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I want to do a quick primer on refreaction

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And our focus here is going to be on seismic

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waves but the principal how things refract when they go from a fast to a slow medium

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or a slow to a fast medium. It`s actually the same as you would see when your doing

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When your studying ligth waves or any type of wave.

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So lets think about it a little bit.

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I have a slow medium rigth over here and lets say I have a fast medium over here.

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lets say just for the, so that we cantravel thorugh both solid and liquid. Lets think,

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about maybe P-Waves and a slow medium maybe

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some type of liquid and our fasr medium could be some type of solid.

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So let me draw the boundry right over here.

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And lets say, so if I have something that, a P-Wave lets say it going through the water

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And going right perpendicular to the boundry

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We`ll then just continue to travel

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in the faster medium in the same direction.

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Going right if it goes right at the boundry

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If it goes right at the boundry and it`l just tracel faster in the faster medium

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And thats because that faster medium is going to be more dense and the molecules

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are going to bump into each other faster.

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In the same amount of time more molucules kind of

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a chain reaction is going to travel further because they are more closely packed and they

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rebound faster than it would in the slow mediums.

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So thats obviusly over is going on

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It has not been deflected.

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And just as a bit of a reminder.

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In general refraction is when a wave gets defelcted.\

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Reflection is when it bounces back refraction

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is when it gets deflected a little bit.

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Let me just make that clearer.

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So I have some type of boundry and I have of

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boundry here and I have a wave that bounces off thats reflection.

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But if the way goes through the boundry.

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and just gets bent a little bit its direction changes that is refraction.

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And thats what we are talking about, So cleary so far this P-Wave.

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Has not been refracted but if this P-Wave comes in

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this angle so lets make this P-Wave

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come in at a angle whats going to happen is,

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the way you should think about it is, the easiest way to think about which direction

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it will be refracted or atleast the way I think about it

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is literally I imagine some type of vehicle

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with wheels on it. This is the top view of my vehicle.

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So if I have some type of vehicle and the wheels will be able to move slowly in this medium.

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Kind of viewed as kind of on mud.

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So it doesn't get fraction and then the fast medium it is a,

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road so it gets good fraction.

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It can move faster, so whats going to happen

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when the vehicle gets to the boundry.

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Well this bottom rigth wheel is going to go on the fast medium.

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Before any of the other wheels do. So it going,

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to get the fraction first these wheels on teh left

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side of the vehicle these wheels right here,

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These are still going to be stuck in the mud so it going to happen is.

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This wheel right over here is moving faster so its essentialy going to be able to turn the,

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vehicle these guys are still stuck in the mud.

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And so you fast foward a little bit,

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the direction of the vehicle will change and so the vehicle will now move in a derection

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something like this the same thing what happened

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in a wave. If the P-Wave is approaching the boundry like this and something,

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annlogus is happening at the molecular level

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You can kind of view it as a even billard balls and maybe even,

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they are kind of hitting each other well I,

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wont go into that because that can kind of get a little confusing depending the diffrent cases and,

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the diffrent boundry`s. But this is the easiest way to think about which direction it will refract.

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And hopefully makes a little bit of a intuitive sence.

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And so when your going from a slow to fast medium or a P-Wave,

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it would kind of, its angle would excentuate in that direction if you would,

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went from the fast medium too the slow medium once again you can just go to same,

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thought expireeement so lets say you have our wave,

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coming like that. Draw! the car visulize the.

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Vizulize the car right here. And you say well,

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look this tire is going to get stuck in the mud,

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because it is not going from, it was on the road now from this top,

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left tire is getting stuck in the mud first,

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so it is going to be moving slower so these tires are going to be,

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able to move faster. So the vehicle is going,

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to turn. So you'll be refracted in a direction,

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like that. When your going from the fast to the slow medium.

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So that is just a primer on refraction generally.

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Now lets think about what would happen when sound waves are travelling in,

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through the earth. And this all will help,

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infrom us of essentially how to we figure,

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out what the actual structure of the earth is.

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So if the earth was just made up of some uniform material and you had,

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You had a earthquake rigth here.

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On earth maybe a little below the surface,

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so like you know its in the actualy happening in the crust bit a little bit below

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the surface of the earth. If earth was a,

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uniform desenty is it was all the saem material how would those, lets just think,

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about the P-Waves how, would cause P-Waves,

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can travel in anything lets think about how,

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those P-Waves would travel. Well they would just go in strait lines there is nothing,

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that would refract the P-Waves. It would,

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just go in straight lines radialy outward,

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from where the where the earth earthquake occured.

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Now at a first approximation know that as,

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we go deeper and deeper in earth there is,

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more and more rock above that the weight of,

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that rock is kind of compressing the rock,

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below. So you get higher and higher pressures,

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and higher and higher densities. So how,

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would, so this is a uniformer.

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But lets imagine a earth that is made up of,

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unifrom material that is all solid.

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A completely solid earth but one where the,

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density is consentantly increasing at as you go,

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down. So lets just think about it in beofre,

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we go into the countinous case cause we,

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are talking about the density as you go,

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deeper it is getting countinously more,

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dense. Lets think about the discreet case,

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where we have we the least dense layer.

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So let me draw right over here.

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So lets say this is the surface of the earth,

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and this is least dense then lets say you,

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have another layer over here that is more,

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dense. So this is more dense. Let say you,

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have another layer that is even more dense,

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so you have another layer over here that is even more dense.

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Then lets do one more layer lets do,

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this layer here. This is the densest layer,

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densest. So in general your P-Wave, your,

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seismic wave is going to travel faster in,

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denser material. So it is going to travel,

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the fastest here than here than here.

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So it is going to the slowest in the least dense material.

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So if your coming in an angle.

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If your coming in an angle lets think about,

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whats going to happen. So lets say have your,

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P-Wave coming in at a angle like this. SO

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it is going straight thorugh the least dense,

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material. So whats going to happen into the,

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let me do a slightly shallower angle so lets,

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say it is like that. What is going to happen ,

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when it goes into the more dense material.

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So once again lets imagine in our little car,

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so this tire is going to be able to go a little,

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faster before the tires on the other side,

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So the car is going to be deflected to the left.

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To the down left so now it is going to travel like,

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this so it is now going travel like this,

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now what is going to happen at this boundry.

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Once again imagine the car this tire right here is,

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going to be able to travel faster before,

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the other tire so it will be deflected even more in that direction.

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Then we go into the densest material.

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Once again tires on kind of the bottom side when we look at it this way.

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We are going to be able to get or move faster before the otehr tires.

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So we are going to get deflected even more.

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So you see as you go least dense material to more,

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dense material your kind of curving outward.

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So if this was continous if we had a countinous,

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kind of countinous structure where as you go down it just gets more and more dense.