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REFRACTION SEISMOLOGY EPSC240: GEOLOGY IN THE FIELD SEISMIC VELOCITY Vibrations can travel through rocks as waves - they travel with a speed called seismic velocity

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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

= 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 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

21

90sinsinvv

ic =2

1sinvvic =

=

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 =

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?

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

CRITICALDISTANCENOREFRACTEDRAYS

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