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.

The answer correct is C

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?

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.

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?

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.

A serious inference question NEVER shows exactly the same information or the same wording in the passage or stem.

Otherwise would not inference

Regards

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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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.

Can someone please provide the answer and explanation for all the question here ?

17. The passage is primarily concerned with

(A) describing an important technique

The passage describes about a technique and how it is performed to deliver a positive result

(B) discussing a new method

wrong

(C) investigating a controversial procedure

wrong

(D) announcing a significant discovery

wrong

(E) promoting a novel application

Basically as B.


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

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.

The number is not mentioned. C is the correct answer

(D) nature of the reflectivity of the rock interfaces
(E) properties of rocks through which the wave train has traveled


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


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.

If all the process described above was similar then would be inaccurate. We do need the differences and then those are adjusted.

(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



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.

The technique to discover the petroleum is asserted, it is still an important element to find new reservoir and how it is connected is explained.

(D) A series of examples is presented, and a conclusion is drawn.
(E) A hypothesis is advanced, and supporting evidence is supplied.

What is the idea behind the seismic recordings? It is to avoid the "hard rock" on the earth crust while drilling for oil reserves, right? Or they want the location of "hard rocks" and drill on that part of the rock?
If its to avoid the "hard rocks", then in Q19, option C, the similarities of the rock characteristics means that the Rock is not hard on the other side and the seismic wave wouldn't record anything, doesn't this in favor for this procedure?,

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

Thank you for taking your time and explaining it. Really Appreciate it.
So, they are using the seismic recordings to avoid the "hard rocks" while drilling, right?

Also, after looking at your explanation, i came to infer that, similar characteristics of a rock does not help.
1) if the first rock is hard, and the second rock is hard as well, the seismic response will be useless.
2) if the first rock is soft, and the second rock is soft as well, the seismic response will be useless.
3) if the first rock is soft, and the second rock is hard, the seismic response will be helpful here, so as to avoid the hard rock while drilling?

This is EXACTLY what they do. It is the perfect example

They DO NOT drill before. AFTER they found something and all the data are analyzed

Read carefully