Multichannel Seismics: Techniques

1
Multi-channel Seismics Techniques
Interpretation

• Andrew M. Goodliffe
• SOEST
• University of Hawaii at Manoa

2
Basic Theory Sound Propagation

• P-Waves propagate through water and are reflected
  from an interface
• How much is reflected depends on the reflection
  coefficient (function of velocity and density).
• The remaining energy continues into the earth,
  being reflected and transmitted
• Some energy refracts and travels along horizons,
  returning to the surface via head waves.

3
Basic Theory The Source

• An ideal pulse convolved with the seafloor
  creates a simple seismogram

4
Convolution

• The output seismogram is a convolution of the
  source signal and the earth (the seafloor)
• Sharp seafloor signal becomes ringy

5
Deconvolution

• Deconvolution returns the seismogram to something
  we can interpret

6
Seismic Sources

• Attenuation High frequencies dont travel as far
  as low frequencies (the bad neighbor syndrome)
• Higher frequency sources have less penetration

7
Vertical Resolution

• Dependant on seismic wavelength
• Wedge tests
• Reflectors clearly resolved when separated by gt
  ?/4
• vf ?
• If v 2000 m/s, and f 30 Hz
• Resolution (66.67 m)/4 16.67 m
• If v 8000 m/s and f 20 Hz
• Resolution (400 m)/4 100 m
• If v 2000 m/s and f 3500 Hz
• Resolution (0.5714 m)/4 0.1428 m
• Energy returned from reflectors lt ?/10

8
Horizontal Resolution

• Partly determined by distance between CDPs
• Also dependant on wavelength
• Energy received within half a wavelength
  interferes constructively to make the reflected
  signal.
• Parts of a reflector separated by less than the
  width of the Fresnel zone will not be resolved
• Wf ? (2z ?)1/2 z depth
• If depth 2000 m, ? 60 m
• Wf ? 490 m
• If depth 100 m, ? 1 m
• Wf ? 14 m

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Electrical Sources Sparker

• A spark is produced by the discharge of a high
  voltage capacitor bank through an underwater
  electrode
• Produces a rapidly expanding bubble of ionized gas

10
Electrical Sources Boomer

• Aluminum plate attached via a spring loaded mount
  to a resin block
• Heavy duty coil is embedded in the resin block
• A capacitor bank is discharged through the coil,
  setting up eddy currents in the aluminum plate
• The currents set up a secondary field that
  opposes the primary field, and the plate is
  repulsed.
• Typically a high frequency source, with
  resolution of 0.1 m
• Depth penetration lt100 m

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High Pressure Air Sources The Air Gun
3
1
2
Ready
Fire!
Fired
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Air Guns

• The most common marine seismic source
• Very Repeatable signal

Bolt Air gun
13
Air Guns

• Airguns suspended from stowed booms

• Single Air gun note air ports

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Tuning An Air Gun Array

• A single airgun creates a ringy signal

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Tuning An Air Gun Array

• Summing the signal of multiple guns creates a
  more desirable signal
• Note the relative scales of the left and right
  plots

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Air Gun Deployment
Guns towed individually from two booms
Guns towed in strings
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Bubble from Air Gun Explosion

• Airguns are suspended from buoys to maintain depth

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GI Gun

• Two air guns in the same body
• The generator is fired and the bubble starts to
  expand
• When the bubble approaches its maximum size, the
  injector is fired into the bubble
• Reduces bubble oscillation

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Other Sound Sources

• Water guns
• Explosives
• Earthquakes
• Echo sounders
• Commonly 3.5 kHz
• Depth penetration of 100 m

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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

21
The Streamer

• Heavy duty plastic sleeve containing cables,
  hydrophones, and strength member.
• Oil Filled for neutral buoyancy.
• Birds keep the streamer at a constant depth
• Compasses record streamer azimuth

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Deploying The Streamer
Tailbuoy
Reel
Streamer and bird
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Ocean Bottom Instrumentation

• Hydrophone or geophone
• Records to an internal hard drive
• Used for primarily for refraction

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Hazards

• Fishing gear
• Other ships
• Too close to land
• Regattas
• Floating Debris
• Reefs and shallow water

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Planning a survey

• Ship speed
• Shot Interval
• Source type and size
• Group Interval (receiver spacing)
• CDP Interval (resolution of sections)

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Meanwhile, in the lab

• Navigation
• Data logging
• Quality control
• Data
• Source signal
• Gun depth
• Streamer depth
• Data recording
• 50 GB/day

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Initial Shipboard Processing

• Read tapes
• Initial quality control
• Geometry
• Trace statistics
• Kill bad traces
• Velocity analysis
• Trace mutes (inside and outside)
• Stack
• Migrate

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Geometry

• Ensure that CDPs are properly populated

• Feathering

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Shot Record
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Velocity Analysis
CDP Gather
Semblance Plot
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Top and Bottom Mutes
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Stacking

• Use stacking velocities to correct for
  Normal-Moveout
• Sum traces within a CDP
• Noise cancels out
• Real signal sums
• Stacking n traces leads to a n1/2 reduction in
  random noise

33
Migration

• Dipping reflector A-B appears in position C-D in
  a seismic section
• Migration corrects this

34
Migration
35
The Realities

• Seafloor and underlying strata are often not flat

• Creates side echoes
• Defies Normal-Moveout rules

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Mariana
37
The Incoming Pacific Plate
Line 53-54
12.5 km
E
W
Subducted Slab
Normal Faults

• Top of downgoing slab is in extension

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The Incoming Pacific Slab
E
W
6.25 km
Normal Fault

• Break-up of seamount as it enters the trench

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Big Blue Seamount
Throat Reflector
6.25 km
S
N
Original Forearc Surface
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Turquoise Seamount
6.25 km
S
N
The Seamount is growing laterally and
overthrusting the original forearc sediments
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Forearc Faulting
W
E
6.25 km
Fault blocks back-tilted to the west
Onlap
42
Forearc Faulting
N
S
6.25 km

• Numerous high-angle faults due to N-S Extension
  as arc balloons

Martinez et al., 2000
43
Mass Wasting
18.75 km
W
E
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Back-arc/active Arc Boundary
Mariana Back-arc Basin
Forearc High
Boundary
Arc Line
6.25 km
E
W
Seafloor Spreading (sedimented)
45
Back-arc/Remnant Arc Boundary
West Mariana Ridge
Mariana Back-arc Basin
6.25 km
W
E
Remnant Arc
Seafloor Spreading
Low-Angle Fault
46
Back-arc Spreading Center
S
N
Side-Swipe?
Magma Lens??
6.25 km
47
Woodlark Basin, PNG
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Woodlark Rift
49
Low-Angle Fault
50
Older Margins
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Synthetic Seismics
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Correlations
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Monohull vs. SWATH

• Small waterplane area, twin hull
• Rides on two submerged pontoons

• Conventional monohull

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Monohull vs. SWATH
R/V Maurice Ewing
R/V Kilo Moana
R/V Melville
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R/V Kilo Moana
fantail
staging bay
main lab
fantail (A-frame)