Seismic Surveying. Dr. Laurent Marescot. Course given at the University of Fribourg (2009) Contact:

Transcription

1 Seismic Surveying Dr. Laurent Marescot Course given at the University of Fribourg (2009) Contact: 1

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. 2

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 3

4 Structure of the Lecture 1. Seismic Waves and Seismic Rock Properties 2. Surveying Techniques 3. Seismic Interpretation and Examples 4. Conclusions 4

5 1. Seismic Waves and Seismic Rock Properties 5

6 Seismic waves We will describe here only two types of waves: P waves S waves 6

7 P waves Compression or primary waves 7

8 S Waves Secondary or shear waves Does not exist in water! 8

9 Rock Velocities Factors that influence rock velocities V: Porosity Depth Age 1 1 V V V rock matrice fluide 9

10 Rock Velocities Classification selon le matériel Matériel Vitesse en m/sec Air 330 Terrain d'altération en surface Graviers, sable sec Sable humide Eau (selon la température et la salinité ) Eau de mer Grès Argiles Craie Calcaire Sel Granite Roches métamorphiques

11 2. Survey Techniques 11

12 Surveying Techniques 12

13 Seismic Acquisition 13

14 Seismic Source: Gun 14

15 Seismic Source: Hammer 15

16 Seismic Receiver: Geophone 16

17 Data Recording: Seismograph 17

18 db Background Noise 0 vertical-component local time [h] 18

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 19

20 Waves Propagation: Snell Law Analogy with optics: the Snell law sin( i ) sin( i ) 1 2 V V 1 2 when i 2 =90, then sin (i 2 )=1 and sin( i ) 1 V V 1 2 In this case, i 1 is called the critical angle i c 20

21 21

22 Seismic Tomography 22

23 T-Dist/2000 [ms] Shot S1 NW m SE

24 Seismic tomography inversion 24

25 T-Dist/2000 [ms] Initial Traveltimes Distance [m] 25

26 T-Dist/2000 [ms] Final Traveltimes Distance [m] 26

27 Depth [m] T-Dist/2 0 [ms] Final Traveltimes Distance [m] Raypaths q2 NW l1 l2 l3 SE Distance [m] 27

28 Refraction Seismic 28

29 Refraction Seismic 29

30 Refraction Seismic 30

31 Interpretation of Refraction Data Solution for two horizontal layers. The unknown parameters are the depth h and the velocities of the two layers. V 1 is given by the slope of the direct arrival segment V 2 is given by the slope of the refracted arrival segment h using the crossover distance: h using the intercept time: h h X b V V 2 V V Ti V V V 1 2 V

32 Reflection Seismic 32

33 Reflection Seismic Acquisition Concept of Common Midle Point (CMP) 33

34 Reflection seismic processing 34

35 Migration and result: the seismic section 35

36 Some Examples Some examples of refraction, reflection and tomography seismic for archeological applications 36

37 MappingtheAncient Port at the Archaeological Siteof Itanos (Greece) Using Shallow Seismic Methods Vafidis et al., 2007, Archeological Prospection, 10, shots 37

38 MappingtheAncient Port at the Archaeological Siteof Itanos (Greece) Using Shallow Seismic Methods Refraction profile Vafidis et al., 2007, Archeological Prospection, 10,

39 Mapping the Ancient Port at the Archaeological Site of Itanos (Greece) Using Shallow Seismic Methods Vafidis et al., 2007, Archeological Prospection, 10,

40 Exploration of the Canal of Xerxes, Northern Greece Reflection profile Refraction profile Jones et al., 2000, Archeological Prospection, 70,

41 Investigation of a Monumental Macedonian Tumulus by Three dimensional Seismic Tomography Polymenakos et al., 2004, Archeological Prospection, 11,

42 Combined Seismic Tomographic and Ultrashallow Seismic Reflection Study of an Early Dynastic Mastaba, Saqqara, Egypt Metwaly et al., 2005, Archeological Prospection, 12,

43 Refraction Metwaly et al., 2005, Archeological Prospection, 12,

44 Reflection profile in time Reflection profile in depth Reflection profile (migration for the first part) Comparison with refraction Metwaly et al., 2005, 44 Archeological Prospection, 12,

45 3. Conclusions for archaeology 45

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 46

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) 47