seismic Flashcards
What is stress
the intensity of the mechanical force applied to a material (force/area)
What is strain
a measure of the deformation (i.e., changes in shape and size) caused by an applied stress. - dimensionless
What is the proportional relationship between stress and strain?
Stress = constant x strain
What is the bulk modulus?
Bulk modulus k relates pressure change ΔP to volumetric strain θ
ΔP = -k θ
What is shear modulus?
Shear modulus (or Rigidity) μ relates shear stress τ to shear strain ε.
Is there volume change with shear stress/strain
No, just shape distortion
Whats the shear modulus of fluids (liquid/gas)
0
What does the elastic wave equation describe?
describes the propagation of mechanical waves through a material
What type of waves travel/propagate through the body (or interior) of a medium
Body waves
What are p-waves
Compressional, longitudinal, primary, have both volume and shape changes
What type of wave’s particle motion is in the same direction as the wave propagates
P-waves
What type of wave’s particle motion is perpendicular to the direction the wave propagates
S-waves
Describe s-waves
Transverse, secondary, only involve shear stress and strain (no volume change)
How do elastic moduli and wave velocities relate to consolidated vs unconsolidated sediments?
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.
Do liquids or gases have a higher bulk modulus?
Liquids.
Are elastic moduli and wave velocities proportional?
Yes
How does changing fluid types (i.e., water to gas) affect s wave velocity
Does not impact shear modulus, so it only impacts it through density change.
what is porosity
the fraction of a rock/soil that is pore space
what happens when you increase porosity (in terms of Vs and Vp)
Increased porosity means fluids are replacing solids. Both Vs and Vp will decrease since moduli for solids is greater than for fluids.
Does the presence of cracks and fracture or spherical pores have a greater effect on the elastic moduli of a rock?
Cracks and fracture.
What is Confining pressure Pc
(overburden or lithostatic pressure)
the pressure applied to the rock grains due to the weight of the overlying solid material
Does confined pressure increase or decrease with depth
Increase
How does confined pressure affect seismic velocities and elastic moduli
Increased confined pressure means stronger grain contacts in unconsolidated sediments, meaning less cracks and fractures. Therefore increase in moduli and v
What is pore fluid pressure?
the weight of the overlying pore fluid as a result of vertical pressure communication through the pore space
How does pore fluid pressure impact elastic moduli and wave velocities
It forces grains apart, lowering em and v
What is effective pressure
Pe = Pc - Pf
how does temperature affect wave velocities
Increased temp decreases velocites, but to a very small degree.
whats a wave front
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).
what is huygen’s principle
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
what is a ray path
the travel paths followed by the small, coherent packets of seismic energy through the body
What is Fermat’s principle
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.
What does the magnitude of the energy and direction of a refracted ray depend on?
Elastic moduli & density of the medium and the direction of the incident ray relative to the interface
What is the angle of incidence
The angle of the incident ray relative to the normal
What principle(s) can snell’s law be derived from
Fermat and Huygen’s
If v1 < v2, which way does a refracted ray bend?
Away from the normal
What is wave conversion
When the body wave type changes during reflection/refraction
How do geophones work
They respond to the motion of the Earth due to mechanical wave in terms of particle displacement, velocity or acceleration.
How do hydrophones work
They measure changes in pressure in the water due to compressional waves
