MAR 110 Natural Hazards and the Oceans Tutorial #2

Ocean Floor Bathymetry

Instructors: Prof. Wendell Brown [email protected] 508-910-6395 SMAST-East Rm 229

TA: M……………… ……[email protected] 508-910-6384 SMAST-East, Rm 222 Bathymetry

• Bathymetry: bathys => depth & metry => measurement (Greek Roots)

• Bathymetric maps: ocean depth as a function of geography topographic maps: altitude (rel. sea-level) as a function of geography

• Map Scale: ratio between map distance & corresponding Earth distance expressed as 1/80,000 or 1:80,000 (natural scale)

• Isobaths: lines connecting points of the same depth on a map or chart

• Slope:ratio ofdepth change of a sea floor feature & horizontal distance of feature

Slope = change in depth/horizontal distance of slope Ocean Floor Technology

echo sounder - a device used to determine the depth of the ocean floor

Echo Sounding Echo Sounding Record

Velocity = distance/time Distance=2*Depth=velocity*time (down & up) (known) (measured) Depth=(velocity*time)/2 Ocean Bathymetric Chart Bathymetry

600m A plane intersecting with seafloor mountains at a depth specific depth => e.g.,600m depth

& at successive 50m depth intervals

… yields Depth contour Contoured Bathymetry Bathymetry

MULTIBEAM ACOUSTICS

many acoustic beams

Map Complex Bathymetry Bathymetry Papua New Guinea

Contour lines follow paths of constant elevation and are closely spaced on steep slopes and widely spaced on relatively flat terrain Contouring Bathymetry

Contour the Sandy Harbor soundings at 1 fathom intervals

Questions What is the depth at point A in

fathoms? meters?

Where is the deepest part of the bay? Remember: 1 fm = ~2 m Bathymetric Slopes

What is the relief (or depth difference) between points A and B?

What is the distance in km between A and B? (draw a line between them, measure it, and then hold ruler up to the legend)

Determine the slope of the bay from points A to B as indicated below: fathom/mi ft/mi m/km

Remember: slope = relief/horizontal distance Use your conversion factors! Contouring Bathymetry: RULES

• Contour lines never split or cross but may merge, indicating a very steep slope

• Contours should never spiral • Contours should never stop in the middle of a map • Contours should always point upstream in valleys • Contours should always point downridge along ridges • Adjacent contours should always be sequential We have just introduced you to Exercise A (Sandy Harbor Chart) Use same techniques for Exercise B (Pacific Ocean Chart which is more challenging).

Homework #2 ………..due next week! Helpful Links! http://oceanservice.noaa.gov/facts/bathymetry.html http://mathinscience.info/public/visualizing_seafloor_adv/ how_to_contour/how_to_bathymetric_chart.htm http://www.dosits.org/science/soundmovement/speedofsound/ http://serc.carleton.edu/eet/seafloor/orig_case_study.html http://www.divediscover.whoi.edu/tools/sonar-singlebeam.html http://www.punaridge.org/doc/factoids/Sound/Default.htm http://adventure.howstuffworks.com/outdoor-activities/hiking/how-to-read-a- topographic-map3.htm