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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]
17. The passage is primarily concerned with

(A) describing an important technique

Yes.

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.

(B) discussing a new method

No. New method of what? Insufficient information in the choice

(C) investigating a controversial procedure

No. There is no description of a controversial procedure in the passage

(D) announcing a significant discovery

No.

(E) promoting a novel application

No.


18. According to the passage, in the seismic-reflection method all of the following have a significant effect on the signal detected by the seismic instruments EXCEPT the

(A) presence of unrelated wave trains

No. This is mentioned in the passage.

(B) placement of the seismic instruments

No. This is mentioned in the passage.

(C) number of sources in the grid pattern

Yes. There is no mention of this in the passage.

(D) nature of the reflectivity of the rock interfaces

No. This is mentioned in the passage.

(E) properties of rocks through which the wave train has traveled

No. This is mentioned in the passage.


19. 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

No.

(B) If the receiver were not positioned directly at the wave-train source

No.

(C) If the rock on one side of a rock interface had similar elastic characteristics to those of the rock on the other side

Yes.

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.


(D) If the seismic records obtained for the different sources in a grid were highly similar to each other

No.

(E) If there were no petroleum deposits beneath the area defined by the grid of wave-train sources

No.



20. Which of the following best describes the organization of the passage?

(A) A method is criticized, and an alternative is suggested.

No.

(B) An illustration is examined, and some errors are exposed.

No.

(C) An assertion is made, and a procedure is outlined.

Yes.

Assertion: 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.

Procedure: "In field practice, a subsurface is mapped by arranging a series of wave-train sources, such as small dynamite explosions, in a grid pattern. " to the end of the paragraph.


(D) A series of examples is presented, and a conclusion is drawn.

No.

(E) A hypothesis is advanced, and supporting evidence is supplied.

No.
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]
1
Carcass wrote:
Basically this is a weaken the argument question.

IF we do know from the information in the passage that X occurs, then if X will NOT occours we will weaken the argument

As rock interfaces are crossed,

The roks are crossed by signals

the elastic characteristics encountered generally change abruptly,

based on the rock elasticity the waves changed direction. I.E the rock is really solid the waves changes is > when a rock is more elastic because the impact is absorbed more

which causes part of the energy to be reflected back to the surface,

hard rock= wave more reflection back. I.E a greater bounce

elastic rock= smaller bounce back


where it is recorded by seismic instruments.

we record all these data


The seismic records must be processed to correct for positional differences between the source and the receiver, for unrelated wave trains,

the hypothetical line between the sender (rock) and the receiver (the radar) is balanced t avoid devisations


(C) If the rock on one side of a rock interface had similar elastic characteristics to those of the rock on the other side

If the different rocks are similar, our waves are similar and our data to process are similar. If all these data are alike in meaning we are not able to spot the differences.

therefore, we will NOT be able to see the differences among rocks and we are not able to find what we are looking for

I hope now is clear.
You must read carefully. Thje answer is there. Pretty straight


Actually, you needn't know how the whole thing works. Just pick the right answer from the passage itself. The GRE is subject specialization agnostic.

You needn't know any subject in-depth to answer the questions.
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]
1
The process is similar to refraction of light as it passes through a lens. There is a change in the medium or a change in the refractive index (as light rays move from air to glass) because of which the light rays bend.

Here instead of light rays we have seismic wave-trains. And instead of change caused by moving from air to glass, the waves move through rocks of different elasticity (refractive indices) causing them to bend and thus are reflected back to the source.
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]
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