Meredith Salmon: Sonars, Sub-bottoms, and Summertime! July 18, 2018

NOAA Teacher at Sea

Meredith Salmon

Aboard NOAA Ship Okeanos Explorer

July 12 – 31, 2018


Mission: Mapping Deep-Water Areas Southeast of Bermuda in Support of the Galway Statement on Atlantic Ocean Cooperation


Weather Data from the Okeanos Explorer Bridge

Latitude: 29.03°N

Longitude: 62.11°W

Air Temperature: 27.5°C

Wind Speed:  6.38 knots

Conditions: Sunny

Depth: 5167.70 meters

Science and Technology Log

EK 60
SIMRAD EK 60 echo sounder readings – 38kHz frequency is not pictured

In conjunction with the EM302 multibeam sonar, the Okeanos Explorer uses five different frequencies of SIMRAD single beam echo sounders to identify biomass in the water column: an 18 kHz, 38kHz, 70 kHz, 120 kHz, and 200kHz. (38 kHz is not pictured because it is not used in conjunction with the EM302 since the frequencies are too similar and they can cross talk). These sonar systems are common on fishing boats for estimating fish abundance and they’re used for other marine research, as well. In deeper waters, lower frequency sonar is used. Since we are surveying in approximately 5,000 meters of water, the 18 kHz will be used.

Knudsen sub-bottom profiler
3.5 kHz Knudsen sub-bottom profiler data

The third piece of important equipment used during this mission is a 3.5 kHz Knudsen sub-bottom profiler. This technology is used to assist in many surveys since these systems identify and characterize layers of sediment or rock under the seafloor. In sub-bottom profiling a sound source directs a pulse towards the seafloor and parts of this pulse reflect off the seafloor while others penetrate the seafloor. The portions of the pulse that penetrate the seafloor are both reflected and refracted as they pass into different layers of sediment. These signals return towards the surface and can be used to determine important features of the seafloor. For instance, the time it takes for the reflected sound pulses to return to the vessel can be used to determine the thickness and positioning (ex. Sloped or level) of the seafloor. The refracted pulses can provide information about the sub-bottom layers. The variability in density can be used to explain differences in composition (ex. greater density is representative of harder materials). Frequency differences can help scientists obtain optimal results that can be used when collecting data during a survey. Lower frequency pulses can penetrate the seafloor but produce a lower-resolution picture while higher-frequency pulses produce the opposite.

The EM 302, EK60, and Knudsen sub-bottom profiler are all used simultaneously during this seafloor mapping operation.

Personal Log 

Throughout the cruise, one of the NOAA Corps Officers is in charge of planning fun morale events for everyone aboard to participate in. Today, we had a cookout complete with delicious food, music, and corn-hole on the fantail. Everyone had a great time! Additional morale events are planned throughout the rest of the mission so I will post about those later on!


Corn Hole!
Competitive Cornhole on the Fantail


NOAA Squad
Some of the Mapping Team aboard the Okeanos Explorer!

Did You Know?

The earliest technique of bathymetry (depth measurement in water) involved lowering a weighted-down rope or cable over the side of a ship, then measuring the length of the wet end when it reached the bottom. Inaccuracies were common occurrences using this technique because of the bending of the rope caused by deflection from subsurface currents and ship movements.

This technique was replaced in the 1920s by echo sounding, in which a sound pulse traveled from the ship to the ocean floor, where it was reflected and returned.

The multibeam echosounder was invented in the 1960’s.



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