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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 =

    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

    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