UNIVERSITY OF CAMBRIDGE
DEPARTMENT OF EARTH SCIENCES – BULLARD LABORATORIES
Early Marine Refraction Seismology and Development of the Cambridge Radio Sonobuoy System
An account of the technology, instrument development and equipment deployments 1937 – 1980.
M Mason Nov 2016
BAGS Ref. 0465 Copyright 2016
Refraction seismology techniques are used to produce seismic velocity profiles of geological strata, which allow scientists to determine the characteristics of the various rock layers comprising the Earth’s crust. Seismic refraction methods had been used on land for some time but by the mid 1930s scientists needed to extend their studies to beneath the oceans and sea bed. In 1937 Teddy Bullard visited the USA and learnt from Maurice Ewing how seismic refraction had been used in a marine environment to determine the thickness of sediments on the continental shelf and he was encouraged to do similar work in the UK. Typically, sea bed layers such as limestone sediments have velocities of 2-3km/sec whilst deeper layers such as basalt and granite have velocities of 5-6km/sec and the Earth’s outer mantle 7.8 – 8.1km/sec. Using this method, layer thicknesses and slopes can also be determined.
The Technique: An explosive charge is detonated, resulting in refracted seismic waves travelling in the rock layers, which in turn are detected by geophones at some distance from the explosion point. The shot instant and geophone signals are recorded against an accurate clock to produce a typical marine seismic refraction record, illustrated below. This illustration shows that following the explosion, the low frequency ground wave is received first because it travels at a faster speed in the rock layers, followed by the slower direct high frequency water wave. At greater range ground waves from the deeper faster rock layers will overtake those travelling in the upper layers and arrive at the geophone first. From these record timings a travel time plot and seismic velocity profile is produced.
Fig. 1 Seismograph record
Equipment Required: Marine refraction seismology had not been done in the UK before so the equipment required for such a study and the logistics of deployment posed a real challenge. Various pieces of equipment were needed for the work, including a geophone housed in a watertight case to detect the sound waves, a six-string photographic galvanometer recorder to record the geophone signals and a radio transmitter and receiver to send and receive the shot detonation instant. A timing trace was produced by a reed-driven valve maintained 100Hz tuning fork. Equipment designed and built by Leslie Flavill for land work in 1936 was to be used for the marine work. Testing and preliminary experiments were done in Lake Windemere, the Wash and Plymouth Sound, which proved valuable in enabling improvements to be made.
Fig. 2 Recording Equipment left to right: Receiver, Galvanometer Recorder and 4 channel Amplifier
Fig. 3 Leslie Flavill testing the transmitter / receiver at the lab
Fig. 4 Circuit of transmitter/receiver
In July 1938 use of the survey ship HMS Jason was granted to carry out a seismic refraction experiment in the English Channel. The recording equipment was installed on HMS Jason and Jason’s launch The Stork was used to fire the shots. Transmitters and receivers on both vessels were used for sending and receiving the shot instant
The photographs below show HMS Jason (850 ton) and ship’s launch the Stork. Leslie Flavill can be seen sitting at the aft of the Stork with a pair of binoculars; the transmitter/receiver aerial wire (faintly visible) is rigged from the top of the mast.
Fig 5 HMS Jason
Fig. 6 The Stork
The Geophones: Geophones mounted in watertight cases with a heavy base plate were used as the ground wave sensors. Initially three geophones were deployed, equally spaced in a line on the sea bed, but two were soon lost through difficulties in keeping the ship on station; thereafter only one geophone was deployed. The geophone was held approximately 6ft off the sea bed and only lowered when the shot was to be fired. During the record time window cable was paid out as the ship drifted to stop the geophone being dragged on the sea bed. The photo shows Tom Gaskell preparing to lower the geophone to the sea bed looking more suitably dressed for a day at Henley Regatta!
Fig. 7 Geophone used in 1938 (overall diameter 10 in.)
Fig. 8 Tom Gaskell on Weather Explorer
Explosive Charges: The charges were made up of 11/4 lb blocks of TNT with a primer and electrical detonators, which were lowered to the sea bed; a maximum charge size of 60lb was only achievable due to the handling limitations of the Jason’s launch. Initially the shot instant signal was generated by a radio interference spike caused by the shaking of the Stork’s aerial; however it was very difficult to distinguish with certainty against general background radio interference. This method was abandoned in favour of a signal generated by the breaking of a signal wire wrapped around the charge which was included in the modulating circuit of the transmitter.
Fig. 9 Explosive Charges on Jason’s Deck
Range Measurement and Timing: The distance from explosion to geophone was measured roughly by taking the mast-head angle of the recording ship from the explosives boat. The travel time of the direct water wave was also used, although this was not always as clear as was hoped. Timing was by means of a power-maintained 100Hz tuning fork producing a time mark on the geophone record. Shown for illustration purposes this photo shows a power maintained tuning fork time mark generator from a later recorder. A contact mounted on one fork
Fig. 10 Timing System ( tuning fork is at the bottom)
switches power to the electromagnet mounted between the forks in order to maintain vibration. A further contact on the other fork provides a power drive signal to a vibratory converter (centre left) which in turn drives a synchronous motor. Mounted on the motor is a disc with slots and a lamp beneath that produces time mark flashes projected onto the record.
The Jason experiment was repeated in June 1939 using two 60ton Brixham sailing trawlers – the Renown and the Arthur Rogers. The plan was to extend The Jason line further west over the continental shelf and fire larger explosive shots from the Arthur Rogers. It was hoped that less time would be lost through bad weather and vessel limitations, but in the event there were no significant gains in this respect.
Fig. 11 The Renown
Fig. 12 The Arthur Rogers
Several improvements were made to the equipment, deployment method and explosive shots.
Geophone: A new geophone was designed that could work in either the upright or inverted position and was less noisy, which allowed greater amplification of the signals. Most significantly the dynamic range of signal amplitude recording was increased by shunting one galvanometer to reduce the sensitivity to 20% of the other so that near range large amplitude signals were recorded without loss of waveform.
Fig. 13 New Geophone Configuration.
Direct Wave: Much clearer records of the direct water wave were achieved using a hydrophone lowered over the side of the Renown to a depth of 20ft.
Explosive Charge: This time blasting gelatine was used, being less expensive than TNT. It was placed in a metal tube packed tight with a clay plug. Greater ground motions were observed but charge size was limited to 160lb so as not to cause damage to the Arthur Rogers.
Transmitters and Receivers: New crystal stabilised equipment working at 7.0 and 7.3MHz with duplex telephony was a great improvement over The Jason arrangement
Fig. 14 Ben Browne – charge preparation
Fig. 15 After the shot
Fig. 16 Teddy Bullard at the helm of the Renown
Signal Comparison between Hydrophones and Geophones
World War 2
As war loomed no further marine refraction work was possible.
After the war further progress to deeper waters was problematic due to the difficulty of deploying geophones and explosive charges on the sea floor. It was suggested that it might be possible to hang instruments from the sea surface and detect the seismic waves travelling from the bottom into the water. Observations during the war showed that seismic waves could in fact be observed in this way. In 1946 with the loan of equipment and M.V.Sabella from the Marine Biological Association at Plymouth and the Admiralty Mining Establishment, Maurice Hill and Pat Willmore set up a test to compare the signals from suspended hydrophones and bottom geophones. Charges of 1-2lb of ‘Polar Geobel’, fired 100ft below the sea surface gave satisfactory results. Initially charges were suspended under a buoy and electrically fired using a clockwork timer and the shot instant was transmitted via radio back to the recording ship. This method was cumbersome, needed elaborate safety precautions and the shot instant could not be accurately controlled. Also the free moving sub-surface explosive charges could be dangerous to other shipping. With a hydrophone suspended from the ship, subsequent shots were fired via a trailing cable with floats every 100yd, but these were limited to short range due to cable handling difficulties. Difficulties also arose from the transmission of mechanical disturbances down the geophone and hydrophone cables. For the geophone this was solved, as before, by lowering it to the sea bed just before the shot, allowing the ship to drift and paying out the cable to prevent the geophone from being dragged on the sea bed. For the hydrophone this was overcome by providing some positive buoyancy and attaching a sinker weight 30ft from the end of the cable to decouple the ship and wave motions. Results were encouraging and there seemed to be no reason why the hydrophone deployment method could not be used for deep-ocean work.
Birth of the Sono Radio Buoy (later known as the Radio Sonobuoy)
For long-range work it was clear that explosive charges, especially for safe working practice, would need to be fired under control from a ship and perhaps the hydrophones remotely deployed in some way. After the war ships were in short supply and even research vessels such as Discovery II were unavailable for scientific work as she was being used for restoring the country’s coastal navigation systems. Faced with this shortage of suitable vessels to carry out scientific work Maurice Hill came up with the idea of single-ship shooting using remote radio buoys with hydrophone detectors. Furthermore, an increase in the number of remote stations would considerably increase the data per shot and significantly improve the interpretation quality. Given the results of previous work on hydrophones, Maurice started a programme of sono radio buoy development: the hydrophones would be suspended beneath surface buoys and a radio transmitter within the buoy would send the seismic signals back to the ship.
After a series of tests and design modifications four buoys were built to the following design:
Hydrophone and Buoy Amplifier: A pressure sensitive double-quartz crystal element with preamplifier, mounted in a watertight tube, was hung 150ft below the buoy. The hydrophone had the same wave decoupling arrangement as previously described. The preamplifier output was fed to the buoy amplifier, which had a dynamic range ratio of 1:500.
Radio Transmitter: Each buoy had an FM transmitter individually tuned to a frequency channel working at the 42MHz range. The radio signal was connected to a 20ft centre-fed dipole aerial mounted on top of the buoy.
Power Supply: Power was provided by a 12volt car battery in a watertight case attached to the bottom pole of the buoy. A DC to DC rotary converter in the buoy produced the high tension (HT= 200volts) power needed for the thermionic valve circuitry. Battery power consumption at 10amps gave a deployment duration of 5 hours.
Fig. 18 Hydrophone Element
Fig.19 Valve Preamplifier (2 x EBC41) – Watertight Tube
The hydrophone end-cap pressure seals were fondly known as ‘Pickle Bottle Bungs’ consisting of a rubber disc sandwiched between two aluminium discs. Four screws are used to tighten the aluminium discs together, squashing the middle rubber disc around the cables and against the pressure-tube walls to form a watertight seal.
Radio Receivers: Four ex-American Army Transmitter-Receivers with slight modification, powered by dry batteries, were used to receive the buoy signals.
Recorder: Initially equipment from previous experiments was used. However a new 8-channel recorder (archive number BAG 0001) shown in this figure7 below was constructed for this work. A detailed description of this recorder has been produced.
Fig. 20 Buoy schematic and Ship’s apparatus schematic
Fig. 21 8-Channel Recording Oscillograph
Fig. 22 sonobuoy stabilisation
Fig. 23 Rigging 0f Sonobuoy
Fig. 24 Weather Explored
In August 1949 an experiment was carried out to determine whether seismic refraction methods developed over the previous three years could be successful in deep water. The area chosen was the Atlantic Ocean, with a water depth of 1300 fathoms. Facilities were provided by the Air Ministry for the work to be undertaken, using the Ocean Weather Ship ‘Weather Explorer’ on one of her routine voyages. Four Sono Radio buoys were deployed, equally spaced in a line approximately three miles long, each buoy
having a quartz hydrophone suspended 150ft below the surface. Depth charges set to fire at 900ft were used as the sound source. A range of 20 miles was achieved before radio reception was lost and two rock layers were detected below the sea bed. The experiments also included seismic reflection shooting using 11/4 lb TNT energy source charges and a hydrophone hung below the ship.
Fig. 25 Buoy recovery
Fig. 26 Maurice Hill with Sono Radio Buoys
Limitations of use: Bad weather with winds of more than 6 knots was the main factor limiting use of the sono buoys, particularly in launching and recovery, but also in the generation of spurious hydrophone signals. John Swallow tried to improve the hydrophone signals by removing the heavy buoy components (namely the battery, rotary HT converter and hydrophone amplifier) and installing them in a separate watertight container to be deployed with the hydrophone at a depth of 200ft. The idea was that this would act as a sea anchor and further decouple the hydrophone from surface wave motion. It gave promising results, if a somewhat unwieldy arrangement to deploy.
On 1st May 1950 HMS Challenger sailed from Plymouth on a 3-year world surveying cruise with a set of ‘Hill Buoys’ on board. Over the next three years 50 refraction lines were completed by John Swallow and Tom Gaskell with Maurice Hill and John Cleverly joining the team at various stages of the cruise.
Fig 27 H.M.S. Challenger
A Novel Way of Deployment: In the very shallow waters over the Seychelles Bank and atolls of Funafuti and Nukufetau a different method of deployment was adopted. The recording was done on the ship moored at anchor. Using the ship’s motor boat the buoys were towed away and moored in 100 fathoms of water, using piano-wire and sinker weights. The motor boat was then used to electrically fire the charges and the shot instant was transmitted back to the ship for recording. By using the high tension voltage of the motor boat transmitter to fire the charges and heterodyning the received carrier to produce an audible note, a clear firing mark was obtained. The buoys remained in position but, with each mooring taking 40 minutes and the short operating duration of the buoy, this method was always going to be limited to fairly shallow waters.
Development of a new set of Small Lightweight Buoys.
In 1954 a new set of buoys were produced primarily for short range work and weighing a 1/5th of the weight of the old buoys at just 50lb, were much easier to handle. Using dry batteries, giving a working deployment life of 24hours and a radiated power of 3watts, a working range of 12-16 miles was achieved, although even at this range it is not possible to detect the deeper crust layers.
Fig. 28 New lightweight buoy
FM transmitters and receivers: Using tried and tested designs from previous systems these worked in the frequency range of 40MHz and 47MHz. The transmitter design used thermionic valves throughout with a frequency modulation of +/- 125KHz deviation. The ship’s receivers had individual tuning and signal strength meters.
Hydrophone: A new hydrophone was developed using a four-section barium titanate piezoelectric transducer encapsulated in ‘Araldite’, which increased the sensitivity by approximately four times. A three-stage amplifier using DF66 miniature valves powered by 1.5v and 30v batteries provided impedance matching to the buoy amplifier. The neutrally buoyant section was also improved by adding a large float to the hydrophone and increasing its length to 100ft.
Fig 29 Hydrophone with pre-amplifier
Fig 30 The rigging arrangement of the buoy showing the neutrally buoyant hydrophone section.
Fig 31 sonobuoy equipment
The photo above shows equipment displayed at the new lab opening in 1967. Left: the internal electronics of the sono-buoy. Centre: a prototype recording buoy. Right: the NEP UV photographic paper 12-channel galvanometer recorder.
Fig 32 Buoy recovery – John Cleverly with back-up grapple iron.
The Francis Recording Buoy
The 1954 buoys were continuously used during the 1950s for marine refraction work by the Department. It was clear from the start that they had limited operating range and were not capable of producing a complete velocity profile of oceanic crust, unless by good fortune of shallow geological depth coinciding with excellent radio propagation. The single-ship system developed at Cambridge had great advantages, so it was highly desirable to extend the operating range in order to obtain data from the deeper strata.
In 1960 using some of the existing designs, Tim Francis developed a long-range buoy that could, when the buoys were beyond radio range, record the arrivals within the buoy on a multi-channel film recorder. This concept was groundbreaking and over the next 15 years internally recording sonobuoys were to be continuously developed and improved. The Francis buoys, built in 1961 and first used in 1962 on a Discovery II cruise achieved results to a range of 57 nautical miles. A crystal oscillator clock was added before the buoys were used again on the International Indian Ocean Expedition in the autumn of 1963.
Fig 33 & 34 RRS Discovery II
Description: At short range the buoys operated in the same way as the small radio sonobuoys transmitting data back to the ship as well as recording internally on a Woodhill film recorder. This was useful in confirming that once launched the buoys were working. It was also used to sound range when retrieving the buoys. Two Francis buoys were produced: cylindrical welded steel cans measuring 1.5ft diameter and 2.5ft in length were constructed to house the internal apparatus. A 10ft stabilising pole with 10lb lead weights was fixed to the bottom of the buoy can and a 11ft fibreglass whip dipole radio aerial to the buoy lid. Weighing 180lb total weight the buoys floated with 9in of freeboard. A car-type tyre valve was mounted in the lid and the buoy pressurised to a few p.s.i. (pounds per square inch) as an additional measure to keep the buoy watertight. Most of the electronic circuitry used transistor technology; however the hydrophone still required a high impedance thermionic valve preamplifier.
Fig 35 The buoy electronics, with the film recorder mounted at the bottom of the frame.
A detailed description of a Woodhill film recorder (BAG archive no. 0008), configured for mounting in a pressure tube, has been produced. The two Francis buoys were named after female members of the department: Kathleen Phillips and Sue Chappell.
Fig 36. One of the Francis Buoys.
Controlling the recorder:
A relatively high-power radio signal in the 3MHz range amplitude, modulated at 800Hz, was transmitted from the firing ship to control the recorder. Modified Pye model Q4 domestic receivers were used to receive the control signals. The recorder remained on whilst the 800Hz modulation was received, which was also
recorded onto the film record. Short interruptions were used to mark the shot instant together with a Morse coded station number. The recorder switched off 10 seconds after the 800Hz modulation ceased, thermal relay interlocks prevented spurious signals switching the recorder on or off at the wrong time.
Internal Record Time Markers: Initially this used a Smiths Electric car clock with a pin attached to the balance wheel operating a contact to produce pulses every 0.2sec. For the Indian Ocean work, an additional digital clock using a 10KHz crystal frequency source with synchronised multi-vibrator dividers was also used. Both clocks were recorded on the film record.
Ship Recorder: Time marks, shot instant and buoy signals were recorded on a 12-channel UV photographic galvanometer recorder made by NEP (New Electronic Products Ltd.)
Pictures from the Indian Ocean Expedition on board the new RRS Discovery:
David Davis operating the radio receivers.The NEP recorder is on the right.
Discovery, painted white for tropical operations, on test trials off Aberdeen.
The Woodhill recorder removed from the Buoy
Tim Francis and Bob Whitmarsh man-handling the Francis buoy after recovery.
Expanding Technical Team at Geodesy & Geophysics
The author joined the Department as an electronics technician in January 1966 and Tim Owen was appointed to the new post of Technical Officer in September 1966. At this time the Assistant Staff numbered six with Leslie Flavill as Chief Assistant.
A New Telemetry System
In 1964 Birmingham University Geophysics department had engaged G&E Bradley of London to design and construct a set of six radio telemetry sonobuoys, based on the Cambridge system, for use in Antarctica. In June 1964 David Davies and John Cleverly joined Birmingham Dept of Geology on board H.M.S Egeria for trials of the Bradley gear in Cardigan Bay to make an assessment of its operation. As with all new designs, modifications to the equipment were needed and once these were implemented it was agreed that the equipment could be a satisfactory replacement telemetry system for the Cambridge buoys. After the Indian Ocean Expedition it was decided that the Cambridge short-range and Francis sonobuoys should be upgraded using the Bradley technology for ease of use and improved performance.
In 1966 Cambridge placed an order for Bradley units, which consisted of six radio transmitters, six hydrophone amplifier sub-carrier generators and a 6-channel ship receiver and demodulator rack.
Fig 41 The Ship Equipment
Fig 42 The Buoy Units
The Buoy Electronics: The hydrophone signals frequency modulated a 3.375KHz sub-carrier, which in turn amplitude modulated the transmitter RF with a radiated power of 700mW working in the 27MHz band. The hydrophone amplifier was the first piece of equipment used in the Department to utilise integrated circuit technology, namely a uA702C operational amplifier. Both the transmitter and hydrophone amplifier sub-carrier generator electronics are housed in aluminium boxes easily mounted within the buoy. Lower power consumption of the buoy system electronics enabled a significant improved deployment duration of seven days.
Radio Transmitter: Six channels operating in the 27MHz band with 25KHz channel spacing and radiated power of 700mW.
Hydrophone Amplifier & Sub-Carrier Generator: The amplifier gain could be adjusted via a front panel switch, the output of which modulated the sub carrier generator by +/- 20%.
The Ship Equipment: This consisted of a rack of six receivers with RF signal preamplifier which in turn is connected to a rack of eight demodulators that reconstituted the hydrophone signals for recording onto the UV paper recorder.
The Bradley Buoy. Since there was commercial interest Bradley were keen to produce a complete sonobuoy package and designed a buoy using a plastic barrel manufactured by Harcaster using the Cambridge hydrophones. Tim Owen and Bradley Ltd participated in a cruise, using the Companie de Generale du Geophysique vessel Andremede in the English Channel to test the Bradley buoy. This resulted in very poor radio signals due to bad earth coupling to the sea. As a result Cambridge decided to use the existing small lightweight steel buoys with added earth braids, which gave reliable radio reception of up to twenty miles.
First scientific use of the new Bradley Telemetry System,
also known as
The Assab Story
The Revamped Buoy System: Following the equipment trials in the English Channel, four short- range and the two Francis buoys were fitted with the Bradley electronics. The Francis buoy film recorder control was redesigned to operate on acoustic command (small explosive shot) in place of the radio command system.
A new way: Since the war, refraction buoy deployments had always been from well equipped comfortable research ships, scientific work usually being shared between the groups on board, which limited time for any one project. Since no research vessel was available Cambridge was keen to test if a survey could be done from other suitably equipped ships using portable scientific kit, installed on board, which would also allow concentrated efforts on a single programme. Such an opportunity arose in 1967 to charter a ship and use the new Bradley system for an urgently needed refraction survey in the Red Sea. Under the leadership of David Davis the Department chartered the MV Assab a 94ton Ethiopian registered cargo vessel more commonly used to carry salt from the Massawa sea-water evaporation beds to Aden. The scientific team consisted of David Davies and student Clive Tramontini with Tim Vertue and Mel Mason providing the technical support. Clive’s brother Mike Tramontini who was employed as a navigation officer on Shell super-tankers was employed to manage ship operations, do all the navigation for the survey and also look after medical requirements. The Assab had been previously used by Gulf Oil, who gave much help and advice in preparing the ship for our use. Lab and accommodation containers were welded to the deck, an explosives shute fitted at the stern and an extra inflatable lifeboat purchased. As there was no radio, Gulf Oil loaned and fitted a VHF radio telephone. There was no navigation equipment apart from the standard binnacle-mounted magnetic compass, which Mike found to have an error of 20 degrees! After adjustments this was reduced to an acceptable 2 degrees. All navigation was done using a sextant, navigation tables and slide rule, a mammoth time-consuming task not only in determining the ship’s course but also the buoy deployment and retrieval positions. Ships’s speed was measured using a spinning ‘Walker Log’ deployed over the stern with an inboard dial readout (much like a car speedometer). All food and bottled water for the two month trip was purchased in Massawa where as well as hiring a cook (Osman) and two scientific helpers (Ali and Farah) were also hired. So now that the ship was fitted out, provisions stowed in the hold and personnel on board the Assab left port for Port Sudan where all equipment and explosives had been shipped from the UK.
Fig 43 MV ASSAB
The Assab had four engines each driving a propeller, a somewhat unusual arrangement. However, despite this potential driving force, it could only manage about six knots speed to Port Sudan which meant that changes to the programme had to be made. Mel Mason would stay for the whole cruise, saving the time needed to sail to Eilat for a planned personnel changeover halfway through the programme. Six days were spent in Port Sudan taking onboard equipment and 15tons of explosives which were efficiently manhandled from a barge by a row of chanting coolies. Assab left Port Sudan for the work area on 7th December.
Shark Attack: On one occasion a hydrophone was lost during deployment, with the resulting bare end of the cable having blood stains. The conclusion was that it had been taken by a shark. Perhaps the reputation of aggressive sharks in the Red Sea is not a myth after all. Alarmingly the cable had wound around one of the ship’s propellers and Mike very bravely dived to untangle it while the rest of us looked out for any sign of sharks. What we would have done if one was spotted is anybody’s guess!
Engine Failure and Discomfort: On another occasion three of Assab’s engines failed, thank goodness it had four, which allowed us to limp back to Port Sudan for repairs. Sailing Port Sudan on Christmas Day (into force 6 seas at 4 knots) not all scientists fully appreciated turkey and Christmas pudding. The general absurdity of the speed of Assab, coupled with a remarkably high output of noise and dirt, was beginning to be more than just an inconvenience. This second leg of the cruise was to complete long refraction lines using the two modified Francis buoys. No sooner had the buoys been deployed than engine failure again prevailed, leaving no option but to lay-to overnight while repairs were made.
A Catastrophic Night: As the ship rolled violently beam onto the sea swell the derrick broke loose and swung all over the deck. Oil drums broke free and rolled about with impunity, the equipment in the lab was hurled all over the place and one of the sets of shelves broke away from the wall releasing most of the equipment. After clearing up and refixing equipment we found that the Francis buoys refused to switch on with acoustic command so we had no means of sound ranging and waited for the programmed switch-on at 1500hrs. Again the buoys failed to switch on. It was claimed by the crew that a ship had been seen in the vicinity the previous night, but no importance could be attached to this as we were near the shipping route to the Port of Suez. During the night search two vessels were sighted, both rather suspicious. One, which came within a mile of us, was clearly not a cargo ship and may well have been military; the second had no navigation lights but was of considerable proportions. Unfortunately in this case the Francis buoys had high intensity ‘Flacon’ flashing lights which lit up the cloud base and could easily be seen at a range of 5 miles. It is entirely possible that the buoys had been picked up by one of these unidentified vessels. The Israeli Egyptian Six-day War had taken place only a few months earlier in 1967 and whilst this did not seem to impinge on our operation we believe that continued surveillance in the area did result in the loss of the two Francis buoys. The remainder of the scientific programme was completed using the small buoys.
Ships Scientific Mess: This picture shows; from left to right Clive, Mike, Mel and David (photograph by Tim Vertue), enjoying the dining arrangement on good weather days. The bottles contain water! Our cook Osman was very inventive with the ingredients to hand and Osman Pie (corned beef and egg) became a firm favourite. Fresh fish (barracuda like) caught shark-attack style by tying a shiny teaspoon onto a sisal line and trailing astern, would command top price at any London restaurant.
Fig 44 Geophex explosives.
Explosives: 25lb sticks of ‘Geophex’ explosive ready for use; the detonator and fuse is only inserted when the shot is to be deployed. Tying the explosives together with sisal was an arduous task, cheerfully carried out by Ali and Farah. The rather superior wooden shipping cases (bottom right hand) were always put to further good use and some are still in use today (2015).
All in all the Assab cruise, despite the difficulties, showed that seismic refraction work could be done from ships of a similar size with basic facilities rather than exclusively from research vessels. If space is available a portable lab equipped for the work in hand, ready to go, is in fact the best option saving a lot of time fixing equipment and cables in a conventional ship’s lab. Following the loss of the Francis buoys a new generation of sonobuoys was now urgently needed for marine seismic refraction work.
New Lab Building and Instrument Workshops
In 1967 the Wolfson Building was opened with much needed larger mechanical, electronic workshops and lab space.
The pictures below show sonobuoy production in the new lab building workshops.
Fig 49 New Electronic Workshop 1967
Left to Right – Jane Sik (PhD student), John Bellamy (work experience), Mel Mason and and Tim Owen
Fig 50 Mechanical Workshop – Left to Right – Roger Theobald, Roger Butler, Leslie Flavill and Tim Vertue
Radio Sonobuoy Mark 2
An important consideration in the design was the need for long deployment duration and ease of running maintenance during use.
Six new buoy cans approximately the size of the Francis buoys were needed to accommodate the electronics, recorder and batteries.
The Bradley radio telemetry system had worked well in the Red Sea so this was retained in its entirety.
It was decided that a ship and buoy synchronous clock system was the simplest way of marking the shot instant and timing the ground wave arrivals.
Magnetic tape FM recording was chosen as the data recorder with some form of run delay to optimize the limited tape capacity.
A two channel (high and low gain) FM sub-carrier as per the Bradley system was chosen as the best option for recording the hydrophone signals on to tape.
FET (field effect transistor) technology was now available and could be used in the hydrophone preamplifier replacing the miniature thermionic valves.
Tape Recorder: Much effort was expended in choosing a suitable tape recorder. A four channel commercial recorder was tested and worked well but was rejected on grounds of cost. The UHER reel to reel 4000 Report L was eventually chosen since it was rugged, used by press reporters in relatively hostile environments and had been successfully used for similar seismic work by groups in the USA. Fitted with a 4 track head, reduced capstan speed at 15/32 inches per second and using
1800 ft triple play tape on 5inch reels a 4 channel recording duration of 12 hours was achieved.
Seismic Signals: The high and low gain seismic signals frequency modulated a 3.375kHz carrier by +/- 40% onto tape track 1 & 2.
Buoy Clock: Fred Gray designed a new 100 hour elapsed time digital clock system using a 10MHz Marconi temperature controlled crystal oscillator and TTL (Transistor Transistor Logic). The ship and buoy clocks were synchronised to zero by a reset push switch on the ship clock. All the clock signals were then recorded on the ship recorder to check that all clocks had synchronised successfully. The buoy / ship clock system needed to have very good stability to keep time drift to better than 1mS per hour. The clock had identifiable second, 10 second and minute time marks by fixing the pulse width to 0.2, 0.4 and 0.8 seconds respectively. Seconds 1,2,3,4,5,6,7,10,11,12,13,14,15,16 following the minute mark are binary coded with the elapsed time. The clock trace was recorded on track 3 of the recorder using a switched 500Hz / 1kHz carrier signal. The clock trace below shows 37 hours 29 minutes of elapsed time.
Fig 48 Clock signal
Direct Water Wave: This was recorded on track 4 using the unmodified original recording electronics of the recorder.
Ship Clock: The ship clock was simply a buoy clock with a digital display powered by mains supply with battery back-up.
Ship Paper Recorder: A small Bell & Howell UV paper recorder, which was more rugged and easier to transport, was purchased to replace the large NEP recorder.
As with most new designs there were teething problems, which needed to be solved before ease of operation was achieved. Modifications and improvements were continuously made from 1972-1974.
Tape Recorder Delay: Initially a simple clockwork timer was used to delay the recorder switch-on, which was set just before the buoy electronics were put into the buoy ready for launch. This could sometimes be a problem if buoy deployment was delayed for any reason, which meant the electronics had to be removed and the timer reset. This was solved by replacing the clockwork timers with a programmable motorised multi-cam timer, which could be set up and switched on with the main buoy electronics switch. Once the delay time was reached the cam timer switched itself off in order to save power.
Buoy Clock: Considerable problems were encountered with the clocks resetting to zero or some other spurious time value. This was a difficult one to fix since we were never sure what was causing the clocks to ‘jump’. We suspected large voltage spikes were generated when the buoys were being launched from the ship where the superstructure could be at some potential above zero volts and the sea at zero. After much testing and fixing trials this was eventually solved by isolating all signal outputs with reed switches and transformers.
Battery Supplies and Power Regulators: By the time a good working system was achieved the buoy had three battery supplies made up from a combination of series/parallel 6volt motor cycle lead-acid batteries. This made the charging arrangement overly complicated and it needed to be simplified for the future. The electronics used several regulated voltage supplies which also needed simplifying.
Fig 51 John Murray in later colours (1980?)
Fig 52 sonobuoy lab gear
The sonobuoys had now become the work-horse equipment for the Lab’s marine seismic refraction surveys, using NERC (Natural Environmental Research Council) research ships such as RRS John Murray pictured. At 440 tons the John Murray could roll by up to 40 degrees in moderate seas and be quite an uncomfortable ship to work from. Most scientists, and even the Captain, would need a few days to recover from sea sickness and on one occasion the author needed to jam his life jacket under his bunk mattress to form a wedge so that he wouldn’t be thrown out of his bunk.
Lab space was limited.
The ship clock sits on top of the Bradley receivers with the small Bell & Howell UV recorder next to the demodulators. All equipment had to be firmly lashed down onto the benches.
Fig 53 Sonobuoy shipboard equipment
Fig 54 The sonobuoy chassis, with space for Uher Repor L or later Report 2000
Fig 55Buoy ready for launch (note no flashing light attached!)
It was important before launch to check, using the radio signals, that everything was functioning correctly; otherwise it could take at least 30 minutes to recover the buoy to remedy any problems. As development progressed the buoy became heavier with additional batteries etc. so a car tyre inner tube was added for extra buoyancy and stability.
Various experiments were undertaken using the Mk2 buoys. ‘AquaSeis’, a cord-type explosive, was tested as an alternative to the standard Geophex, which could be electrically detonated using synchronised pulses from the ship clock. The idea was to increase the number of short-range shots fired per line. However, it was not really powerful enough to get decent refractions.
On one occasion in Famagusta Bay, Cyprus, the buoys were nearly lost for want of a flashing light fitted to help with location on retrieval. Although the planned test deployment was for only a few hours, delays meant that the buoys could not be retrieved in daylight, and without a flashing light fitted it is very difficult, if not impossible, to locate them in darkness. The ship returned to Famagusta overnight and the buoy radio signals were continuously monitored, but with the prevailing currents the fear was that they would be washed ashore to the north. Fortunately they were all recovered the next day; a few more hours delay and they would have been lost. Thereafter flashing lights were always fitted.
By 1975 it was decided to update the electronics to reduce power consumption, increase deployment duration and reduce running and maintenance effort. The logistics of deployment and recovery needed optimising using items such as quick-release clips on the bottom pole weight.
It was decided that all the electronics, apart from the Bradley transmitter, should be rebuilt using the Lab’s standard format small plug-in pc (printed circuit) boards mounted in a single enclosure. Key requirements were to lower the power consumption, for the design to use a common ground connection throughout and most importantly to provide a versatile tape recorder run time programmer.
The Mk2 thin-walled aluminium hydrophone preamp tube, using piston ‘O’ rings, was prone to leaks and this needed to be rectified. A new hydrophone housing was designed using 10mm thick walled anodised aluminium tube, the barium zirconate pressure-sensitive element being potted directly onto the tube at one end. Both piston and end ‘O’ ring seals used at the cable end cap proved to be rugged and reliable. New armoured Polyurethane cable was purchased, which could stand up to rough use – a great leap forward in helping with the reliability of the system.
Fig 56 & 57 Hydrophone Preamplifier:
The transistorised Mk2 unit above
was replaced with a single integrated
Hydrophone Amplifiers: Dual IC (integrated circuit) low-power version with gain control.
Frequency Modulators: Single board with switched modulating frequency control.
Clock: Rebuilt using low power CMOS (Complimentary Metal Oxide Silicon) logic ICs and a low -power 5MHz TCXO (Temperature Compensated Crystal Oscillator).
Tape Recorder: As per the Mk2 version.
Tape Recorder Timer: Driven by and synchronised to the buoy clock providing five programmable start times that initiated plug-in fixed period cycle timers. Once initiated the cycle timer ran continuously until the next programme time was reached. Typically, two cycle options were used: (10 mins. ON – 10 mins. OFF) and (20 mins. ON – 10 mins. OFF). This gave a good degree of flexibility and optimised use of the limited tape capacity.
Battery Supply: Single 12-volt supply using two 6-volt lead-acid gel batteries, made maintenance and charging much easier.
Signal Monitor: All signals including the clock BCD (Binary Coded Decimal) display could be monitored from the control panel without the need to open the electronic board housing.
Flashing Light: COSALT buoy light with battery pack and light level detector.
Portable Replay: Small unit with integral replay tape recorder for use on board ship.
Lab Replay: 19-inch rack with extra filtering facility.
Paper Recorder: Siemens 8-channel jet pen recorder.
Fig 58 Sonobuoy chassis, Bradley receiver and Uher Tape playback system
Above: The buoy showing the control panel with monitor sockets and tape recorder below. The sealed lead-acid battery was fixed below the recorder so that all components of the system were contained within the single frame. In the centre are the Bradley receivers, demodulators and hydrophone. On the left the portable replay unit.
Fig 59 recorder system
The electronic boards:
Left – the modulators and power regulators; Centre – the tape run time programmer.
Right – the clock circuits.
Ship Clock: A new unit with high performance crystal oscillator and elapsed time LED read-out was constructed. Built as a modular design, the system also had an air gun timer, trigger hydrophone signal processor and radio tone break decoder for two ship work.
Fig 60 ship clock
Fig 61 8 Channel Jet Pen Recorder:
The Bell & Howell machine, used with the Mk2 buoys, was replaced by a Siemens Ink Jet recorder, which used ordinary Z-fold plain paper that produced a permanent low-cost record. Originally developed for medical use it was ideal for displaying seismic refraction waveforms. The sintered ceramic (porous) roller directly in front of the jet pens blotted the ink trace as the paper passed below, providing a dry, permanent record.
Under the leadership of Drum Matthews the sonobuoys were continuously used by the Marine Group in the 1970s in the Atlantic Ocean, – Mediterranean Sea – and on one occasion the Indian Ocean. Many of the deployments were from RRS Shackleton, a ship and crew we all became very fond of working with over the years. Unusually, equipment handling was from the fore deck, which worked well; explosive charges were deployed from the after deck.
Fig 62 & 63 Shot Firing from Shackleton’s afterdeck:
Charges of up to 900lb were assembled and placed on a tipping table for jettisoning over the stern whilst the ship maintained a speed of 10 knots when shooting the line. Sometimes the fuse and detonator could get ripped out of the charge when it hit the water so generally, for charges over 300lb, two fuses were used to make sure the shot was successful. Geophex was the preferred explosive; it was reliable and having a marzipan consistency was easy to cut with a wooden knife. The 25lb tubes were held together with a standard plastic-strap banding system. Fuse lengths were measured to give a 2.5 minute burn time, which allowed the charge to sink to approximately 300ft at detonation. Handling the actual explosive material was kept to a minimum as shot-firers would sometimes suffer from severe headaches by absorbing nitro-glycerine through the skin.
End of an Era
The Sonobuoys had served the Department well, but by 1979 scientific study of the ocean crust had become more complex and greater numbers of sea-bed instruments were needed. Sea-bed instruments had the advantage of not moving during the refraction line shooting, they were in a quieter environment and could accommodate additional sensors such as a 3-component geophone package. The sonobuoys were last used in 1980. A series of sea-bed instruments were developed over the next 20 years and used by the Department until the instrument development team retired in 2002.
Developed for military use in the late 1950s disposable sonobuoys were launched from RAF AVRO Shackleton aircraft to detect submerged submarines. When the buoy hit the sea surface a hydrophone was released to detect the submarine’s propeller sound, which in turn was sent back to the aircraft using VHF (Very High Frequency) radio signals. By deploying a number of sonobuoys the position, direction of travel and speed of the submarine could be determined. After a pre-set time the buoy transmitter would switch off. After a few hours a salt-water soluble plug mounted, in the side of the buoy casing dissolved, causing the buoy to sink. By the 1960s this type of buoy became commercially available and the Marine Group used this technology to obtain seismic refraction profiles of the sea bed layer during reflection profiling surveys.
Fig 64 AVRO Shackleton
The Archive’s example was manufactured by Ultra Electronics Ltd and has the following specification:
Radio Transmitter: Frequency modulated in the 165MHz range.
Radio Receivers: 2 Lafayette Domestic Communication receivers with fixed crystal controlled frequency tuning option allowed two buoys with different radio frequencies to be deployed without causing radio interference.
Hydrophone Deployment Depth: Selectable 60 or 450ft. The hydrophone amplifier gain was reduced by 10dB for seismic refraction use.
Operation Duration: Selectable 1, 4, or 8 hours.
Fig 65 The Ultra Electronics Disposable Sonobuoy: Small and lightweight the buoy was simply deployed by hand over the side of the ship. The lead-shot weighted hydrophone string was allowed to fall to the sea surface before the buoy was released.
The two Lafayette receivers provided an effective low-cost
Fig 66 The G+G Scanning Jet Pen Recorder
Disposable sonobuoy signals were displayed using the above recorder, running in simple line mode, which was also used for reflection profiling in variable area mode. The sound source used was a Bolt Associates air gun towed from the ship at a depth of 30metres. The
Fig 67 Lafayette
scan interval was controlled by the ship clock, which also triggered the air gun that fired every 10 seconds.
Fig 68 The Seismic Refraction Demonstration Machine
This glorious ‘Heath Robinson’ machine restored to working order shows with a series of lamps how seismic waves travel through rock strata and detected by the sonobuoy hydrophones. The lamp illumination sequence is started by pressing the black button (above right of the ship). At short range, as illustrated by the ship position, waves travelling the red path (labelled 5 inches per click indicating slow seismic velocity) reach the sonobuoy first. The next sequence, long-range starting at the right hand side, shows the faster waves travelling the yellow path (labelled 10 inches per click indicating fast seismic velocity) reaching the sonobuoy first.
The light sequence is controlled by a uniselector relay, a multi contact device with contacts arranged in banks of four, which are selected by a rotary switch arm. The arm is advanced once a second and moves through 180 degrees to complete the switch sequence.
A micro switch, driven by a clock motor, produces the sequence drive pulses.
Bullard E C and Gaskell T F. Submarine Seismic Investigations. Proc.R.Soc.Lond. A (1941) 177, 476 – 499
Davis D. Assab Cruise Report (1968). Dept. of Geodesy & Geophysics, Cambridge University
Francis T J. Seismic Observations at sea with Long Range Recording Buoys. PhD Thesis (1964) Dept. of Geodesy & Geophysics, Cambridge University
Gaskell T F and Swallow J. Seismic Refraction Experiments in the North Atlantic. Nature 167 (1947) p723
Gaskell T F. Seismic Refraction Work by HMS Challenger in the Deep Oceans. Proc.R.Soc.Lond. A (1954) 222, 356 – 361
Hill M N and Willmore P. Marine Seismic Prospecting. Nature 157 (1947) p 207
Hill M N. Refraction Shooting at Sea. PhD Thesis (1950) Dept. of Geodesy & Geophysics, Cambridge University
Hill M N and Swallow J. Seismic Experiments in the Atlantic. Nature 165 (1950) p198.
Hill M N. Single Ship Seismic Refraction Shooting. The Sea Vol. 3 p39 – 46. Interscience Publishers (1963)
Mason M. Sonobuoy Mk2 & Mk3 Handbooks (1979). Dept. of Geodesy & Geophysics, Cambridge University
Tramontini C. Seismic Refraction Experiments at Sea. PhD Thesis (1969) Dept. of Geodesy & Geophysics, Cambridge University
Further information on people referred to in this article and earth crust studies can be found in the following publications:
Lamb S and Sington D. Earth Story (1998) BBC Books ISBN 0 563 38799 8
Williams C A. Madingley Rise and Early Geophysics at Cambridge (2009) Third Millennium Publishing ISBN 978 1 90650718 3
People’s position in the Department at the time of referral in this article:
Demonstrator (later Head of Department 1948 – 1960)
Bullard Teddy (Sir Edward)
Demonstrator (later Head of Department 1960 – 1980)
Senior workshop technician
Senior Assistant in Research
Instrument technician (later Chief Technician)
Assistant Director of Research
Assistant Director of Research, Marine Group Leader
Electronics workshop technician
Reader, Marine Group Leader
Technical Officer (in charge of instrument development and workshops)
Listed sequentially from page 1.
Page BAG No Type Description
1,3,5,6 386 Video Experiments at sea 1938 & 1939
2 298 Photo Radio telephone test
2 300 Photo Radio telephone internals
3 309 Photo Lowering geophone from HMS Jason
4 301 Photo Shot charges HMS Jason
4,9 001 Inst 8-Channel recording oscillograph
8 094 Inst Sonobuoy hydrophone
8 394 Inst Parts Sonobuoy hydrophone pre amplifier
11 310 Photo M N Hill with sonobuoys
12 *** Photo John Cleverly sonobuoy recovery
12 434 Photo Sonobuoy and associated equipment
14 346 Photo RRS Discovery II
16 463 Video International Indian Ocean Cruise 2
17 016 Inst Bradley sonobuoy ship receivers & demodulators
17 045 Inst Parts Bradley hydrophone amp & sub-carrier modulator
17 *** Inst Parts Bradley buoy transmitter
19 *** Photo MV Assab
22 292 Photo Photograph album Lab opening 1968
23 342 Photo RRS John Murray Mk2 sonobuoys
24 347 Photo RRS John Murray sonobuoy launch
25 014 Inst Mk3 Hydrophone
26 015 Inst Mk3 Sonobuoy
26 128 Inst Mk3 Sonobuoy portable replay
27 047 Inst Mk3 Ship clock
27 066 Inst 8-Channel Jet Pen recorder
28 313 Photo RRS Shackleton
28 *** Photo Shot firing
30 070 Inst Disposable sonobuoy
30 069 Inst Disposable sonobuoy Lafayette receivers
30 026 Inst G+G seismic jet pen recorder
31 390 Equip Seismic refraction demo machine
Images from the Internet:
The following images were obtained from Internet Sites.
Ocean Weather Ship Explorer
RRS Discovery II
RRS John Murray