seismic velocity refraction seismology - …eps.mcgill.ca/~courses/c240/w9_l2.pdf · refraction...
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R E F R A C T I O N S E I S M O L O G Y
E P S C 2 4 0 : G E O L O G Y I N T H E F I E L D S E I S M I C V E L O C I T Y
Vibrations can travel through rocks as waves - they travel with a speed called seismic velocity
S E I S M I C V E L O C I T Y D E P E N D S O N T H E P H Y S I C A L P R O P E R T I E S O F R O C K
K = bulk modulus (1/compressibility)
= shear modulus
S E I S M I C V E L O C I T YC R O S S S E C T I O N T H R O U G H T H E C R U S T
R AY S & W AV E F R O N T S
Rays are perpendicular to wavefronts
Rays show direction wavefront is traveling
We use rays to show wavepath, but remember wave fronts can be diverging or straight
W AV E L E T S Rays are not real - they are just an easy way to understand
and quantify waves
Wave fronts are what is really happening
Huygens wavelets explains wavefronts: each point along a material is acts like a point source of waves
S N E L L S L A W The ratio of thesinesof
the angles of incidence (1) and refraction (2) is equivalent to the ratio ofphase velocities in the two media
C H A N G E I N V E L O C I T Y M E A N S C H A N G E I N R AY A N G L E A N D I N W AV E F R O N T A N G L E
Notice: Change in wavelength
W AV E S AT A N I N T E R FA C E
3 possible outcomes for a wave meeting an interface
R E F R A C T E D W AV E S
Wavelets from propagating refracted wave are continually emitted - they constructively interfere to form HEAD waves above and below interface
Head waves propagate to the surface to be recorded.
Recorded rays are called the refracted ray
S E I S M I C R E F R A C T I O N S U R V E Y
In a seismic refraction survey, a recorded ray can come from three main paths
The direct ray
The reflected ray
The refracted ray
Because these rays travel different distances at different speeds, they arrive at different times
The Direct Ray Arrival Time: Simply a linear function of
the seismic velocity and the shot point to receiver distance
D I R E C T W AV E
The Reflected Ray Arrival Time: is never a first arrival Plots as a curved path on t-x
diagram Asymptotic with direct ray Y-intercept (time) gives thickness
Why do we not use this to estimate layer thickness?
R E F L E C T E D R AY The Refracted Ray Arrival Time: Plots as a linear path on t-x diagram
Part travels in upper layer (constant) Part travels in lower layer (function of x)
Only arrives after critical distance Is first arrival only after cross over
distance Travels long enough in the faster layer
ic ic v1
R E F R A C T E D R AY
R E F R A C T E D W AV E FA S T E R T H A N D I R E C T W AV E A F T E R C R O S S O V E R D I S TA N C E
D I R E C T, R E F L E C T E D , A N D R E F R A C T E D W AV E S
R E F R A C T I O N R E F L E C T I O N
S E I S M I C E N E R G Y T O I L L U M I N AT E T H E S U B S U R FA C E1. ATTENUATION - seismic waves lose energy as they travel.
2.The initial energy of seismic source limits how far you can see.
Sledge hammer source = 100 m
shotgun = few 100s m
thumper truck = km
explosives = km - 10 km
airgun (in water) = few km
natural earthquakes = 1000s km
3. Spatial resolution ~ 1/2 wavelength
T I M E O F W AV E A R R I VA L S Very close to
source, the direct wave arrives first
But, refracted wave travels faster!
So, at some crossover distance, the refracted wave overtakes the direct wave and arrives first.
Reflected wave arrives later
R A W D ATA
M A K I N G A T- X D I A G R A M
A dipping interface produces a pattern that looks just like a horizontal interface! Velocities are called
What do we do?
What if the critically refracted interface is not horizontal?
In this case, velocity of lower layer is underestimatedunderestimated
Shoot lines forward and reversed
If dip is small (< 5o) you can take average slope
The intercepts will be different at both ends Implies different thickness
Beware: the calculated thicknesses will be perpendicular to the interface, not vertical
To determine if interfaces are dipping
Dipping Interfaces If you shoot down-dip Slopes on t-x diagram are
too steep Underestimates velocity
May underestimate layer thickness
Converse is true if you shoot up-dip
In both cases the calculated direct ray velocity is the same.
The intercepts tint will also be different at both ends of survey
S I M P L E S T C A S E
Material closer to surface has lower seismic velocity than deeper material lower density at surface - usually true more rigid at depth - usually true
Each layer is homogeneous, with sharp boundaries If velocity changes gradually, rays curve instead of
changing angle sharply - much harder to resolve