Monday, September 24, 2012

Deep-C Geomorphology/Benthic Studies Cruise - Fall 2012

Ship’s Blog: Weatherbird II (WB1305)

24 September 2012

The Google Earth Viewer allows you to see the planned route and sampling sites for Deep-C cruises. You need to download Google Earth.

We had a 24 hour transit from Tampa to our first sampling site, AC1, arriving at 23:00 on 23 September.  The first operation was a CTD cast to get a speed of sound profile in the water column and to give Chrissy and Samantha a chance to practice picking where they will collect water samples on the next sampling site. 

Chris flies the MILET 2-3 m above bottom
by reeling cable in or out from the winch.
After the CTD, we tried out the MILET camera platform for the first fully operational deployment.  The seas were flat calm, which helped our launching and towing.  We use a winch and armored cable with embedded electrical and fiber-optic cables to power the platform and send data back and forth.  Typically, we want to have the platform between 2 and 3 m above the sea bottom and moving along at the ship’s slowest speed. 

MILET has a real time video feed to the surface which lets us see the bottom.  It also has instruments that constantly update depth, height above bottom, forward speed and lots of information about the electrical and instrument systems.  We track MILET with the USBL pinger.  The basic operation is that we tell the USBL navigation system (IPS software) what the platform depth is.   Every 2 sec, we command the pinger to send an acoustic pulse, which the hydrophone mounted on the pole over the side of the ship listens for.  We know the time between the pulse command and when the hydrophone hears the pulse arrive at the ship.  The integrated speed of sound through the water (which we measured with the CTD cast) tells us the distance between the ship and MILET (range).  The hydrophone also determines the direction that the signal is coming from (bearing).  Range and bearing are analyzed by the IPS software to give the position of MILET relative to the ship and the final step is to make precise calculation of the location of the hydrophone relative to the ship’s GPS antenna to determine MILET’s real-world location on the seafloor.

On our first deployment, the system worked very well.  We lay out survey lines that follow the contours of the bottom depth.  We want to avoid going up steep slopes or running into mounds or other obstructions.  When the Weatherbird tows MILET even at its slowest speed, pressure of water on the cable makes the MILET trail behind the ship so we have to pay out extra cable to keep MILET at the desired height off the bottom.  We end up with more cable played out than the water is deep.  Last night during one of our survey lines MILET was 2.5 m off the bottom at a depth of 463 m with 714 m of cable played out.  The total drag on the winch was 447 kg.  

Plot of MILET track during survey of AC1 as recorded by the USBL system and IPS software.
Overall the system worked very well and we got many good pictures of the soft muddy bottom with numerous burrows, fish, crabs, sea urchins, and sea cucumbers.  However, we made a mistake at the end of the third survey line and did not haul in enough cable when the ship turned around to go the other way.  MILET dropped to the bottom and dragged along for a over a minute before we figured out what was happening.  Everything seemed to be okay, but we pulled the system back on board to inspect it.  There was no damage.  MILET completed over 4 hours of video survey and recorded almost 1000 digital pictures.

Whiteboard in lab where we keep track
of sites and sample numbers.
The final operation at AC1 was to collect sediment samples with the multicorer.  We need to make three separate drops of the corer for a total of 24 cores.  Each core is taken off the corer and placed on work rack.  Then the cores are photographed and a description of each core is recorded in our log.  Record keeping is a very important part of our operations.  We need to know where each core was taken, how it was processed, and what scientist will get samples from each collection.  We have to check each other constantly to  be sure we are recording all the information  correctly.

Arvid, Mau, Samantha, and Nikki remove core tubes
full of sediment from the multicorer as the sun comes up.

Samantha logs core descriptions when the core tubes are on the rack.

Nikki processes sediment samples from the  core tubes.
Next site will be AC2, where we will start all over again.  Except this time we will collect water samples for Chrissy and Samantha.

Post Author:
Dr. Ian MacDonald

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