4/5/2014
Based on observing the passage of soundwaves through Earth
Methods used in industry
Refraction Seismology
Uses wave arrival times as a function of distance
Based on changes in wave speed/direction with depth
Reflection Seismology
Based on reflection of waves off discontinuities in the subsurface
PROS CONS
Less equipment; cheaper
Less processing
Easier to model results
Distance between source and receiver must be larger
Only works where wave speed increases with depth
Limited to simple subsurface geometry
PROS
CONS Distance between source can be small
Can accommodate any subsurface velocity structure
Can deal with complex subsurface geology
Data set contains more information
Directly image subsurface
Needs more equipment; more expensive
Data processing is complex and computer intensive
Data interpretation requires more sophistication and knowledge
REFRACTION REFLECTION
Used for small scale projects
Used in environmental consulting, geotechnical work
Used by oil industry
Pulses of strain energy that propagate in solids and fluids
Elastic = distortion (deformation) not permanent
Travel at specific velocities
Can be described as waves
Two ways (frames of reference) of looking at waves
In time
Rising and falling of boat on the ocean
In space
Waves on a pond
Space
Time
Space Time
Amplitude (A) height of wave (1/2 trough to peak)
Wavelength (Ξ») Peak to peak distance (one cycle)
Period (T) Time that it takes to go through on cycle
Frequency Number of cycles in a given period of time 1/period Units of Hertz (Hz) = 1/s
Speed of wave (c) Rate at which wave travels
π = π/π
Body waves β move throughout the body of the medium it propagates in
P-wave
S-wave
Surface waves β motion of the surface of the material
Love
Rayleigh
Highest wave speed, arrives first
Consists of train of compression and expansion
Particle motion is parallel to direction of travel
Can travel in both solid and fluid media
Lower wave speed, arrives after P wave
Consists of train of shear motion
Particle motion is perpendicular to direction of travel
Can travel only in solid media
Lower velocity than body waves
Disturbance primarily at surface
Love waves β lateral motion of surface
Rayleigh waves β elliptical particle motion at surface
Amplitude decreases with depth
Raypath β lines that show the direction that the wave is propagating
Wavefront β position of the seismic wave that are doing the same thing at the same time.
The wavefront is always locally perpendicular to the raypath
Reflection
Transmission
Refraction
change in direction of propagation of any wave as a result of its traveling at different speeds at different points along the wave front.
Diffraction
Direction of bending of ray paths depends on
change in wave velocity across interface
Angle between interface and ray path
Going from lower velocity to higher velocity ray path shallows
Going from higher velocity to lower velocity ray path steepens
fast
slow
fast
slow
slow
V1<V2
Wave front (plane wave)
Wave fronts of new waves stimulated at surface
Resulting plane wave front
Described by Snellβs law
π ππ π1
π£1=
sin π 2π£2
Waves traveling from a single source travel outwards in all directions
Leads to more complex looking wave front
Slower
Faster Direct wave
refracted wave
reflected wave
Depend on physical properties of rock
Density
Elasticity
Each wave type has its own velocity
Depend on physical properties of rock
Density (Ο)
Elasticity
Resistance to compression
Bulk modulus (K)
Resistance to shearing
Shear modulus (ΞΌ)
ππ =π
π
ππ =
43
π + πΎ
π
ππ
ππ