What’s inside an OBS

Until this cruise, my experience with seismology fieldwork had been entirely on land. The group at Bristol has deployed seismometers in some of the harshest conditions on Earth, including the Canadian Arctic and the Afar region of Ethiopia and Eritrea. Operating a seismometer at the bottom of the ocean presents a whole new set of challenges. Ocean bottom seismology goes back to early work by Maurice Ewing in the late 1930’s, but it was not until the era of nuclear test ban monitoring that the incentive for ocean bottom seismology really picked up – forensic seismology is one of our best methods for monitoring such activities. The lack of seismic monitoring stations on 70% of the Earth’s surface – i.e. beneath the oceans – is one of our biggest limitations in imaging the Earth’s interior, especially in the southern hemisphere. Permanent ocean bottom seismic stations are still being developed, but currently the deployment of temporary networks or arrays is the most economical way to study a region.

Most early applications of OBS technology were to either record human-made sources (seismic reflections / refraction) or local seismicity (e.g. at Ridges). Through the 1980’s and early 90’s a number of experiments surreptitiously recorded distant (teleseismic) earthquakes, presenting the possibility of using OBS arrays to image the Earth’s interior. In the 1990’s the bandwidth of the sensors increased (broadband), as did the length of deployments. This has allowed seismologists to image the deeper earth beneath the sea-floor using teleseismic earthquakes. A 3-month experiment in the Tonga-Lau Basin (1994) and then the 6-month MELT experiment on the East Pacific Rise (1995/6) ushered in the beginning of a new era.

We are fortunate to have three different OBS groups on the cruise – IPGP, Lamont and Scripps – so I have an ideal opportunity to learn all about this exciting technology and the challenges working on the sea floor present. To kick things off, I interviewed Ted, Carlos and Peter – the research engineers from the Lamont Doherty Earth Observatory at Columbia University. The Lamont OBS was first developed when Spahr Webb moved to Lamont from Scripps in 2001. The team is led by Dr. Andrew Barclay.

In some ways, the guts of an OBS are the same as that of a land station. You need a sensor, datalogger, power and timing – but there the similarities end. OBSs sit in highly corrosive sea water and need to contend with crushing pressures in excess of 500 bars (50 MPa) at 5 km depths. The sensor in the LDEO OBSs is a Nanometrics Trillium Compact – a sensor we routinely use on land. The choice of sensor is difficult, as you want something robust and easy to deploy, but you would also like a very sensitive instrument with high dynamic range that can record everything from very long-period (surface waves) to very high frequency (local microseismicity) signals. Many seismometers require the springs to be unlocked before they will work, but this is not practical at the bottom of the sea. Also, seismometers need to be deployed on a level surface. On the deployment cruise a site survey was used to find an area with little sea-bottom topography. To some extent the seismometers can internally self-level and a motorised gimbal system is used with these instruments. Occasionally, the OBS lands on uneven terrain and cannot level itself, in which case the data are unusable.

Another sensor that is routinely included is a pressure gauge, but these too come in many flavours. Most of the instruments we are using have a Differential Pressure Gauge (DPG), which measures changes in pressure within the instrument with respect to the ambient pressure of the surrounding water. They record seismic waves very clearly, and have the added advantage that their signals can be added to the displacement signal from the seismometer to cancel out reverberations in seismic energy within the water column. This is now a routine procedure in offshore seismic exploration for oil and gas. Some of the Lamont instruments are unique in that they use an Absolute Pressure Gauge (APG). These measure pressure changes looking at the frequency of resonance in a quartz fibre as a pressure wave passes through it. This reminds me a bit of the old Warden gravity meters that used a zero-length quartz spring. The APG also has a simple hydrophone attached – these use piezoelectric sensors to measure pressure change and are the workhorse sensors in the long streamers towed behind ships in seismic reflection experiments.

The data from the sensor are digitised and stored on a 24-bit logger, which has a 64GB storage disk. This is plenty of space to record four channels of data with a sample rate of 100 samples-per-second for a year’s deployment. On land, power comes from solar panels trickle charging large 12V batteries, but these OBSs get their power from 60 DD-cell lithium batteries. Lithium batteries have something like 5 times the energy density of regular alkaline batteries, but are a lot more difficult to ship by air.  The total current drawn by the system over the year is 377 AmpHours. The electronic are cased in aluminium-alloy tubes. The APG housing, which is unique to the LDEO instruments, is shown.

On land, we use GPS antennas to get accurate positioning and timing for the station. We require timing accuracy on the order of a few milliseconds. GPS isn’t possible on the sea floor. The locations of the instruments are surveyed using acoustic ranging. The timing comes from accurate clocks. In the LDEO instruments the clock is a quartz oscillator, similar to that used in many satellites. The instruments drift by roughly 1 second over the entire year and a linear correction is applied to the timing. Other timing issues include accounting for leap-seconds whilst recording – all pretty nitpicky, but important stuff. There is some discussion of using atomic clocks in OBSs, but they are currently too power hungry … and very expensive and often unreliable.

Communicating with the instruments is obviously very important. We need to know where they are, and to be able to tell them to start rising. This is done by acoustics, and each instrument has its own encoded signal. One frequency is used for speaking to the instrument (11 kHz) and another is used to listen (12 kHz). If we know the time it takes the signal to get from the ship to the instrument, and back, and we know the speed of sound in the water, we can determine the range to the instrument. This is also helpful to confirm that the OBS has started rising to the surface. The entire process from first communication, releasing the anchor, and confirming that the instrument is rising takes roughly 15 mins.

On land, the entire OBS unit weighs over 350 kg, but in water there is a delicate balance of flotation to anchor weight. The LDEO package has a pair of 30 kg cylindrical anchors, which are roughly 15 cm in diameter. A thin stainless-steel wire holds them in a vertical sleeve that is attached to the frame. An acoustic signal is used to trigger a burn command: an electrical current is passed through the wire, which leads to hydrolysis and the rapid corrosion (oxidation) of the wire. Flotation comes from large glass spheres, made from two hemispheres with precision ground edges that are held together by evacuation. A sphere is shown – they are 30 cm in diameter. The LDEO instruments rise (or sink) at a rate of roughly 40 m per minute. They can be deployed to a maximum depth of 5 km and can be left out for 400 days. So, we are really pushing them to the limit. Pictured below is the LDEO instrument coming out of the water just as the sun is rising. The sensor is hanging below the frame.

Magnetic attractions at the equator

The weather remains good, the sea is calm and we are about to cross the equator for the third time. We are steaming westward away from Africa and have some long transits between stations. Time to enjoy the sunsets and sunrises. But, this has also given us some time to start acquiring magnetic data.

Electromagnetic induction is behind many geophysical methods. Movement of a magnet can induce an electric current, and vice versa – this is the basis of the magnetotelluric method … but more on this later. The rapid movement of liquid iron in the outer core – by rapid, I mean as fast as a snail moves – gives rise to the Earth’s magnetic field. This field shields us from harmful stellar winds, but also provides us with a means of navigation, as the needle on a compass points towards the North Magnetic Pole.

Ships have played an important role in our study of the Earth’s magnetic field. Historical records of declination (the angle between geographical north and magnetic north) and inclination (the angle between horizontal and the dip of a compass needle) of the magnetic field have allowed geomagnetists to model how the field has varied in time, which in turn tells us something about dynamic mechanisms of the Earth’s outer core that lead to a magnetic field. The position of the magnetic pole is constantly changing and there is a westward drift of anomalies at the equator. As a historical note, the first maps of magnetic declination were produced in 1702 by Edmond Halley, based on extensive measurements by ships in the Atlantic.

One of the crucial bits of evidence that lent support to the idea of plate tectonics involved evidence of geomagnetic pole reversals in the geologic record. These reversals occur at irregular intervals roughly every few 100’s thousands to millions of years, but happen very rapidly (over 1000s of years).  The rocks of the sea floor are strongly magnetic and their remnant magnetism after formation acts like a time stamp on the new crust produced at an oceanic spreading centre (in our case, the Mid-Atlantic Ridge). Magnetic surveys as early as the 1950s showed clear linear patterns in magnetic anomalies on the sea floor, but it was not until the 1960s that these observations were tied to the idea of new plates being formed at mid-ocean ridges, which are more correctly termed oceanic spreading centres.

Even though this is now a very routine measurement at sea, for me, it is exciting given the historical context of the technique. So, I was up at 3am to help Martin, the research technician from the National Oceanography Centre, deploy the instrument with Johnny Mac, the scientific Bosun.

We are using a type of proton precession magnetometer, which is towed over 300m behind the ship. As a rule of thumb, magnetometers are towed 3 ship lengths behind the winch to get away from the magnetic signal of the metal in the ship. A current is passed through a coil that is wrapped around a proton-rich canister of fluid (e.g. water or kerosene). The hydrogen protons align with the field induced by the coil. The current is then turned off and the protons precess back to the direction of the Earth’s magnetic field. The frequency of precession is proportional to the strength of the magnetic field. Our magnetometer uses something known as the ‘Overhauser effect’, which I think removes the need for orienting the sensors – but as we have virtually no access to the internet, I cannot check.

Our magnetometer is towed off the port-side of the ship and flies at a depth of roughly 6m when we are doing 10 knots. The manufacturer describes the instrument rather flowerly as a ‘surveyor’s best friend’, with ‘unfailing data, durable hardware, and an easy-going disposition’. I must admit it is easy to use and the data look reasonable. Looking a bit like a fish, sea-borne magnetometers often return with shark’s teeth in them.

What to pack

If you have been to sea many times, packing is no doubt a fairly simple task and you seldom forget something. However, for many of us on DY-072, this is the first time on a scientific cruise. I am happy to have remembered all the cables and adaptors that I need, but brought too many warm clothes. I am getting a lot of wear out of my steel-toed work boots and hard hat, but not my coveralls and safety goggles. Also, I could have really done with more gym clothes and T-shirts, but my flip-flops have been unnecessary as you cannot wear them around the ship. And even though I remembered the box set of “The Office (USA)”, which a friend kindly lent me for the cruise, I forgot my DVD/CD player – fortunately, as Senior Scientist, my room has a TV and DVD player! On any future cruise, I would definitely bring some good coffee and a coffee maker. I have been saved by the MT guys, who make some of the best filter coffee I have tasted in years.

One thing I really wished I had brought was a small collapsible fishing rod. Chris from the MT lab fashioned a jig for squid fishing from wire and electrical tape and Kiwi Dan managed to actually catch one. Apparently, lots of tasty fish have been caught on previous cruises, which the cooks are normally happy to prepare. The squid (pictured) was saved because no one knew how to kill it.

To see if I am not alone in my ‘haves’ and ‘have-nots’, I have asked my shipmates three questions: what have they forgotten; what did they bring too much of or is unnecessary; and what are they really happy to have remembered. I got this idea from a recent blog by Bristol colleagues, Laura Robinson and Kate Hendry, who were on a cruise in the Southern Ocean a year or so ago. I’ll start with the most experienced sailors (Ted has been going to sea for nearly 50 years) and end with the novices. 

Name (times at sea)ForgottenToo much or not neededEssentials
Ted (>70)NothingSpare safety PPE (hard hat, boots, etc.)Microwave popcorn
OBS Martin (>50)More work shirtsPeanut butterKindle
Wayne (> 50)Nail clippers; French chocolate; Good work bootsRunning socksChocolate (duty free – not as good as French); Earl Grey tea
Chris (>50)More booksLong trousersCoffee
ResTech Martin (>40)Hammock; sunglassesToo many jumpersTea
Jake (>30)Nail clippersT-shirtsCoffee
Sean (>25)Gigantic bag of pretzels; More power stripsLong trousersStarburst and Sour Patch Kids candies
Carlos (>10)Booze (but it is not allowed anyway)Party mix of nuts, corn, etc.Books
Dan (6)Plug adaptors2.5 decades of seismic reflection / refraction dataNZ Chocolate
Simone (4)Steel toed boots (still in container of previous cruise)NothingToiletries (forgot them on previous cruise)
Matthew (2)SpeakersTowelsWork boots
Kate (1)NothingSunscreenCoffee and Aeropress; Maps, software, previous cruise reports
Owain (first)More shortsToo many jumpersLaptop
Océane (first)Steel toed safety boots; good cheeseSocksBooks and music
Sai (first)Internet (poor connection); booksSocksLaptop; maps, programmes, etc.
Michaela (first)Chocolate; non-gym clothesExtra computer peripheralsWarm fleece for the lab
Peter (first)Only has one pair of shoes – his work boots; not enough musicToo many shirts, as he wasn’t sure about laundry.Books
Mike (first)Not enough gym wear; DVD/CD player; fishing gearWarm clothes; SunscreenPower cables; Books; DVDs

A general theme is that most of us brought too many warm clothes. Coffee and chocolate seem to be common creature comforts. Our French colleagues all have issues with work boots – Wayne’s self-destructed on their first station-servicing and there is still a trail of rubber around the lab. Forgetting electronics and peripherals is common, but also apparently nail clippers are easily forgotten.

In a second survey, I asked ‘what are you missing most?’, but I gave the condition that it could not be a person. Food on the ship is much better than I expected, but is a common theme in what people miss: Matthew is missing fresh fruit and veg; Chris is missing Mexican food; Océane is missing cheese, charcuterie and other delicatessen food; Kate misses lightly cooked fresh vegetables. I am missing good red wine, Jake is missing his wine cellar and Carlos is missing booze in general. Some people are missing activities that they cannot do on the ship: Kate is missing running outside, as am I (but the gym is better than I expected); Sean is missing surfing; Ted is missing cooking. Dan is missing Sky Sports and OBS-Martin is missing his dogs (I am missing Stanley). Peter is missing Central Park in New York. Matthew is missing the internet, but Ted and I are happy to be away from the relentless coverage of current politics. There is no need to carry money on the ship and rooms are generally unlocked. It will be a shock going back to carrying keys, money, credit cards, etc. – not a way of modern life that I miss.

The scientific crew:


Over 12 days into the trip and instruments are being recovered fast and furiously. We are just finishing the densest of the 3 lines and at one point we were pulling in a station every 6 hours, working 24 hours around the clock. The transit from L20D to S19D marked half-way and we have crossed the equator twice already, hopping back and forth between the Nubian (Africa) and the South American plates. We will soon make it to S17D, our deepest station, which lies in over 5.2km of water. The OBMT will take well over 4 hours to rise to the surface at this station.

Finding an instrument when it pops up at the surface can be a bit of an adventure. All instruments are fitted with a bright orange flag, a strobe light and some sort of radio communication that indicates it is on the surface. And, the transducer can be used to monitor the distance to the instrument as it rises. Despite all this, things can go wrong. We have had instruments where the strobe light fails to turn on, which makes instrument location in the dark a real challenge. The radio or transducer can be used to get a rough estimate of range and the ship then tries to triangulate based on measurements in a few locations. The ship has good search lights and the Bridge has some night-vision goggles. In the daytime, everyone looks for the red flag on the instruments, but even this can be difficult if it drifts a long way. The worst scenario is radio failure, a broken strobe and a snapped off flag – fortunately we have not experienced this. The OBMT is this easiest to find as it also has a GPS locator on it. If an OBS and OBMT rise at similar times, we always go for the OBS first. The ship’s crew are amazingly eagle-eyed and are usually the first to spot the instrument from the Bridge. Pictured is a Lamont OBS coming on board just as the sun is rising at 6.30am.

Communication during station retrieval is crucial, and each group – the Bridge, deck crew, main lab (picture below), OBS lab and OBMT lab – has a walkie-talkie. The Bridge makes all decisions about the ship and the use of its equipment. The main lab decides things like the order of pickup, when to turn on and off the swath bathymetry and to what depth the SVP (sound velocity profile) is conducted. Our cruise is a bit unusual in that we have both OBS and OBMT instruments at each station. It took us a few stations to iron out the bugs and to develop a standard procedure for recovery, which is slightly different from the previous blog:

  • When we arrive at the area, the ship moves into hold on the edge of a circle centred on the drop position of the OBS and with a diameter equivalent to the water depth. The ship lowers a keel with the hull transducer in it (this ensures clear acoustic communication with the instruments).
  • The OBMT guys establish contact with their instruments, often even before we get to the circle. They then send a signal to release the instrument (a burn wire holds the instrument to the anchor). They confirm lift off, establish an ETA and then disable communication.
  • Next the OBS team establishes contact with their instrument.
  • A 270-degree circle is sailed around the circle, ranging to the instrument. The OBS location is later established using simple triangulation and knowledge of the sound-speed of the water column.
  • When finished, the ship holds in ‘Dynamic Position’ or DP, whilst the OBS is released from the bottom. An ETA at the surface is calculated.
  • Once it is confirmed that both instruments are rising, a Sound Velocity Profile (SVP) is taken. This measures the P-wave velocity and the temperature of the water as a function of depth and takes anywhere from 30 mins (for 700m) to 90 mins for our deepest SVP, which is 2.5 km. Here is a picture of the SVP logger being brought on board as the sun rises – it is attached to a 500 kg weight that looks like a giant bath plug.
  • The ship then sails to the estimated position that the OBS will pop up.
  • The OBS is retrieved.
  • Then the ship sails to the estimated surface position of the OBMT, where it is retrieved.
  • The drop-keel is raised, the swath bathymetry is turned on and the ship sets a course for the next station.Sometimes the instruments are recovered but then have data problems. Thus far, we have seen a flooded seismic sensor, a ruptured battery cable, a frozen gimbal that stopped the seismic masses releasing, and two data logger failures. We are pushing technology to the limit. It is a bit like leaving your computer on at home unmonitored to record something for a year – but, only using batteries for power and at the bottom of the ocean where pressures are 500 times that on the surface.

Our first returned instruments

Finally, after nearly 6 days of sailing we arrived at our first site – L39D – where we retrieved one of the Lamont ocean-bottom seismometers (OBS) and a Scripps ocean-bottom magnetotelluric (OBMT) instrument. We arrived on site at almost 17:00 and we finished at midnight. Much of this time was spent trying to communicate with the instruments, which have been peacefully lying on the sea floor for a year and at first seemed reluctant to wake up.

A transducer is used to locate and communicate with the instruments. It can send commands to tell the instrument to release its anchor and rise to the surface. The OBMT takes roughly 4 hours to rise in 4000m of water, whilst the OBS takes 90 minutes. When they reach the surface a signal is sent to receivers on the ship, but they are also equipped with a strobe light and a bright flag. Additionally, the OBMTs have a GPS locator on them.

So, the procedure is to stop the boat at a distance equivalent to roughly half the depth of the water, which is normally a reliable distance to start communicating with the instruments. Our instruments are in water that varies in depth from 2900m to 5200m. With our experiment, we first communicate with the OBMT and send it on its way to the surface. We then do a sounding survey at various points roughly equidistant from the drop-site of the OBS. This is to find and locate the OBS (the deployment drop location is used for the OBMT, as station-location accuracy is less important for these instruments). Ideally one would sail in a circle around the likely site of the OBS and triangulate to locate it. In this first pick-up, we used the overboard transducer at four points of a diamond. When we finish surveying in the location of the instrument, the OBS was sent a signal to release its anchor and return to the surface.

The diagram, left, shows in orange the circle with a radius of half the water depth, the proposed ship trajectory (dashed line) and the location of the drop point of the OBS (in the centre) and the OBMT (to the left) – the smaller circles are 500m around the OBMT and OBS. The white line shows the actual ship track, which starts near the OBS, then shows the diamond shaped survey used to locate the OBS and then the final area where the instruments were picked up. Due to the 1 knot current, the OBS and OBMT respectively drifted roughly 500m and 1000m away from the deployment location.

The transducer transmits the time it takes a signal to propagate from the ship to the instruments and back. To determine how far away they are, one needs to know the speed of sound in the water column. In the ocean, this varies as a function of pressure, temperature and salinity, in a somewhat complicated manner. Furthermore, the profile of soundspeed varies both temporally and spatially in the ocean. A well-known low velocity region lies between 500m and 1000m (the SOFAR channel), which serves as a wave guide that transmits energy great distances. For this reason, submarines try to avoid detection by staying out of this channel and whales use it to communicate. Due to the variability in soundspeed, we acquire what is known as a soundspeed vertical profile (SVP) to a depth of 700m. This is where all the action is – the deeper parts of the oceans are a bit more predictable. This picture shows the SVP probe attached to a 500kg weight – the Bosun uses a winch and crane to lift and lower into the water. This procedure takes roughly 30 mins.

As with any geophysics fieldwork, there are always unexpected problems in the field. With our cruise, the ship’s transducer is currently being awkward. We could not establish communication with the instruments from the lab and the reason for this was not readily apparent. We then tried to circumvent the ship’s wiring by patching into the transducer cable at the point it enters the ships wiring – this was done in a tiny, hot, crowded little room called the ‘hole’. Ship noise and bubbles will also interfere with the signal. We therefore turn off the other equipment that surveys water depth (e.g., the swath bathymetry) and we can get the Bridge to even turn off the propeller, thrusters and dynamic positioning on the ship. Furthermore, the transducer is housed in a keel that can be dropped below the ship. Finally, if this doesn’t work we can lower a transducer over the side of the ship to a depth of 20-60m. Thus far, we have tried all the above in a variety of configurations. Going forward, the best solution, which optimises speed and accuracy, seems to be to work in the ‘hole’ with the keel dropped. With each instrument the procedure will become more and more routine.

Recovering an instrument is exciting. There is regular communication between the Bridge, the main lab, the OBS or OBMT labs and the deck – everyone has walkie-talkies. The transducers can be used to interrogate the depth of the instruments as they rise. When they hit the surface, everyone looks for the strobe light (or flag in the daytime) on the beacon, but in practice the Bridge is always the first to see it. Our first recovery was in the dark, which made it a bit easier to see the light. The crew control the ship with amazing accuracy and using the various thrusters they can slide up alongside the instrument. 2-3 people then use hooks attached to poles to pull the instrument to the side of the boat and the instrument is attached to a crane. The Bosun (or Boatswain) uses the crane to hoist the instrument on to the deck. In general, he helps us with most of the logistics on deck. The whole procedure only takes 10-15 minutes – but this can take a lot longer in rough water.

The pictures below show the OBMT (left) and OBS (right) coming on board. 2 down – 76 more to go.



The RRS Discovery

The Royal Research Ship (RRS) Discovery is our home for the month. We are currently about 200 nautical miles off the coast of Sierra Leone and will arrive at our first site tomorrow afternoon. The weather has been good – calm seas and the temperature is climbing towards 30oC. The past 4 transit days have given me lots of time to get to know the ship.

The Discovery is operated by the UK’s National Oceanographic Centre (NOC). The ship we are on was built in Spain and launched in 2012, but the name ‘Discovery’ goes back centuries to the British East India Company. The use of the name for exploration and research starts at the beginning of the last century when Robert Scott commanded the National Antarctic Expedition of 1901-1904. At one point, Scott and Shackleton sailed together to the Antarctic on the Discovery. In 1925 she was designated a ‘Royal Research Ship’ and the name has transferred to new research ships – in 1929 and 1962 and then our ship in 2013.

The Discovery is something of a tardis. It is nearly 100 m long and 20 m wide and is carrying 16 scientists, 2 research technicians, 21 crew members and 1 cadet, but it doesn’t seem crowded. You see most people at mess times, but otherwise you can spend a lot of time on your own. The ship has 9 decks, which have confusing names. Only 3 of the lower decks extend the full length of the ship. The upper decks cover only the front of the ship.

Starting at the bottom of the ship, the Tank Top lies above the fuel and ballast tanks and houses the engines. A discontinuous Lower Deck houses various control areas for running the engines and other things. A lot of water is carried for ballast and the ship has a clever system of sloshing water from side to side to mitigate roll or to provide counterbalance when using the cranes to load the ship. We are hoping to get a tour of this deck toward the end of the trip, but Andy – the Chief Engineer – was kind enough to show me around part of it this morning.

The Main Deck is defined as the lowest deck above the water line that extends the full length of the ship. On the Discovery it houses the gym, laundry room, gravimeter and most of the accommodation (cabins) for the scientists. There are also a number of workshops down on this deck.

Most of the scientific labs are on the Upper Deck, which is where we spend the majority of our time. Chemistry labs and wet rooms are where samples from the ocean, dredging, and so forth can be examined, but we are not using these facilities in this cruise. The OBS and OBM labs for this trip are towards the stern, with easy access to the open part of the deck where the instruments will be hauled onto the ship and into the main hangar area for storing. The computer and server rooms are housed in well-chilled areas and some crew cabins and the sick bay are on this deck. The garbage treatment and storage is also on this level – fortunately, we have little contact with this area.

The decks above the Upper Deck do not extend the full length of the ship. The Mezzanine Deck has a nice conference room, library, video room, and bar and lounge. We are allowed two tins of beer or one small (175ml) bottle of wine a day. Most importantly, this level is where you find the galley, servery and mess. This is the deck you enter the ship from the gangplank.

The Boat Deck is where, as Senior Scientist, my room is and some of the officers’ cabins, including the Purser’s. The two life boats are stored on this level. Moving up a level is the Forecastle – pronounced fo’c’sle – Deck and it is primarily living quarters for the Captain, Principal Scientist (Kate Rychert), Chief Engineer, and other high-ranking officers. In case of emergencies, the muster station is at the aft of this deck.

Finally, the Bridge is the highest deck and where the Captain and Officers control the ship. There is actually a deck of limited height below the Bridge that houses electrical and navigation equipment. This is the Navigation Bridge Deck, nicknamed ‘Middle Earth’, as only hobbits can stand up comfortably in the space.

Everything on the ship is fastened down. You soon learn that anything left unstable will end up rolling across to floor to a corner of the room.

There are monitors everywhere in the labs, showing our location and speed, but also sea conditions, water depth, the weather, and the way points. CC-TV also broadcasts video from the deck and winch areas.  In a later blog I will describe more about the equipment on board this impressive ship.


Seaboard gravimeter

Anyone who has done a gravity survey on land knows that this can be a labour of patience – a bit of wind or uneven terrain makes things difficult. So, even though gravimeters have been used on ships and planes routinely since the 60s, I am still impressed with this technology. The RRS Discovery is equipped with a Micro-g LaCoste Air-Sea gravity meter, which is essentially a LaCoste-Romberg instrument on a gimbaled platform.

Gravimeters were first used at sea in submarines in the 1950s and were modified for planes in the late 1950s. Most early applications were by the U.S. Military, but exploration companies quickly bought into the technology. They are now routinely used on research vessels and planes. For example, the NERC Twin Otter that flies in the Antarctic is equipped with one.

The instrument has its own air-conditioned room in the center of the ship on the Main Deck (which contrary to its name is almost the lowest deck, just above the Tank Top level where the engines are). It is housed in a sensor bucket, which sits on a stabilised platform. The pitching and rolling of the ship is counteracted by two orthogonally-oriented torque motors that are controlled by fibre-optic gyroscopes. Oil shock absorbers and shock cords also help to damp movement.

Gravity meters or gravimeters measure the value of gravitational acceleration, g, which is nominally 9.81 m/s2. We are interested in variations in this value that are due to variations in the density of the Earth’s crust and mantle. For example, a magma chamber will be less dense than the surrounding rock. Gravimeters can measure very small changes in g; changes in elevation on the order of a few meters will affect a gravimeter reading. The unit we measure g in is a Gal – named after Galileo – which is 10-2 m/s2, and mGal anomalies are normal (10-5 m/s2), but a modern survey can detect even variations down to microGals (mGal).

Gravity corrections: Because the Earth can be approximated as an oblate spheroid (the shape of a flattened orange), g will be larger near the poles, because you are closer to the centre of the Earth, and less near the equator. Correcting for this is known as the latitude correction. To give you an idea of how important this is, the variation between the equator and poles is over 5000 mGal, and yet variations in gravity measured last year over the Chain Fracture Zone are only ±50 mGal. There is also a need to correct for variations in elevation and topography. On a mountain side, you are further away from the centre of the Earth, but the mass of the mountain, both above and below, will exert gravitational attraction on the instrument. With land data, these corrections and the latitude correction are used to calculate the well-known Bouguer anomaly. At sea we are on the Earth’s equipotential surface – sea level – so there is no elevation correction, but there is a need to correct for variations in water depth. Furthermore, gravity measurements will vary on a moving object, like a ship or plane, as the movement affects the gravitational force of attraction. Moving against the spin of the Earth increases gravitational attraction, and vice versa. The correction for this is named after the Hungarian scientist Eötvös, and is dependent on the speed of the ship, the direction of travel and the latitude. The latitude and Eötvös corrections combine to give what is known as the Free Air Anomaly, which is often what is reported with surveys over the sea. We will also be applying a Bouguer correction for variations in water depth. Another common correction on land or at sea accounts for the effects of tidal variations, but this affect will be small in the middle of the ocean.

Finally, the instrument will drift a bit due to temperature variations, so repeat measurements are taken at a common spot or measurements are related to an absolute gravity reading at a known spot (these are known as tie points). In our case the tie point is on Tenerife.

To the left is the display output for the gravimeter on the Discovery.

Time is money

We are now firmly in the tropics and making good time to the first site. The wind is at our backs and the sea is still calm. The biggest issue has been problems with our satellite links, which has led to a lack of internet and inability to receive our email and the like.

One of the most expensive items with a ship is fuel, so minimising distance travelled is an important consideration. However, another reason to pick the most efficient route is to maximise the science that can be done. A final consideration is that we need to be back in port for a set date. Hence, there is a lot of discussion about routes and, in our case, the best schedule to pick up the instruments, without spending too much time in the pirate infested waters of the Gulf of Guinea (bit of an exaggeration). Here, Kate and Sai discuss routes on the bridge with Colin, the second officer (mate).

The route we have chosen is shown to the left. We are hugging the coast of Africa in transit to our first station, which is the most northeasterly station. To give you a sense of time, it will take 5.5 days of sailing to get to the first site. An ocean bottom seismometer and magnetotelluric instrument have been deployed at each site (marked by a red triangle). Cutting the diagonal path just above the 15oW, rather than sailing west across the top 4 stations, will save us 12 hours.

Saving time will enable us to survey the Chain Fracture zone. In comparison to the much larger Romanche Fracture
Zone, relatively little is known about the Chain. The deployment cruise on the Langseth mapped part of the FZ using side-scan sonar and swath bathymetry. We are hoping to have enough time to finish the mapping. The proposed swath lines over the Chain FZ are shown, right.



Safety at sea

We are now underway. The ship left Tenerife at 9pm last night and we are heading down the west coast of Africa. Fortunately, the weather is good and we were rocked to sleep by a gentle swell as the RRS Discovery steamed towards the equator travelling at 12-13 knots.

Safety is of paramount importance at sea. Fires or flooding take on a whole new meaning when you can’t simply run away from them.

We have had 2 safety and security talks and one practice drill so far, and will have a lockdown drill in a few days’ time. In our case, a lockdown is in the event of pirates boarding the ship. Should this happen, we lock ourselves in the citadel – a secure location on the ship – which is stocked with food, water and other survival equipment. The ship can be navigated to a safe port from this location. I guess parley is not an option.

First aid is important on a ship, as there is no nearby hospital. It is easy to trip or fall on ship – there are steps up and down all over the place, steep stairways, low-hanging beams, and the ship is continually pitching, rolling, heaving and yawing. I have already cut my shin on a door bolt.

In the first drill, we tried on the lifejackets and learned where the survival suits are located. The latter are donned when you are likely to end up in the sea and not a lifeboat or raft.

The ship is equipped with two lifeboats, each of which can carry over 50 people – here is a picture of us sitting in the lifeboat – it felt a bit like the scene in the film Aliens where the crew are about to drop to a planet in a landing craft.

There are a number of liferafts on the ship, which will self-inflate in the event of the life boats being inoperable. In comparison, a lifeboat seems luxurious.

What is a plate?

Tectonic plates are jigsaw-like pieces of the thin rigid outer-shell of our planet, which move around colliding and sliding past each other. They are comprised of the crust and the outermost part of the mantle, and are somewhat simpler in nature beneath the oceanic parts than they are beneath the continental parts. Plates are a manifestation of the Earth trying to cool itself –  material upwells from the deep mantle and cools forming a rigid shell (a ‘thermal boundary layer’ or plate). We are going to a place on the planet where new plates are formed – the Mid-Atlantic Ridge. New plates – in our case the Nubian and South American plates – rift away from such mid-ocean ridges, thickening as they cool. Based on this idea, the base of a plate can be defined by a given temperature.

Another way of thinking of a plate is in terms of a hard shell overlying the more ductile mantle. The rigid part – the lithosphere – cracks in a brittle manner causing earthquakes, whilst the less viscous part – the asthenosphere – deforms aseismically (without earthquakes). Moving away from a mid-ocean ridge, earthquakes occur deeper and deeper as the plate thickness. This provides a mechanical definition of a plate.

Because the plate is rigid, it will flex and warp in response to bending (e.g. at subduction zones), or as loads are placed on the plate (e.g. oceanic islands like Hawaii). The shape and amount of flexure allows geophysicists to define the thickness of the elastic part – usually called the Effective Elastic Thickness of the plate. This varies from place to place around the Earth.

Another measure of the thickness of the plate comes from seismic imaging. Seismic waves radiating from large earthquakes propagate along the surface of the Earth and are sensitive to the plate and underlying mantle. These seismic ‘surface’ waves propagate faster in the more rigid plate and slower in the underlying asthenosphere. This transition from high to lower velocities defines the seismic thickness of the plate.

A more recent tool for imaging the base of the plate comes from the observations of seismic waves that propagate deep through the body of the Earth (body waves). These are the P-waves and S-waves: P-waves travel faster than S-waves. At abrupt changes in the elastic properties (seismic wavespeeds and density), the seismic waves can change from a P-wave to an S-wave, and vice versa, as they travel upwards toward our ocean bottom seismometers. In the last 10 years, seismic studies have increasingly observed these seismic wave conversions from the base of the plate. This suggests that the bottom of the plate might be much sharper than the thermal models or surface wave results suggest.

A final method for detecting the base of the plate is to use electromagnetic methods to detect changes in the conductivity of the lithosphere and asthenosphere. These methods are particularly sensitive to the presence of fluids, such as pockets of molten rock (termed partial melt). A long held idea is that partial melt may exist in the mantle below the plate, reducing its viscosity and seismic wavespeeds. More recently, it has been proposed that this melt may preferentially accumulate at the lithosphere-asthenosphere boundary, thereby enhancing the seismic phase conversions.

Unfortunately, the thickness of oceanic plates obtained from each of these approaches are often inconsistent. This implies that there are components of the puzzle that we are missing. We need to better understand the distribution of partial melt, the nature of boundaries in the uppermost mantle, and how the minerals, the building blocks of rocks, align in response to mantle convection. The PiLAB experiment will explore these ideas through obtaining an unprecedented seismic and electromagnetic data set from an oceanic plate setting.