Great George

The Bell

Great George arrives

Named ’Great George’, the bell in the Wills Tower is, on one hand, the very embodiment of solidity and strength and a cultural object of high status and historical value, but on the other hand is just another synthesis of rock and iron that is worn by sun and wind and will tremble with vibration from natural forces. If you haven’t already been on the weekly Saturday tour, go, and see for yourself.

Top of the Tower view

Bells have been important in Western and other cultures through the centuries – tolling for celebration, emergency and ritual. The importance of Great George was made clear to all passers by on a day in 1925 when all 9.5 tons of copper and tin (13 parts to 4 parts) was hauled up the 65.5 m (or 215 feet) tall Wills Tower to adorn the then newly built university buildings.

Prison bell chimed for executions at People’s Palace , Glasgow

The oldest bell in Bristol is number 11 or C# in St Mary Radcliffe Church bell and was cast in 1622 by Roger Perdue. It was rung at the time of the Great Plague, the Great Fire of London and the English Civil War and has been peeling over Bristol ever since then.

In our discussions we talked about the sound heard in the air – the Eb from Great George and the 11th bell in St Mary Radcliffe. Those who have the memory of the sound of those two bells in Bristol must run to an incalculably huge number. In regards remembering sound, where sound is a memory, the imagination can take steps back into history. The bells of a city have a gravitas that comes from the collected memory of the listeners. In addition the material of the metal bell holds a history of the ground in its metal, how it was forged, where it was forged and by whom.

Cornish Giant Bolster straddles 6 miles

The Cornish Giant Bolster straddles only 6 miles, as illustrated by George Cruikshank. The sound of giant bell Great George when its’ clapper is used straddles a 12 mile radius (depending on the wind).

Bell – tests in the tower

Tests in the Tower

Wills Tower

With the help of James Wookey and Anna Horleston a seismometer has been set up in the Wills Tower so we can record what seismic or urban activity is vibrating and shaking the bell in the 215 feet tall Wills Tower.

‘A Day In The Life Of Great George ’ could be shown, we thought, with recordings of the seismic activity affecting the bell in the Tower, the sounds heard and recorded in the tower and images could show this.

With the data from the seismometer we could see clearly the vibration of the chimes of the bell affecting the seismometer.

The waveform image showed the chiming of the bell, but in addition to this we could see there were other vibrations. We thought they might be vibrations of the wind on the tower and perhaps from the impact of the urban traffic up Park Street showing up on the image of the waveform.

Can we see evidence of vibration from other continents?

Setting up the seismometer

We are looking to see if seismic activity across the globe would show up in the bell tower seismometer recordings. We have chosen a date and time when there was seismic activity of 6.4 on the Richter Scale – in the sea near Indonesia 25th March 2018.

We are now looking at the data from the seismometer in the tower and comparing it with data picked up in the workshop seismometers down stairs. The seismometers in the workshop I’m told show up more clearly the long distance seismological events.  Taking the date and time of the events near Indonesia on the 25th March, we can compare read outs from the tower seismometer and the workshop seismometer to check if the vibration is evident in the tower wave form – i.e. is the seismic activity from afar affecting the tower and bell and can we see it?

We also set up a camera – a Go-pro – to see what it’s like to experience time passing through the day. Our first tests were every 30 seconds. Next we will try a continuous video and sound recording.

We also set up a camera – a Go-pro – to see what it’s like to experience time passing through the day. Our first tests were every 30 seconds. Next we will try a continuous video and sound recording.

Students from the university working with Alison Rust have measured the reach and strength of the bells’ low frequency acoustic sound waves. More to come on that ….

The Indonesian quakes on gallery screen
Waveform of the bell chiming

The unsettled planet

The Unsettled Planet is a project funded by the Brigstow Institute, that will bring together a diverse group of researchers from the arts, humanities, and science – addressing the Brigstow theme of ‘living well with uncertainty’.  Michael Kendall will be working with Shirley Pegna, whose artistic research and practice is concerned with sound as material, together with Tamsin Badcoe (Department of English), Daniel Haines (Department of Historical Studies) and Lucy Donkin (History / History of Art).

Work has already started on an art and science installation: ‘A day in the life of a bell – a study in sound and vibrations’. Bells have a rich history in announcing natural hazards, such as earthquakes. We will instrument the Great George bell in the Wills Memorial Building with a seismometer, amplifier, camera, and data logger. The bell will record cultural noise in Bristol, but also distant earthquakes from as far away as Japan and other seismically active regions, but with regular punctuation from its own bell clapper.

The shipshape crew of the RRS Discovery

We have been at sea for nearly a month, as the guests of the crew of the RRS Discovery. This group of 23 seafarers operate under the leadership of Captain Antonio Gatti. In general, it has taken me some time to get used to the many nautical terms on the ship, but especially the crew members’ titles and jobs. This is not helped by the fact that there are differences between titles in the UK and USA, and between military and civilian ships. The crew are employees of the research council NERC, and are part of the National Marine Facility at the National Oceanography Centre in Southampton.

There are essentially two pathways of careers on merchant or scientific ships – the ‘officers’ and the ‘ratings’. Officers can study navigation or engineering, and all need to train for 3 or 4 years to get their ‘ticket’, which allows them to get a job on a ship. The Maritime and Coastguard Agency awards the training certificates. The qualification is taken in phases and includes at least 24 months of classroom training at a nautical college and 12 months at sea. At this stage officers can become Officers of the Watch, of which we have three navigation Officers who man the Bridge (Evelyn, the Chief Officer or First Mate, Colin, the Second, and Tom, the Third). To move up to Chief officer takes another set of training and exams and at least 24 months at sea. Finally, becoming Captain (or Master) requires additional training and experience, including an oral exam and at least another 12 months at sea – and of course being lucky enough to find a job opening on a ship. The Captain runs the ship and has final say on everything.

Engineering is another career stream for Officers. The training path is similar to that for the navigation Officers, with a resulting qualification of Engineering Officer of the Watch. There is a cadet in our crew – Taylor – who is midway through his training. Chris is the Second Engineer, but has a budding career in pantomime as Queen Neptune. Angus and Nick are Third Engineers and Tom is the Electronics Officer (pictured at the end of the line in the photo above). These guys seem like they can take apart any part of the ship and put it back together, but are also good to hang around with for spotting wildlife from the front deck. Yesterday, Andy – the chief Engineer – kindly gave us a tour of the ship, showing us the areas that contain the engines, bow thrusters, propellers, drop keel and winches. It is amazing that essentially five people run all of this.

The ‘ratings’ are men or women who work on deck and have trained as an EDH or efficient deck hand, where they learn everything from winch operation to fire safety. A course and 12 months of sea time is required to obtain an EDH ‘ticket’. The deck crew or Able-Bodied Seaman can be SG1 or SG2, depending on experience – all members of this deck crew are the more experienced SG1. Stuart, the Bosun or Boatswain, is also called the Chief Petty Officer and is in charge of the deck crew. Nathan – the Petty Officer on Deck – is his assistant. The scientific Bosun – Johnny Mac – looks after the scientific crew, which is not easy when a bunch of Pollywogs are running around the deck as instruments are coming on board. He generally runs the winches, whilst cursing at us – but in a friendly way. Andy, Simon, Gary and Chris are ABs who seem to be in a constant state of Brownian motion. They help with the winches, pulling our equipment on board, and running the various instruments off cables. The ship is continuously being painted, which starts with removing rust with a needle gun and disc. A member of this group also works in the engine room – Emlyn is the ERPO (Engine Room Petty Officer).

The Purser – Graham or King Neptune – is the Officer who looks after the accounts, makes sure that we have enough food and supplies, runs the bond (where we can buy t-shirts, sweets, etc.), deals with the port agents, and is head of the stewards and catering team. We have two cooks, Peter and Wally – who are both formally trained in maritime cooking. The food has been really good, especially the soups and desserts. Early in the trip they kindly made Kate a cake for her birthday. Kevin and Tina are the two stewards who keeps things clean and in order on the ship and have basic training in food hygiene and maritime Personal Safety Training. There is never a rest day for this group whilst at sea, as we need to eat.

I have been struck by not only how professional everyone is, but how friendly and welcoming the crew has been. They knew our first names well before we knew theirs. Academics can be real prima-donnas, but the crew take it all in stride. Everyone enjoys their job and there seems to be little friction – below is a picture of them on break or a ‘smoko’ as it is known as sea. The atmosphere is a lot less stressful than that in many conventional offices on land. The maritime profession is dominated by men, but the NMF employs more women than most, including a female Captain, Chief Officer, and 2nd engineer.

Crossing the line

This cruise has been a bit unusual in that we transited the equator four times, and, given our proximity to the vernal equinox, we jumped between all four seasons. As exciting as this is from both a geographical and celestial point of view, I hadn’t appreciated the naval significance of ‘crossing the line’. I soon learnt of the seriousness of my hapless transgression when I was summoned to the Court of Neptune to answer to the charge of crossing the equator without King Neptune’s permission.

Those who have previously crossed the equator are known as Shellbacks, those who have not are known as Pollywogs. I was one of 14 Pollywogs on this cruise – 4 crew members and 10 members of the science party. Interestingly, this was also the first time the RRS Discovery had sailed across the equator. We received subpoenas and threatening notes posted on our cabin doors a few nights before the court date. Immediately we formed a resistance party and kidnapped the Master’s (Captain’s) mythical pet cat. The rhetoric flew, with most of us a bit unclear as to what was going on.

At 2pm we scattered to hiding places around the ship, although Tom – one of the officers – was locked in his bathroom at noon. I ‘hid’ on one of the upper aft decks with a drink, cushion and a book to read. In a Keystone Kops fashion, we were each found and thrown in a severely chilled ‘maximum security holding facility’ (aka a locked room). At 3.15 we were paraded out on deck and into the hangar, first past the cadet Taylor, who was strapped to a dolly with a face mask on, looking a bit like Hannibal Lecter.

Neptune’s police kept us in order as the Master and Chief Engineer, dressed in full uniform, prepared to welcome Neptune and his Queen on deck. The Royal Bailiff announced the ‘Queen’ and then the King. Each rose ceremoniously on the lift from the main deck to the hangar, shrouded in smoke and accompanied by suitably royal music (‘You Sexy Thing’ and the theme music to ‘Pirates of the Caribbean’). To say that we were shocked with what we saw would be an understatement.

The Judge then read our charges and we were given an opportunity to argue our innocence. Everyone was found guilty. Amongst my charges was posing as a serious scientist to get a better room – an accusation I could not deny. King Neptune delivered the sentence, usually adding a few deriding comments about the defendant. Most sentences involved kissing or licking the Queen’s navel or the royal couple’s child, a flying fish. Kneeling four in a row, we had our heads partially shaved by the blind (and seemingly drunk) barber.  We were made to eat disgusting biscuits of dubious origin, before being sent to the surgeon. The medical treatment involved a healthy dousing in flood slops from the galley, which managed to end up everywhere, especially in Jake’s boots. In a surprise turn of events, the barber, Ted, was also tried and sentenced, receiving the mother lode of slops. As a final, but welcome, insult we were cleaned with the fire hoses. The King and Queen then returned to the deep on their chariot and, after a long hot shower, I joined a pleasant evening BBQ on the aft deck.

This tradition goes back at least 200 years. There are similar designations for crossing the international dateline, and the polar circles. We missed becoming ‘Royal Diamond Shellbacks’ by 6.6 degrees – this is the rare status of crossing the equator at the prime meridian. The ship’s crew did a fantastic job in preparing for the event, building the set and props and making the costumes. The actors in this farcical pantomime played their parts almost too believably. A truly memorable occasion. I am happy to be a Shellback, and I have the certificate to prove it.

Syntactic flotation: the Scripps way

Today I interviewed the OBS group from the Institute of Geophysics and Planetary Physics (IGPP) at the Scripps Institution of Oceanography, which is part of the University of California San Diego. Scripps has a long history of making OBSs. In the early 1990’s, I was a postdoctoral fellow at IGPP and at that time there were three OBS groups at Scripps.

With early Scripps instruments (e.g. the OBS88), the OBS package was housed in a giant sphere mounted on a tripod. In the 90’s, a decision was made to modularise the instruments, separating the sensor from the recording unit and moving various components into aluminium-alloy tubes. The so-called ‘L-Cheapo’ carried a short-period sensor, and from 2000 onward the equipment was incrementally upgraded to broad band sensors. The instruments we are using deploy a Nanometrics Trillium 240.

The Scripps group is led by Jeff Babcock and is part of John Orcutt’s research group. Scripps is part of the USA ocean bottom seismometer instrument pool, or OBSIP, which also includes instruments from the Lamont Doherty Earth Observatory and the Woods Hole Oceanographic Institute. An advantage of OBSIP is that it has led to common data formats.

The Scripps instruments are a bit different from the LDEO and IPGP instruments in that they use syntactic foam for buoyancy. The foam is made from microscopic air-filled beads set in an epoxy matrix. The Scripps engineers have adopted a moulding technology that helps to make the instrument more streamlined and reduces the manufacturing cost. The bright-yellow foam is rated to depths of 6000m and rises at a much faster rate (50m/minute) than that of an OBS using spheres. The foam is slightly heavier than the glass spheres and is more expensive, but is more durable and has a longer life span. At 500 kg, the Scripps OBS is ~100 kg heavier than the other OBSs on the ship.

Otherwise, the Scripps instrument has very similar components to the other instruments. The clocks, acoustics, and memory storage are the same, and, as with the other groups, they have developed their own data logger. The anchor is a large metal platform, released by a spring and burnwire assembly. Their instruments draw 800 mWatts and are powered by 132 DD-cell lithium batteries, which have a mass of 50 kgs. Power for the strobe light, radio frequency modem and acoustics comes from ordinary alkaline batteries. They can operate for up to 14 months and 6000 m deep. Here is a picture of the OBS lab on the ship and a picture of Martin and Sean, the Scripps engineers, astride their OBSs on the aft deck.

As with the other groups, Scripps is continually looking to improve their instruments. The new ‘Abilone’ has a hydrodynamic shield over the Trillium sensor, which should reduce noise by minimising its profile on the seafloor.

Another aspiration is real-time monitoring, especially as we move to more permanent ocean bottom observatories. The limitations of acoustic communication in water make it difficult to transmit data with a high sample rate. Scripps has been experimenting with using autonomous gliders to relay data back to the shore, a ship, or satellites.

I asked each of the instruments groups what are the biggest challenges they face. As we noticed first hand, inconsistencies with the hull transducers on ships can be a real headache. We ended up circumventing the ship’s internal wiring to get a good signal. The same problem was even worse on the deployment cruise on a different ship. Keeping anything at a depth of over 4000m for a year is a challenge. The syntactic foam is more reliable and lower maintenance than glass spheres, but more expensive. Electronics, cables and connectors will continue to improve. We saw an imploded battery connector and evidence of leaks in a few instruments. Funding constraints in general are a common theme and mean that staffing can be an issue with equipment facilities. This cruise has presented a nice opportunity for 4 different research groups to work together and compare notes. I have been impressed with the hard work and dedication of the engineers working in all 4 groups – and how collegial everyone is.

The wildlife at sea

I have been at sea for over 3 weeks now and have seen very little of anything beyond the ship. As our internet connection is barely working, I am feeling a bit cut off from the outside world. In contrast, I have never been so aware of my immediate surroundings. We are constantly monitoring things like: geographical position; water depth and the nature of the seafloor; temperature of the air and sea surface; air pressure and humidity; relative and absolute wind speed and direction; ship speed and bearing; any evidence of other ships in the area; wave effects, including pitch, roll and heave; ETA to our next waypoint; sunrise and sunset; and even cable length, speed and tension, if a winch is working.

On breaks, I spend a lot of time watching the sea (… or watching episodes of ‘The Office’). My shift starts at 5am, so I see the sun rise every day (07:15), and most people go out to watch the sunset after dinner (19:15). We have seen a few fishing ships and tankers, but also saw the French research ship, the ‘Pourquoi Pas ?”. On a good day, from the Bridge you can see roughly 12 NM or over 22 km.

Any sighting of animals is exciting. On the way south, near the Cape Verde Islands, we often saw turtles. A few birds have been seen and one was even a stowaway for a few days. Flying fish are ubiquitous. A few whales have been spotted, including a pod of pilot whales, one of which came very close to an OBS on the surface. They may have been attracted to the ‘chirping’ noise from the acoustic signal.

Dolphins are great fun to watch. We have seen them racing the ship off the front bow, playing in the surf. Today we saw 40-50 frolicking in front of the ship, while we were stationary trying to communicate with an OBS on the seafloor. Their clicking and squeaking was clearly audible from the acoustic box in the main lab.

At night time, when we are running the sound velocity profiler (SVP) off the winch, squid and fish are attracted to the lights. We have seen squid up to a metre in length.

But, probably the most common sighting is a duck. The Brunel duck is the mascot of the Bristol Geophysics Group and helps with field work wherever we go. It has played a key role in navigation, marking our progress across the instrument array. Not content with this function, the duck volunteered to help with the SVP survey. A pinhole was drilled in its tail – a pintail duck, perhaps – and the duck was tie-wrapped to the SVP. It travelled to a depth of 1000m and returned unscathed. The duck also helped retrieve one of the magnetotelluric instruments – here is it attached to the grappling hook.

Fishing for OBSs

Sometimes things don’t always go as to plan. We arrived at site I04D at 15:00, quickly establishing communication with the OBMT and the OBS. The OBMT was on its way to the surface with an ETA of 17.50 – all was going well. After surveying in the location of the OBS and sending an acoustic signal for the instrument to release its anchor and start rising, things took a turn for the worse. The OBS showed no sign of rising. After waiting 3.5 hours for some sign of motion, we moved to Plan B.

Because the instrument was responding to the acoustic pinging, it could be located to within 25m. The ship positioned itself east of the instrument location and a 500kg anchor was attached to a winch with 7000m of cable. The weight descended nearly 4000m to the sea floor – after 75 minutes, a slight release in tension indicated that the weight had reached the seafloor. The ship then steamed at 0.2knots 200m south, 200m west, 200m north and 100m east, letting out another 1500m of cable. The ships route is the white line shown in the figure above. The whole process took over 3 hours. The idea was to drag the cable across the seafloor, hopefully nudging the OBS into motion. The plan seemed a bit of a reach – literally – we were trying to lasso a 1m wide object that lay 4000m away with 5500m of rope.

The winch was locked and the ship steamed northward at 0.2 knots. This sort of manoeuvre is a bit tense as the cable can snag on a rock outcrop and get stuck – the cable tension was monitored carefully. Throughout the process the OBS was monitored for any sign of motion. Some 30 minutes into the procedure, Simone cautiously muttered “I think it is rising”. It is always a bit difficult to confirm lift-off when the ship is moving, but eventually the depths were shallower than the water depth, and a sense of jubilation swept through the lab.

Some 90 minutes later the OBS arrived on deck, joyful and triumphant. By 2am the cable had been retrieved and we were en-route to the next station. The entire process had taken 11 hours, but another station – and more data – had arrived. A post-mortem revealed that the burn-wire had been removed, but the spring that holds the anchor had failed to release. The cable must have jostled the instrument enough to release it. Below is a picture of the relieved IPGP group and their instrument.


France has a rich history of working in the oceans, collecting some of the longest tidal records available, for example. During our voyage, we happened to pass the French research ship the “Pourquoi Pas?” off the west coast of Africa. The Institut de Physique du Globe in Paris (IPGP) runs geophysical observatories that monitor earthquake and volcanic activity both on land and in the oceans. Their OBS group – led by Wayne Crawford – is a National Equipment Facility. They have provided 9 instruments for the PiLAB experiment. IPGP has been a key player in broadband ocean bottom seismology, providing some of the earliest broadband instruments in an experiment led by Jean-Paul Montagner at 23oN on the Mid-Atlantic Ridge in 1992.

The IPGP sismomètre fond de mer (OBS) is based on the design developed by Scripps. Their instruments use a Nanometrics Trillium 240 broadband sensor. Below (left) is the sensor without its protective sphere, and to the right is one of the electronics busses, which include the CPU, the internal clock and hard-disk. Bristol owns 6 similar instruments for land stations. In comparison to the Compact in the LDEO instrument, the ‘240’ consumes more energy and has a slightly higher profile, which means that it is more sensitive to noise caused by ocean currents. But, these instruments are much more sensitive at longer periods. There are three primary uses for the long period signals that the 240s record: (1) surface wave studies, (2) ambient noise interferometry, (3) seafloor compliance measurements.

Seafloor compliance is still a relatively new way of measuring the shear modulus structure of the oceanic crust. It is particularly sensitive to variations in porosity in the shallow crust and the presence of melt in the deeper crust. The technique was first developed in the 1980’s by Japanese scientists, and in the early 1990’s Wayne and colleagues adapted the method to the deep ocean. The signal comes from very long period gravity waves that travel across the oceans. The amplitude of these waves is only a few millimetres and the results seafloor deformation is a few micrometres. Such studies are particularly useful near seafloor spreading centres, where melt and hydrothermal fluids control the generation of new oceanic crust.

Surface waves are the longer period seismic phases that propagate along the surface of the Earth and decay in strength exponentially with depth. They are particularly good at resolving variations with depth in the velocity structure of the outermost few 100s of kilometres of the Earth. To the right is a seismogram of a large earthquake in Chile, recorded on one of the IPGP instruments nearly 8000km away. The first obvious signal is the P-wave arrival, followed by S-wave signals. The large and long duration signal at the end of the seismogram is the packet of surface wave energy.

Seismology has seen a bit of a revolution in the past decade in the use of seismic noise as a signal to image the Earth. Rather than fighting to get rid of noise, this technique embraces it, looking at the correlation between noise at stations to infer the seismic signal between stations. In its simplest application, so-called ambient noise tomography inverts the correlated noise structure for Earth structure, using techniques similar to those used in the analysis of surface waves.

Above is an IPGP OBS as it comes aboard and the entire IPGP team, Wayne (scientists and facility director), Simone (technical engineer), Océane (PhD student) and Michaela (Masters student). Their instruments can be deployed for up to 13 months and at depths up to 5000m. Flotation comes from 8 flotation spheres, 33cm in diameter, and they rise at a rate of roughly 31m/min, which means that it takes a little over 2 hours to rise from a depth of 4000m. The anchor is a large metal grate that attaches to the bottom of the instrument. They are next being used in the Mediterranean as part of a large European experiment focused on the Alps.

Zen and the art of magnetotelluric measurements

PiLAB is a unique experiment in many ways, one of which is that ocean bottom seismometers and magnetotelluric instruments are co-located. This allows the comparison of two very different physical properties of the plate and underlying mantle – elasticity and the conductivity – which together offer insights into the nature of tectonic plates, how they form and how they evolve. Today I talked with Jake, Chris and Dan (L to R) who have come to recover the magnetotelluric instruments. Jake and Chris are the research technicians from Scripps. Dan recently left a postdoctoral research position at Scripps for a position at IGNS in his home country New Zealand. This cool-headed group ensures that the instruments are working well, but also offers life-supporting coffee every morning. This group is the brain-child of Steve Constable at Scripps.

Geophysicists have developed a diverse set of techniques for measuring electrical conductivity in the Earth. These techniques are used in a wide range of applications such as mineral exploration, archaeological investigations, detecting military ordnance and munitions, and ground water studies. With PiLAB we are interested in the conductivity structure of the crust and upper mantle beneath the seafloor. Like most geophysical tools, electromagnetic techniques can be divided into active and passive methods. With active methods, electrical and magnetic fields are human-made, where passive methods exploit naturally occurring electromagnetic fields. The Ocean Bottom Magnetotelluric (OBMT) instruments deployed in this experiment measure electrical and magnetic fields induced by natural phenomena.  Solar radiation interacting with charged particles in the ionosphere induce a magnetic field in the Earth’s subsurface, which in turn induces electrical or telluric currents. Lightning strikes also work in a similar manner, but at higher frequencies. A secondary magnetic field generated by these telluric currents can be detected using sensitive instruments comprised of coils and electrodes. The strength of the induced field and its phase provide information about the depth and strength of the conductive body. Making such measurements on the sea floor is especially difficult, as sea water is highly conductive and acts like a high-frequency filter. It takes a patient operator – and a precision instrument – to record electromagnetic fields in the solid Earth under these conditions.

The Scripps Institution of Oceanography, which is part of the University of California San Diego, started making ocean bottom magnetotelluric measurements in the 1960’s. Chip Cox and Jean Filloux pioneered this work and we are benefiting from their legacy. Through the 80’s and 90’s a number of institutions, including Scripps, WHOI, Tokyo, Southampton, Cambridge, Toronto, Adelaide, and others, continued to develop both MT and controlled-source EM instruments. More recently, interest from the oil industry in controlled source electromagnetic methods has led to improvements in instrumentation and has made active-source sea floor EM measurements more routine. EM methods are well-suited to imaging conductive bodies that lie beneath more resistive salt, carbonate or volcanic layers. These layers can cause imaging problems with conventional seismic reflection surveys.

This generation of long-term passive deployments started in 1996 with the MELT experiment on the East Pacific Rise. Significant improvements came from innovations in reducing power consumption, improving the signal to noise, and general advancements in electronics. But equally important has been advances in the mathematical treatment of the data and the inversion for 3D conductivity structure – a non-trivial problem. Scripps now has a pool of over 150 MT instruments.

The current Scripps OMBT instruments look like giant 4-legged crabs. Silver-silver chloride electrodes sit at the four ends of the arms – 10 m apart. Two magnetic coils cross-cut the central unit and are wrapped around something called mu-metal, a nickel-iron alloy used in magnetic shielding. The original use of mu-metal was to improve communication in submarine telegraph cables using inductance. The signal from the coils needs to be amplified and the induced electric field is also measured simultaneously. Unlike the OBSs, most of the power in an OBMT is used for data logging. Power comes from 40 alkaline D-cell batteries. These and the electronics are housed in an aluminium-alloy tube (below, left). This is a high-strength, but light, alloy and is commonly used in aircraft components. The tubes are evacuated, and desiccant is used to absorb any moisture.

It is important to know the orientation of the electrodes and coils. Unlike the OBSs, the MT instruments are equipped with compasses. Once a day a Honeywell electronic compass (above right) logs the instrument heading, roll and pitch. It is mounted high up on the frame, to be clear of any magnetic parts of the instrument. One of our stations has shown changes in orientation, suggesting that it landed on a slope and moved a bit with time.

The OBMT clock is similar to that used in the OBS stations, as are the acoustics for communicating with the instrument. The acoustics have slightly better firmware, in that an operator can continue to communicate with the instrument during the burn and release sequence. A 120 kg anchor is made of a concrete slab reinforced by a stainless-steel frame. It acts as a steady 80cm x 80cm base for the instrument to sit on, and is nearly 9cm thick. Hooks from the frame attach it to the instrument in such a way that the burn wire and a spring mechanism release the instrument from the frame when it receives an acoustic signal to rise. As with the LDEO OBSs, buoyancy comes from evacuated glass spheres, which are 30cm in diameter. Some of the instruments have slabs of syntactic foam (more on this later) on them, which adds further buoyancy, but cannot be used in depths greater than 4km. They rise at a rate of 20-25 m a minute, so at some of our very deep sites we wait well over 4 hours. The deepest they can be deployed in the ocean is 6km.

Of the five spheres, one serves as a beacon that floats a short distance from the main instrument package. It contains a strobe light, a radio frequency modem, and a GPS transmitter, which are used to find the instrument when it pops up at the surface. When the instrument is near the ship, a plastic grappling hook is thrown over the tie line between the instrument and the beacon sphere. This is used to pull the beacon out of the water and to pull the instrument toward the ship so that the crane can hoist it out of the water. Here is Kate throwing the hook, and then a shot of the OBMT on the end of the crane hook.

These are the longest deployments these instruments have seen, both in terms of recording time and time on the sea floor. This should mean that these instruments will probe the conductivity of the Earth well into the sublithospheric mantle.