Seafloor Mapping

Ocean Ecology designed a towfish system to carry out seafloor mapping which used a single-beam sounder transducer mounted in a highly streamlined towfish that was towed at 1.5 m depth from an A-frame at the stern of our ship. This technique reduced the effects of pitch and roll on the transducer output, which can create significant artifacts in hull-mounted systems.

Ocean Ecology’s single beam sounder towfish.
Deployment of the towfish.

The signal from the transducer was processed by a JFC-130 echo sounder, which extracted both depth and bottom hardness components from the signal (to learn more about how bottom hardness and roughness components are extracted from the returned echoes, click here). The output from the echo sounder was combined with the output from a GPS unit using Franson GPSGate, and this combined signal was sent to a laptop for further processing, data logging, and realtime display.

Raw NMEA sentences generated by the echo sounder and DGPS unit were logged to a laptop. Realtime tracking and logging of the boat’s position was done using ArcGIS. Translation of the NMEA sentences logged by Franson GPSGate into latitude, longitude, depth, and hardness values was done using some of our own in-house software. This data was then imported directly into ArcMap or other GIS programs for further data analysis.

After all bathymetic data was collected for a site, the data were corrected for towfish depth (usually 1.5 m) and tidal height in ArcMap. The corrected data were exported from ArcMap, and used to generate contours in Surfer. These contour maps were imported back into a GIS program for further analyses.

Shown below is a typical example of the type of data which can be generated by our seafloor mapping system as viewed in ArcScene.

3D bathymetry.

This image shows a three-dimensional depth plot with purple colors representing deeper depths and green colors representing shallower depths. Below is an interactive 3-dimensional model of the bathymetry shown above.

Interactive 3-dimensional model of bathymetry. Left mouse button rotates, right mouse button (or wheel) zooms, middle mouse button (or left mouse button + Ctrl) pans. Press “r” to reset the view and “a” to show all.

3D bottom hardness.

The image above shows bottom hardness overlain on the three-dimensional depth plot. Reds and browns represent rock bottom, whereas blues and purples represent softer sediments. Note the presence of softer sediments around the edges of the depression (which was located at the center of the bay). These softer sediments represent areas of sand which accumulated in the shallower regions of the bay to form beaches. Below is an interactive 3-dimensional model of the bottom hardness shown above.

Interactive 3-dimensional model of hardness. Left mouse button rotates, right mouse button (or wheel) zooms, middle mouse button (or left mouse button + Ctrl) pans. Press “r” to reset the view and “a” to show all.

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