Marine
Geophysics Exercise
SO231, Fall
2000
This
lab (and its continuation in two weeks) is designed to teach you:
1.
The appearance of major bathymetric features like ridges, trenches, transform
faults, fracture zones, and seamounts.
2. The age depth curve for ocean crust.
3.
The pattern of marine magnetic anomalies, both in map view
4.
The pattern of gravity anomalies over ridges, trenches, and fracture zones.
5.
The difference between a transform fault (active plate boundary) and fracture
zone.
6.
Geoid (mean sea level) patterns over major bathymetric features
We will be looking at three geophysical data sets (bathymetry, gravity,
and magnetics) for three areas of the world code named ALE, ICE, and ORE.
You will select data sets ALEBATH, ICEBATH, and OREBATH for the three
areas. The three regions selected each have a significant feature associated
with them. You will be working to
determine what that feature is, and how the gravity and magnetics compare with
the feature. You will also develop
an appreciation for what the seafloor looks like on the very large scale.
Bathymetry is the measurement of sea floor depths.
The bathymetry (and topographic elevations above water) are expressed in
meters. The gravity and magnetic
values are expressed as anomalies, or departures from the expected values.
This is because the differences in the earth's gravity or magnetic field
are very small, and it is much easier to perform the subtraction from the
expected value and just compare the differences.
A positive anomaly means that the gravity (or the magnetic) field is
larger than would be expected, while a negative value means it is less.
Over the oceans the patterns are consistent and meaningful, and help
verify the predictions of plate tectonics.
The anomalies are expressed in terms of the field values at sea level.
Both gravity and magnetics are important to the Navy, and both have been
measured by satellite. The Navy has
funded the GEOSAT satellite which had a classified mission to measure the
earth's gravity field; all data from that satellite is now available to
scientists to compare with the older SEASAT mission.
The gravity is measured in tenths of a milligal; each of these units
corresponds to 10-4 cm/sec˛. Since
the normal value of gravity is about 980 cm/sec˛ (minor variation with
latitude, and variation with elevation), we are measuring gravity at about one
part in 107.
These differences are large enough to cause ICBMs to miss their targets.
The magnetic values for field intensity are measured in tenths of gammas
in these data sets. The earth's
field varies from 25,000 gammas at the equator to 70,000 gammas at the poles, so
again the anomalies are very small compared to the values of the field.
The
program you need is called MICRODEM, and it is located in the GUTHPROG folder
(=directory). Start it with the MGT
icon on the start menu under Oceanography.
This gives you the correct menu structure (if you have the MICRODEM
program, use the Options, Menu to select the MGT menus).
Using
the MGT menus of the MICRODEM program, you can do the following operations with
the data:
1.
Select the area you want. The
computer will show maps with the bathymetry, gravity, and magnetic data sets.
Note the magnetic data set is derived from ship data, and you can follow
the paths taken by survey ships.
2.
View the data in multiple formats.
The
text book will be a helpful reference for this exercise.
See especially Fig. 2-1, 2-3, 2-16, and 2-20.
You
can use the coordinates from the program and the maps in the book to locate
where these data sets are located which may help you to get oriented.
We are not looking to study specific geographic features, but are trying
to understand the basic processes and relationships of plate tectonics to these
geophysical parameters.
All
coordinates given in these instructions are approximate; if you get in the
general vicinity (±15'), it should be clear what major features are being
referred to. You can also use the
Overlay, Feature IDs option which will show the locations identified by question
number.
The
program has an integrated help file, which has examples of the graphics output
with discussions of the results. You
can view the help file during the lab, or download it from the SO231 web page.
You can also download the entire program and its data files if you want,
but there should be no requirement for you to do this.
AREA ALE:
Your instructor should guide you through the following operations of the MGT program:
a. Use the mouse to locate, on
the maps, the points referred to in questions 1-3 below, so you have a general
idea where on the map they are located. You
should also use the Overlay, Feature IDs to see where they are located.
b. Then use the View, 3 Profiles
command to see how all three fields (bathymetry, gravity, and magnetics) vary
along one profile; select one that runs from about N55°56' W162°11' to N50°3'
W160°17' (note this profile runs through two of the points you are asked about
(#2 and #3), and perpendicular to another (#1)).
c. After the profile, select the
OBLIQUE command and set the front left corner at about N55°09' W164°46' and
the right front corner at about N51°01' W161°8' (note that feature #1 runs
front to back in the three dimensional view, and #3 is to the right of center
about midway back). Crank up the
vertical exaggeration a little to see the features more clearly.
d. Use the Calculate, Age from
depth command to move about the map and observe the results.
e. You can now select your own
views to help answer the questions.
1.
What feature is located between N52°27' W164°45' and N54°17' W156°8'?
How does gravity correspond with this feature (larger or smaller than
normal)?
2.
What features are located at N55°24' W162°05', and at N56°8' W159°22'?
Note these features are above water; how are they related to the feature
from question 1?
3.
What feature is located at N52°53' W161°9'?
Looking at the three dimensional picture of the feature (oblique
diagram), what is about to happen to it?
4.
Note the pattern in the magnetic anomalies from N51°52' W164°15' to N51°25'
W158°17' to N47°40' W155°50'. What
does it say about the ridge that created this seafloor? What is the age of this seafloor (use the age-depth curve in
the book to verify what the program says)?
If the spreading rate were 60 mm/yr (half rate, the amount of crust added
to each plate), how far away should the ridge that created this seafloor be now?
In which direction should be ridge be?
AREA
ICE:
1.
Note the major feature from N62°21' W25°14' to N55°52' W34°48'.
What is this feature? Note
that it comes above sea level to the NE; does this make sense?
How does gravity correspond with this feature? How does the magnetic pattern correspond with this feature?
2.
What is the feature from N52°34' W35°1' to N52°8' W30°28'? A
three dimensional oblique view might help to visualize the feature.
AREA
ORE:
1.
Note the major feature running east-west from N40°55' W145° to
N40°15' W125°.
a. Note
the pattern of magnetic anomalies in this region; what was the orientation of
the ridges that created the seafloor?
b. North
of this feature, in which direction does the seafloor get shallower?
younger? Where is/was the
ridge that created it?
c. South
of this feature, in which direction does the seafloor get shallower?
younger? Where is/was the
ridge that created it?
d. What
is this feature, is there currently motion along this feature, and is it a plate
boundary?
e. How
does gravity correspond with this feature?
Why?
2.
Note a subtle feature from N47°43' W128°57' to N45°15' W130°11', and
then offset to continue from N42°56' W126°23' to N40°15' W127°18'.
What is this feature? Can
you see it best on the gravity, magnetics, or bathymetry?
(This question may require some thought and careful selection of the data
views.)