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Refraction seismic

Jacques JENNY

Geo2X Genève

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Seismic refraction principes

V 1

V 2

V 3

R efraction

R eflexion

R efraction tota le

S in

S in

V itesse

V itesse

1

2

R éfraction to ta le S inVitesse

Vitesse

1

2

b a

a

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Applications

• Oil in 1920 th• Geotechnic• Water prospecting• Mineral research• Landslide study• Wheathering zone

determination (for reflection seismic statics)

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Unfavourable geological setting with refraction seismic

Seisimic line

A

Seisimic line

B

Red ray pathes are always hidden by shorter black rays

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Record example V1 V2 V3

Distances

T ime

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Record example

Refraction

Reflexion

Air wave

Surface wave

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Refraction data acquisitionFor a efficient processing, you need at least:• 2 Offset shots (half spread distance)• 2 End shots• 1 Center shot

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Geophone Shot 1/1 Shot 1/2 Shot 1/3 Shot 1/4 Shot 1/5 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 3/5123456789101112131415161718192021222324252627282930313233343536 E

nd s

hot

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Off

set

12

End s

hot

12

Off

set

12

Off

set

1

End s

hot

1

Centr

e s

hot

Off

set

12

Off

set

1

End s

hot

1

Centr

e s

hot

Off

set

1

End s

hot

1

Centr

e s

hot

E

nd s

hot

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Example of a complex acquisitionwith 12 channels

Shot points

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Equipment

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

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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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Sledge hammer

You can add (stack) few shots to improve signal/noise ratio

Produce a good energy with high frequencies,Possible investigation depth 10-50 m

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Explosives

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.

To buy and use explosive is often difficult, impossible in some countries. A miner licence is required

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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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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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Picking FBP on good data (Wiggles are clipped) Noise

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Picking FBP on poor data

?

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Travel time assembly

Offset 1

End Shot 1

Offset 24

Center shot

End Shot 24

X axis = distance Y axis = time Slope=1/velocity

Steep slope= low velocity gentle slope = high velocity

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Bedrock velocityUsing 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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Bedrock velocityA

B

C

D

E

F

G

H

Pente= 1 / Vitesse réfracteur

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PhantomingMove offset shot to end shot to determine

which part corresponds to bedrock arrivals

Intercept time 2

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Velocity determinationVelocity = 1 / slope

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Seismic velocity of some rocks

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Rock velocity examples

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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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Intercept time (IT) method

If you know all velocities and Intercept times, thickness can be computed below all shots, except offset shots

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What is an Intercept time

Distance = 0 m

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IT Formula

Z1= thickness layer 1T1= intercept 1V1=layer 1 velocityV2=ayer 2 velocity

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Plus minus Method Principle

A

B C

D

E F

G

Tim e C D E = Tim e AB C D + D E FG - Tim e A BC EFG

Tota l tim e

a

S in = V 1/V 2a

V 1

V 2

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Plus Minus 1Total time

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Plus Minus 2ABC method depth computation

DEMO2.WS4

Velocity meters/s

50004800460044004200400036003200280024002200200018001600140012001000800600400

Depth is computed below all receivers

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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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Generalized Reciprocal Method

XY = Optimal distance

Red = plus-minus path Blue = plus-minus path

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GRM 1 XY optimal distance selection

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GRM 2: Depth with differents XY

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GRM 3

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GRM Seismic profilGRM method depth computation

DEMO2.WS4

Velocity meters/s

36003200280024002200200018001600140012001000800600400

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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.

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Example 1 Tomography on a gallery

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Example 1

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Example 2 Landslide in Swiss Alps

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Example 2

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R oute

Sentier

Profil 3

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FIN END