In 1922 a historic moment in cartography and oceanography occurred when the USS Stewart used a Navy Sonic Depth Finder to draw a continuous profile across the bed of the Atlantic, 68 years after Maury had published his map. But whereas Maury’s had involved large amounts of guesswork, the Stewart’s involved none, aside from certain problems of interpretation. Otherwise the soundings were fast, accurate and simple to take. The principle of sonar is straightforward. A pulse of sound is bounced off the sea floor and received by the vessel which sent it. Having made small allowances for such things as the speed of the vessel, it is a matter of the simplest arithmetic: divide the time elapsed by two and multiply the result by 4,800, which gives the depth in feet, 4,800 feet per second being the mean speed of sound in water. (The layman is astonished only that, with a principle as thoroughly understood and long turned to practical use, it should have taken so long to develop radar, that apparently magical World War II conceptual breakthrough. Since then, with a good deal of technical wizardry but zero intellectual daring, radio pulses and even laser beams have been bounced off the Moon, ranging it to a matter of a few centimetres.)
There are, however, certain difficulties to this kind of bathymetric depth sounding. One can be intuited from the phrase ‘mean speed of sound in water’, since in practice the speed of sound in seawater varies. It travels faster, for instance, if the temperature, pressure or salinity increase. This is to leave aside for the moment all the difficulties in interpreting the returning signals, which may be severely affected by shoals of fish and other powerful sources of scattering. A further drawback is that a series of pulses focused vertically downwards beneath a ship’s keel will give only a series of depths. In order to build up a contour map of how the sea floor actually looks a ship using this method would have to make a laborious succession of very closely spaced passes. Otherwise, only a narrow cross-section of the seabed being traversed is possible.
A way around this difficulty was pioneered at the Institute of Oceanographic Sciences in Surrey. Instead of using vertical echo sounding, they developed a sidescan sonar in which the sound pulses are emitted sideways in two slanting fans, one on either side of the ship and at right angles to its course. The outer edges of these fans brush irregularities on the seabed as far away as 30 kilometres. The returning echoes, since they do not come merely from a single point directly beneath the vessel’s keel, are complex and take skill and experience to interpret. Yet the profile which emerges from the plotters provides by far the clearest imagery yet possible of a continuous swathe of seabed, certainly on a cost-effective scale. GLORIA’s efficiency is striking. In good conditions and with the ship travelling at 8 or more knots it can reveal a strip of seabed 60 kilometres wide. In twenty-four hours this amounts to mapping more than 20,000 km2, or an area the size of Wales.
This cruise in the Farnella is one of a series at the end of a great project on the part of the US to map the 200-mile EEZ around all 19,924 km of its coastline. The EEZ of the continental US has already been mapped, including Alaska and the Aleutian Islands. Now Farnella is working away at the Hawaiian Chain. As the scientists aboard keep telling me, only the GLORIA system could have covered such an area, and even that has taken ten years.
Such things are explained as the ship bounces and judders through seasonal Pacific rollers on her way to the survey area. On and off a tropic sun blazes the sea to a luminous indigo across which flying fish skip and glide. In the intermittent bursts of sunlight there is a stampede of scientists up on to the deck where they grill themselves on towels, redolent of coconut oil. December! Hawaii! Field trips sometimes throw in for free the costly ingredients of other people’s holidays. They are watched by two boobies clumsily slithering on the button atop the radar mast while a small albatross flies elegant, mournful rings around Farnella. On successive days of watching I never once saw it move its wings, only soar and tilt in any direction and at any speed into the headlong breeze while keeping pace with the ship. It looked as though it knew by heart a map of the ocean’s surface, a map no man will ever make.
Time was spent checking the various instruments which were soon to be lowered into the sea and towed behind us. First of all GLORIA itself: a large yellow torpedo lined inside with banks of transponders precisely angled to give the correct fan-shaped pulse. It sits in a hydraulic cradle directly over the ship’s stern. From time to time technicians climb up to tighten a nut and pat it protectively. Even without its cable it is worth nearly half a million pounds. There is spare cable but only one GLORIA aboard. ‘We don’t even like to think about that,’ is the response to the obvious ‘What if?’
Amidships, two little ‘fish’ like fat yellow bombs wait on wooden trestles. These are the 10 kilohertz and 3.5 kilohertz sonars. The first will act more or less like an old-fashioned depth sounder to give the distance to the seabed immediately beneath us. The second will send pulses up to 400 metres into the ocean floor to provide some idea of the underlying geology. It is explained that its readout, taken by itself, would be fairly meaningless since in physics, as elsewhere, one cannot get something for nothing and the deeper penetration gained by using a higher frequency is at the expense of detail and spread. But taken in conjunction with information coming from other instruments this data will be usefully corroborative.
Over the starboard stern will go the air gun. This is a heavy pot of machined steel, something like the body of a pneumatic drill and similar in principle. Connected to a high-pressure air hose it will ‘fire’ itself every ten seconds with a loud detonation and send sound waves capable of penetrating up to 1 kilometre into the ocean floor. This is the oceanographer’s equivalent of seismologists’ ‘thumper trucks’ which used to drive around the deserts of the Middle East, dropping huge blocks of concrete and listening for signs of oil-bearing strata. The air gun is a safer and simpler alternative to throwing overboard dynamite charges at timed intervals. Its echoes are received on a hydrophone ‘streamer’, hundreds of metres of transparent plastic tube filled with bundles of multicoloured sensor wires and light oil. The streamer may be towed well over a kilometre behind the ship and is of all the equipment the most susceptible to damage. (At the end of the cruise when the streamer is wound aboard there is a section leaking oil with a broken shark’s tooth embedded in the gash.)
Over the port stern will go a magnetometer to measure magnetic variability in the Earth’s crust, and down in the lab is a gravimeter to record differences in its gravitational field. This machine looks, and is, expensive. It is suspended in a cradle mounted in computer-controlled gimbals, dipping and tilting so it appears to be the one thing in the lab which is constantly in motion, whereas it is really the only thing aboard remaining utterly still while the ship gyrates about it. At supper the conversation turns to where might be the best place on Earth for setting high-jump records, a particular spot with significantly weaker gravity. All the best ones seem to be covered by a couple of miles of water. In response to a remark of mine which betrays real ignorance about gravity, Roger says kindly:
‘I suppose one always imagines the surface of the oceans as basically flat. Ignoring waves and local storms, of course – they’re just “noise”. But apart from its being curved to fit the surface of the globe, one thinks of the sea as having to be flat because at school we’re told water always finds its own level so as to be perfectly horizontal. On a small scale that’s pretty much true, though when I was about ten I remember being surprised when someone pointed out that all rivers are tilted, and if you row upstream you’re also rowing uphill as well as against the current. Anyway, since gravity varies from place to place it acts variably on the sea, too. When you start using instruments like the ones aboard this ship you really appreciate how the ocean surface actually dips and bulges all over the place. It shows up best from space.’
He explains that by having enough satellites in orbit making passes over the same area, day after day for months on end, it was possible to build up a mean reading for the height of the
sea’s surface at that spot. It took a long time because there was a good deal of ‘noise’ to be discounted: wind heaping, sudden areas of low atmospheric pressure which could suck the sea upwards as if beneath a diaphragm, even very low-frequency waves with swells so long they might take half a day to pass. But if the satellites went on measuring the same spot for long enough such fluctuations would even out and a geodetic point be established: a mean distance to the sea’s surface as measured from the centre of the Earth. By building up enough geodetic points it soon became clear that the oceans were anything but flat.
‘What’s more, if you match this up with the underlying features on the seabed, you’ll find that the surface of the sea broadly mimics the topography underneath. And the reason for that is fluctuations in gravity, which depends on the density of the crustal material.’
It is a pretty notion, that the sea follows the Earth’s crust like a quilt laid over a lumpy mattress. It is also odd to think that to some extent the depths of the oceans can be read from space. Over a plate of steak and kidney pie (the galley makes no concessions to the tropics) I presume this means that sometimes a ship has to go uphill and downhill.
‘Certainly. But the “hills” are so slight you’d never know. We’re talking a few tens of centimetres here, spread out over kilometres. Sure, a ship often has to go uphill, though it won’t be using any more energy. All points on the hillock’s surface have the same gravitational potential, obviously.’
This is not obvious to me, and nor is it any more so after Roger has explained it several times in different ways. I tell myself that physics is humiliating not when it defeats the intellect but when it confounds the imagination. This makes me feel better. Giving up on me, he reverts to a sort of ‘Ripley’s Believe It or Not’ mode suitable for lay company. Roger is himself a geologist and in describing the planet gives the impression of talking about a beach ball under-inflated with water: labile, plastic, sagging and crinkling and bulging. It is not only the oceans which respond tidally to the Sun and the Moon; the Earth’s surface does as well, rising and falling twice a day. When the Moon is directly overhead it is pulled up by half a metre.* What is more, this elastic crust seems to have a frequency of its own at which it resonates. A Russian geologist, S. L. Soloviev of the Moscow Institute of Oceanology, recently made seismograms of micro-earthquakes under the Tyrrhenian Sea. Using bottom seismographs (developed from nuclear explosion detectors originally designed to enforce the Test Ban Treaty), Soloviev began picking up a distinct, ultra-low frequency oscillation which he thought was most likely the fundamental frequency of the Earth’s crust itself.
That night I go to bed with my head full of marvels. In the course of the evening I had also learned that the sea levels at either end of the Panama Canal were different by nearly half a metre, and the same went for the sea on either side of the Florida Peninsula. This was caused by things such as the heaping effect of wind and the Coriolis force. But I am most captivated by the idea of the Earth’s crust vibrating at an ascertainable frequency since it would theoretically be possible to calculate the precise note. True, it would probably not be a pure tone because there would be all sorts of harmonic interference from irregularities such as mountain ranges. Yet it ought to be possible to determine the fundamental note of the planet, the music of our spheroid.
I also wonder at the notion of the sea’s surface modelling the plains and mountains, chasms and basins beneath the keel. It is not hard to believe this at the moment since we are all being thrown about our bunks by the Farnella’s plungings as if she were ploughing across rough country. We have reached the foothills of the Necker Ridge. More than 2 miles below us a mountain chain thrusts steeply upwards. I bang about in my wooden trough and tell myself this is just ‘noise’.
Next morning the sea is quieter. The ship heaves to and a certain tension comes over the scientists as one by one the precious instruments are carefully deployed. Launchings and retrievals are the moments when damage is most likely and although there are several workshops aboard for mechanical and electronic repairs our sailing patterns are planned to the nearest nautical mile for the next several days. Worries revolve around personal responsibility for the correct functioning of machines. The spectre of disgrace and delay flits about the ship until everything is safely in the water and the test readings are monitored. In the end these anxieties are probably rooted less in codes of professionalism than in the huge expense of modern oceanography. Great sums of money are being lowered delicately into the ocean. Any delays would be tantamount to damage, chunks of money becoming dislodged and drifting down to the seabed where they would dissolve and be lost for ever. Even the crew seems less laconic while all this is going on. When we are under way again there is a feeling that the Farnella is more in the hands of scientists than seamen and the crew can now be found in odd corners reading copies of the Hull Daily Mail which were flown out in bundles to the shipping agent in Honolulu. Shipboard life settles into routine. It is still curious to be in the Pacific in a British ex-trawler with a television in the lounge showing video-cassettes of highlights from last season’s Hull Kingston Rovers matches. Not to mention the cuisine. At the same time we are, as an IOS zealot proudly says, ‘at the leading edge of geophysical seabed surveying’.
*
The issue of who is really in command of the ship is interesting, as is the whole idea of a joint survey paid for by the US Government using a significant proportion of British equipment and scientists. In a legal sense the Captain has full and final responsibility for the ship. Yet it soon becomes plain that his actions are largely determined by the exigencies of the survey, which is costing the American taxpayer such a pretty penny. It is the USGS which has chartered the vessel and hired GLORIA and so calls the tune. On the other hand the scientist formally in charge of this particular cruise is a Briton, one of GLORIA’s original developers at IOS. At the same time one of the young American women aboard is responsible to her government for completing this leg of the survey. … All this interweaving of authority is glossed for me as ‘A joint effort. Absolute cooperation and consultation. Democracy in action like you wouldn’t believe.’ This is emphatically not science for the sake of science, a matter of drifting about the Pacific like the old Challenger in the 1870s, sounding here and dredging there at whim. This is time-and-motion science, with a given area of blank map to be filled in a given time. And the whole issue, for very cogent reasons of physics, hinges around the matter of navigation.
As is all too clear to anybody swimming in circles looking for a lost boat in the middle of the ocean, one has no position in water. When mapping the seabed from a moving ship, therefore, accurate navigation is of crucial importance. Without the ship’s position being known from one second to the next the most beautiful chart of peaks, ravines and plateaus would be useless. The only thing known would be that they were down there somewhere. Establishing the ship’s course along lines as straight as possible (always allowing for the Earth’s curvature) requires much work, not least because the swathes GLORIA maps must lie next to each other without gaps or wasteful overlapping. On the chart table in the lab is the dot of Johnston Island, a pencil circle whose diameter represents 400 nautical miles inscribed about it. High up in its top left-hand quarter a chord shows the first leg we have just started. Next to it is written the estimated time at which we should come about for the return pass, each leg getting longer as we eat downwards into the circle. If all goes well, by the end of a fortnight we should have hatched off about a quarter of the total area.
While the lab computers flicker with the instruments’ returning signals, various repeater gauges give the ship’s speed through the water, its speed over the ground, the wind speed and any consequent degree of yaw. If to remain on a straight course against a quartering wind and current the Farnella needs to sail crabwise, GLORIA’s angle will also be fractionally oblique to its correct path. The result is that its signals will no longer be exactly at right angles to this course and
the map will be distorted. Information on all these factors is fed into the computers, which correct for them. In order to determine the ship’s position at any moment the Farnella uses GPS or Global Positioning System. This depends on satellites and eventually, provided there are still spare slots in an already overcrowded geostationary orbit, the system will cover the Earth and in theory allow a person anywhere on the planet’s surface to determine his position to within a few metres. This would not be of the slightest use to a lost swimmer looking for his boat.
Bored with the sight of bright red digital figures flickering their decimal points on display panels, I wander off in search of sound. Down in the forward hold, above the banging of the ship’s forefoot into wave troughs, the chaffinch-like chinking of the 10 kilohertz ‘fish’ can be heard through the steel hull. Up on the stern deck there is a sharp cracking sound every ten seconds, the higher frequencies of the air gun’s detonations being transmitted back up the compressed air hose. In the water astern white puddles dimple and churn to mark the boilings of released air. They follow the ship with the measured pace of footsteps.
Seven-Tenths Page 2