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- I want to do a quick primer on refreaction
- And our focus here is going to be on seismic
- waves but the principal how things refract when they go from a fast to a slow medium
- or a slow to a fast medium. It`s actually the same as you would see when your doing
- When your studying ligth waves or any type of wave.
- So lets think about it a little bit.
- I have a slow medium rigth over here and lets say I have a fast medium over here.
- lets say just for the, so that we cantravel thorugh both solid and liquid. Lets think,
- about maybe P-Waves and a slow medium maybe
- some type of liquid and our fasr medium could be some type of solid.
- So let me draw the boundry right over here.
- And lets say, so if I have something that, a P-Wave lets say it going through the water
- And going right perpendicular to the boundry
- We`ll then just continue to travel
- in the faster medium in the same direction.
- Going right if it goes right at the boundry
- If it goes right at the boundry and it`l just tracel faster in the faster medium
- And thats because that faster medium is going to be more dense and the molecules
- are going to bump into each other faster.
- In the same amount of time more molucules kind of
- a chain reaction is going to travel further because they are more closely packed and they
- rebound faster than it would in the slow mediums.
- So thats obviusly over is going on
- It has not been deflected.
- And just as a bit of a reminder.
- In general refraction is when a wave gets defelcted.\
- Reflection is when it bounces back refraction
- is when it gets deflected a little bit.
- Let me just make that clearer.
- So I have some type of boundry and I have of
- boundry here and I have a wave that bounces off thats reflection.
- But if the way goes through the boundry.
- and just gets bent a little bit its direction changes that is refraction.
- And thats what we are talking about, So cleary so far this P-Wave.
- Has not been refracted but if this P-Wave comes in
- this angle so lets make this P-Wave
- come in at a angle whats going to happen is,
- the way you should think about it is, the easiest way to think about which direction
- it will be refracted or atleast the way I think about it
- is literally I imagine some type of vehicle
- with wheels on it. This is the top view of my vehicle.
- So if I have some type of vehicle and the wheels will be able to move slowly in this medium.
- Kind of viewed as kind of on mud.
- So it doesn't get fraction and then the fast medium it is a,
- road so it gets good fraction.
- It can move faster, so whats going to happen
- when the vehicle gets to the boundry.
- Well this bottom rigth wheel is going to go on the fast medium.
- Before any of the other wheels do. So it going,
- to get the fraction first these wheels on teh left
- side of the vehicle these wheels right here,
- These are still going to be stuck in the mud so it going to happen is.
- This wheel right over here is moving faster so its essentialy going to be able to turn the,
- vehicle these guys are still stuck in the mud.
- And so you fast foward a little bit,
- the direction of the vehicle will change and so the vehicle will now move in a derection
- something like this the same thing what happened
- in a wave. If the P-Wave is approaching the boundry like this and something,
- annlogus is happening at the molecular level
- You can kind of view it as a even billard balls and maybe even,
- they are kind of hitting each other well I,
- wont go into that because that can kind of get a little confusing depending the diffrent cases and,
- the diffrent boundry`s. But this is the easiest way to think about which direction it will refract.
- And hopefully makes a little bit of a intuitive sence.
- And so when your going from a slow to fast medium or a P-Wave,
- it would kind of, its angle would excentuate in that direction if you would,
- went from the fast medium too the slow medium once again you can just go to same,
- thought expireeement so lets say you have our wave,
- coming like that. Draw! the car visulize the.
- Vizulize the car right here. And you say well,
- look this tire is going to get stuck in the mud,
- because it is not going from, it was on the road now from this top,
- left tire is getting stuck in the mud first,
- so it is going to be moving slower so these tires are going to be,
- able to move faster. So the vehicle is going,
- to turn. So you'll be refracted in a direction,
- like that. When your going from the fast to the slow medium.
- So that is just a primer on refraction generally.
- Now lets think about what would happen when sound waves are travelling in,
- through the earth. And this all will help,
- infrom us of essentially how to we figure,
- out what the actual structure of the earth is.
- So if the earth was just made up of some uniform material and you had,
- You had a earthquake rigth here.
- On earth maybe a little below the surface,
- so like you know its in the actualy happening in the crust bit a little bit below
- the surface of the earth. If earth was a,
- uniform desenty is it was all the saem material how would those, lets just think,
- about the P-Waves how, would cause P-Waves,
- can travel in anything lets think about how,
- those P-Waves would travel. Well they would just go in strait lines there is nothing,
- that would refract the P-Waves. It would,
- just go in straight lines radialy outward,
- from where the where the earth earthquake occured.
- Now at a first approximation know that as,
- we go deeper and deeper in earth there is,
- more and more rock above that the weight of,
- that rock is kind of compressing the rock,
- below. So you get higher and higher pressures,
- and higher and higher densities. So how,
- would, so this is a uniformer.
- But lets imagine a earth that is made up of,
- unifrom material that is all solid.
- A completely solid earth but one where the,
- density is consentantly increasing at as you go,
- down. So lets just think about it in beofre,
- we go into the countinous case cause we,
- are talking about the density as you go,
- deeper it is getting countinously more,
- dense. Lets think about the discreet case,
- where we have we the least dense layer.
- So let me draw right over here.
- So lets say this is the surface of the earth,
- and this is least dense then lets say you,
- have another layer over here that is more,
- dense. So this is more dense. Let say you,
- have another layer that is even more dense,
- so you have another layer over here that is even more dense.
- Then lets do one more layer lets do,
- this layer here. This is the densest layer,
- densest. So in general your P-Wave, your,
- seismic wave is going to travel faster in,
- denser material. So it is going to travel,
- the fastest here than here than here.
- So it is going to the slowest in the least dense material.
- So if your coming in an angle.
- If your coming in an angle lets think about,
- whats going to happen. So lets say have your,
- P-Wave coming in at a angle like this. SO
- it is going straight thorugh the least dense,
- material. So whats going to happen into the,
- let me do a slightly shallower angle so lets,
- say it is like that. What is going to happen ,
- when it goes into the more dense material.
- So once again lets imagine in our little car,
- so this tire is going to be able to go a little,
- faster before the tires on the other side,
- So the car is going to be deflected to the left.
- To the down left so now it is going to travel like,
- this so it is now going travel like this,
- now what is going to happen at this boundry.
- Once again imagine the car this tire right here is,
- going to be able to travel faster before,
- the other tire so it will be deflected even more in that direction.
- Then we go into the densest material.
- Once again tires on kind of the bottom side when we look at it this way.
- We are going to be able to get or move faster before the otehr tires.
- So we are going to get deflected even more.
- So you see as you go least dense material to more,
- dense material your kind of curving outward.
- So if this was continous if we had a countinous,
- kind of countinous structure where as you go down it just gets more and more dense.