Lecture 4

Question 1

What are the main applications of seismic methods

Answer 1

• hydrocarbon exploration
• mineral exploration
• research into Earth’s structure

Question 2

What are the seismic reflection and refraction methods

Answer 2

Measuring the time taken for a seismic wave to travel from a source, down into the ground where it gets reflected/refracted back to the surface where it is recorded.

Question 3

What do ‘travel-times’ allow to be calculated

Answer 3

Allow wave velocities to be calculated and wave paths to be inferred

Question 4

What is snells law?

Answer 4

sini/sinr = v1/v2

Question 5

What is the angle of incidence equal to?

Answer 5

Angle of refraction

Question 6

What does reflection depend on?

Answer 6

Acoustic impedence

Question 7

Why can surface waves complicate seismic surveys?

Answer 7

As they are also recorded by geophones on the Earth’s surface

Question 8

Define critical refraction

Answer 8

When V2>V1, there will be an incident angle for which the refracted angle is 90 degrees.

Question 9

Define Acoustic impedence contrast

Answer 9

Contract between layers determines the amplitude pf reflected seismic phases

Question 10

What is acoustic impedence the property of?

Answer 10

Density and velocity

Question 11

If acoustic impedence = 0, will refraction

Answer 11

Refraction can still occur.

Question 12

If Z1 and Z2 are the same, reflection can occur. True or False

Answer 12

False. Reflection will not occur.

All energy will be transmitted along the interface.

Question 13

What are seismic refraction surveys designed to do?

Answer 13

Acquire information about the subsurface from seismic waves that are refracted and post-critically refracted. When incident upon a boundary where there is a constant velocity.

Question 14

What are seismic reflection surveys designed to do?

Answer 14

Acquire information about the subsurface from seismic waves that are critically reflected when there is no acoustic impedance contract.

Question 15

wave fronts that propagate back to the surface are called what?

Answer 15

Head waves

Question 16

The relative proportions of the seismic wave energy that are reflected and transmitted are known as what?

Answer 16

Zoepritt’s equations

Question 17

Define Zoepritts equations

Answer 17

The relative proportions of the seismic wave energy that are reflected and transmitted

Question 18

What can zoepritts equations be written in terms as?

Answer 18

Can be written in terms of the acoustic impedance of the two layers

Question 19

Define the reflection coefficient

Answer 19

The reflection coefficient is the ratio of the reflected ray (A1) to the amplitude of the reflected ray (A0)

Question 20

Define propagation velocity

Answer 20

The velocity with which the seismic energy travels through a medium

Question 21

is the reflected phase the first to arrive?

Answer 21

No, it never is, it comes sooner once it is beyond the critical distance

Question 22

Define crossover distance

Answer 22

The offset at which the arrival times of critically refracted waves preceded those of the direct waves

Question 23

Define critical distance

Answer 23

At the critical distance the reflected arrival is coincident with the first critically refracted arrival, the travel times of the two are identical.