Transcript Lecture 9 and 10

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
- 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.
Gravity Anomalies
A gravity anomaly is the
difference between the measured
value and an expected value.
Are calculated:
g = gmeasured +/- correction - ggeoid
= gravity anomaly
Whenever a
gmeasured may be corrected for:
•Bouguer
•Free Air
•Topography
and others
measured value
departs from an
expected value,
an anomaly
exists.
Corrections are applied to the
measured value, depending on
one’s interest.
Free Air (=Elevation) Corrections
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.
Mountain
gm
Geoid
Ocean
go
go = gmeasured (1 + 0.00031 h)
g in gal, height in meters
Free Air (=Elevation) Corrections
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.
Mountain
gm
Geoid
Ocean
go
go = gmeasured (1 + 0.00031 h)
g in gal, height in meters
Bouguer (=Mass) Corrections
Mountain
gh
Geoid
Ocean
go
The Bouguer correction accounts for the additional gravitational attraction
between the material that lies between the gravimeter and the geoid.
Bouguer (=Mass) Corrections
Mountain
gh
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...
Bouguer (=Mass) Corrections
Mountain
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
Geoid
Ocean
go
go = gmeasured (1 - 0.00004 h•density)
g in gal, height in meters, density in g cm-3
Bouguer (=Mass) Corrections
Mountain
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
Geoid
Ocean
go
go = gmeasured (1 - 0.00004 h•density)
g in gal, height in meters, density in g cm-3
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.
Mountain
gh
This correction is often made in nearshore gravity
surveys that extend onto land.
z
Geoid
Ocean
go
gcorr = gmeas [1 + 2Gz(dseafloor-dseawater)]
g in mgal, height in meters, density in g cm3
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
Mt. Blanc, France
48 mgal
123 mgal
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
Mt. Blanc, France
48 mgal
123 mgal
Free-Air Anomaly Over a Subduction Zone
Central Aleutian Trench
+200
0
-200
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
0
300
Free-Air Anomaly Over a Subduction Zone
Central Aleutian Trench
+200
0
-200
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
0
300
Free-Air
Gravity
Anomalies
Japan Plate
Pacific Plate
Triple Junction
Japan
Trench
Free Air Anomaly For Atlantic
Continental Margin
Upper
Continental Crust
Oceanic Crust
Rift Stage
Crust
Lower
Continental Crust
Coastline
Upper Mantle
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.
Free Air Anomaly For Atlantic
Continental Margin
Upper
Continental Crust
Oceanic Crust
Rift Stage
Crust
Lower
Continental Crust
Upper Mantle
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.
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)
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
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.
Free-air gravity tracks above provide very high resolution information about the
dimensions of a small offshore sedimentary basin.
Gravity (mgal)
Many layers of low density sediments in the basin result in lower gravity
anomalies over the basin’s center.
Distance (km)
The image above shows the Bouguer anomaly map for the continental US.
The effect of topography and elevation removed by the Bouguer 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.
The image above shows the Bouguer anomaly map for the continental US.
The effect of topography and elevation removed by the Bouguer 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.
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”.
Evidence of Isostacy
• Free Air Gravity Anomalies for most of
Earth’s surface are close to zero
There must be an equilibrium state ... ISOSTACY
gm
Mountain
Geoid
Ocean
go
Implies that the mass
excesses at surface are
balanced by
mass deficits at
depth.
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
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.