Lecture 4 Flashcards
Seismic Methods
What are the main applications of seismic methods
- hydrocarbon exploration
- mineral exploration
- research into Earth’s structure
What are the seismic reflection and refraction methods
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.
What do ‘travel-times’ allow to be calculated
Allow wave velocities to be calculated and wave paths to be inferred
What is snells law?
sini/sinr = v1/v2
What is the angle of incidence equal to?
Angle of refraction
What does reflection depend on?
Acoustic impedence
Why can surface waves complicate seismic surveys?
As they are also recorded by geophones on the Earth’s surface
Define critical refraction
When V2>V1, there will be an incident angle for which the refracted angle is 90 degrees.
Define Acoustic impedence contrast
Contract between layers determines the amplitude pf reflected seismic phases
What is acoustic impedence the property of?
Density and velocity
If acoustic impedence = 0, will refraction
Refraction can still occur.
If Z1 and Z2 are the same, reflection can occur. True or False
False. Reflection will not occur.
All energy will be transmitted along the interface.
What are seismic refraction surveys designed to do?
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.
What are seismic reflection surveys designed to do?
Acquire information about the subsurface from seismic waves that are critically reflected when there is no acoustic impedance contract.
wave fronts that propagate back to the surface are called what?
Head waves
The relative proportions of the seismic wave energy that are reflected and transmitted are known as what?
Zoepritt’s equations
Define Zoepritts equations
The relative proportions of the seismic wave energy that are reflected and transmitted
What can zoepritts equations be written in terms as?
Can be written in terms of the acoustic impedance of the two layers
Define the reflection coefficient
The reflection coefficient is the ratio of the reflected ray (A1) to the amplitude of the reflected ray (A0)
Define propagation velocity
The velocity with which the seismic energy travels through a medium
is the reflected phase the first to arrive?
No, it never is, it comes sooner once it is beyond the critical distance
Define crossover distance
The offset at which the arrival times of critically refracted waves preceded those of the direct waves
Define critical distance
At the critical distance the reflected arrival is coincident with the first critically refracted arrival, the travel times of the two are identical.
