4/7/2010
GG450
Today’s material comes from p.
117-162 in the text book.
April 8, 2010
Please read and understand all
of this material!
Seismic Reflection II
Data Acquisition
3
Seismic Reflection Acquisition
Source
Receivers
• Key Measurements
• Two Way Travel Time From Surface to
Layer Interface (“Reflector”) and Back to
Surface
A seismic survey consists of a source and
receivers sequentially moved along the surface.
• Amplitude - “Strength of Return”
3
SEISMIC SOURCE:
In reflection seismology, the source used is
governed by the problem - large low-frequency
sources for deep penetration, high-frequency small
sources for shallow targets.
Marine seismic survey geometry
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The vibroseis truck is jacked up with a hydraulic jack
and a radio signal controls the vibration of the truck.
Many vibroseis trucks can be used in series to
make a combined source signal.
Air Guns
Bolt Air gun
Most common marine
source, repeatable
Vibroseis trucks can be used in urban areas.
air
air
Variable
chamber
size
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Tuning An Air Gun Array
Air Gun firing video
A single airgun creates a “ringy” signal= bubble
pulse (often seafloor parallel reflections)
Airgun Deployment
Tuning An Air Gun Array
Old method: Guns towed individually from booms at
the stern, suspended from buoys to maintain depth
Summing the signal of multiple guns creates a
more desirable signal
Bubble from Air Gun explosion
Gun Array
New method: Guns towed in linear arrays
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Basic Theory: Listening
Hydrophone
Piezoelectric material
Pressure changes in the water generate
small currents which are amplified
Geophone
Mechanical
Motion of coil relative to magnet
generates a small current which is then
amplified
The Streamer
Basic Theory: Listening
Multiple geophones are usually grouped
together electrically to form one recording
channel
This helps to cancel noise – towing noise in
marine surveys and ground roll (surface
waves) in land surveys
Seismic Reflection Field Methods
Heavy duty plastic sleeve containing cables,
hydrophones, and strength member.
Used to be Oil filled for neutral buoyancy,
but now they are solid
Arrays of hydrophones create a directional
filter
Know your goals:
What is the target depth? This is probably the
most important question for definition of
survey parameters.
For reflection, your spread lengths should be
about the same length as the target is deep.
Digitizers convert analogue signal to digital
“Birds” keep the streamer at a constant
depth
Compasses record streamer azimuth
Deploying The Streamer
Reel
Tailbuoy
Streamer and bird
Paravanes for towing multiple streamers
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Streamer layout
Most streamers now have 12.5 m groups
M/V Nordic Explorer (PGS)
4 streamers, each 4500- 6000 m long
Ramform Sterling:
22 streamers, each 8 km long
Shot gather
All the data recorded
on all the channels
by a single shot.
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Common Mid Point (CMP)
This is the standard for seismic reflection acquisition
today.
The idea is to "gather" all data that reflect from the
same point below and use these data for moveout
correction and stacking.
While it requires considerable rearranging of traces,
computers can do this quite easily.
Stacking - Summing
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
Shotpoint # 1
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
#1
Midpoints
Shotpoint # 2
#1
Midpoints
Separation between midpoints is
1/2 separation between hydrophone groups
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 3
#6 #5 #4 #3 #2
#1
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
#6 #5 #4 #3 #2
Shotpoint # 4
#1
Midpoints
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Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 5
#6 #5 #4 #3 #2
#1
Midpoints
Common Midpoint Method (CMP Method)
Shotpoint # 7
#6 #5 #4 #3 #2
Shotpoint # 6
#6 #5 #4 #3 #2
#1
Midpoints
Hydrophone groups
Common Midpoint Method (CMP Method)
Hydrophone groups
#1
Midpoints
Common Midpoint Method (CMP Method)
Hydrophone groups
Shotpoint # 8
#6 #5 #4 #3 #2 #1
Midpoints
CMP #6
Common Midpoint Method (CMP Method)
3 Shots
Midpoints
#6
#5
#4
#3
Shotpoint #2
#1
#6
#5
#4
#3
Receiver #2
#1
Midpoints
Common Midpoint Method (CMP Method)
Why do this??
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Source & Receiver Navigation
4 streamers
150m separation
2 Gun arrays (alt firing)
8 CMP Lines @ 37.5 m
3D acquisition geometry
3D acquistion – overlapping tracks
Streamer
Feathering
8