15' Reflection Seismology William Wilcock

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15. Reflection SeismologyWilliam Wilcock
OCEAN/ESS 410
Fall 2007
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Reflections from Interfaces
When a downgoing P-wave meets an interface, a
portion of the wave is reflected.
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Amplitudes of Reflections for vertical rays
Reflected Amplitude
The amplitude of the reflected and transmitted
phase depends on the seismic velocity, V and the
density, ? in each layer. Larger contrasts in
velocity and density result in large amplitude
reflections
A0
V1, ?1
V2, ?2
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Marine Reflection Seismology - Airgun Sources
Reflection data is relatively easy to acquire in
the oceans. Seismic sounds (shots) can be
generated with arrays compressed air guns
(airguns) towed behind the ship
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Marine Reflection Seismology - Hydrophone
Streamers
The airgun shots are recorded by arrays of
hydrophones towed behind the ship in a streamer.
The seismic streamers contain 1000s of
hydrophones and can be gt5 km long. A modern 3-D
seismic ship may tow several streamers.
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Marine Reflection Seismology - Geometry
The streamer records waves reflected from
interfaces
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Marine Reflection Seismology - Data
The seismic data recorded for a particular shot
will look display a geometric effect termed
normal moveout (NMO) which reflects the
increased distance the wave travels for as the
source-receiver offset increases
Offset
X
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Time, s
Time
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Marine Reflection Seismology - Sorting Records
The records are sorted so that they all have the
same mid-point (Common Mid-Point - CMP)
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Marine Reflection Seismology - Airgun Sources
The seismic records can be corrected for
geometric affects and stacked (summed) to produce
a single record for the reflections below each
each point
Before Geometric Correction
After Geometric Correction
Stacked (summed)
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Marine Reflection Seismology - Filled Wiggle Plots
Stacked records are plotted on the same plot with
the horizontal axis showing position along the
profile. Rather than showing lines for each
record the plots often show filled regions for
positive (or negative) displacements
Time, s
Position
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A reflection profile across the East Pacific Rise
Reflections come from the seafloor, the base of
layer 2A (pillow basalts), the axial magma
chamber (AMC) and the Moho (M)
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Intersecting Record Sections from the East
Pacific Rise