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# Because of its accuracy in outlining the Earth's subsurface,

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Because of its accuracy in outlining the Earth's subsurface, [#permalink]  22 Aug 2019, 08:46
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Question Stats:

58% (03:47) correct 41% (05:47) wrong based on 34 sessions
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 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.
17. The passage is primarily concerned with

(A) describing an important technique
(B) discussing a new method
(C) investigating a controversial procedure
(D) announcing a significant discovery
(E) promoting a novel application

[Reveal] Spoiler: OA
A

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
(B) placement of the seismic instruments
(C) number of sources in the grid pattern
(D) nature of the reflectivity of the rock interfaces
(E) properties of rocks through which the wave train has traveled

[Reveal] Spoiler: OA
C

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
(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

[Reveal] Spoiler: OA
C

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

(A) A method is criticized, and an alternative is suggested.
(B) An illustration is examined, and some errors are exposed.
(C) An assertion is made, and a procedure is outlined.
(D) A series of examples is presented, and a conclusion is drawn.
(E) A hypothesis is advanced, and supporting evidence is supplied.

[Reveal] Spoiler: OA
C

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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  06 Oct 2019, 11:43
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Question 18

Why not B?.

The very essence of the seismic instrument is to be arranged in a series form, not a single one, without which the signal cannot be detected. Therefore, the number of sources can be very important. The reading also states "Then the data acquired at each of the specific source locations", implying that there is no single source, but a number of that.

Does the passage talk about the nature of the reflectivity? Here are the information about the reflections stated in the passage; "multiple reflections from the rock interfaces" and "part of the energy to be reflected back." Does either of these show the nature of reflectivity?
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  06 Oct 2019, 13:22
Expert's post
In the following sentence, we do have both the wrong option and the right option.

In field practice, a subsurface is mapped by arranging a series of wave-train sources, such as small dynamite explosions, in a grid pattern.

(B) placement of the seismic instruments

a subsurface is mapped by arranging a series of wave-train sources

(C) number of sources in the grid pattern

The sentence above says that we arrange the sources in a grid. What we DO NOT know is the number. It is not specified.

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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  13 Feb 2020, 23:40
Carcass wrote:
In the following sentence, we do have both the wrong option and the right option.

In field practice, a subsurface is mapped by arranging a series of wave-train sources, such as small dynamite explosions, in a grid pattern.

(B) placement of the seismic instruments

a subsurface is mapped by arranging a series of wave-train sources

(C) number of sources in the grid pattern

The sentence above says that we arrange the sources in a grid. What we DO NOT know is the number. It is not specified.

Regards

However, the passage does say "As each source is activated, it generates a wave ... reflected back to the surface, where it is recorded by seismic instruments"

Would it not be true then that each source creates its a wave which is then detected by the instrument, so the number of sources will have an affect on the signal?
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  14 Feb 2020, 01:01
Expert's post
Sorry,

is it a question ??
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  14 Feb 2020, 09:06
Carcass wrote:
Sorry,

is it a question ??

Yeah sorry,

Why is 18C correct? Why does the number of sources not affect the detected signals?
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  14 Feb 2020, 10:49
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greprepper2 wrote:
Carcass wrote:
Sorry,

is it a question ??

Yeah sorry,

Why is 18C correct? Why does the number of sources not affect the detected signals?

Because it's neither implied or stated that NUMBER of sources would would have significant effect on signal detection.
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  16 Feb 2020, 09:45
Can someone explain q19?
I eliminated answer choice to narrow down to B & C & D but ended up choosing the wrong answer B. Is this the portion used to answer this question "As each source is activated, it generates a wave train that moves downward at a speed determined uniquely by the rock's elastic characteristics." If so, can someone explain?
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  16 Feb 2020, 14:13
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mind wrote:
Can someone explain q19?
I eliminated answer choice to narrow down to B & C & D but ended up choosing the wrong answer B. Is this the portion used to answer this question "As each source is activated, it generates a wave train that moves downward at a speed determined uniquely by the rock's elastic characteristics." If so, can someone explain?

Yes, and that this line: 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.

If the elastic characteristics are similar on one interface of a rock to the other, then the change in elastic characteristics would be close to zero. if that happens, then net energy would be zero; therefore no energy would be reflected back to the surface, where it is recorded by seismic instruments.
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  16 Feb 2020, 18:25
theBrahmaTiger wrote:
mind wrote:
Can someone explain q19?
I eliminated answer choice to narrow down to B & C & D but ended up choosing the wrong answer B. Is this the portion used to answer this question "As each source is activated, it generates a wave train that moves downward at a speed determined uniquely by the rock's elastic characteristics." If so, can someone explain?

Yes, that this line: 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.

If the elastic characteristics are similar on one interface of a rock to the other, then the change in elastic characteristics would be close to zero. if that happens, then net energy would be zero; therefore no energy would be reflected back to the surface, where it is recorded by seismic instruments.

Great answer, this particular question seems to require a good dose of inference, not like some other inference questions which oftentimess show the answer right in the passage.
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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  16 Feb 2020, 18:31
Expert's post
A serious inference question NEVER shows exactly the same information or the same wording in the passage or stem.

Otherwise would not inference

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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  16 Feb 2020, 18:45
I agree, this one is not like other inference ones that I’ve encountered. The passage just gives information about elastic characteristics of rocks and the difference of elasticity level causes the changes in seismic record. We do have to infer that if the rocks having the same elasticity, then nothing will be shown in the seismic records. This question has only 34% test takers answer correctly (stat in the book), no doubt about its difficulty at least for me

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Re: Because of its accuracy in outlining the Earth's subsurface, [#permalink]  29 May 2020, 12:07
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Carcass wrote:
In the following sentence, we do have both the wrong option and the right option.

In field practice, a subsurface is mapped by arranging a series of wave-train sources, such as small dynamite explosions, in a grid pattern.

(B) placement of the seismic instruments

a subsurface is mapped by arranging a series of wave-train sources

(C) number of sources in the grid pattern

The sentence above says that we arrange the sources in a grid. What we DO NOT know is the number. It is not specified.

Regards

Also, I found the statement "positional differences between the source and the receiver" to support the notion that the placement of the seismic instruments is important, thus eliminating answer choice B.

C was tricky because the passage does mention "grid pattern" but does not mention any information about the number. Remember, GRE is trying to trick you when they include a term like "grid pattern". Just because the answer has "grid pattern" in it doesn't mean it's correct. But GRE is hoping you skim through that option or are not able to distinguish a half-correct statement from a fully correct or incorrect one.
Re: Because of its accuracy in outlining the Earth's subsurface,   [#permalink] 29 May 2020, 12:07
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