Marine Geophysics Exercise
Draft SO461 Lab, Fall 2008
This lab 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. Young seafloor is thermally buoyant, typically about 2˝-3 km deep at the ridge. Old seafloor (>70-100 Ma) subsides to abyssal plain depth, 5˝-6 km, and remains there until it approaches a trench. Within this range, depth provides a reasonable first estimate for seafloor age, and depth trends point out the direction of the ridge that created the seafloor.
3. The pattern of marine magnetic anomalies, both in map view (long thin strips of alternating positive and negative anomalies) and profile (periodic increases and decreases in the measured intensity of the magnetic field). The anomalies represent small variations in the intensity of the earth's magnetic field, caused by a vector addition of the seafloor's small magnetic signature with the earth's overall field.
4. The pattern of gravity anomalies over ridges, trenches, and fracture zones. Close objects contribute the most to the value of local gravity; when rock replaces water (at a ridge or seamount) the value of gravity goes up, and when water replaces rock (at a trench) the value of gravity goes down.
5. The difference between a transform fault (active plate boundary) and fracture zone.
6. Geoid (mean sea level) patterns over major bathymetric features. Sea level typically provides a subdued mirror of features on the sea floor, with geoid relief of about 1 to 10 m for each km of sea floor relief. Radar altimeters easily measure these changes in sea level.
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. (1 gamma = 1 nanotesla)
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.
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.
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?
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.
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.)
For this lab, we will look at three kinds of data, all of which have been described in the book and lectures:
Radar altimeter data, from the TOPEX/Poseidon Mission, covering the ALE and ICE regions
TOPEX/Poseidon Radar Altimeter DATA
To access this data, run the program MGT, and select the ALEBATH data set. Select the bathymetric map, and then pick the "Overlay, Altimeter tracks, All" option.
Note the diamond pattern on the map formed by the satellite passes. The satellite has ascending passes (moving southeast to northwest) and descending passes (moving northeast to southwest), which cross the equator at a consistent angle.
To determine the coverage for a single pass, use the "Overlay, Altimeter tracks, Single" option, which will let you pick the color.
When graphing the altimeter data (under View, Altimeter graphs), you will display: Geoid (short wavelength): the corrected geoid (satellite measurement plus environmental corrections), plus removal of the long wavelength geoid changes using a 10°x10° grid of the geoid from the Defense Mapping Agency. This emphasizes the tectonic changes in the geoid. To see the changes in the sea surface resulting from geostrophic currents which we will discuss after the first test requires additional processing, since they are an order of magnitude smaller than the changes due to plate tectonics.
From west to east, the ascending passes over the trench are: 132, 208, 30, 106, 182, 4, 80, and 156.
Pass 106 shows the same coverage for cycles 83 and 84, 10 days apart. How does the geoid determined on these two days compare when you graph the data (i.e. how big is the difference between the two passes, and what could account for the difference)? (You may have to rescale the graph to answer this question; the quick answer will probably be wrong.)
Pass 30 has an ascending pass over a seamount. Can you see a geoid anomaly over this seamount? Is there an explanation for this based on what you read about geoid anomalies?
Would the ascending or descending passes provide a better picture of what the geoid does over the Aleutian trench? Why?
How large is the anomaly over the Aleutian trench? Why does the anomaly change from one end to the trench to the other? How does the magnitude of the anomaly compare with the depth of the trench relative to the adjacent seafloor?
From west to east the ascending passes over the ridge are: 172, 96, 20, 198, 122, and 46.
Pass 122 has repeat data for both cycles. What is the difference between the two days? Why would scientists want the orbit to repeat like this?
Pass 198 provides a good ascending pass over the ridge. What is the difference in sea surface measured by the altimeter? (The ridge crest is at about 58°N; you can go back to the map to verify this.) What is the elevation difference of the ridge at that point compared to adjacent seafloor?