新概念英语听力mp3下载第三册lesson 47

Satellites orbiting round the earth have
provided scientists with a vast amount of
inf.mp3ation about conditions in outer
space. By comparison, relatively little is
known about the internal structure of the
earth. It has proved easier to go up than
to go down. The deepest hole ever to be
bored on land went down 25,340 feet--
considerably less than the height of
Mount Everest. Drilling a hole under the
sea has proved to be even more difficult.
The deepest hole bored under sea has
been about 20,000 feet. Until recently,
scientists have been unable to devise a
drill which would be capable of cutting
through hard rock at great depths.
This problem has now been solved. Scientists have developed a method which
sounds surprisingly simple. A new drill which is being tested at Leona Valley
Ranch in Texas is driven by a turbine engine which is propelled by liquid mud
pumped into it from the surface. As the diamond tip of the drill revolves, it is
lubricated by mud. Scientists have been amazed to find that it can cut through
the hardest rock with great ease. The drill has been designed to bore through
the earth to a depth of 35,000 feet. It will enable scientists to obtain samples of
the mysterious layer which lies immediately below the earth's crust. This layer
is known as the Mohorovicic Discontinuity, but is commonly referred to as
'the Moho'.
Before it is possible to drill this deep hole, scientists will have to overcome a
number of problems. Geological tests will be carried out to find the point at
which the earth's crust is thinnest. The three possible sites which are being
considered are all at sea: two in the Atlantic Ocean and one in the Pacific. Once
they have det.mp3inded on a site, they will have to erect a drilling vessel which will
not be swept away by ocean currents. The vessel will consist of an immense
platf.mp3 which will rise to 70 feet above the water. It will be supported by six
hollow columns which will descend to a depth of 60 feet below the ocean surface
where they will be fixed to a huge float. A tall steel tower rising to a height of
nearly 200 feet will rest on the platf.mp3. The drill will be stored in the tower
and will have to be lowered through about 15,000 feet of water before operations
can begin. Within the tower, there will be a laboratory, living accommodation
and a helicopter landing station. Keeping the platf.mp3 in position at sea will
give rise to further problems. To do this, scientists will have to devise methods
using radar and underwater television. If, during the operations the drill has to
be withdrawn, it must be possible to re-insert it. Great care will therefore have
to be taken to keep the platf.mp3 steady and make it strong enough to withstand
hurricanes. If the project is successful, scientists will not only learn a great deal
about the earth, but possibly about the nature of the universe itself.