VẬT lý địa CHẤN 03 geometry

Applications for shallow high resolution Reflection seismic• Hydrogeological studies of acquifers • Engineering geology • Shallow faults • Mapping Quaternary deposits • Ground investigat

Reflected, Refracted and Diffracted

waves

• Reflected wave from a horizontal layer

• Reflected wave from a dipping layer

• Refracted wave from a horizontal layer

• Refracted wave from a dipping layer

• Diffracted waves

Applications for shallow high resolution Reflection seismic

• Hydrogeological studies of acquifers

• Engineering geology

• Shallow faults

• Mapping Quaternary deposits

• Ground investigation for pipe and sewerage

tunnel detection

Applications for Refraction

seismic

• Depth of groundwater level

• Depth and location of hardrock

• Elastic medium parameters

• Permafrost

• Glaciology

Refraction seismic

• Refracted Waves

• Mainly horizontal Wave propagation

• Only refracted waves are used (Lower layer must have higher velocity than upper layer)

• Distribution of velocity as well as the depth and

orientation of interfaces between layers

Reflection seismic

• Reflected Waves (“Echo lot principal”)

• Mainly vertical wave propagation

• Complete seismic recording is used

• Distribution of the velocity variation

Direct wave

Geometrical situation

Traveltime curve

Source Receivers

Receivers

x v

Velocity of direct wave is derived from the distance and travel time

o x x x x

Direct wave

x v

t = 1

t x

v =

Reflection: Horizontal reflector

2 2 2

v

x t

t

v

x

h t

2

, 4

0

2

2 2

0 2

2

2

2 2

Reflection: horizontal reflector

t = for x >> h

Difference in travel time t(x 1 ) und t(x 2 ):

90+Θ

h h

1 2

(Roth et al., 1998)

Headwave Propagation of seismic waves

Direct wave

Traveltime curve

( )

1 2

2 1

cos 2

tan

2 cos

x

v

i h

x i

v

h

T T

T T

T T

c

c c

AB SA

BG AB

SA SG

+

=

− +

=

+

= +

+

=

h

v 1 x

v 2

t i

t x

v2 - 2h 2

2

v12–

v1v2

+

-=

h

t x

v2 - t+ i

Refraction: horizontal reflector

v

1

v1 -

1 1

1 1

cos 2

) sin(

cos 2

) sin(

v

z v

x

t

v

z v

x

t

c b

c u

c a

c d

θα

θ

θα

1 1

1 1

cos 2

) sin(

cos 2

) sin(

v

z v

x

t

v

z v

x

t

c b

c u

c a

c d

θα

θ

θα

Huygens’ Principle:

Every point on a wavefront can be considered as a secondary

source of spherical waves

Surface

19 29 39 50 59 62 65 68 72 76 78 83

15 30 45 60 75 90 105 120 135 150 165 180

T(ms)

X(m)

To find the depth to bedrock in

a dam-site survey, traveltimes

were measured from the

shot-point to 12 geophones laid out

on a straight line through the

shotpoint The offsets x range

from 15 to 180 m

Determine the depth of

overburden from the data

in the Table