Seven-Tenths Read Free Page B

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 mimicsthe 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.