1. I. Basic Techniques in Structural Geology
Field measurements and mapping
Terminology on folds and folds
Stereographic projections
From maps to cross-sections
Growth Strata
Fault related folds
Seismic Imaging

2. Twiss and Moores, ‘Structural geology’, Chapter 2.
C.M.R. Fowler, ‘The Solid Earth, An introduction to Global Geophysics’
Shaw, Connors and Suppe, ‘Seismic Interpretation of Contractional Fault-related Folds’ (AAPG Seismic Altlas, #53)

3. ‘Snell’s laws’
- There is no energy refracted if i>ic, where the critical angle is defined by
ic= sin-1(V1/V2)
Rock Vp (km/s)
Granite
Basalt
Limestone 6.0
Sandstone 4.2
Shale 2.5

4. Seismic Imaging Techniques
Source
geophones
Direct ic ic
time
Reflected
Refracted
Seismic reflection
Seismic refraction

5. Seismic Imaging Techniques
Travel time of P wave
geophones
Source V1 ic ic V2
V1 < V2
Seismic refraction
Seismic reflection
Critical distance: xc
Crossover distance: Xcross

6. Seismic Imaging Techniques
Travel time of P wave
geophones
Source V1 V2
V1 < V2
Seismic refraction
Seismic reflection
Critical distance: xc
Crossover distance: Xcross

7. Seismic Imaging Techniques
Travel time of P wave
geophones
Source V1 V2
V1 < V2
Seismic refraction
Seismic reflection
Critical distance: xc
Crossover distance: Xcross

8. Seismic Reflection Source geophones Reflection coefficient
A typical value for R is 0.001
Reflectors reflect contrasts of acoustic impedance:
Polarity of reflected wave depends on sign of reflection coefficient

9. Simple ‘zero-offset’ Reflection survey
An ‘image’ of the subsurface is obtained by plotting seismograms side by side.
Reflections are generally faint
The ‘image’ obtained this way is in two-way time, not depth. (to convert to depth the velocity needs to be determined).
The cost scales with the number of sources
For these reasons it is advantageous to deploy lines of geophones (with a range of ‘offsets’)

10. Seismic Reflection Source geophones Two-way travel time is: A x C B zOr
t0 is the two-way normal incidence travel time:
An horizontal reflector generates an hyperbola in time
Velocity, V1, and depth,z. can be determined by plotting t2 as function of x2. t0

11. Seismic Reflection Source geophones Two-way travel time is: A x C B zOr
t0 is the two-way normal incidence travel time:
An horizontal reflector generates an hyperbola in time
Velocity, V1, and depth, z. can be determined by plotting t2 as function of x2. t0

13. Common Mid- Point (CMP) Stacking
The seismograms corresponding to the various offsets can be corrected to account for the effect of the offset on the arrival time (Normal Move Out), and then stacked to simulate a ‘zero offset’ seismograms with enhanced signal to noise ratio.
The Normal Move Out is : 𝛥𝑡

14. Common Mid- Point Stacking
In case of multiple layers the t2-x2 plot yields the ‘Root Mean Square velocity’,VRMS, (also called stacking velocity):
The equation is used to correct for NMO before stacking.
VRMS relates to interval velocity according to Dix’s equation
Interval velocities and thicknesses are determined from

15. Unmigrated Seismic Reflection Profile
Seismograms are plotted side by side.
Vertical axis is the two-way travel time
A Common Mid-Point (CMP) stacked profile show records as if shots and geophones were coincident

16. Migration
In a stacked profile all reflections are plotted as if they were coming from vertical ray paths. This is a ‘distorted’ view of the sub-surface.
Diffractions
Migration aims at correcting these distortions and diffractions (assuming that all reflections are in the plane of the vertical section along the geophones line).

17. Distortions

18. Distortions
Buried focus

19. Distortions
An example with Synthetic seismograms

20. Diffractions

21. Most Common ‘Artifacts’
Multiples (Sediment/Basement interface or water/sea bottom interface in marine survey)
Sideswipes (reflections out of the plane of the section) can mess up the migration process.
Incorrectly migrated diffractions (they look like anticlines but are not)
Pull-up and Pull-down (not really artifacts)

22. Multiples Source geophones Primary reflection t1 First multiple 2.t1
Primary reflection t1
First multiple
2.t1
Time

23. Reflection seismic Line DLC9708 (Hopper et al. , 1997)
Reflection seismic Line DLC9708 (Hopper et al., 1997). Extent of corresponding sparker seismic lines marked by thick line at top. Three first multiples can be seen lower in the section

25. Still not directly an image of the subsurface.
Unmigrated Seismic Reflection Profile
Migrated Seismic Reflection Profile
Still not directly an image of the subsurface.

27. Shortcomings in seismic images of folds
Folds can be distorted or only partially imaged in seismic sections. Two common shortcomings are:
(1) Overlapping reflections in un-migrated or under-migrated sections;
(2) lack of imaging of steeply dipping fold limbs.
NB: Note also pullup.

28. Deformation since Sueyi Time