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Dr. Laurent Marescot

Seismic Surveying

Course given at the University of Fribourg (2009)

Contact:

laurent@tomoquest.com

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Introduction

Seismic surveying…

Investigation based on the propagation of man-made

seismic waves through the subsurface. The parameter of

interest is the propagation velocity of these waves in the

subsurface. This propagation depends on the elastic

properties of the rocks.

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Application

• Exploration of fossil fuels (oil, gas, coal)

• Exploration of bulk mineral deposits (sand, gravel)

• Exploration of underground water supplies

• Engineering/construction site investigation

• Archaeology

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Structure of the Lecture

1. Seismic Waves and Seismic Rock Properties

2. Surveying Techniques

3. Seismic Interpretation and Examples

4. Conclusions

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1. Seismic Waves and Seismic Rock

Properties

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Seismic waves

We will describe here only two types of waves:

• P waves

• S waves

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P waves

Compression or primary waves

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S Waves

Secondary or shear waves

Does not exist in water!

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Rock Velocities

Factors that influence rock velocities V:

• Porosity

• Depth

• Age

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rock matrice fluideV V V

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Rock Velocities

Classification selon le matériel

________________________________________________________________

Matériel Vitesse en m/sec

________________________________________________________________

Air 330

Terrain d'altération en surface 300 - 600

Graviers, sable sec... 500 - 1000

Sable humide 600 - 1850

Eau (selon la température et la salinité ) 1430 - 1680

Eau de mer 1460 - 1530

Grès 1800 - 3500

Argiles 2750 - 4250

Craie 1850 - 3950

Calcaire 2100 - 6100

Sel 4250 - 5200

Granite 4580 - 5800

Roches métamorphiques 3000 - 7000

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2. Survey Techniques

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Surveying Techniques

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Seismic Acquisition

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Seismic Source: Gun

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Seismic Source: Hammer

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Seismic Receiver: Geophone

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Data Recording: Seismograph

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Background Noise

0

-5

-10

-15

-20

-25

-30

-35

9 12 15 18 21 24 3 6

dB

vertical-component

local time [h]

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Seismic Interpretation

Three techniques:

• Refraction seismic: based on the direct and refracted waves

• Reflection seismic: based on the reflected waves

• Seismic refraction tomography : based on the direct and

refracted waves

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Waves Propagation: Snell Law

Analogy with optics: the Snell law

1 2

1 2

sin( ) sin( )i i

V V

when i2=90 , then sin (i2)=1 and 1

1

2

sin( )V

iV

In this case, i1 is called the critical angle ic

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Seismic Tomography

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Shot S1

0 100 m

T-D

ist/

200

0 [

ms]

0

100

150

50-50NW SE

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Seismic

tomography

inversion

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Initial Traveltimes

50

40

30

20

10

00 50 100 150 200 250

T-D

ist/

20

00 [

ms]

Distance [m]

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Final Traveltimes

50

40

30

20

10

00 50 100 150 200 250

T-D

ist/

20

00 [

ms]

Distance [m]

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Final Traveltimes504030201000 50 100 150 200 250T-Dist/2000 [ms]

Distance [m]

Raypaths q2NW

20

25

30

35

Dep

th [

m]

40

45

500 50

Distance [m]150 200100

l1 l2 l3SE

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Refraction Seismic

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Refraction Seismic

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Refraction Seismic

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Interpretation of Refraction Data

Solution for two horizontal layers. The unknown parameters are the depth h and the velocities of the two layers.

• V1 is given by the slope of the direct arrival segment

• V2 is given by the slope of the refracted arrival segment

• h using the crossover distance:

• h using the intercept time:

2 1

2 12

bX V Vh

V V

1 2

2 2

2 12

Ti V Vh

V V

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Reflection Seismic

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Reflection Seismic

Concept of Common Midle Point (CMP)

Acquisition

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Reflection

seismic

processing

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Migration and result: the seismic section

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Some Examples

Some examples of refraction, reflection and tomography

seismic for archeological applications…

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MappingtheAncient Port at the Archaeological Siteof Itanos (Greece)

Using Shallow Seismic Methods

Vafidis et al., 2007, Archeological Prospection, 10, 163-173shots

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MappingtheAncient Port at the Archaeological Siteof Itanos (Greece)

Using Shallow Seismic Methods

Vafidis et al., 2007, Archeological Prospection, 10, 163-173

Refraction

profile

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Mapping the Ancient Port at the Archaeological Site of Itanos (Greece)

Using Shallow Seismic Methods

Vafidis et al., 2007, Archeological Prospection, 10, 163-173

40Jones et al., 2000, Archeological Prospection, 70, 147-170

Exploration of the Canal of Xerxes, Northern Greece

Reflection

profile

Refraction

profile

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Investigation of a Monumental

Macedonian Tumulus by Three dimensional Seismic Tomography

Polymenakos et al., 2004, Archeological Prospection, 11, 145-158

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Combined Seismic Tomographic and Ultrashallow Seismic Reflection Study

of an Early Dynastic Mastaba, Saqqara, Egypt

Metwaly et al., 2005,

Archeological Prospection, 12, 245-256

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Refraction

Metwaly et al., 2005, Archeological Prospection, 12, 245-256

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Reflection profile in time

Reflection profile in depth

Reflection profile

(migration for the first part)

Comparison with refraction

Metwaly et al., 2005,

Archeological Prospection, 12, 245-256

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3. Conclusions for archaeology

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Advantages

• Seismic used mainly to explore the geology related to

archeological features (canal, port, bedrock)

• For results in the first meters, very high resolution seismic

is used (distance between the geophones from a few cm to

a few meters)

• Useful complement to resistivity

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Drawbacks

• Sensitive to noise in urban areas

• Reflection seismic needs important processing steps

• The velocity must increase with depth for refraction!

• Velocity contrast must exist (void, slab, wall, soil)