Refraction seismology

Question 1

What is critical refraction?

Answer 1

Because seismic sources radiate waves in all directions some ray must hit interface at exactly the critical angle, ic. This critically oriented ray will then travel along the interface between the two layers.
If more oblique than critical, all wave energy is reflected

Question 2

What is Snell’s law?

Answer 2

sin(i1)/sin(i2)=v1/v2

Question 3

What is meant by wavelets?

Answer 3

When any particle oscillates it is a tiny source of waves. Every point on a wave front acts as a small source that generates waves. The waves have circular (spherical) wave fronts and are called wavelets. Wavelets constructively interact (reinforcement) to produce the wave front.
Because of wavelets, a wave front that encounters an obstacle will travel through the open space and after the barrier diffracts, bend into an area.

Question 4

How is it possible to record refracted waves?

Answer 4

The wave front just above the interface produces a continual stream of critically refracted rays. The wave front just below the interface does the same.
These stream of critically refracted rays form wavelets and they combine to form head waves.
The head waves propagate up to the surface and can be recorded. The recorded rays are called the refracted rays

Question 5

Which potential paths can rays take in a refraction survey?

Answer 5

As a direct ray: Simply a linear function of the seismic velocity and the shot point to receiver distance

As a reflection ray: Is never a first arrival. Plots as a curved path on t-x diagram. Asymptotic with direct ray. Y-intercept (time) gives thickness

As a refracted ray: Plots as a linear path on t-x diagram. Only arrives after critical distance. Is first arrival only after cross over distance.

Question 6

What are the negatives of seismic refraction surveys?

Answer 6

• Only works if each successive layer has increasing velocity. Cannot detect a low velocity layer
• May not detect thin layers
• Requires multiple (survey) lines

Question 7

Which different types of surveys can be done?

Answer 7

Hammer seismic survey: a sledgehammer is wacked into a steel plate. The simplest and cheapest survey type.

Explosion seismic survey: gives a strong signal.

Marine surveys: explosives, compressed air, high voltage charges, or many other types can be done

Question 8

What is meant by critical distance?

Answer 8

The distance from shot point that is required to have refracted waves. Closer than this distance no refracted waves can be recorded