How Shallow Earth Structure Is Determined

A Classroom Exercise Demonstrating Seismic Refraction Use in the Real World

NSTA, St. Louis, 2007John Taber, IRISMichael 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?!?!

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 WaveReflected Wave

Refracted Waves

Direct WaveReflected Wave

Time

Distance

Direct wave: Slope is 1/v1

Time

Distance

Direct wave: Slope is 1/v1

1. Measure Direct Wave Slope and you get v1 !!!!

v1

v2

i1

i2

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!!!

Time

Distance

Direct wave: Slope is 1/v1

Head wave: Slope is 1/v2

Time

Distance

Direct wave: Slope is 1/v1

2. Measure Head Wave Slope and you get v2 !!!!

Head wave: Slope is 1/v2

Time

Distance

Direct wave: Slope is 1/v1

3. Measure the cross-over distance, xd , to get the DEPTH to the boundary !!!!

Head wave: Slope is 1/v2

xd

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)

Depth d

3. Measure v1 , v2 , and xd , and you can get the Depth to the boundary!!!!

Example:

Example:

Step 1. Find the slope of the Direct wave.

Example:

Step 2. Find the slope of the Head wave.

Example:

Step 3. Find the slope of the Head wave.

xd

Example:

Step 3. Find the slope of the Head wave.

Step 4. Compute the Depth to the boundary.

xd

What about multiple layers?

What about dipping layers?

0 100 km

Mid-Atlantic Ridge off South America

Computer model

Ray trace diagram

What about variable layers?

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