SO231, General Oceanography I

Fall 2000

Radar Altimeter & Gloria-Sidescan Imagery LAB

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 Aleutian trench and Iceland.

GLORIA sonar imagery of a region off the coast of Oregon.

Side scan sonar imagery from an EG&G sonar (identical to our system) in the Chesapeake Bay near Annapolis.

TOPEX/Poseidon Radar Altimeter DATA

To access this data, run the program MGT, and select the ALEBATH data set, which we used two weeks ago.  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.

Area ALE

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 near the one we looked at two weeks ago.  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?

Area ICE

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?

GLORIA IMAGERY,  BLANCGLR.IDX

Use the Satellite and GLORIA Imagery program.

From "File, New scene" pick the blancglr.idx data set, which is located about N44.5°, W130° off the West Coast.  You will be looking at every third pixel in every third row, indicated by the Zoom out factor listed in the window title.  You can continue with this overall view, or zoom in on parts of the image by selecting the "File, New Window, Subset option".  If you want to look at Full size or blow up imagery you should select small regions; otherwise the computer may run out of memory or you will run of patience as it takes forever to draw.

Find the points listed below by moving the mouse over the image.  There are linear streaks from about x=800,y=0 to x=780,y=615, from about x=0,y=927 to x=1200,y=1010, and from about x=210,y=0 to x=180, y=600.  Do these look like features on the seafloor? What could they be?

What is the grain of the seafloor on the top half of the image? What do you think could cause it, in terms of a major feature on the seafloor?  What would be the orientation of the feature?

What happens in the bottom portion of the image?  What do you think could cause it?

EG&G Side Scan Sonar Image

Use the SIDESCAN program.  Select "File, New image", and pick the wreck4.egg image.  This image was collected with a sonar identical to ours, from a Corps of Engineers boat.  The image may be better than what we observed due to time constraints during our labs.  This image was one of about a dozen passes taken over a two hour period with a smaller boat that is more maneuverable than YP686. 

The first time you display the image, it has been geometrically corrected to display horizontal distances rather than slant range.  When it finishes displaying, turn slant range geometric corrections off and redisplay.  Note the line down the center, which separates the port and starboard channels, and which corresponds to the ship track.  You can use the distance function to estimate the size of features in the image.

Are strong returns shown in white or black?  How can you tell?

How big is the wreck (length and width)?

Note the thin strong return close to the track.  How far from the fish is it?  How much time after the sound pulse is emitted does this represent (use a sound speed of 1500 m/s, a figure you should know)?  What could the return be?

Where is the bottom return (how far from the fish)?  How did you recognize it?

How far from the fish is the bowsprit of the wreck?

How far from the fish is the shadow of the bowsprit?

How well does the bowsprit show up on the image when you turn the slant range corrections back on?  Why?

Draw a sketch (approximately to scale) showing the following relations: the fish, the water surface, the bottom, the bowsprit, the shadow of the bowsprit.  Label all the distances that you can.