SEISMIC VELOCITY REFRACTION SEISMOLOGY - …eps.mcgill.ca/~courses/c240/W9_L2.pdf · REFRACTION...
Transcript of SEISMIC VELOCITY REFRACTION SEISMOLOGY - …eps.mcgill.ca/~courses/c240/W9_L2.pdf · REFRACTION...
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)
• µ = shear modulus
• ρ = density
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 the sines of
the angles of incidence (θ1) and refraction (θ2) is equivalent to the ratio of phase 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
21
90sinsinvv
ic =2
1sinvvic = ÷÷
ø
öççè
æ=
2
1arcsinvvic
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
DirectRay
ic ic
ShotPoint(i.e.theSource)
v1
v2
Layer1
Layer2
• The Direct Ray Arrival Time: – Simply a linear function of
the seismic velocity and the shot point to receiver distance
1vx
tdirect =
DirectRayShotPoint Receiver
v1
v2
Layer1
Layer2
Time(t)
Distance(x)
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?
ShotPoint Receiver
v1
v2
Layer1
Layer2
1
12vh
Time(t)
Distance(x)
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
v2
Layer1
Layer2
ic
“CRITICALDISTANCE”ßNOREFRACTEDRAYSà
ic
criticaldistance
crossoverdistance
22
21
1112vv
h −
22
21
12
112vv
hvx
t −+=
Time(t)
Distance(x)
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
v1=1/slope
v2=1/slope
Y-intercepttofindthickness,h1
22
21
12
112vv
hvx
t −+=RefractedRayArrivalTime,t
M A K I N G A T- X D I A G R A M
Dipping Interfaces
• A dipping interface produces a pattern that looks just like a horizontal interface! – Velocities are called
“apparent velocities”
• What do we do?
• What if the critically refracted interface is not horizontal?
In this case, velocity of lower layer is underestimatedunderestimated
Dipping Interfaces
• 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