seismic surveying - tomoquest.com

1

Dr. Laurent Marescot

Seismic Surveying

Course given at the University of Fribourg (2009)

Contact:

laurent@tomoquest.com

2

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.

3

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

4

Structure of the Lecture

1. Seismic Waves and Seismic Rock Properties

2. Surveying Techniques

3. Seismic Interpretation and Examples

4. Conclusions

5

1. Seismic Waves and Seismic Rock

Properties

6

Seismic waves

We will describe here only two types of waves:

• P waves

• S waves

7

P waves

Compression or primary waves

8

S Waves

Secondary or shear waves

Does not exist in water!

9

Rock Velocities

Factors that influence rock velocities V:

• Porosity

• Depth

• Age

11

rock matrice fluideV V V

10

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

11

2. Survey Techniques

12

Surveying Techniques

13

Seismic Acquisition

14

Seismic Source: Gun

15

Seismic Source: Hammer

16

Seismic Receiver: Geophone

17

Data Recording: Seismograph

18

Background Noise

0

-5

-10

-15

-20

-25

-30

-35

9 12 15 18 21 24 3 6

dB

vertical-component

local time [h]

19

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

20

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

21

22

Seismic Tomography

23

Shot S1

0 100 m

T-D

ist/

200

0 [

ms]

0

100

150

50-50NW SE

24

Seismic

tomography

inversion

25

Initial Traveltimes

50

40

30

20

10

00 50 100 150 200 250

T-D

ist/

20

00 [

ms]

Distance [m]

26

Final Traveltimes

50

40

30

20

10

00 50 100 150 200 250

T-D

ist/

20

00 [

ms]

Distance [m]

27

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

28

Refraction Seismic

29

Refraction Seismic

30

Refraction Seismic

31

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

32

Reflection Seismic

33

Reflection Seismic

Concept of Common Midle Point (CMP)

Acquisition

34

Reflection

seismic

processing

35

Migration and result: the seismic section

36

Some Examples

Some examples of refraction, reflection and tomography

seismic for archeological applications…

37

MappingtheAncient Port at the Archaeological Siteof Itanos (Greece)

Using Shallow Seismic Methods

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

38

MappingtheAncient Port at the Archaeological Siteof Itanos (Greece)

Using Shallow Seismic Methods

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

Refraction

profile

39

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

41

Investigation of a Monumental

Macedonian Tumulus by Three dimensional Seismic Tomography

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

42

Combined Seismic Tomographic and Ultrashallow Seismic Reflection Study

of an Early Dynastic Mastaba, Saqqara, Egypt

Metwaly et al., 2005,

Archeological Prospection, 12, 245-256

43

Refraction

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

44

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

45

3. Conclusions for archaeology

46

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

47

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)