refraction

Question 1

What is the direct wave

Answer 1

The direct wave travels from the source to the receiver along interface 1 just below the surface

Question 2

What is the critical distance?

Answer 2

minimum offset distance before the critical refraction arrives at the surface

Question 3

What is the crossover distance?

Answer 3

where the travel times of the direct wave and critical refraction are equal

Question 4

What conditions must be met for the refraction technique to work in a multilayer scenario?

Answer 4

Velocity must increase with depth?

Question 5

What causes a hidden layer?

Answer 5

Velocity does not increase with depth

Question 6

Is the position of receivers different for updip and downdip surveys?

Answer 6

No

Question 7

How is apparent velocity related to the true velocity for updip vs downdip surveys

Answer 7

Apparent velocity is higher than true for updip and lower for downdip

Question 8

What is the principle of reciprocity

Answer 8

the travel time along a specific path is the same regardless of the travel direction (i.e., the source and receiver positions are interchangeable)

Question 9

What two assumptions are made in first arrival data?

Answer 9

The seismic wave velocities increase downward across each interface

The critical refraction along each interface produces first arrivals over some range of source-receiver offset distances.

Question 10

What are the applications of the refraction method?

Answer 10

useful in the shallow subsurface due to the general trend of velocity increase with depth
- unsaturated material overlying saturated material
- unconsolidated sediments overlying consolidation bedrock

engineering and environmental issues require the mapping/characterization of these interfaces and layers.

• estimation of crustal thickness and structure.