Heidi Wigman: The STEM of Mapping Operations, June 2, 2015

NOAA Teacher at Sea
Heidi Wigman
Aboard NOAA Ship Pisces
May 27 – June 10, 2015

Mission: Reef Fish Survey
Geographical area of Cruise: Gulf of Mexico (28°58.91’N 085°29.87’W)
Date: June 2, 3015

Weather: 82° @ surface, SE winds @ 5-18 knots, seas 1ft, chance of showers, average depth 72m

Science and Technology Log:

So far, I’ve talked about the daytime ops aboard the Pisces, and the different ways in which sample sites are surveyed, but once the sun goes down, something else happens.  After the daytime series of drops of the CTD, camera rig, and bandit reels; a different deployment commences.  During the evening, mapping operations are underway utilizing the technology of the sidescan sonar towfish.  This little guy does exactly what it says – it gets towed and uses its sonar to scan laterally and map the ocean floor.  The acoustic imaging can be used for mapping of geologic features, hazard surveys (for pipeline and cables), archaeological sites, sunken ships and downed aircraft. It can be deployed from the surface (via Pisces) or incorporated on a remote AUV (Autonomous Underwater Vehicle).  By mapping a predetermined feature, or area, in a linear transect array, the sidescan relays data from 20% above the depth of the ocean floor.  So, if we are at 87m, the sidescan would be at an altitude of about 70m (210ft).

Math at Sea: One of the tasks of the scientists is to determine the amount of “layback” or distance between the tow point and the lateral distance of the towfish from the vessel – making sure that this is in the range of the shipboard GPS transmission.  Typically line is laid out at 3 times the depth at which the towfish will be cruising.  By looking the diagram below, you can see that all three points create the vertices of a right triangle . . . get ready for some real-life applications of the Pythagorean Theorem. Math Question of the Day: If the Pisces is cruising at 5 knots @ 96m above the ocean floor, what is the measure of layback? An extension to this problem has to do with “catenary” (red parabolic line) or the amount of bend in the tow cable due to the speed of the vessel, cable length and the drag of the towfish/cable. Usually this is determined by subtracting 5% of the layback value.  Based on the problem above, what is the total amount of layback in ft, to account for the catenary in the tow cable? Previous Answers: Trigonometry of Navigation post: 18 m/s @ 34°SE Bandit Reels post: about 14.6 nautical miles Coming soon . . . Now Hear This! Underwater Acoustics