Vehicle for Marine Perlustration

In a previous post we focused on one of our main instruments, the CTD. But that isn’t the only device we’re frequently sending on dives into the ocean. Now it is time for our other scientific submersible to have some time in the spotlight!

The vertical microstructure profiler (VMP) is designed to take measurements of temperature, conductivity (from which salinity can be calculated) and velocity over small length scales. Dissipation of turbulent flows typically occurs over distances of a few millimetres to tens of centimetres, so in order to observe their effects the instrument must take measurements within this range. The microstructure measurements can be used to infer information about mixing in the ocean due to turbulence and convection.

The VMP ready for an early morning deployment.

In contrast to the CTD, which is attached to a cable and lowered into the sea by winches on the ship, the VMP is a freefalling instrument. Its torpedo-like shape minimises the resistance it will encounter on its journey to the deep ocean. Before deployment, weights are attached to the end of the VMP. Without these weights, the device is buoyant enough to float to the surface.

The weights attached to the end of the VMP before deployment.

The VMP is an expensive and important instrument, so great care is taken to minimise any risk of losing it to the abyss. The weight release mechanism receives instructions from the instrument’s main computer, and can be triggered by several different conditions. The first release trigger, the one we ideally want, is the instrument reaching a pressure which is calculated and programmed in for each individual dive. This pressure will relate to the depth we want the VMP to reach, usually around 50m above the seabed in order to get as much data as possible without risking a collision with the ocean floor. A weight release can also be triggered by reaching the time limit calculated for each dive, the fall rate dropping below 0.4ms^-1 or the main computer in the instrument shutting down. The weight release mechanism, which runs from a separate computer, will also be triggered if no signal is received from the main computer for over 4 hours. The final failsafe, in case of complete electronic failure, is the ‘burn wire’ used to attach the weights to the VMP. The wire will completely dissolve after being submerged for around 24 hours.

After the weights have been released, the VMP returns to the surface.

When the VMP returns to the surface, it must be recovered. Its position is tracked by the ship using a USBL beacon, and when at the surface it can be identified by the flag and flashing lights attached to the top. However, it can be surprisingly difficult to spot a bright orange flag in the ocean!

A little game we like to call 'spot the VMP'!

Once all the data collected from the VMP dives is processed, we will have a better idea of the level of mixing which occurs across the Panama basin. When used in conjunction with other data, this could tell us a lot about how hydrothermal vents are contributing to circulation in the ocean.

Wildlife update

Over the last couple of weeks the CTD has been picking up some very strange objects. After a little Wikipedia research, we have decided they are probably a type of pyrosome. These bioluminescent tubular structures are actually colonies of small creatures called zooids. An individual zooid is only a few millimetres long, but they are embedded in a gelatinous substance that holds together colonies which can be several metres long! The ones we’ve found haven’t quite been that large, but it’s still very difficult to imagine them being made up of thousands of tiny creatures.

One of the pyrosomes found on the CTD, before being thrown back to the ocean.