GEOL- 270: Issues in Oceanography Developed by Jessica Kleiss, Lewis & Clark College

Transcription

1 GEOL- 270: Issues in Oceanography Developed by Jessica Kleiss, Lewis & Clark College Lab 02: Ocean floor bathymetry. Material for students to bring: A calculator A ruler Pens and pencils Objectives: Interpret 2D bathymetric maps as a 3D physical model. Construct a 3- dimensional model of a feature of the ocean floor Simulate the experience of gathering depth data from an ocean vessel Construct a contour map from individual depth soundings The ocean floor was once thought to be a featureless, flat terrain, due to the constant rain of marine snow. Ships originally determined the depth of water by lowering a weighted line, capturing a bit of sediment as proof that they hit bottom, and marking the length of line needed to reach bottom. In the 1950 s, scientists and naval officers started using sonar to determine the depth of water. They would send a ping of sound from a transmitter, and measure the time for the sound pulse to echo off the ocean bottom, and return to the receiver. This dramatically decreased the time needed to obtain a depth sounding! Later technological developments resulted in the side- scan sonar, which allowed a boat to determine a swath of ocean bottom contours to the side of the vessel. This created a more descriptive picture of the ocean floor than single point measurements could offer. This lab will consist of a bit of puzzle- making, and puzzle- reconstruction, where the puzzle is the shape of the ocean floor! Part I: Making the 3D model Materials: One rigid small stowaway box with holes punctured through the lid Many sheets of corrugated cardboard, of uniform thickness Heavy- duty scissors, razor blade, or exact- o knife. Glue or double- sided tape Tracing paper Masking tape and scotch tape sharpie marker Each group will be provided with a bathymetric map of a seafloor feature. Your job is to convert this map into a properly scaled, 3- dimensional model of this sea floor feature inside of your shoebox.

2 The NOAA maps provide a great deal of detail! I suggest that you use the tracing paper to make your job easier. Choose some sub- set of contours, and trace them onto your tracing paper. Then cut these shapes out of the cardboard, and glue them into your model. I recommend shooting for about 10 layers of cardboard in your model. The NOAA maps also cover a great deal of terrain. I suggest that you focus on a small region that strikes you as particularly interesting. Add horizontal and vertical scales to your model. Be as accurate and precise as possible. You ll need to determine a vertical scale for your model: each centimeter of height corresponds to how many meters of depth in the real world? You ll want to make a scale, and tape it to the outside of your box. Note that the scale of your seafloor feature will not be the same as the scale of the distance from the box lid (eg. ocean surface), since you can t adjust the height of the lid. Try taking a couple sample soundings with your box lid closed, and indicate the depth of the ocean to the top of your scale for your bathymetric feature. Also be sure to include the horizontal scale of your map, which you can obtain directly from your NOAA chart. Indicate the Latitude & Longitude of your model and a simple compass rose. Write this clearly on the top of your shoebox. Just identify a single Lat / Lon at approximately the center of your model. TIPS: Sometimes the bathymetric charts have contours that are not regularly spaced. Make sure that you select around 10 contours to trace that are equally spaced in depth. You ll certainly want to divide labor for this lab, but have everyone do each job at least once. That way everyone gets to experience the different parts of the lab. 1 degree of Latitude is equal to roughly 111km. There is no such relationship for Longitude. (Why?) Checklist for 3D model construction: Layers of cardboard correspond to contour layers in the bathymetric map Lat / Lon written on box Horizontal scale attached (and double- checked for accuracy) Vertical scale attached (and double- checked for accuracy) Distance to ocean surface attached Lid closed! Names included on inside of box, as well as the seafloor feature name.

3 Part II: probing the sea floor Materials: A couple skewers or wooden dowels Graph paper (the same kind that is on the shoebox lid). A global map, with labeled Latitude & Longitude (in the classroom) Now exchange boxes with another group. Be sure to keep the lid closed! Imagine you are steaming across the ocean in a research vessel and you want to know the topography (shape) of the seafloor below your boat. Your boat does not have the latest technology for continuous mapping of the seafloor, but you can make echo- soundings to measure the water depth at some discrete points. Address the following prompts in your write up (see the ANSWER SHEET below) 1) Determine your research vessel navigation plan. Ship time is expensive: a fully equipped research vessel such as Woods Hole s Oceanus costs $40,000 per day. Normal cruising speed is 7-12 knots. How do you propose to probe the ocean floor to determine the underlying bathymetry? How long will it take, and how much will it cost? Describe your ship track here. (include a sketch if desired) 2) Now follow your ship s research track, and take depth soundings at each drill hole opportunity. [Tip: Make sure your dowel is perpendicular! You may want to use a 90 o rigid corner to help, such as your ruler.] Record your measurements on graph paper as you go. Use a pencil! It s easy to make mistakes. After you have sampled the ocean, remove the grid paper from the box. You may wish to duplicate your sounding measurements, since creating a good contour map can sometimes take a couple tries. Draw contours on your graph paper, aspiring to re- create the terrain in your box. Draw contour lines directly on this grid paper. To be handed in: The sheet of graph paper indicating depth soundings at each sampled drill hole. Contours should be clearly labeled. Be sure to include units! (you don t need to write units for every number, but it needs to be clearly indicated.) Completed Answer Sheet

4 ANSWER SHEET Names: A. Hypothesis: Given the tools available to you, state below what you think you will need to do to create a two dimensional contour map of the object in your box without opening the box and looking inside. B. Ship time is expensive! Estimate how much your survey would cost, and how far your research vessel traveled. Use the following assumptions: Your research vessel is cruising along at 8 knots (the echo- soundings take no time) You work around the clock, 24- hour days. The ship costs $40,000 / day to run. Steaming out to the research location and back to port is magically accomplished, for free. Show your work CLEARLY. Be sure to check your units and conversions! (hint: do the work on a separate sheet of paper, and only include a neat write- up here) B.1) What is the distance between sampling locations, in nautical miles? B. 2) What is the total distance of your research track, in nautical miles? B. 3) How much would this expedition have cost?

5 Once you have created your contour map do the following: C. Without opening the box, guess at what type of seafloor feature is in your box? D. Using the coordinates written on your box, find your seafloor feature on the global map in the room, and/or using the internet. What type of seafloor feature was in your box? What is the name of your seafloor feature? E. Look in your box, and consult the NOAA bathymetric map from which your model was made. Are you surprised by what you see? F. Name at least three possible sources of error in this experiment. G. Name at least three sources of error in a real- world bathymetric survey. Indicate whether these errors are similar to the errors in our class exercise. E. Now that you ve seen your real sea floor feature, How many more data points would you have wanted to gather in order to see better? How would you have designed your ship s route?

6 E. Now go around to other groups and look at their contour maps, at the 3D models, and the original NOAA maps. Find out what coordinates are written on other boxes and find those sea floor features as well. Write down what each seafloor feature is H. Do any of the undersea features look like topography we see on land? Give an example? I. What geologic process do you think created each of the features? Reflections: Comment about your experience in this lab. This is free- writing, and you may address some subset of the questions below. Write on the back of this sheet of paper. Did you learn about how the seafloor is mapped during this lab? Do you have a better appreciation for oceanographic cruises, and echo- sounding data? Were you surprised at the scales and dimensions of the maps and the 3D model? What aspect of this lab was particularly challenging to you? How could you apply the concepts of this lab to other areas of your life and your studies? Did your group work effectively together?