How Shallow Earth Structure Is Determined

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Presentation transcript:

How Shallow Earth Structure Is Determined A Classroom Exercise Demonstrating Seismic Refraction Use in the Real World NSTA, St. Louis, 2007 John Taber, IRIS Michael Wysession, Washington University, St. Louis

Objective(s) Explore how refracted seismic waves are used to determine earth structure. Reinforce the concepts of reflected and refraction of waves through a real world example.

The deepest drill hole was 12.7 km. So…how do we know all this?!?!

Seismic Refraction The deepest drill hole was 12.7 km. So…how do we know all this?!?! Seismic Refraction

Identify: Direct wave Head wave Refracted wave Reflected wave

Direct Wave

Direct Wave Reflected Wave

Direct Wave Reflected Wave Refracted Waves

Direct wave: Slope is 1/v1 Distance Direct wave: Slope is 1/v1 Time

Direct wave: Slope is 1/v1 Distance Direct wave: Slope is 1/v1 Time 1. Measure Direct Wave Slope and you get v1 !!!!

i1 v1 i2 v2

The first head wave is a reflected wave at the critical angle ic ic

ic ic The Head wave moves along the surface at the SPEED OF THE LOWER LAYER!!!

Direct wave: Slope is 1/v1 Distance Direct wave: Slope is 1/v1 Time Head wave: Slope is 1/v2

Direct wave: Slope is 1/v1 Distance Direct wave: Slope is 1/v1 Time Head wave: Slope is 1/v2 2. Measure Head Wave Slope and you get v2 !!!!

Direct wave: Slope is 1/v1 Distance xd Direct wave: Slope is 1/v1 Time Head wave: Slope is 1/v2 3. Measure the cross-over distance, xd , to get the DEPTH to the boundary !!!!

How do you get the depth to the boundary from the cross-over distance, xd ?

How do you get the depth to the boundary from the cross-over distance, xd ? Direct wave: TD(x) = x / v1

How do you get the depth to the boundary from the cross-over distance, xd ? Direct wave: TD(x) = x / v1 Set TD (x) = TH (x)

How do you get the depth to the boundary from the cross-over distance, xd ? Direct wave: TD(x) = x / v1 Set TD (x) = TH (x) Depth d

How do you get the depth to the boundary from the cross-over distance, xd ? Direct wave: TD(x) = x / v1 Set TD (x) = TH (x) 3. Measure v1 , v2 , and xd , and you can get the Depth to the boundary!!!! Depth d

Example:

Step 1. Find the slope of the Direct wave. Example: Step 1. Find the slope of the Direct wave.

Step 2. Find the slope of the Head wave. Example: Step 2. Find the slope of the Head wave.

Example: xd Step 3. Find the slope of the Head wave.

xd Step 3. Find the slope of the Head wave. Example: xd Step 3. Find the slope of the Head wave. Step 4. Compute the Depth to the boundary.

What about multiple layers?

What about dipping layers?

What about variable layers? Mid-Atlantic Ridge off South America Computer model Ray trace diagram 100 km

Q: How do they know the compositions?

A: Laboratory experiments on real rocks!

North American Head P-wave Speeds http://earthquake.usgs.gov/research/structure/crust/nam.php

North American Direct P-wave Speeds

North American Crustal Thicknesses

Global Crustal Thicknesses