seismic

Question 1

What is stress

Answer 1

the intensity of the mechanical force applied to a material (force/area)

Question 2

What is strain

Answer 2

a measure of the deformation (i.e., changes in shape and size) caused by an applied stress. - dimensionless

Question 3

What is the proportional relationship between stress and strain?

Answer 3

Stress = constant x strain

Question 4

What is the bulk modulus?

Answer 4

Bulk modulus k relates pressure change ΔP to volumetric strain θ
ΔP = -k θ

Question 5

What is shear modulus?

Answer 5

Shear modulus (or Rigidity) μ relates shear stress τ to shear strain ε.

Question 6

Is there volume change with shear stress/strain

Answer 6

No, just shape distortion

Question 7

Whats the shear modulus of fluids (liquid/gas)

Answer 7

0

Question 8

What does the elastic wave equation describe?

Answer 8

describes the propagation of mechanical waves through a material

Question 9

What type of waves travel/propagate through the body (or interior) of a medium

Answer 9

Body waves

Question 10

What are p-waves

Answer 10

Compressional, longitudinal, primary, have both volume and shape changes

Question 11

What type of wave’s particle motion is in the same direction as the wave propagates

Answer 11

P-waves

Question 12

What type of wave’s particle motion is perpendicular to the direction the wave propagates

Answer 12

S-waves

Question 13

Describe s-waves

Answer 13

Transverse, secondary, only involve shear stress and strain (no volume change)

Question 14

How do elastic moduli and wave velocities relate to consolidated vs unconsolidated sediments?

Answer 14

unconsolidated sediments have relatively lower elastic moduli and wave velocities due to their weak uncemented grain contacts. Conversely, consolidated sedimentary rocks have higher elastic moduli and wave velocities than loose sediments due to their stronger cemented grained.

Question 15

Do liquids or gases have a higher bulk modulus?

Answer 15

Liquids.

Question 16

Are elastic moduli and wave velocities proportional?

Answer 16

Yes

Question 17

How does changing fluid types (i.e., water to gas) affect s wave velocity

Answer 17

Does not impact shear modulus, so it only impacts it through density change.

Question 18

what is porosity

Answer 18

the fraction of a rock/soil that is pore space

Question 19

what happens when you increase porosity (in terms of Vs and Vp)

Answer 19

Increased porosity means fluids are replacing solids. Both Vs and Vp will decrease since moduli for solids is greater than for fluids.

Question 20

Does the presence of cracks and fracture or spherical pores have a greater effect on the elastic moduli of a rock?

Answer 20

Cracks and fracture.

Question 21

What is Confining pressure Pc

(overburden or lithostatic pressure)

Answer 21

the pressure applied to the rock grains due to the weight of the overlying solid material

Question 22

Does confined pressure increase or decrease with depth

Answer 22

Increase

Question 23

How does confined pressure affect seismic velocities and elastic moduli

Answer 23

Increased confined pressure means stronger grain contacts in unconsolidated sediments, meaning less cracks and fractures. Therefore increase in moduli and v

Question 24

What is pore fluid pressure?

Answer 24

the weight of the overlying pore fluid as a result of vertical pressure communication through the pore space

Question 25

How does pore fluid pressure impact elastic moduli and wave velocities

Answer 25

It forces grains apart, lowering em and v

Question 26

What is effective pressure

Answer 26

Pe = Pc - Pf

Question 27

how does temperature affect wave velocities

Answer 27

Increased temp decreases velocites, but to a very small degree.

Question 28

whats a wave front

Answer 28

the surface (or loci of all points) in the Earth that a compact seismic pulse would reach at a given travel time t (e.g., the leading edge of the wave).

Question 29

what is huygen’s principle

Answer 29

Huygens’ Principle states that all points on a wave front act as point sources for new waves that travel away from these points. The envelope where these new waves constructively interfere forms the new wave front

Question 29

what is a ray path

Answer 29

the travel paths followed by the small, coherent packets of seismic energy through the body

Question 30

What is Fermat’s principle

Answer 30

Fermat’s Principle states that actual ray path used between two fixed points is the one which has the shortest travel time compared to similar possible paths (i.e., the minimum travel time path). Hence, Fermat’s Principle is sometimes referred to as the Principle of Least Time.

Question 31

What does the magnitude of the energy and direction of a refracted ray depend on?

Answer 31

Elastic moduli & density of the medium and the direction of the incident ray relative to the interface

Question 32

What is the angle of incidence

Answer 32

The angle of the incident ray relative to the normal

Question 33

What principle(s) can snell’s law be derived from

Answer 33

Fermat and Huygen’s

Question 34

If v1 < v2, which way does a refracted ray bend?

Answer 34

Away from the normal

Question 35

What is wave conversion

Answer 35

When the body wave type changes during reflection/refraction

Question 36

How do geophones work

Answer 36

They respond to the motion of the Earth due to mechanical wave in terms of particle displacement, velocity or acceleration.

Question 37

How do hydrophones work

Answer 37

They measure changes in pressure in the water due to compressional waves