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

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.


Shotpoint # 1Hydrophone groups#1#2#3#4#5#6

Separation between midpoints is 1/2 separation between hydrophone groups

Midpoints



Midpoints



Midpoints



Midpoints



Midpoints



Midpoints



Midpoints



Midpoints



Midpoints

Common Midpoint Method (CMP Method)Hydrophone groups

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

Midpoints

Shotpoint # 1


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

Midpoints

Shotpoint # 1Shotpoint # 2



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

Midpoints



Shotpoint # 3

Shotpoint # 3


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

Midpoints



Shotpoint # 3

Shotpoint # 3

Shotpoint # 4

Shotpoint # 4


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

Midpoints

Shotpoints # 1-8

12 3

4 5 6 7 8 138


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


Midpoints1

2 34 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 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


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


A shotpoint gather samples different reflecting points at a variety of anglesMidpoints

Reflecting points


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


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




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)




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




SEGY data

SEGY dataOne line at a time

400 byte tape header

3200 byteEBCDIC header

240 byte trace header



DATA

DATA

DATA




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.


•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




Listen Time

….Twice target time to be sage




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?


K=?


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




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




Cross-line geometry

Matlab code

Spatial aliasing

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

Land and Marine Seismic Acquisition from 2D to 3D

Documents

hydrophone groupsa shotpoint

shotpoint gathershotpoint

recording groups

hydrophonesmidpoint

shotsmidpoint separation

number of geophones

different types

receiver group