2017年7月29日雅思阅读考试回忆及解析

　　D

　　The project used two remote sensing techniques. The first is magnetometry which works on the principle thai magnetic fields al the surface of the Earth are influenced by what it buried beneath. It measures localised variations in the direction and intensity of this magnetic field. "The Earth's magnetic field can vary from place to place, depending on what happened there in the past." says Branting. "if something containing iron oxide was heavily burnt, by natural or human actions, the iron particles in it can be permanently reoriented, like a compass needle, to align with the Earth's magnetic field present at that point in time and space." The magnetometer detects differences in the orientations and intensities of these iron particles from the present-day magnetic field and uses them to produce an image of what lies below ground.

　　E

　　Kerkenes Dag lends itself particularly well to magnetometry because it was all burnt at once in a savage fire. In places the heat was sufficient to turn sandstone to glass and to melt granite. The fire was so hot that there were strong magnetic signatures set to the Earth's magnetic field from the time - around 547 BC - resulting in extremely clear pictures. Furthermore, the city was never rebuilt, "if you have multiple layers confusing picture, because you have different walls from different periods giving signatures that all go in different directions," says Branting. "We only have one going down about 1.5 meters, so we can get a good picture of this fairly short-lived city”.

　　F

　　The other main sub-surface mapping technique, which is still being used at the site, is resistivity. This technique measures the way electrical pulses arc conducted through sub- surface soil. It's done by shooting pulses into the ground through a thin metal probe. Different materials have different electrical conductivity. For example, stone and mudbrick arc poor conductors, but looser, damp soil conducts very well. By walking around the site and taking about four readings per metre, it is possible to get a detailed idea of what is where beneath the surface. The teams then build up pictures of walls, hearths and other remains. "It helps a lot if it has rained, because the electrical pulse can get through more easily," says Branting. "Then if something is more resistant, it really shows up." This is one of the reasons that the project has a spring season, when most of the resistivity work is done. Unfortunately testing resistivity is a lot slower than magnetometry. "If we did (r the whole site it would take about 100 years," says Branting. Consequently, the team is concentrating on areas where they want to clarify pictures from the magnetometry.