1. Refraction
seismic
Jacques JENNY
Geo2X Genève
1

2. Seismic refraction principes
R e fle x io n a V1
R e fr a c tio n
V2 a
S in
=
V ite s s e 1
S in β V ite s s e 2
b
V3
V ite s s e 1
R e fr a c tio n to ta le R é f r a c t io n t o ta le S in α =
2
V ite s s e 2

3. Applications
• Oil in 1920 th
• Geotechnic
• Water prospecting
• Mineral research
• Landslide study
• Wheathering zone
determination (for
reflection seismic statics)
3

4. Unfavourable geological setting
with refraction seismic
Seisimic line S iclin
eisim e
A B
Red ray pathes are always hidden by shorter black rays
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5. Record example V1 V2 V3
T
i
m
e
Distances 5

6. Record example
Refraction
Reflexion
Air wave
Surface wave
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7. Refraction data acquisition
For a efficient processing, you need at least:
• 2 Offset shots (half spread distance)
• 2 End shots
• 1 Center shot
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8. Geophone Shot 1/1 Shot 1/2 Shot 1/3 Shot 1/4 Shot 1/5
Example
Shot 2/1 Shot 2/2 Shot 2/3 Shot 2/4 Shot 2/5
Shot 3/1 Shot 3/2 Shot 3/3 Shot 3/4 Shot 3/5
1
2
3
4
5
of a
End shot 12
complex
6
7
8
Centre shot
End shot 1
9
acquisition
Offset 12
Offset 1
10
11
12
with 12
13
14
15
16
17
18
19
channels
20
End shot 12
Centre shot
End shot 1
21
Offset 12
Offset 1
22
23
24
25
26
27
28
29
30
31
32
End shot 12
Centre shot
End shot 1
33
Offset 12
Offset 1
34
35
36
8
Shot
points

9. Equipment
• Geophones
• Seismograph
• Battery
• Cables
• (Blasting box)
• Radio
• Portable drill
9

10. Energy sources
• Sledge hammer (Easy to use, cheap)
• Buffalo gun (More energy)
• Explosives (Much more energy, legal problems)
• Drop weight (Need a flat area)
• Vibrator (Uncommun use for refraction)
• Air gun (For lake / marine prospection)
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11. Sledge hammer
Produce a good energy with high frequencies,
Possible investigation depth 10-50 m
You can add (stack) few shots to
improve signal/noise ratio 11

12. To avoid aerial projection and improve energy,
explosives can be burried into a small drilling
(1-1.5 m) using a portable mechanic drill or a
jumper.
Explosives
To buy and use
explosive is often
difficult, impossible
in some countries.
A miner licence is
required
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13. Processing
• Few specific softwares are found to
process seismic refraction
• Most of them use conventional
methods like Intercept Time (IT),
ABC, GRM
• New inversion softwares can
produce tomography interpretation
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14. First Break Picking
• This is the most important operation,
good picking on good data !!!!
• A commun problem is the lack of
energy, for far offset geophones
• Seismographs produce SEG2, SEGY or
special file format, generaly they must
be converted to another file format, like
Seismic Unix.
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15. Picking FBP on good data
(Wiggles are clipped) Noise
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16. Picking FBP on poor data
?
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17. Travel time assembly
Offset 1
End Shot 1
Offset 24
Center shot
End Shot 24
Steep slope= low velocity gentle slope = high velocity
X axis = distance Y axis = time Slope=1/velocity 17

18. Bedrock velocity
Using only offset shots, we can compute the true velocity of the bedrock,
even if the bedrock is dipping. If the points are not straight, it means that
bedrock is not homogenous (fault, lithological boundary)
Save the Velocity as layer 3 velocity
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19. Bedrock velocity
A
E
Pente= 1 / Vitesse réfracteur
B F
C G
D H
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20. Phantoming
Move offset shot to end shot to determine
which part corresponds to bedrock arrivals
Intercept time 2
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21. Velocity determination
Velocity = 1 / slope
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22. Seismic velocity of some rocks
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23. Rock velocity examples
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24. Rock rippability
• Rippability (bulldozer) is related with
rock velocities.
• A refraction profil can be used to
determine if explosives use will be
necessary instead bulldozer
• Refraction survey can also be used to
caracterize soils: soft, hard to set
excavation rates
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25. If you know all velocities and Intercept
times, thickness can be computed
below all shots, except offset shots
Intercept
time (IT)
method
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26. What is an Intercept time
Distance = 0 m
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27. IT Formula
Z1= thickness layer 1
T1= intercept 1
V1=layer 1 velocity
V2=ayer 2 velocity
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28. Plus minus Method Principle
T o ta l tim e
S in a = V 1 /V 2
T im e C D E = T im e A B C D + D E F G - T im e A B C E F G
A G
D
a
V1
B C E F
V2
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29. Plus Minus 1
Total time
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30. A B C m e t h o d d e p t h c o m p u t a t io n
Plus Minus 2
Velocity meters/s
5000
4800
4600
4400
4200
4000
3600
3200
2800
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
D E M O 2 .WS 4
Depth is computed below all receivers 30

31. GRM principle
• Generalized Reciprocal Method (see
PALMER papers) is a variant of the ABC
method. It takes in account noncoincidence of
the stations used for calculating plus values
• GRM requires more receivers than IT or ABC
• Different distances are used to compute time
to bedrock, geophysicist must select the
optimal distance (XY)
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32. Generalized Reciprocal Method
XY = Optimal distance
Red = plus-minus path Blue = plus-minus path
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33. GRM 1 XY optimal distance selection
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34. GRM 2: Depth with differents XY
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35. GRM 3
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36. GRM Seismic profil
G R M m e t h o d d e p t h c o m p u t a t io n
Velocity meters/s
3600
3200
2800
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
D E M O 2 .WS 4
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37. Tomography
If you input all geometrical data and first break picks, computer
can build a theoretical model as close as possible to field data
using different algorithm.
A very precise picking and closer shots are required to give
accurate results. 37

38. Example 1 Tomography on
a gallery
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39. Example 1
39

40. Example 2 Landslide in
Swiss Alps
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41. Example 2
T1
1300
5000
T2
R o u te
P r o fil 3
1280 4500
T3
4000
1260
T4
3500
T5
1240
3000
T6
1220
T7 2500
T8
1200 2000
S e n tie r
T9 1500
1180
1000
T10
1160
500
0 20 40 60 80 100 120 140 160 180
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42. FIN END
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