Weather: Calmish

Location: Surveying near Loch Broom

Food: Mussels, Pork Loin

Hello!!

Ok, last night we conducted a pretty comprehensive survey of the seabed. The survey itself has now gone on for well over 36 hours. During this time the RRS James Cook has been steaming up and down “survey lines” which are around 5km in length. The survey has revealed some interesting information about the area and some features that may be worth investigating later in this cruise or on future cruises. As nothing really interesting happens during swath I will explain a little about swath and surveying!

A little bit about Swath!!!

RRS James Cook is equipped with a EM120 and EM710 swath system – this will probably mean nothing to you. The EM710 is used for surveying in shallow water and the EM120 is used for deeper water, usually water deeper than 200m. We won’t go into the reasons why! Just understand we have a shallow and deep water system! This cruise we have been using the EM710 because the water depth has been under 200m nearly the entire duration of the cruise – in fact it’s usually be well under 100m!!!

Each of these two systems has a transducer that both transmits and receives sound waves. The EM120 transducer is located on the ships hull and is very big. The EM710 is located on the ship’s drop keel –more about drop keels later!!

Transducers send a cone of sound down to the seafloor, which reflects back to the ship. Just like a flashlight beam, the cone of sound will focus on a relatively small area in places where the ocean is shallow, or spread out over the size of a football field when water depths reach 3,000 meters. The returned echo is received by the transducer, amplified electronically, and recorded on graphic recorders – the screens in the main lab.

Image showing sound being emitted by the transducer and returning to the ship after hitting sebaed

(http://www.divediscover.whoi.edu/ArticleImages/Blog - JC015/sonar_swath_top.gif)

EM120 Transducer Array

( www.geo.su.se/.../projekt/oden_mb)

The time taken for the sound to travel through the ocean and back is then used to calculate water depths. The faster the sound waves return, the smaller the water depths and the higher the elevation of the seafloor. Echo sounders repeatedly “ping” the seafloor as a ship moves along the surface, producing a continuous line showing ocean depths directly beneath the ship.

There are lots of different things that need to be considered when conducting a survey of the seabed. In bad weather (high winds, storms), however, turbulence along the hull surface produces a layer of bubbles which interferes with sound propagation. These bubbles confuse the transducer recording system and results in bad data. Usually during bad weather the ship’s motion increases – this also affects data as the transducer moves around with the motion of the ship.

In addition to weather the ship’s speed is important – powering through the water at 30 knots also creates a lot of turbulence and this also affects data quality – it also means that by the time the beams return from the seabed (especially in deep water) your miles away!!! Tides can affect data collection as the distance from the transducer to the seabed varies between low and high tide!

As discussed previously in the blog the speed of sound in water changes based on a number of factors. Whilst salinity is fairly constant throughout the worlds ocean’s. However, in estuaries where fresh water is being dumped from rivers into salt water the concentration of salt will be slightly less (and there may other suspended particles in the water -e.g. mud)– this will effect the speed that sound travels through the water.

The heating and cooling of water also affects the speed of sound. Because water is not perfectly transparent, almost all sunlight is absorbed in the surface layer, which heats up. Wind and waves circulate the water in the surface layer, distributing heat within it somewhat, and the temperature may be quite uniform for the first few hundred feet. Below this mixed layer, however, the temperature drops very rapidly – the layer where this rapid transition occurs is called a thermocline! This rapid change in water temperature affects the sound waves travelling through the water which causes the beams to bend. It is therefore necessary to measure this change in temperature and ultimately speed of sound using a Sound Velocity probe. The data from this probe is then loaded into the EM710 and EM120 which then do the necessary calculations to ensure that the sound waves emitted by the transducer take into account the variations in sound and result in the best quality data!

In addition to an SVP it is good standard practice to calibrate the system before any swathing is started. This ensures that the data collected is as correct as possible and discussed previously requires us it map the same survey line more than once and the compare differences in the data.

The survey area is usually compiled by scientific staff and submitted prior to the cruise although ad-hoc surveying does happen!!. The crew then check these areas for hazards, shipping lanes and other things that will affect operations – it has been known for scientists to submit survey lines that go through islands! It is Also is necessary to obtain permission to survey an area – we can’t just send our research ship into an area and survey it as we please!

Surveying itself involves the ship moving up and down survey lines to get a detailed map of the survey area. The survey lines are usually spaced at distances so that there is a 20% overlap of the beams from one line to another. Surveying large in shallow water (remember the beams can’t spread out as much) takes a long long time!! Someone usually has to sit in front of the system whilst the ship is surveying to ensure that the system is healthy, we are receiving good quality data and to communicate with the bridge in the event of problems.

So there you go – a crash course in surveying!!!!

Mapping the Seabed! What we see!!

Window One: Beam intensity. This tells us information about the sound beams used to map the seabed

Window Two: The Waterfall Display - This is a 3d representation of the seabed

Window Three: The "Cross Track" display shows the measured depths from each of the sound beams.

Window Four: This shows the swath data. The colors change with depth. The darker the colors - the deep it is!

Window Five: A computer generated image of the seabed!

Julia infront to the swath system.

Another shot of Julia using the swath. the top screen shows the EM710 in operation during a survey.

CLICK HERE FOR THE RECENTLY ADDED PANORAMIC GALLERY FOR JC015

Some Pictures from the Day

Above: Dave and Kev working on the Vibro-Core

Above: Dave is all smiles as the Vibro-Core gets load tested!