Planet Earth International 2009

T
he polar regions and the deep oceans
Understanding the
are among the most difﬁ cult places on
Explor
extremities of the planet
Earth to explore, yet they hold the key to
and the effects this has
both understanding the workings of the helps scientists with
planet and ﬁ nding the resources we need
further discovery.
for the future. Technology has changed not
only how we can reach these inaccessible
regions, but also how we explore and ation and disco
understand them; and, while some of
that essential technology was developed
precisely for this purpose, much of it came
from entirely different ﬁ elds.
Modern medicine has tools that provide
clear, colourful and informative images
of the human body – a wonderful aid to
diagnosis, surgery and fundamental
biological understanding. Similar techniques
can help to reveal more of the planetary
body on which we live. X-ray images show
what’s going on under the skin, especially
if bones are broken, because they interact
with the varied tissues they encounter in
their passage through the body. A powerful
v
reﬁ nement of x-ray imagery is computer
tomography, a data analysis technique
Alvin
eries
that produces three-dimensional images by
combining information from sequential two
Alvin was the pioneer
Seismic tomography does not use the
dimensional scans – like reconstructing a
submersible that explored the
tidy ‘slices’ of a medical scan, produced by
loaf by combining slices of bread. X-rays are
deep sea. An immensely strong
machines that track along the body – it has
measured in many directions in each slice,
ball just two metres across,
with engines to manoeuvre,
to use the data observed, which generally
to build up each two dimensional image
arms to take readings and
comes from whatever earthquakes happen.
prior to combining all. It takes formidable
baskets for samples, Alvin took
That’s why the earliest seismic tomography
computer power, but such power is easily
two scientists and a pilot to
images came from regions with frequent,
available.
the deep sea ﬂ oor, where they
powerful earthquakes. Seismic waves
explored the undersea volcanic
Seismic waves
mountains that make up the mid-
radiate in all directions from the source
ocean ridges - and some of their
of an earthquake. When a seismometer
Similar techniques are used to explore the
surprises.
detects the signals, it provides information
subsurface and interior of the Earth, which on the rock between instrument and source.
are otherwise difﬁ cult to reach. Mines In this way many different signal paths are
and boreholes rarely reach even as far as one that stays in the less dense surface combined to map the three-dimensional
10 kilometres down, not even scratching layers, for example. structure of the Earth.
the surface of a planet more than 6000 Many earthquakes and many detectors The result is a three-dimensional picture
kilometres from surface to centre. But x- enable geophysicists to pick out major of the otherwise inaccessible depths of the
rays are not especially useful for this; they changes in rock density, such as the Earth showing, among other things, how
do not travel far in rock, so geoscientists boundary between the outer layer of the the large-scale circulation of rock associated
use seismic waves that spread round the planet – the crust – and denser mantle rock with plate tectonics works inside the planet.
world in an earthquake. These are pressure beneath. These layers are like the layers of Earth sciences in its modern form came
waves, like sound waves, although the tissue and bone within the human body; the into being following the International
notes they make would be far too low for techniques of computer tomography can be Geophysical Year (1957–1958), which kick-
us to hear. Seismic waves generally travel applied to parts of the Earth to produce started global observation of the Earth
more quickly in denser rock, which is, again three-dimensional images of the interior – and especially the ocean ﬂ oor. This led to
in general, found further below the surface provided that there is enough information. the revolutionary ideas of plate tectonics
of the Earth. So seismic waves from an Seismic tomography did not emerge until – namely that the surface of the Earth is
earthquake that penetrate more deeply into computing advances made it possible to made up of essentially rigid plates of rock
the Earth, and then return to the surface, record fully the signals from earthquakes, constantly moving with respect to each
will reach the detectors more quickly than and manipulate the vast datasets collected. other. The ocean ridges are the places
www.yearofplanetearth.org PLANET EARTH 117
PE27.Exploration.rky.vp.indd 117 16/2/09 12:12:26 pm
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