GRE Reading Comprehension
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Source: CHP
Because of its accuracy in outlining the Earth's subsurface, the seismic-reflection method remains the most important tool in the search for petroleum reserves. In field practice, a subsurface is mapped by line arranging a series of wave-train sources, such as small dynamite explosions, in a grid pattern. As each source is activated, it generates a wave train that moves downward at a speed determined uniquely by the rock's elastic characteristics. As rock interfaces are crossed, the elastic characteristics encountered generally change abruptly, which causes part of the energy to be reflected back to the surface, where it is recorded by seismic instruments. The seismic records must be processed to correct for positional differences between the source and the receiver, for unrelated wave trains, and for multiple reflections from the rock interfaces. Then the data acquired at each of the specific source locations are combined to generate a physical profile of the subsurface, which can eventually be used to select targets for drilling.
It can be inferred from the passage that the seismic-reflection method would be likely to yield an inaccurate physical profile of the subsurface in which of the following circumstances?
- A If the speed at which the wave train moved downward changed
- B If the receiver were not positioned directly at the wave-train source
- C If the rock on one side of a rock interface had similar elastic characteristics to those of the rock on the other side
- D If the seismic records obtained for the different sources in a grid were highly similar to each other
- E If there were no petroleum deposits beneath the area defined by the grid of wave-train sources