrophone groups

#1#2

#3

#4

#5#6

Midpoints

Shotpoints # 1-8

12

34 5 6 7 8 138


Midpoints1

23

4 5 6 7 8 138

FoldFold 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


Midpoints1

23

4 5 6 7 8 138

Maximum Fold is achieved after the 6th shotMaximum Fold is achieved after the 6th shot

Fold


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

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 Gather

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 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 angles

Midpoints

Reflecting points


Hydrophone group #4

Common source-receiver offset and

common receiver, shotpoints 1-8

#1#2#3

#4

#5#6

Hydrophone group #4

Common 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


Hydrophone group #4

Common mid-points and

shotpoints 1-8

#1#2#3

#4

#5#6

Midpoints

Hydrophone group #4

Common mid-point and

shotpoints 1-8

#1#2#3

#4

#5#6

Midpoint #6

group

12345678

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

group

12345678

CMP gathers sample varying angles but a common geological midpoint

#1#2#3

#4

#5#6

Midpoint #6

group

12345678

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-1


Seismic sensors

•geophones

•hydrophones


•accelerometers

Sources

•Explosives

• Vibroseis

SEGY data

Geophones

Convert ground motion into electricity

at a rate of about 1 Volt/inch/sec

GS-100 from Geospace

Natural Resonance Frequency 100 Hz

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


Seismic sensors

•geophones

•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-1


Seismic sensors

•geophones

•hydrophones


•accelerometers

Sources

•Explosives

• Vibroseis

SEGY data

SEGY data

One line at a time

400 byte tape header

3200 byte

EBCDIC header

240 byte trace header



DATA

DATA

DATA

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•Time Sample Rate

•Offset Range

•Listen Time




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Sample Rates

What is the fewest number of times I need to sample this waveform per second?

?

?

?

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)

Am

plit

ud

e

Reconstructed frequency

(f -aliased)

ff

Oversampled waveforms

= True frequency (f -true)

Am

plit

ud

e

Reconstructed frequency

frequency is unaliased

Nyquist frequency

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

Oversampled waveformsA

mp

litu

de 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



•Offset Range

•Listen Time


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

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•Time Sample Rate

•Offset Range

•Listen Time




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Listen Time

….Twice target time to be sage

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•Time Sample Rate

•Offset Range

•Listen Time




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•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

1

2( )kN 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

4sin

lambdaCMPinterval

Liner, 9.7,p.192

Spatial aliasing

4sin

lambdaCMPinterval

x

V t

limitsinV t

x

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

Spatial aliasing

limitsin2

VT

x

lim 2sinit

VTx

Geophone Spacing and Spatial Aliasing

K=0

1/4 wavelength shift per trace

total shift across array=3/4 wavelength

K=+ or -ve?



K=?



K=0


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

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•Time Sample Rate

•Offset Range

•Listen Time




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•cross-line

Fundamental Parameters for land 3D shooting

( , , )

( , , )

geophone g g g

shot s s s

X x y z

X x y z

UUUUUUUUUUUUUU

UUUUUUUUUUUUUU

Common Midpoint

1( )2

CMP shot geophoneX X X UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU

Source-Receiver Offset

( )offset shot geophoneX X X UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU

2D

offset shot geophoneX X X UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU

3D

Azimuth (3D)

1tan shot geophone

shot geophone

x x

y y

Inline geometry

Matlab code

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•Time Sample Rate

•Offset Range

•Listen Time




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•cross-line

Cross-line geometry

Matlab code

Spatial aliasing

•Degrades (“string of pearls”) stacked sections

•Degrades migration

Land and Marine Seismic Acquisition from 2D to 3D

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Transcript of Land and Marine Seismic Acquisition from 2D to 3D