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Finding thickness of upper layer

  1. Feb 9, 2014 #1
    My task is

    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. jcsd
  3. Feb 9, 2014 #2

    mfb

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

    No.
     
  4. Feb 9, 2014 #3
    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.
     
  5. Feb 9, 2014 #4

    mfb

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    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.
     
  6. Feb 9, 2014 #5
    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
     
  7. Feb 9, 2014 #6

    mfb

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    I think you don't need the critical time here.
     
  8. Feb 9, 2014 #7
    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?
     
  9. Feb 9, 2014 #8

    mfb

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    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.
     
  10. Feb 10, 2014 #9
  11. Feb 10, 2014 #10

    mfb

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    This is not the critical distance.
     
  12. Feb 10, 2014 #11
    confused, taskes says ''Based on this information, what is the thickness of the upper layer?''
     
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