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ries that experience sudden, heavy rainfall, and can also be triggered by earthquakes and even water erosion.
Landslides start when a few particles of soil or rock within a slope start to move, but the early stages can be hard to spot. Following this initial movement, "slopes can become unstable in a matter of hours or minutes," says Nell Dixon at Southborough University1, UK. He says a warning system that monitors this movement "might be enough to evacuate a block of flats or clear a road, and save lives. "
The most common way to monitor a slope for signs of an imminent landslide is to watch for changes in its shape. Surveyors can do this by measuring aside directly, or sensors sunk into boreholes or fixed above ground can be used to monitor the shape of a slope. Slopes can. however, change shape without triggering a landslide, so either method is prone to causing false alarms. Now Dixon's team has developed a device that listens for the vibrations' caused when particles begin moving within a slope.
The device takes the form of a steel pipe dropped into a borehole in a slope. The borehole is filled in with gravel around the pipe to help transmit high-frequency vibrations generated by particles within the slope. These vibrations pass up the tube and are picked up by a sensor on the surface. Software analyses the vibration signal to determine whether a landslide may be imminent.
The device is currently being tested in a 6-metre-tall artificial clay embankment in Newcastle2, UK. Early results suggest it should provide fewer false positives than existing systems. Once it has been carefully and thoroughly tested, the device could be used to create a complete early-warning system for dangerous Slopes.
" Locations with a significant risk of landslides could definitely benefit from a machine like this," says Adam Poulter, an expert at the British Red Cross. "As long as it doesn't cost too much, " But, Poulter adds that an early-warning system may not be enough on its own. "You need to have the human communication," he says. "Making systems that get warnings to those who need them can be difficult. "
41. What does "Such natural disasters" in the first paragraph refer to?
A Sudden, heavy rainfall.
B Earthquakes.
C Water erosion.
D Landslides.
42. Which of the following statements is true of landslides?
A The initial movement is hard to spot.
B They start with a movement of a few particles of soil or rock.
C They can be destructive in a matter of hours or minutes.
D All of the above.
43. Why do researchers develop a new device to monitor signs of landsides?
A Because the new device can measure the site directly.
B Because the new device can be sunk into boreholes or fixed above ground.
C Because the common methods .can cause false alarms.
D Because the common methods are useless.
44. Which of the following statements is NOT true of the device, according to Paragraph 4?
A It is filled in with gravel.
B It consists of a steel pipe.
C It is dropped into a borehole filled in with gravel.
D It is connected to a sensor on the surface.
45. According to the context, what does the word "positives" in the fifth paragraph mean?
A Positive electric charges.
B Evidences.
C Warnings.
D Predictions.
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