We have been working in the Porcupine Seabight (hence the PO of PORO-CLIM) for the past week (with a brief stay in Bantry Bay) and have just finished our second profile which marks the end of data collection for the trip!
We previously talked about the OBS deployment and retrieval process (see earlier posts), but also deploy other instruments which we’ll be focusing on in this post, as well as providing insight into the stages of running a profile.
The sun setting on our data acquisition period
The two other instruments we deploy overboard are:
Magnetometer
XBT (Expendable Bathythermograph)
What is a magnetometer?
A magnetometer does what it says on the tin - it measures the magnetic field. Marine magnetometers are used by geophysicists to detect magnetic variations in the oceanic crust. They can also be used to survey the seafloor for shipwrecks.
The magnetometer is towed in the water behind the ship and measures the background magnetism. When the instrument detects ferrous (containing iron) items, for example a sunken ship’s hull, this registers as a change in magnetism on the display. This way the tool can be used to find shipwrecks, even if they are buried.
Why are we using a magnetometer?
We are interested in examining the changes in magnetism of the oceanic crust, as we can determine the age of the rocks.
Oceanic crust is made of basalt, which contains ferrous minerals including the highly magnetic magnetite. When basalt forms along the mid-oceanic ridges, for example the Mid-Atlantic Ridge, magnetite crystals in the basalt record the alignment of Earth’s magnetic field. Thus, basalt can provide us with a record of the strength and direction (polarity) of the Earth’s magnetic field at the time the rocks were formed. Changes in the Earth’s polarity can be observed in a striped pattern on the seafloor.
What are seafloor stripes?
The Earth is made up of tectonic plates, which move over geological time, and are constantly being recycled. The plates spread apart at mid-oceanic ridges as new oceanic crust is produced. If you were to walk along the seafloor away from the mid-oceanic ridge the age, density, and thickness of the oceanic crust increases.
Throughout geological history the Earth’s magnetic polarity has experienced reversals. These reversals are recorded in basalt. Magnetometers reveal a striped pattern on the seafloor of alternating normal and reversed polarity. As the plates spread out from the mid-oceanic ridges, the polarity stripes widen on either side of the ridge. When the Earth’s magnetic field reverses, a new polarity stripe begins. Rocks of the same age have the same polarity. As such, you can observe rocks of the same age and polarity on either side of mid-oceanic ridges.
When spreading centres, such as the Mid-Atlantic Ridge, produce new oceanic crust the Earth’s polarity is recorded in the rock. Changes in polarity result in “seafloor stripes” on either side of spreading centres. (Image source: NOAA)
It was actually by observing the changes in magnetism of the oceanic crust around mid-oceanic ridges that helped geologists identify the process of sea-floor spreading in the 20th century. Seafloor spreading remains as the strongest piece of evidence for the Theory of Plate Tectonics.
We monitor the magnetometer from the container. The magnetometer is attached to the orange cable seen through the window.
What is an XBT? An XBT, or Expendable Bathythermograph, is a probe that measures the sea temperature as deep as 2000 + meters. We use it to give us a better understanding of the structure of the water column, which helps with processing seismic data later on.
It plots the thermocline really well and we have even observed the “Salty smarties”, or Meddies, within the water column. Mediterranean Water eddies (meddies) are pockets of higher salinity found in the North Atlantic, which are outpourings from the Mediterranean Sea.
Are there any issues with an XBT?
We did have a couple of issues with the first few XBT deployments. We were getting readings of 37°C, which is certainly much warmer than the North Atlantic! It turns out the probe was hitting off the magnetometer in the water. We now deploy the XBT from the port (left) side, whilst the magnetometer is deployed off the starboard (right) side of the ship.
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Imaging the subsurface takes more time than you might think! During the PORO-CLIM expedition, we’ve been collecting 2D seismic data, which means than we image cross-sections of the crust and sedimentary cover along lines drawn on a map (seismic lines). These cross-sections are called seismic profiles. Several operations must be conducted to complete each profile and that requires us to do three runs along the same transect. The first profile was 380 km long and took over a week to be completed. There are three main stages to running a profile:
Deployment
Acquisition
Retrieval
First, we deploy an OBS every 14 km. We drop them overboard, and they slowly sink down to the seafloor. This process of stop & go takes approximately 24 hours to complete. For the first profile we deployed 27 OBS’s, and 20 for the second profile.
We deploy the OBSs 24 hours a day
Acquisition
Once we’ve reached the end of the line, we turn back and head in the opposite direction to start shooting the seismic waves. This process can only start after the marine mammal observer has given the all clear. We must wait 60 minutes after the last marine mammal is spotted in the area surrounding the ship before seismic imaging can start.
At this point, the technicians on board the ship deploy the streamer (a series of hydrophones held within a kilometer-long tube), and we start our watches in Per’s container.
After a ramp-up protocol, during which the emitted sound waves slowly increase in strength, the proper data acquisition starts and we keep going until we’ve reached our original starting point. This is the longest stage and it lasts 2 to 3 days, as the ship must travel very slowly during this process (~ 3.5 knots).
It is also the most critical stage, as it very much depends on weather conditions. If the waves are too high, we cannot continue data collection. We were faced with quite a big storm during the acquisition of our first line. We had to heave to for an entire day.
Retrieval At the end of the line, when all the seismic data is collected, we pull the streamer back into the ship and we must now do a third pass on the same line to retrieve all the OBS’s. This also takes quite some time and depends highly on weather conditions.
Fortunately for us, this stage went quite well on the first profile and we even made it a competition to see who could the most OBS’s! We managed to find all the OBS’s in record time and recovered them all safely. This can take as long as 3 to 4 days in bad conditions, but we managed to complete the process in roughly 2 days.
Bringing an OBS back onboard is always a welcome sight.
The last stage is processing of the data. A lot of steps are required for the data to be readable, and this is done using computer software. This can be started during the cruise, but is usually completed at a later time once the expedition has ended. We will have a future post on data processing.
As you can imagine, the time necessary for seismic data acquisition can be highly variable, especially in areas where the weather can be rather unpredictable, and many things can go wrong. Despite our many delays due to the weather, we have managed to run two profiles.
Written by Niamh Faulkner & Ben Couvin
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