Home >> Oceanography >> Observation Systems
This is the nitty gritty of ocean science. In order to fully understand something we need to measure and collect data. Given the vastness of the ocean and the inhospitable environment at sea this is a major challenge. We will be learning about, and hopefully working with, some cutting edge technology. Even apparently simple measurements of depth, temperature and salinity are difficult. Students will do some practical work to try to measure some of these variables using homemade devices based on what we have at school.
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Hydrostatic Pressure
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Buoyancy
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Robots
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4.1 - Introduction
Objectives:
- To understand and know the challenges facing marine scientists.
- To appreciate just how difficult and expensive it is to collect oceanographic data.
Data collection from the ocean is a challenge for many reasons. The primary one is that the samples we need are at sea and usually under water! Until recently this has meant that oceanographers have needed to go to sea on expensive research ships. Ships often cost around $1000 per hour to run, which includes down time and relocation. This all makes data very expensive and time consuming to get. Problems for ocean scientists include:
All of these factors mean that the oceans remain the most unexplored part of the planet. So why and how can we measure the important aspects of the ocean? Science depends on high quality, accurate and reliable data to enable understanding. If the data is good, then we can derive conclusions based on it. For example, the world's longest continuous oceanic time series data has come from a location to the southeast of Bermuda and is called the Bermuda Atlantic Time Series (BATS). In the old days, it was believed that the ocean was unchanging and therefore we could just create a snapshot and understand everything. Now, thanks to measurements repeated over a long period of time we know that the oceans (and atmosphere) are changing. The BATS time series shows, among other things, that the percentage of dissolved carbon dioxide is steadily increasing in conjunction with the increase in atmospheric carbon dioxide. This shows us something that people and politicians don't want to acknowledge - that the climate is slowly and remorselessly changing due our fossil fuel emissions and attacks on the world's forests.
- We cannot breathe under water.
- Water absorbs light, which makes the ocean dark and it is impossible to see deep down.
- Water absorbs radio and microwaves, which affects the usual methods of remote sensing and communication.
- The oceans are very deep and the pressure at depth is extremely high, which can crush hollow instruments or find a leak through any potential seal.
- Biofouling. Any instrument left in shallow water will quickly become fouled with algae and other marine organisms such as barnacles.
- Seawater is corrosive and will degrade metals and electrical connections.
- Seawater is conductive so making electrical connections challenging to insulate against.
- The ocean is dynamic, tides and currents constantly move the water from place to place.
- Waves mean that there cannot be a steady observation platform and any instrument on the surface could be destroyed by the motion. They can also be life threatening and make life aboard a ship dangerous and unpleasant.
All of these factors mean that the oceans remain the most unexplored part of the planet. So why and how can we measure the important aspects of the ocean? Science depends on high quality, accurate and reliable data to enable understanding. If the data is good, then we can derive conclusions based on it. For example, the world's longest continuous oceanic time series data has come from a location to the southeast of Bermuda and is called the Bermuda Atlantic Time Series (BATS). In the old days, it was believed that the ocean was unchanging and therefore we could just create a snapshot and understand everything. Now, thanks to measurements repeated over a long period of time we know that the oceans (and atmosphere) are changing. The BATS time series shows, among other things, that the percentage of dissolved carbon dioxide is steadily increasing in conjunction with the increase in atmospheric carbon dioxide. This shows us something that people and politicians don't want to acknowledge - that the climate is slowly and remorselessly changing due our fossil fuel emissions and attacks on the world's forests.
Graph of the percentage of dissolved carbon dioxide in the ocean at the BATS site, just off Bermuda. Notice how the amount of carbon dioxide has increased since the 1980s. The red line is the percentage of carbon dioxide in the atmosphere measured at Hawaii. (from BIOS)
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A CTD (conductivity, temperature and depth) rosette after recovery from the deep ocean. The bottles have been closed and contain samples of water from different depths. (Photo from BIOS)
4.2 - Hydrostatic Pressure
Objectives:
- Understand that pressure increases with depth.
- Be able to calculate the pressure at a given depth, by either rule-of-thumb or direct calculation.
- Understand the problems caused by the huge hydrostatic pressures in the deep ocean.
4.3 - Buoyancy
4.4 - The CTD
The most well known and standardised method of sampling the water column remains a large rosette frame loaded with instruments and water sampling bottles, usually just simply called the CTD, which stands for the basic intruments; conductivity, temperature and depth sensors. Surrounding the large frame are special water bottles, called Niskin bottles, that is the standard equipment for sampling the water column. You could in theory use the amount of cable paid out by the winch for the depth, but as the cable gets longer, it gets heavier and starts to stretch – so is not a reliable measuring device. Other sensors such as florescence (to detect photosynthesis) and dissolved oxygen are usually included.
The hydrostatic pressure essential is linear with depth, so correlates well. The conductivity of seawater varies as a function of salinity and temperature, so an accurate temperature reading is required to determine the salinity. The variation of temperature and salinity affects the density of the seawater, so is the major cause of convection and currents. Ocean physics depends on accurate vertical density profiles. The Niskin bottles are open tubes, so that they don’t crush under the pressure, with spring loaded end caps that can be ‘fired’ off by the CTD controller at ‘bottle stops’. As water cannot be compressed the sealed bottles will not explode as the CTD is brought up.
To prep the CTD, the deck crew open and cock the bottles on their trigger levers, check the top air and bottom sampling valves are closed and remove the buffering water syringes from the two conductivity sensors. To deploy can be tricky and dangerous in rough seas. The ship is hove to with the starboard bow in to wind so that that ship is lying about 60 deg to the waves. The cable is taken up to lift the CTD off the deck. It is held under control by two deckhands with ropes to stop it swinging out of control. The crane is then swung outboard and the CTD allowed to swing slowly over the water. Then it is lowered as the guide ropes are released. Once below the surface it is more stable. The danger for the ship is that the CTD barely wobbles as the ships rolls due to the weight of the cable and the drag through the water, so that the loads on the cable vary as the ship rolls. The ship slowly drifts with the wind and waves as the CTD is lowered and raised by the winchman. The winch is a complex piece of machinery that has to pay out the cable under heavy load and bring it back in, without jamming! It is hydraulically powered. The scrolling mechanism on the front enables a neat and tight lying of the cable on the drum – otherwise it would be impossible to stow 6000 m of cable!
The bridge must always be connected with as the ship must be hove to and cannot maneuver while the CTD is over the side. Given that the maximum winching speed is 50 m/min, and the depth on this trip is usually 1000 m, it takes a while to winch it back up! *** how long? **** So, the bridge needs to ensure that there is no shipping that could cause issues.
The closing of the Niskin bottles is controlled by the main computer and the depths of the relevant bottle samples is recorded.
Once the CTD is recovered and secured to the deck, the scientists rush out to take the samples from the Niskin bottles to test for a variety of sciency stuff. The sampling is wet and fun, especially with the ship rolling and water occasionally sloshing over the side and under the deck grills. Put the tube on, open the air vent, place tube in bottle and open valve. Rinse twice using the Niskin water jet and then fill the bottle. Easy stuff. There are a wide variety of sampling containers to use. Typical uses for the water samples are testing for:
The hydrostatic pressure essential is linear with depth, so correlates well. The conductivity of seawater varies as a function of salinity and temperature, so an accurate temperature reading is required to determine the salinity. The variation of temperature and salinity affects the density of the seawater, so is the major cause of convection and currents. Ocean physics depends on accurate vertical density profiles. The Niskin bottles are open tubes, so that they don’t crush under the pressure, with spring loaded end caps that can be ‘fired’ off by the CTD controller at ‘bottle stops’. As water cannot be compressed the sealed bottles will not explode as the CTD is brought up.
To prep the CTD, the deck crew open and cock the bottles on their trigger levers, check the top air and bottom sampling valves are closed and remove the buffering water syringes from the two conductivity sensors. To deploy can be tricky and dangerous in rough seas. The ship is hove to with the starboard bow in to wind so that that ship is lying about 60 deg to the waves. The cable is taken up to lift the CTD off the deck. It is held under control by two deckhands with ropes to stop it swinging out of control. The crane is then swung outboard and the CTD allowed to swing slowly over the water. Then it is lowered as the guide ropes are released. Once below the surface it is more stable. The danger for the ship is that the CTD barely wobbles as the ships rolls due to the weight of the cable and the drag through the water, so that the loads on the cable vary as the ship rolls. The ship slowly drifts with the wind and waves as the CTD is lowered and raised by the winchman. The winch is a complex piece of machinery that has to pay out the cable under heavy load and bring it back in, without jamming! It is hydraulically powered. The scrolling mechanism on the front enables a neat and tight lying of the cable on the drum – otherwise it would be impossible to stow 6000 m of cable!
The bridge must always be connected with as the ship must be hove to and cannot maneuver while the CTD is over the side. Given that the maximum winching speed is 50 m/min, and the depth on this trip is usually 1000 m, it takes a while to winch it back up! *** how long? **** So, the bridge needs to ensure that there is no shipping that could cause issues.
The closing of the Niskin bottles is controlled by the main computer and the depths of the relevant bottle samples is recorded.
Once the CTD is recovered and secured to the deck, the scientists rush out to take the samples from the Niskin bottles to test for a variety of sciency stuff. The sampling is wet and fun, especially with the ship rolling and water occasionally sloshing over the side and under the deck grills. Put the tube on, open the air vent, place tube in bottle and open valve. Rinse twice using the Niskin water jet and then fill the bottle. Easy stuff. There are a wide variety of sampling containers to use. Typical uses for the water samples are testing for:
- Bacteria
- pH
- Salinity – to calibrate the instruments if required.
- Dissolved solids
- Carbon isotopes
- Other stuff that I don’t know about yet!
- On the Atlantic Explore cruise, some water is bled off through a filter to support the captured copepods while experiments are done on their pooping habits.
Figure 10 - Bottles cocked and ready for deployment. Note that the bottle caps are pulled to the side by the fishing line. The mechanism in the centre is the trigger for each bottle.
Figure 11 The caps on the bottom of the bottles are cocked open and held in place by a clip leading to the sprung loaded top cap line. The white things are the sample valves.
Figure 12 The CTD control centre. The winch control is above the CTD winch itself, but both can perform an emergency stop. The computer monitor shows the direct output from the instruments that is sent up a cable that is in the centre of the CTD cable.
Figure 13 The recovered CTD, note how the trigger levers are up and the bottle tops have closed down. The bottles contain 12 litres of water and have thick walls for insulation. The water from the deep ocean comes out COLD!
Figure 14 CTD winch gear. The winchman's cabin is to the left. Note that the drum as a scrolling mechanism to lay the wire flat as it is winched back up. Hard to imagine that there is over 6000 m of wire on the drum.
Me filtering water from a Niskin bottle for the copepod pooping experiment. (photo by Kaitlin Noyes)
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A spare Niskin bottle mounted on a bulkhead in the CTD garage. Note the long springs that close the end caps when the trigger is released.
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4.5 - Deep Sea Moorings
One of the ways to overcome one of the problems of the shipboard CTD is to lay a deep sea mooring, so that, while it is still a single point in the ocean, it is possible to see how the oceanic variables change as a function of time. I was heavily involved in the deep sea moorings for the RAPID MOC project in 2009/10. The moorings are covered with instruments and lowered to the seabed and are usually recovered a year or so later. The RAPID moorings were up to 4500 m long and used old locomotive wheels as anchors.
4.6 - Using Sound Waves
4.7 - Robots
Argo Floats
Gliders
Boaty McBoatface article - BBCNews 18 June 2019
AUVs
ROVs
Surface Drifters