# Homework Help: Seismic reflection question

1. Nov 18, 2015

### RJLiberator

1. The problem statement, all variables and given/known data
The question is set up by a diagram.

It has:

Layer 1 velocity = 6km/s
Layer 2 = calculated velocity of 7.58 km.s
Layer 3 = reflection

The angle of the source in layer 1 is 34 degrees, the angle in layer two is then 52.33 degrees
The source then reflects off of layer 3.

Question: Do these reflections tell you anything about the velocities in layer 3?

2. Relevant equations
Snells law

3. The attempt at a solution

I am thinking two things:
1) Either the answer is NO, it does not tell us about the velocity, it only tells us about the composition in that the source does not travel through it, but reflects off of it.
2) YES it does tell us about the velocity of layer three because it does not go through it, so layer three must be significantly slower.

I am tending to agree more with argument one, any help here?

2. Nov 19, 2015

### davenn

hi there

is there an actual diagram you can post ?
just struggling to visualise the location of the source relative to the 3 layers and also the orientation of
the layers to each other

Dave

3. Nov 19, 2015

### RJLiberator

Thank you :)

4. Nov 19, 2015

### RJLiberator

Wouldn't reflection mean that layer 3 is much slower velocity than layer two and so the wave chooses to go through layer 2?

Or is entirely the composition of the layer that dictates it.

5. Nov 19, 2015

### davenn

Thanks for the diagram

I will be honest with you, its been 20 yrs since I have done any seismic reflection/refraction stuff a way back in my uni days
an pretty rusty. a lot easier for me to help in your other thread on seismic waves

or consider a 3rd option

Either the answer is NO, it does not tell us about the velocity, it does not tell us about the composition, the source does travel through it, but is dissipated within the layer (3) not reaching any other layer to reflect off. Also the angle of any signal reflected off the L2/L3 boundary is not picked up by the receiver ( when dealing with the angles specified)

have a read of this link --- it may well help and it will also give me a refresher course

https://en.wikipedia.org/wiki/Reflection_seismology

cheers
Dave