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Seismic Reflection II
Data Acquisition
Today’s material comes from p.
117-162 in the text book.
Please read and understand all
of this material!
3
Seismic Reflection Acquisition
- Key Measurements
- Two Way Travel Time From Surface to
Layer Interface (“Reflector”) and Back to
Surface
- Amplitude - “Strength of Return”
3 Source Receivers
A seismic survey consists of a source and receivers sequentially moved along the surface.
Marine seismic survey geometry
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.
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.
Vibroseis trucks can be used in urban areas.
Bolt Air gun
air
Variablechamber size
air
Air Guns
Most common marine
source, repeatable
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
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
The Streamer
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 Digitizers convert analogue signal to digital “Birds” keep the streamer at a constant depth Compasses record streamer azimuth
Seismic Reflection Field Methods
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.
Deploying The Streamer
Tailbuoy
Streamer and bird
Reel
Paravanes for towing multiple streamers
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.
Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 5 #6 #5 #4 #3 #2 #
Midpoints
Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 6 #6 #5 #4 #3 #2 #
Midpoints
Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 7 #6 #5 #4 #3 #2 #
Midpoints
Common Midpoint Method (CMP Method) Hydrophone groups Shotpoint # 8 #6 #5 #4 #3 #2 #
Midpoints
Common Midpoint Method (CMP Method) 3 Shots
Midpoints Common Midpoint Method (CMP Method) Why do this??
CMP
Shotpoint
Receiver
Midpoints
CMP
Shotpoint
Receiver
CMP
Shotpoint
Receiver
CMP
Shotpoint
Receiver
CMP
Shotpoint
Receiver
CMP
Shotpoint
Receiver
4 streamers 150m separation 2 Gun arrays (alt firing) 8 CMP Lines @ 37.5 m
3D acquisition geometry
Source & Receiver Navigation
3D acquistion – overlapping tracks
Streamer
Feathering
