2/5/2014 – Multibeam, Multichannel, and Maybe Multidredge

Katy writes:

Mertz Trough sea ice

Mertz Trough sea ice (click to enlarge)

When we started our shift, the multichannel seismic streamer had just been deployed to start its 16-hour survey. This is the maximum continuous operational time for this instrument because our marine mammal observers, Tasha Snow (USF) and Andrea Walters (University of Tasmania), have to be on watch while it’s in the water, and each of them is only permitted to do an 8-hour shift. We anxiously awaited the data to be processed by Bruce Frederick and Sean Gulik (University of Texas at Austin), our resident seismic experts on the night shift.

These data are very important not only for determining core and dredge sites on our cruise but to add to the support of an IODP (Integrated Ocean Drilling Program) proposal for this area. The proposed drill cores will be approximately 80 meters in length will help scientists to observe and analyze how climates (particularly ice sheets) respond to increased CO2. The specific goals of the proposal are to address the timing of “the Eocene-Oligocene ice advance (~34 Ma), the mid-Miocene climate transition (~14 Ma) and the earliest Pliocene warmth and climate fluctuations (~5 Ma).” Everyone was very excited about the data that we collected and now it was time to pick dredge locations!

PIs reviewing the processed seismic data

PIs reviewing the processed seismic data

Although our watch was dominated by this seismic data and activity, I had a lesson with Caroline Lavoie (Universidade de Aveiro), one of our multibeam specialists. We discussed how to process the data that we were receiving hourly from the multibeam system and about the CARIS system in general, which we use to process the data. We use the multibeam on the ship to see a high-resolution profile of the seafloor. To do this, the multibeam system, EM 120, sends a continuous wave pulse down to the bottom with defined amplitude, frequency (12 kHz), and a time limit. This pulse is known as a “ping” sent by 191 beams creating a swath of an area. Because this ping is sent as a swath, the information that is received by the system after these beams hit the seafloor contains more, but less-resolved data for deeper areas and less, but more-resolved data for shallower areas due to the width that the beams’ pulse could reach before hitting the seafloor (this is why the lines are varying sizes when plotted on a map).

The multibeam system and the CARIS software both correct for certain parameters of the ship and the water to produce more accurate data. For example, the software corrects for the sound velocity profile of the water that is affected by changing temperature, salinity, density, pressure, etc of the water. The EM 120 system also corrects for the heave, pitch, roll, and heading of the ship when collecting data. To process data, tide information is also needed, but in our case tides are not a factor based on our location and due to the fact that the resolution we are seeing is between 25-100 meters.

Multibeam data in the CARIS software

Multibeam data in the CARIS software

After learning the basics of how the data is collected, Caroline showed me how to process the raw data in CARIS, allowing for a more accurate reading of the imagery.  The multibeam data are useful for us to see in real time on the ship for many reasons, including an accurate depth reading when our Knudsen 3.5 kHz CHIRP system is having a hard time reading the seafloor, and to easily see prominent features such as megascale glacial lineations and other glacial erosional features to briefly analyze what these features mean in terms of glacial history. However, multibeam data are very useful afterwards in programs such as ArcGIS and for future analysis of the seafloor in other media. The only other time multibeam data have been collected in this area was during the NBP01-01 cruise, so many of the paths that we are crossing are collecting multibeam data and depth profiles for the first time.

Selecting dredge locations using the multibeam data in ArcGIS

Selecting dredge locations using the multibeam data in ArcGIS

After the rest of the seismic data were processed, the PIs and Chief Scientist started to pick ideal dredge locations based on the reflectors they saw within these data. Six sites were chosen, and Bruce and I were assigned to determine the latitude, longitude, and depth of these sites, which we fixed using the processed multichannel data. When our shift ended, there were six potential dredge sites in hand, with hopes that the wind would die down enough to make dredging possible.

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