Heidi Wigman: Underwater Acoustics, June 4, 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 (26°33.512’N 083°43.064’W)
Date: June 4, 2015

Weather: 82° @ surface, NE winds @ 5-10 knots, seas 0-2 ft, chance of showers and Tstorms, average depth 75m

Science and Technology Log:

The science behind underwater acoustics play a huge role in the operations of the Pisces.  Each of the five survey types (CTD, camera rig, sidescan, bandit reels and AUV) need accurate data about the depth and contours of the ocean floor.  Most people are familiar with the idea of how radar sends out a “ping” and waits for a return in order to determine a distance of an object.  This is not a new, or even a human invented design — bats, dolphins, and some whale classes use “echo location” to get information on food sources and predators.  As a pulse is emitted from the transmission source, it travels through the water at a certain speed, and as it encounters objects, returns as an echo.

ping transmit and return
“ping” transmit and return provided by C. Thompson

 As data is received, it can be read as a function of voltage output to time in seconds, but this type of information generally is not useful for operational purposes.  This two-way travel data needs to be converted to provide a graphical representation of the contour of the ocean floor, and the location of objects in the water. An algorithm turns all of this into usable data, that gives the viewer a depiction of what is under the vessel, and at what depth.

sonar imagery provided by Charles Thompson
sonar imagery provided by C. Thompson
echosounder depth measurement, provided by C. Thompson
echosounder depth measurement, provided by C. Thompson

In order to get depth (Z), you need to know about how fast sound travels (c) – and this can vary with environmental factors such as temperature, salinity, depth, turbidity, etc. The third variable is the time (t) in seconds that it takes from ping to return. The formula that is used to calculate the depth is Z = c*t/2.

speed of sound graphDuring our cruise, the sound speed value we are using (1540 m/sec) is the mean value of the measured sound speed vs. depth profile, with slight margin of error on the minimum values.  Therefore, any miscalculations based on the constant will provide a reading more shallow, rather than more depth.

The EK60 echosounder emits a frequency of 18kHz, with most of the power in an 11° conic sector directed downward(see diagram).  In order to find the area covered by the pulse, we first need to find the diameter (d) and the vertical depth (Z) or the max beam range (R).

sonar effective area; provided by C. Thompson
sonar effective area; provided by C. Thompson

Math question of the day: What is the area covered by one sonar ping from the Pisces? If you know that your vertical depth is 75m, and the bisect on the beamwidth (11°) angle, use some trigonometry to help find your radius. [Tan 5.5 = r/75].  Once you have the value of r, use the formula for area [A=3.14(r*r)]

Previous Answers:

Trigonometry of Navigation post: 18 m/s @ 34°SE

Bandit Reels post: about 14.6 nautical miles

The STEM of Mapping post: layback = 218m, layback w/ catenary = 207m

Coming soon . . . A trip underwater – A closer look at NOAA dive tables

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