Transcript Seismic Reflection and Ocean Crustal Structure

OCEAN/ESS 410
8. Ocean Crustal Structure
and Seismic Reflection
William Wilcock
(via Michelle W.)
Lecture/Lab Learning Goals
• Know the basic structure of the oceanic crust as
determined from ophiolite studies
• Understand how oceanic crustal structure is linked to
mantle melting and volcanic processes
• Physics: Understand what seismic impedance is and how
it controls the amplitude of seismic reflections
• Data collection: airguns & LOTS of hydrophones
• Data processing: Be able to explain how reflection data is
stacked and converted into a seismic record section
• Interpretation: What can seismic records tell you? (LAB)
Structure of oceanic crust:
what’s down there?
Geophysics VS Geology
• What is the difference??
– Geology: a science based on direct
observation of terrestrial formations
– Geophysics: finding things underground
without digging. Or diving.
Drilling to the mantle
The Geology part: Ophiolites
• Ophio = Snake
(i.e. green)
• Lithos = Stone
What if the Juan de Fuca Ridge collided
with the North American Plate?
North American Plate
I’m so hungry! I
think I’ll eat the
whole Juan de
Fuca plate.
Nom nom nom
Oh no! Please
don’t eat me!
What if the Juan de Fuca Ridge collided
with the North American Plate?
North American Plate
I’m so hungry! I
think I’ll eat
that Mid Ocean
Ridge.
You would get an ophiolite!
-Often happens when ocean basins close
nom nomcrust
-You get “obduction”, or upthrustNom
of oceanic
and mantle onto continental plates
Oh no! Please
don’t eat me!
Ophiolites around the world
Melting beneath mid-ocean ridges
The melt rises out of the mantle because it is buoyant (less
dense than the mantle)
Ophiolite Sequence
Mantle
Crust-Mantle Interface (“Moho”)
Gabbro layer
Depth, km
0
3
6
Gabbros (layered) - they can also be un-layered (massive)
Sheeted dike layer
- “feeder channels” for
basaltic lava rising
from below
- The cracks extend
over large distances
beneath ridges
Sheeted dike layer
Sheeted dike layer
Pillow basalt layer
Pillow basalt layer
- Lavas extruded onto ocean floor at a spreading center
Sediment layer
Pelagic sediments
deposited above oceanic
crust basalts
Seismic Reflection
Reflections from Interfaces
When a downgoing P-wave meets an interface, a portion of the wave is reflected.
Characteristic acoustic impedance
- An inherent property of the medium
- Depends on sound speed (or seismic velocity) and
density
- Pressure generated by vibration of molecules of a
particular medium at a given frequency
Amplitudes of Reflections for vertical rays
The amplitude of the
reflected and transmitted
phase depends on the
seismic velocity, V and the
density,  in each layer.
A0
V1, 1
V2, 2
Larger contrasts in
impedance result in large
amplitude reflections
Marine Reflection Seismology - Airgun Sources
Reflection data is relatively easy to acquire in the oceans. Seismic sounds (shots) can
be generated with arrays of compressed air guns (airguns) towed behind the ship
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 1000’s of hydrophones and can be >10 km long.
A modern 3-D seismic ship will tow several (the records is 20) streamers.
Marine Reflection Seismology - Geometry
The streamer records waves reflected from interfaces
Normal moveout drawing on board…
Marine Reflection Seismology - Data
The seismic data recorded for a particular shot will display a geometric effect termed
“normal moveout” (NMO) which reflects the increased distance the wave travels as
source-receiver offset increases
0
Offset X
Time
Marine Reflection Seismology - Sorting Records
The records are sorted so that they all have the same mid-point (Common Mid-Point CMP)
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)
Marine Reflection Seismology - Filled Wiggle Plots
Time, s
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
Position
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)
Reflection profile - Sumatra
Endeavour
segment of
Juan de Fuca
Ridge
TWTT (s)
Endeavour
segment of
Juan de Fuca
Ridge
2A/2
B
AMC
Intersecting Record Sections from the East Pacific
Rise