Keary & Brooks – Chapter 5

Seismic refraction surveys

• Small-scale: weathering layer, high-resolution sequence stratigraphy (onlap/offlap), sedimentary stuctures (salt domes, reef complexes, deltas),

facies.• Medium-scale: sedimentary basin architecture, depth to basement

•Large-scale: Structure of the crust and mantle, Moho.+ depth conversion of seismic reflection data!

Uses :

Seismic refraction surveys use controlled sources to generate sound waves that are refracted back to Earth’s surface from density and velocity discontinuities

at depth.

Two horizontal layers

TSA =TBD =z

Cosθ1V1

x =AB+ 2 tanθz

This is the travel time of the refracted wave. The refracted wave propagates along a buried interface at the velocity of the lower medium. They are normally the first phases

to arrive at a receiver and hence are called head waves.

     TSABD =xV2

+ 2zCosθV1

Sinθ =V1

V2

,        Cosθ = 1−Sin2θ( ) = 1−V1

V2

⎛

⎝⎜⎞

⎠⎟

2⎛

⎝⎜

⎞

⎠⎟

                 TSABD =xV2

+2zV1V2

V22 −V1

2( )1/2

TSABD =2z

Cosθ1V1

+x−2 tanθz

V2

=xV2

+ 2z1

CosθV1

−tanθV2

⎛

⎝⎜⎞

⎠⎟ =

xV2

+2z

CosθV1

1−V1SinθV2

⎛

⎝⎜⎞

⎠⎟=xV2

+2z

CosθV1

1−Sin2θ( )

TSABD =TSA +TAB +TBD

Time-Distance Plot

Intercept time =2zV1V2

(V22 −V1

2 )1/2

At the the cross-overdistance, xcros, travel

times of the direct andrefracted arrivals are

equal.

xcros

V1

=xcrosV2

+2zV1V2

(V22 −V1

2 )1/2

        xcros =2zV2 +V1

V2 −V1

⎡

⎣⎢

⎤

⎦⎥

1/2

      ∴xcros always> 2z

The thickness of the upper of the two layers, z, can be determined from the cross-over distance and the velocities or the intercept time and the velocities.

T

Direct, reflected and head wave fronts

Elapsed time after shot (s)

Dep

th (

m)

Geo

phon

e nu

mbe

r

Offset (m)

Multiple layers

ABCDEF is the refracted ray path through the bottom layer of a three layer model. The traveltime curve for the direct and two head waves are shown above.

Dire

ct w

ave

Head wave

Head wave

This gives the travel time, Tn of a ray critically refracted along the top surface of the n th horizontal layer

  TSD =xV2

+ 2zCosθV1

,             V2 =V1

Sinθ   

By analogy:

    TABCDEF =xV3

+ 2z1Cosθ1

V1

+ 2z2Cosθ2

V2

           Tn =xVn

+2ziCosθi

Vii=1

n−1

∑where

                       θi =sin−1 Vi

Vn

⎛

⎝⎜⎞

⎠⎟

The velocity V3 can be estimated from the slope of the second head wave. V1 and V2 can be estimated from the direct and first head wave and z1 and z2 from the intercept

times

Mendips field data

V2 =1.89 km/s

V3=5.84 km/s

Common shot point gathers from 3 streamers (6,15,6 km)

3.1 km/s

1.5 km/s

Seismic refraction using Ocean Bottom Seismometers (OBSs)

4-channel: hydrophone + 3 componentseismometer

Data logger + batteries + GPS clockBallast weights (for coupling with seabed)

Hydro-acoustic releaseTitanium tubes for > 6000 m

Operation: 10-360 days

OBS DataReduced time Vs. distance plots (A 6m/s refractor will appear flat)

Dipping layersShoot down-dip Shoot up-dip

Down-dip: t2 x( ) =xsin θ +γ( )

v1+

2zcosθv1

t2' x( ) =

xsin θ −γ( )v1

+2z'cosθ

v1Up-dip:

θ and γ can be estimated from the velocities V1, V2u and V2d and hence z and z’ and h and h’ calculated.

See Keary & Brooks (Chap 5) + Practical 4

θ =1

2(sin−1(V1 / V2d ) + sin−1(V1 / V2u ))

γ =1

2(sin−1(V1 / V2d ) − sin−1(V1 / V2u ))

               ti = 2z cosθ / V1

z = V1ti / 2cosθ ,       z ' = V1ti' / 2cosθ

      h = z / cosγ ,       h ' = z '/ cosγ

Offsets in the travel time Vs. distance plot for head waves from opposite sides of a fault

Δt

Δz = ΔtV1V2

(V22 − V1

2 )1/2

A thin layer that does not generate a head wave that is a first

arrival

A low velocity layer that does not generate a head wave

Thin and low velocity layers

Non-planar refractor geometry

Reference (dashed lines) show the planar case

M (e.g.) is nearer the surface than the reference interface, the actual travel time to M’ plots below the reference line. Conversely, that for N’ is above it. These

observations can be quantified using the concept of delay time.

The concept of delay time

We can think of the travel time of a refracted wave being made up of 3 parts: the timeit takes to travel between the source and receiver, SvRv, at velocity V2 , plus a

term at the source, δS, to equal the time it takes to go from S to C at velocity V1,and an equivalent term, δR, at the receiver.

where δS and δR are called the delay times

tSR =δS +SRV2

+δR

 t f + tr =ttotal + 2δR

δR =12(tf + tr −ttotal )

hR =δR

V1V2

(V22 −V1

2 )1/2

tf, tr and ttotal can be read off from a travel time Vs. distance plot and the delaytime calculated. The depth to the interface at R can then be calculated from

the delay time and the velocities.

Determining lateral variations in layer thickness

hR

The time, tf, to go from one end to a receiver (SfCDR), and then on to the otherend,tr, (REFSr), is longer than the total time, ttotal, to go from end to end (SfCDEFSr),

because of the extra times to travel from the interface to the receiver, along DR and ER.

Generalized velocity structure of continental and oceanic crust

Velocity Vs. depth

White et al. (1992)

Velocities increase gradually through the oceanic crust (difficult to fit straight lineson Time Vs. distance plots). Moho is usually marked by a velocity jump

to > 8.0 km/s

Moho

Amazon margin, NE Brazil: Line B - CDP Stack

SW NE

Late Miocene - Pleistocene

Fan channel-levee system

Late Albian - Mid-MioceneOceanic crust

Seafloor multiple

OBS 315

Moho??

Reflectors showingaggradation and fan deposition

Line B: Refraction and velocity model

Iso-velocity contoursshowing progradation and facies

Moho

OBS surveys and the Continental/Ocean Transition (COT) at conjugate rifted margins