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Processing of Shear Waves from VSP Data at the Forsmark Site Investigation

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Processing of Shear Waves from VSP Data at the Forsmark Site Investigation Institutionen för Geovetenskaper – Geofysik UNIVERSIDAD SIMÓN BOLIVAR Earth Science Department – Geophysical Engineering Processing of shear waves from VSP data at the Forsmark site investigation Master's Thesis Gwendal Georges Salmon Chatelet Uppsala, Spring 2008 TABLE OF CONTENT 1. Introduction....................................................................................................................................7 2. Geology of the area 2.1. Regional Geology....................................................................................................................9 2.2. Local Geology; The Forsmark site........................................................................................13 3. Seismic theory and VSP 3.1. Bases of Seismic Theory.......................................................................................................15 3.2. The Wave Equation...............................................................................................................16 3.3. Vertical Seismic Profile.........................................................................................................17 3.4. VSP processing.....................................................................................................................18 4. Seismics in anisotropic media 4.1. Anisotropic mediums ...........................................................................................................20 4.2. Shear waves splitting analyzing methods.............................................................................24 4.3. Real anisotropy in the subsurface.........................................................................................30 5. Equipment and data acquisition process 5.1 The seismic equipment...........................................................................................................34 5.2. The acquisition process.........................................................................................................35 6. Data Processing............................................................................................................................36 6.1. Polarization estimation..........................................................................................................44 6.2. Velocity analysis....................................................................................................................52 7. Results and interpretation.............................................................................................................54 8. Discussions and conclusions........................................................................................................57 9. Further work.................................................................................................................................59 2 10. Acknowledgment.........................................................................................................................60 11. References....................................................................................................................................61 12. Internet Sources............................................................................................................................63 3 LIST OF FIGURES 1. Figure 1.1. Location of borehole KFM01 and the 10 shot points. Cosma et al. (2005)................8 2. Figure 2.1.1. Main Precambrian units in southern Fennoscandia. Åhäll K., Connelly J. (2008)..9 3. Figure 2.1.2. Major tectonic units in the Fennoscandian Shield. Hermansson et al (2008)........10 4. Figure 2.1.3. A) Bedrock map of the Fennoscandian Shield. Bergman et al. (2008)...................11 B) Bedrock map of south-central Sweden and Southern Finland....................11 5. Figure 2.1.4. a) Tectonic domains in the western part of the Svecofennian orogen....................12 b) Structural style of the Forsmark area. Hernansson et al. (2008)........................12 6. Figure 2.2.1. The Forsmark investigation site. Cosma C. et al. (2005).......................................13 7. Figure 3.1. Representation of axial stress in a cylinder. http://physics.uwstout.edu/..................15 8. Figure 4.1.1. Medium with vertical hexagonal symetry and horizontal propagation. Caicedo M., Mora P., (2004)........................................................................................................22 9. Figure 4.1.2. Birefringence for the shear waves in two bodies with horizontal and vertical fracture planes. Caicedo M., Mora P., 2004.............................................................23 10. Figure 4.1.3. Polarization of the shear wave depending of the relation between incidence polarization and fractures direction. Juhlin C., (1990).................................24 11. Figure 4.2.1. Wave propagating through an anisotropic medium and recorded by a geophone. MacBeth C. and Crampin S. (1991)...................................................................25 12. Figure 4.2.1.1. Polarization diagrams. MacBeth C. and Crampin S. (1991)...............................27 13. Figure 4.2.2.1. Particle motion discriminants. MacBeth C. and Crampin S. (1991)...................28 14. Figure 4.2.3.1. Ellipsoid of motion described by the particle motion. MacBeth C. and Crampin S. (1991).............................................................................................29 15. Figure 4.2.4.1. Propagator matrix between horizontal components of two adjacent 4 geophones. From MacBeth C. and Crampin S. (1991)................................................................30 16. Figure 4.3.1.1 Splitting of shear waves in two anisotropic medium with different direction of the fracture's planes. MacBeth C. and Crampin S. (1991).......................................................31 17. Figure 4.3.2.1. a) Polar map with the direction of the fast and slow shear waves......................32 b) Three receptor at different angles with the source. MacBeth C. and Crampin S. (1991)................................................................................................32 18. Figure 5.1.1. One of the VIBSIST-1000 sources used for the at Forsmark. Cosma et al. (2005)......................................................................................................................34 19. Figure 6.1. Shot S01 without (a) and with (b) spherical divergence correction .........................37 20. Figure 6.2. Horizontal and vertical components..........................................................................37 21. Figure 6.3 a) Seismic traces for the vertical (V) and horizontal components from receiver 37 in S06......................................................................................................38 b) Hodograms before and after azimuth and incident angle rotation from receiver 37 in S06......................................................................................................39 22. Figure 6.4 a, b, c, e, f and g. Shots S01, S02, S03, S05 S06 S08 and S09 respectively after rotations of horizontal and vertical components............................................................40-41 23. Figure 6.5. Frequency range in S06.............................................................................................42 24. Figure 6.6. Shot S05 before and after balancing..........................................................................43 25. Figure 6.7. Seismograms from shot S06. Fill holes process........................................................43 26. Figure 6.1.1a, b, c, e, f and g. Shots S01, S02, S03, S05 S06 S08 and S09 respectively. Hodograms and seismograms with LMO..........................................................44-50 27. Figure 7.1 Mid Ocean Ridge. Minster et al. (1974).....................................................................55 28. Figure 8.1. Shot S02, Receiver 75. Representation on particle motion for a shear wave in the two horizontal components................................................................................................57 5 LIST OF TABLES 1. Table 2.2.1. Main groups of rocks at Forsmark. T. Hermansson et al. (2007).............................14 2. Table 4.1. Crystalline structures and the number of independent coefficient. Modified after Caicedo M. and Mora P., 2004.............................................................................20 3. Table 4.2.1. Applications of several methods in different areas. MacBeth C. and Crampin S. (1991).................................................................................................................26 4. Table 5.2.1. Coordinates of borehole KFM01A. Cosma et al. (2005).........................................35 5. Table 5.2.2. Coordinates of the source positions for the VSP survey at KFM01A. Cosma et al. (2005)......................................................................................................................35 6. Table 6.1.1 Rotation angles from hodograms for every shot.......................................................51 7. Table 6.1.2 Incidence angles at the first and last receptor for every shot....................................51 8. Table 6.2.1. Velocities in both shear wave components measured directly in t he seismic sections.......................................................................................................................52 9. Table 6.2.2. Shear waves velocities for H1 and H2 components using the method described by Li et al. (1988)........................................................................................................53
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