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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]

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Updated on: 24 Dec 2018, 21:40
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Question 1
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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 (petroleum reserves: oil 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.

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

Originally posted by adiraju on 07 Jul 2009, 03:14.
Last edited by workout on 24 Dec 2018, 21:40, edited 2 times in total.
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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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07 Jul 2009, 14:28
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 (petroleum reserves: oil 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.

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

But Why not B ?

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. Why not answer B?

Seismic records is the key here.

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 rock on one side of a rock interface had similar elastic characteristics to those of the rock on the other side then no change in elastic characteristics will be encountered and nothing will be reflected back to the surface. No reflections would be recorded by seismic instruments.
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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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17 Sep 2015, 00:06

The reason why B is incorrect is that the error resulting from positional differences between the wave-train source and the receiver can be processed and corrected, see:

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.
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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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06 Jan 2019, 22:44

+1 kudos to the posts containing answer explanations of all questions

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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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07 Jan 2019, 07:32
C vs D anyone?

really confusing!
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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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07 Jan 2019, 10:12
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1 min 41 secs to the correct answer.

The passage talks about the seismic-reflection method's importance in the field of oil reserves due to its accuracy to map the subsurface of the earth. Wave trains ( created by dynamite reflection) are positioned and seismographs are used to read the reflection of these wave trains. "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. " is the crux of the method. Certain inaccuracies are needed to be dealt with and are summarized at the end of the passage.

Inference type - we need to ensure 100% support for the option we choose from the passage
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 Nah. The speeds are characteristic of the rock type but we don't care about the exact speed as such. Discard.
(B) If the receiver were not positioned directly at the wave-train source Nowhere in the passage it is mentioned that the receiver is placed directly above. There are multiple sources and multiple reflections which are recorded by multiple receivers. Discard.
(C) If the rock on one side of a rock interface had similar elastic characteristics to those of the rock on the other side Bingo - as highlighted in the summary - the fact that different rock types have different elastic properties is the reason behind a part of the wave train getting reflected off the boundary, if different rocks had identical elasticities reflection would not occur and hence method would fail
(D) If the seismic records obtained for the different sources in a grid were highly similar to each other Nowhere the passage mentions that the records are different for different sources - so we cannot conclusively say that such an occurrence would cause the results to be inaccurate. Discard.
(E) If there were no petroleum deposits beneath the area defined by the grid of wave-train sources Irrelevant to the discussion. Discard.

Hope the explanation makes the doubts clear between C & D.
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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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14 Jan 2019, 08:42
Gladiator59 Can you clear this doubt, Question asks 'likely to yield inaccurate physical profile' in case of c there might be no report made at all cuz there is no reflection which doesn't mean it will make an inaccurate report and in the passage there is nothing mentioned about this scenario all we can infer from it is this method will fail but not that it will give inaccurate report. Even though I am confused between opt b and c but I think both are not close to correct ans maybe if opt b changed to refer there might be positional difference calc error it would be much better opt. Is this a poor quality question or my thinking is wrong?
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Re: Because of its accuracy in outlining the Earth's subsurface,  [#permalink]

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14 Jan 2019, 08:53
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Hi akhil101, welcome to the GMAT Club!

Your main concern is Option (B) vs. Option (C) and you feel none of those two deserve to be the OA.

Let's consider option (B) - we want to look for a way in which we could determine that the results would be inaccurate. The passage does not mention a straight line positioning at all as necessary, for all we know the multiple sensors are positioned in a web-like manner. But again, since nothing is mentioned we cannot opt for this.

Option (C) on the other hand is different. You are correct to point out that this case is not mentioned directly but the passage clearly indicates that the difference at the boundary lead to reflection and this reflection is what we measure. If there was no difference there would still be a boundary but we would just not be able to measure it. So, the report would still be "inaccurate" - we don't need to worry about "how much is this inaccuracy" for in Option (C) clearly we will have an inaccuracy but in Option (B) we don't know and we can't conclude.
That is why Option (C) is the clear answer here.

Does all this make sense?

akhil101 wrote:
Gladiator59 Can you clear this doubt, Question asks 'likely to yield inaccurate physical profile' in case of c there might be no report made at all cuz there is no reflection which doesn't mean it will make an inaccurate report and in the passage there is nothing mentioned about this scenario all we can infer from it is this method will fail but not that it will give inaccurate report. Even though I am confused between opt b and c but I think both are not close to correct ans maybe if opt b changed to refer there might be positional difference calc error it would be much better opt. Is this a poor quality question or my thinking is wrong?
NO REPORT AT ALL IS PRETTY INACCURATE - say for example one goes to a doctor and does a blood report for a possible case of malaria. If one has malaria but the report does not show it at all would that report be inaccurate? Think on these lines...
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Re: Because of its accuracy in outlining the Earth's subsurface,   [#permalink] 14 Jan 2019, 08:53
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