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In the film Kong: Skull Island, scientists investigate an island's structure by dropping explosive charges from helicopters:

Kong: Skull Island clip
Source: YouTube trailer

In reality, geophysicists do something similar using buried explosives.

Seismic charge detonation
Source: fondasol.fr

In simple terms, how does detonating explosives in the ground help us to understand the subsurface structure?

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  • $\begingroup$ This question would be a better fit for movies.stackexchange.com , since it's not about real-world seismology, but rather what some fictional explosives were supposed to do in a fictional world involving 30-metre gorillas and three-headed dragons. If you want to know how controlled-source seismology works in the real world, this Wikipedia page might be a good place to start, but I doubt it has much in common with Hollywood "science" :). $\endgroup$ – Pont Mar 12 '17 at 15:10
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    $\begingroup$ Having now watched the film trailer on YouTube, I'd like to clarify that, outside of the magical realm of action films, nobody performs seismic surveys by lobbing bombs out of low-flying helicopters. $\endgroup$ – Pont Mar 12 '17 at 15:18
  • $\begingroup$ Can you tell us a bit more about what exactly was being done or proposed in the film? It might be that what they did is completely contrary to physics, in which case it would be off-topic here. From talking to moderators over on Movies your question may be on-topic there if there is an in-movie explanation for what or how. $\endgroup$ – gerrit Mar 13 '17 at 18:36
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    $\begingroup$ @gerrit AFAIK we don't currently have a high-level question on "how do seismic surveys work" and I think such a question would be good to have here. But it's still unclear to me whether OP is interested in how this stuff works in the real world (good fit for this site), or how it supposedly worked in the film (good fit for movies.se). $\endgroup$ – Pont Mar 16 '17 at 8:24
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    $\begingroup$ @Pont: I am interested in how this stuff works in the real world. $\endgroup$ – avito009 Mar 17 '17 at 13:53
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Explosives are just one way we can produce seismic waves in the ground. By placing geophones on the surface, which measure the rate of change of displacement of the surface, we can actually guess where there are boundaries between different rock types in the subsurface. When we detonate our chosen 'seismic wave generator' the geophones begin recording this change in displacement. As the seismic waves propagate and hit boundaries between rocks of different acoustic impedances (resistance to propagation of seismic waves) some of that wave energy will reflect back to the surface and some will be transmitted downward (http://petrowiki.org/Seismic_wave_propagation#Reflection_coefficients). The amplitude and sign (positive/negative) of those recorded wiggles tell us about how big the contrast in acoustic impedance is as well as if acoustic impedance is increasing/decreasing downwards. This allows us to produce a picture of the subsurface where the x-axis is offset of the geophones from the wave source and the y-axis is the time it takes for the wave to travel down and back up as shown in this picture:

Example seismic section

However it is never that simple. A lot of processing has to be done to make sure our initial image is the most accurate picture of the subsurface we can make and even then there is still uncertainty. It must also be stressed that we don't actually know about the properties of the rocks themselves just that boundaries exist between these rocks. If we want to estimate that we can use processes such as seismic inversion.

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I think I have the answer. Charges are dropped on the ground. This generates a wave underneath the surface of the earth. Analysing the time the waves take to return provides valuable information about rock types and possible gases or fluids in rock formations. This is similar to the use of ultrasound in medicine.

There are two primary means of generating these waves: with dynamite and with a process called vibroseis. These charges in the movie must have been dynamite.

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  • $\begingroup$ You don't have to use dynamite. Any powerful explosive will do. $\endgroup$ – bon Mar 16 '17 at 9:04
  • $\begingroup$ I think the waves used are sound waves. $\endgroup$ – avito009 Mar 16 '17 at 11:01

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