8. Ocean Crustal Structureand Seismic Reflection

William Wilcock

OCEAN/ESS 410

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

• Understand what seismic impedance is and how it controls the amplitude of seismic

• Know how seismic reflection data is collected• Know how reflection data is stacked and converted into a

seismic record section• What can seismic record sections tell you? - LAB

Melting beneath mid-ocean ridges

The melt rises out of the mantle because it is buoyant (less dense than the mantle)

Ophiolites

What will happen when the Juan de Fuca Ridge collides with the North American Plate?

30

Ophiolites around the world

Ophiolites - Remnants of oceanic crust

•The spreading center collides with a subduction zone.

•As the two plates converge the spreading center is thrust on top of the downgoing plate to form the ophiolite.

Oman

Ophiolite Sequence

Mantle

Crust-Mantle Interface (“Moho”)

Gabbro layer

0

6

3

Dep

th, k

m

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 lavas

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.

Acoustic (Characteristic) 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

A0

V1, 1

V2, 2

The amplitude of the reflected and transmitted phase depends on the seismic velocity, V and the density, in each layer.

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 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

Marine Reflection Seismology - Geometry

The streamer records waves reflected from interfaces

Marine Reflection Seismology - DataThe 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

Tim

e, sTime

X0

Marine Reflection Seismology - Sorting RecordsThe records are sorted so that they all have the same mid-point (Common Mid-Point - CMP)

Marine Reflection Seismology - Airgun SourcesThe 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 PlotsStacked 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

Tim

e, s

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)

Intersecting Record Sections from the East Pacific Rise

0

6

3

Dep

th, k

mStructure of a mid-ocean ridge crust

molten

2A

Moho

Layered Gabbro

Peridotite (Mantle)

Endeavour segment of

Juan de Fuca Ridge

TW

TT

(s)

AMC

2A/2B