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