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Interpretation of Seafloor Gravity Anomalies. Gravity measurements of the seafloor provide information about subsurface features. For example they help resolve : -the structures that exist at the boundary between oceans and continents

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Presentation Transcript
slide1

Interpretation

of

Seafloor

Gravity Anomalies

slide2

Gravity measurements of the seafloor provide information about subsurface features.

  • For example they help resolve :
  • -the structures that exist at the boundary between oceans and continents
  • - the dimensions of mid-ocean ridge magma chambers
  • - the presence and dimensions of offshore sedimentary basins
  • Gravity surveys of continents reveal additional information about the processes that lead to the rifting of continents and the formation of ocean basins.
slide3

A gravity anomaly is the difference between the measured value and an expected value.

Gravity Anomalies

Are calculated:

g = gmeasured +/- correction - ggeoid

= gravity anomaly

gmeasured may be corrected for:

•Bouguer

•Free Air

•Topography

and others

Whenever a measured value departs from an expected value, an anomaly exists.

Corrections are applied to the measured value, depending on one’s interest.

slide4

Free Air (=Elevation) Corrections

gm

Mountain

Geoid

Ocean

go

The free-air correction accounts for the difference in elevation between the gravimeter and the geoid.

For measurements at sea, this is really small! Recall that the difference between sea level and the geoid is slight.

go = gmeasured (1 + 0.00031 h)

g in gal, height in meters

slide5

Free Air (=Elevation) Corrections

gm

Mountain

Geoid

Ocean

go

Most free-air gravity anomalies are in the range of a few hundred milligal, while most shipboard corrections are close to one milligal.

If the gravimeter is below sea level, the correction must be subtracted.

go = gmeasured (1 + 0.00031 h)

g in gal, height in meters

slide6

Bouguer (=Mass) Corrections

gh

Mountain

Geoid

Ocean

go

The Bouguer correction accounts for the additional gravitational attraction between the material that lies between the gravimeter and the geoid.

slide7

Bouguer (=Mass) Corrections

gh

Mountain

Geoid

Ocean

go

The meaning of the term “free-air correction” becomes more apparent in relation to the Bouguer correction.

The free-air correction assumes only air lies between the gravimeter and the geoid; the Bouguer correction assumes material other than air lies between them...

slide8

Bouguer (=Mass) Corrections

One assumes for the Bouguer correction that the mass between the gravimeter and the geoid is that of an infinite plate of uniform density and thickness.

The Bouguer correction on land is in the opposite direction of the free-air correction; the added attraction of the extra mass increases the observed gravity.

gm

Mountain

Geoid

Ocean

go

go = gmeasured (1 - 0.00004 h•density)

g in gal, height in meters, density in g cm-3

slide9

Bouguer (=Mass) Corrections

To apply a Bouguer correction to gravity measurements, the composition and density of the slab must be known, or inferred.

Terrestrial data that are corrected for both elevation and mass (i.e., free air and Bouguer corrections) should approach the same value of g (gravitational attraction) as that of the geoid, provided the local relief is not great.

gm

Mountain

Geoid

Ocean

go

go = gmeasured (1 - 0.00004 h•density)

g in gal, height in meters, density in g cm-3

slide10

Bouguer (=Mass) Corrections at Sea

The Bouguer correction at sea substitutes for seawater a layer with the same density as the seafloor.

This removes the effects of variations in bottom topography from the gravity data, and makes the data useful for studies of the subsurface.

This correction is often made in nearshore gravity surveys that extend onto land.

gh

Mountain

z

Geoid

Ocean

go

gcorr = gmeas [1 + 2Gz(dseafloor-dseawater)]

g in mgal, height in meters, density in g cm3

slide11

Topographic Corrections

Topography also affects gravity. A gravimeter next to a mountain is attracted to the mountain. The outward directed (upward) component of the attraction decreases the gravitational attraction experienced by its mass.

gh

Mountain

Geoid

Ocean

go

For land areas with large variations in topography, this correction is important.

Examples:

Pikes Peak, CO 48 mgal

Mt. Blanc, France 123 mgal

slide12

Topographic Corrections

For data collected at sea, this type of correction is incorporated in what is known as a 3-dimensional Bouguer correction. It differs from a simple Bouguer correction in that the gravitational attraction of nearby seafloor is also considered.

gh

Mountain

Geoid

Ocean

go

Examples:

Pikes Peak, CO 48 mgal

Mt. Blanc, France 123 mgal

slide13

Free-Air Anomaly Over a Subduction Zone

+200

Central Aleutian Trench

0

-200

0

The large positive anomaly (more than expected gravitational attraction) above the island arc represents a mass excess (the descending slab). The large negative anomaly (less than expected gravitational attraction) above the trench represents a mass deficiency (low density overlying sediments and the trench itself).

Kilometers

300

slide14

Free-Air Anomaly Over a Subduction Zone

+200

Central Aleutian Trench

0

-200

0

The free air gravity anomaly is near zero away from the plate boundary. This indicates the oceanic crust is in isostatic equilibrium.

Isostacy= Mass excesses at the Earth’s surface are balanced by mass deficiencies, below the surface.

Kilometers

300

slide15

Japan Plate

Free-Air

Gravity

Anomalies

Japan

Pacific Plate

Triple Junction

Philippine Plate

Japan

Trench

slide16

Free Air Anomaly For Atlantic

Continental Margin

Upper

Continental Crust

Oceanic Crust

Upper Mantle

Rift Stage

Crust

The large positive anomaly near the shelf edge (more than predicted gravity) occurs because high density basalts lie underneath the shelf. This high density “basement” rock formed during the initial stages of rifting between North America and Africa.

The large negative anomaly seaward of the shelf edge is evidence of a large volume of accumulated sediment.

Lower

Continental Crust

Coastline

slide17

Free Air Anomaly For Atlantic

Continental Margin

Upper

Continental Crust

Oceanic Crust

Upper Mantle

Rift Stage

Crust

Lower

Continental Crust

The location of the boundaries between continental and ocean crust are poorly known.

Subsurface geology such as that above is a “best fit” solution to gravity and seismic surveys.

Coastline

Only for a few continental margin locations,

interpretations have been drawn from drilling data.

slide18

The decrease in the gravity anomaly along the transect suggests a large mass of low density material beneath the ridge crest. The shape of the magma chamber (seen in cross section) is estimated from the gravity data.

Bouguer Anomaly

Mid-Ocean Ridge (Magma Chamber)

slide19

From: J. R. Ridgway, M.A. Zumberge, and J.A. Hildebrandat Scripps Institute of Oceanography

Source: http://spot.ucsd.edu/towdog/towdog.html

The resolution of small, subsurface seafloor features in gravity data improves as the gravitometer is towed closer to the features.

Sour

From

Submersible gravitometer system named Tow Dog

slide20

Three Tow Dog transects over ~10 km of seafloor.

This information, along with ship’s speed data, is needed to correct for gravity variations resulting from Tow Dog’s depth in the water column and vertical acceleration.

slide21

Free-air gravity tracks above provide very high resolution information about the dimensions of a small offshore sedimentary basin.

Many layers of low density sediments in the basin result in lower gravity anomalies over the basin’s center.

Gravity (mgal)

Distance (km)

slide22

The image above shows the Bouguer anomaly map for the continental US.

The effect of topography and elevation removedby theBouguer correction.

The reds are positive anomalies (higher than expected gravity) and the blues are negative anomalies (lower than expected gravity).

Many of the red areas are the result of ancient rift systems that contain denser basalts.

slide23

The image above shows the Bouguer anomaly map for the continental US.

The effect of topography and elevation removedby theBouguer correction.

The reds are positive anomalies (higher than expected gravity) and the blues are negative anomalies (lower than expected gravity).

Many of the red areas are the result of ancient rift systems that contain denser basalts.

slide24

Red along the Atlantic and Gulf coast margins results from subsurface basalt formed during the break-up of Pangea and the birth of the Atlantic ocean.

The blue areas of the Rockies and Sierra suggest that these mountains are in isostatic equilibrium, meaning that they have deep, low-density granitic “roots”.

slide25

gm

Mountain

Geoid

Ocean

go

Evidence of Isostacy

• Free Air GravityAnomalies for most of Earth’s surface are close to zero

There must be an equilibrium state ... ISOSTACY

Implies that the mass

excesses at surface are balanced by

mass deficits at

depth.

slide26

In this model, mountains have deep roots. The dashed line is an isostatic level; along this line, the weight of the overlying material is the same.

Airy Isostacy

Crust

2.7 g /cc

Mantle

>3.3 g /cc

slide27

Can you name a major feature of the seafloor that is an example of Pratt isotacy?

Pratt Isostacy



Crust



















Mantle

Another type of balance (model), known as Pratt isostacy.

The density of the overlying material varies throughout a topographic feature.

The isostatic level here is the boundary between crust and mantle.