As the above image shows our primary work area on the vessel has looked rather empty for quite a while now, as every seabed instrument that the group has is now sitting on the seabed – 58 in total.
So that’s given us a chance to tidy up after ourselves, and even jet wash the floor which is now sparkly clean despite Nigel the Egret’s best efforts – see the Teacher At Sea’s blog – https://teacheratseablog.wordpress.com – for the latest update on Nigel.
With the seismic reflection surveying now half done, we are planning the next activities which will be Autosub dives interleaved with recovering our instruments.
This interleaving process is unusual for us, as normally we just recover all the instruments in one batch lasting several days on the trot. But not this time. For this recovery we have to plan quite carefully so that we start where we deployed Autosub and recover instruments for the next 20 hours of its dive time, and make sure we are back at the Autosub recovery point as it arrives at the surface after a total dive length of 24 hrs. Then we have to plan the continuation of this recovery for the next 20 hrs it will take to charge Autosub’s batteries and get it ready for redeployment, making sure we are back at the next Autosub deployment site at the right time so as not to waste valuable ship and dive time.
So we’ve got the Boss on the case planning this, and she does that using two primary factors:
a) the time between Autosub recoveries vs the time it takes the vessel to transit between OBS and Autosub sites
and perhaps more importantly
b) choosing a pattern through the sites that also matches the seabed topography.
Why is that important you ask? Well its all about sound shadows.
Our instrument’ acoustic releases can hear us ping at them from the vessel for many nautical miles. The group’s record currently stands at 7 nm which is almost 13 km, and at that distance not only did we manage to communicate with a release but we also managed to do it clearly enough for it to understand the release signal and start its journey to the surface.
In the 13N work area our instruments will take about 1 hr 15 mins to get to the surface once released, so its a careful balancing act between talking to them along way away and releasing them to save precious time, and being able to get to their deployment site in time for them to arrive at the surface. So normally we have a 5 nm “rule” unless the water is very deep indeed.
The seabed topography is also quite severe in the work area so to get communication at distance its best to run up and down the topography, like going up and down valleys on land, rather than trying to talk to our instruments over the ridges in between.
So the map below shows our intended tracks through the grid of instruments over three successive days during which Autosub will either be roaming about or having its batteries recharged. We may not get all those marked on each day (thick red, green and blue trails) and any we don’t make on any day, we will “sweep up” at the end. The pastel green-to-yellow east-west lines in the plot below are the Autosub surveys that we have to interleave with and arrive at the end points of at the right time.
It is also looking like we will have some spare time to fill at the end, and so we are thinking about doing one further considerably smaller OBS survey of some other interesting features in the work area. So as the banner image shows, we have started to lay out our last 12 pieces of ballast ready to have 12 of the returning instruments placed straight on top ready to be turned around for redeployment. The Boss is also planning this survey in parallel.
We’ll keep you posted!