IODP Expedition 312: seismic velocities in the deep ocean crust - - PowerPoint PPT Presentation

IODP drilling vessel JOIDES Resolution

IODP Expedition 312: seismic velocities in the deep ocean crust

Presentation for RPI Natural Science II March 13th, 2007 Anahita Tikku

What is IODP?

• The Integrated Ocean Drilling Program (IODP) is an

international marine research drilling program dedicated to advancing scientific understanding of the Earth by monitoring and sampling subseafloor environments.

• IODP science focuses on four principal themes: the deep

biosphere and the subseafloor ocean; environmental change; Earth processes and effects; and solid earth cycles and geodynamics. (for more info see: http://www.iodp.org)

How does ocean drilling work?

How does ocean drilling work?

“Tripping pipe” at the surface. Re-entering drill hole.

Drill cores: what do you get?

Expedition 312: drilling objectives

• The goal of the Superfast Spreading Rate Crust

mission (spanning ODP Leg 206, and IODP Expeditions 309 and 312) was to core through the entire sequence of extrusive rocks, the underlying sheeted dike complex, and into the upper gabbros at a site in the eastern Pacific Ocean (Site 1256D).

• Though gabbros constitute the majority of

the oceanic crust, they have not been sampled in situ previously.

What are gabbros?

• Gabbros are dark,

coarse-grained, intrusive igneous rock chemically equivalent to basalt. It is a plutonic rock, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass.

Scientific goals of expedition:

• Provide the first sampling of a complete section of oceanic

crust to the gabbros.

• Confirm whether ocean crust formed at a superfast spreading

rate conforms to classic ophiolite stratigraphy. (Ophiolites are slivers of oceanic crust that have been obducted -

• verthrust and accreted to the continental crust.)
• Confirm the nature of high-level axial magma chambers.
• Define the relationship between magma chambers and their
• verlying lavas and the interactions between magmatic,

hydrothermal, and tectonic processes.

• Provide in situ calibration of seismic velocity and magnetic

measurements made from surface ships so that these regional-scale geophysical measurements can be related to geology.

Mid-ocean ridges: where new

• cean crust is created

Artist's cross section illustrating the main types of plate boundaries. From This Dynamic Planet -- produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)

Textbook oceanic crustal structure

Figure from: Kennett (1982)

Obduction & formation of ophiolites

Ocean drilling site distribution

Deep ocean drilling sites are few

Site 1256D: location of Exp 312

P-wave seismic velocities in rocks (generally between 4-7 km s-1)

Seismic velocity is a function of:

• Porosity (more porous - slower velocity)
• Density (more dense - higher velocity)
• Microcracks (more cracking - lower

velocity)

• Mineralogy (more complicated)
• ?

Solid, dense rocks transmit signals faster and with less dispersion.

Seismic velocity as a function of density: experimental results

Figure from Carlson & Herrick (1990)

Wireline Tool Strings

IODP Logging Services IODP Logging Services

USIO Science Services - LDEO USIO Science Services - LDEO

Dipole Shear Sonic Imager (DSI)

The DSI uses a combination

• f monopole and dipole

transducers to make measurements of sonic wave propagation in a wide variety of lithologies.

• The monopole transducer excites compressional

and shear waves at high frequencies.

• A low-frequency source enables Stoneley

waveforms to be acquired.

• These “guided” waves are associated with the

solid-fluid boundary at the borehole wall and their amplitude decays exponentially away from the boundary in both the fluid and the formation.

Seismic layers in oceanic crust

Seismic layer 2 constitutes the upper crust and is characterized by a rapid increase in seismic velocity with depth, and a thicker lower crust Seismic layer 3 constitutes the thicker lower crust and is distinguished from layer 2 by both a higher P-wave velocity & a much smaller vertical velocity gradient.

Downhole Logging data

Seismic velocities from cubes at 1256D

Seismic velocities downhole at Site 1256D

Seismic velocity anomaly at sheeted dike-gabbro boundary?

Experimental setup for high pressure seismic velocity measurements

Seismic velocity as a function of pressure

microcracks

New downhole seismic velocity profile

Shipboard ambient P velocities 1000 bar velocities Sonic log downhole velocities

Fun at sea!