Marine Reflection Seismology - Geometry

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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
0
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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Reflection Profile
1 km
300
400
600
700
500
900
800
1000
CMP Number
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Fast/Intermediate Spreading-Rate Ridges
Sinton Detrick, 1992
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Ophiolites
Water (1.5 km/s 1000 kg/m3)
Reflector
Basalts Layer 2A (2.5 to 4 km/s 2200 kg/m3)
Reflector
Sheeted Dikes - Layer 2B (5.5-6.5 km/s 2800
kg/m3)
Gradational (No reflection except when magma
chamber is present)

• 4 major geological units
• 3 units form the crust
• Basalts - basaltic rocks that erupted on the
  seafloor as pillow lavas and sheet flows
• Sheeted Dikes - Magma solidified in 1-m-wide
  vertical cracks extending along the ridge axis.
  These cracks transported lavas from the magma
  chamber to seafloor eruptions.
• Gabbros - Course grained rocks formed by slow
  cooling in the crustal magma chamber
• 1 unit forms the mantle
• Harzburgite - Mantle rocks which partially melted
  to produce the crust.

Gabbro Layer 3 (6.5 to 7 km/s 2800 kg/m3)
Reflector
Moho
Mantle (8 km/s 3400 kg/m3)
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gabbro
Moho
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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