Finding thickness of upper layer

1. Feb 9, 2014

hopelesss

Let's consider a model made up of two horizontal layers: the upper layer has a velocity
Vp1=3.0 km/s and the lower layer (or half space) has a velocity Vp2= 5.0 km/s. Aseismic experiment is conducted at the surface and showsthat the P wave refracted in the lower layer has a critical distance of 7.5 km and a critical time of 4.17 s. Based on this information, what is the thickness of the upper layer?

1. can i solve this by useing ti= (2z(v2^2 -v1^2)^1/2) / (v1*v2) and then
z= (ti*v1*v2) / 2(v2^2-v1^2)^1/2?
2. Is critical distance the thickness of layer 2?

greatful for any help

2. Feb 9, 2014

Staff: Mentor

Check where the formula comes from, that should answer the question.
Where did the critical distance go there?

No.

3. Feb 9, 2014

hopelesss

It only say ''two-layer case with horizontal interface'' and the intercept time is given by .... and solving for refractor depth. And i cant find any equation with xcrit in it. tryed to google it but did not find any equation with Xcrit in it.

4. Feb 9, 2014

Staff: Mentor

Searching for "critical distance seismics" gives the Wikipedia article about seismic refraction as one of the first hits, and this article has a formula. I think it is hard to miss that.

5. Feb 9, 2014

hopelesss

I only find seismic refraction when i google that.
I still dont get it. i would still miss the T0 if i use that equation. And the critical distance and critical time info would not have been used

6. Feb 9, 2014

Staff: Mentor

I think you don't need the critical time here.

7. Feb 9, 2014

hopelesss

can i do this
ic= sin^-1 3/5 = 36,8
tan 36,8 = (7,5/2)/b = 0,75*3,75 = 2,8
And say upper layer is 2,8meters?

8. Feb 9, 2014

Staff: Mentor

Meters? Certainly not.

I don't understand what you calculated there (please add a sketch), in particular I don't see how this is supposed to include the path of the waves at the critical distance.

9. Feb 10, 2014

hopelesss

10. Feb 10, 2014

Staff: Mentor

This is not the critical distance.

11. Feb 10, 2014

hopelesss

confused, taskes says ''Based on this information, what is the thickness of the upper layer?''