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5.2: Seismic Refraction (Single Layer)

  • Page ID
    3542
  • One of the most important uses of seismic waves is for seismic surveys. Seismic surveys can be used to determine shallow (~100s of meters) crustal structure. The surveys shed light on the layers of rock, including each ones thickness and seismic velocity (and from seismic velocity we can estimate rock type). We can also determine structures such as faults and basins, history of deposition, and look for oil and gas. Primarily, we use P-waves for seismic surveys as they are faster and easier to see on seismograms. There are two types of seismic surveys

    1. Refraction ie 'critical refraction' which is useful for determining velocities of layers
    2. Reflection, which is useful for determining layers and structure

    In seismic surveys we can use geometry and travel time of waves to determine structure and velocity. Today we are going to focus on refraction surveys. First, we are going to quickly review what we learned in the last section: slowness/Snell's Law and Reflection and Transmission 

    FIGURE lecture 2 review snells law

    \[\frac{\sin\theta_i}{v_1}=\frac{\sin\theta_r}{v_2}\]

    \(v_2>v_1\) is the most common interface in the earth, and \(\theta_r>\theta_i\) is due to \(v_2>v_1\).

    Now that we've reviewed, let's first look at two sample ray paths.

    FIGURE Direct Ray

    The direct wave travels horizontally along the surface. The Rs represent receiving stations. What is the arrival time at each station? 

    \[distance=velocity\cdot time\]

    \[x=v_1\cdot t\]

    Solve for t.

    \[t=\frac{x}{v_1}\]

    The above equation is the direct ray travel/arrival time. The equation is a line whose slope is \(\frac{1}{v_1}\) and intercept=0.