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
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
• 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)
• http://principles.ou.edu/seismic_explo/reflect/reflect.html
‘Snell’s laws’ - There is no energy refracted if i>ic, where the critical angle is defined by
ic= sin-1(V1/V2)
43
K
vp
Rock Vp (km/s)Granite 5.0 Basalt 5.5Limestone 6.0Sandstone 4.2Shale 2.5
Seismic Imaging Techniques
• Seismic reflection• Seismic refraction
geophonesSource
Direct
Reflected
Refracted
time
icic
Seismic Imaging Techniques
• Seismic refraction• Seismic reflection
geophonesSource
V2
V1
Travel time of P wave
Critical distance: xc
Crossover distance: Xcross
V1 < V2
icic
Seismic Imaging Techniques
• Seismic refraction• Seismic reflection
geophonesSource
V2
V1
Travel time of P wave
Critical distance: xc
Crossover distance: Xcross
V1 < V2
Seismic Imaging Techniques
• Seismic refraction• Seismic reflection
geophonesSource
V2
V1
Travel time of P wave
Critical distance: xc
Crossover distance: Xcross
V1 < V2
Reflection coefficient
2211
1122
vv
vvR
11v
22v
geophonesSource
A typical value for R is 0.001
Seismic Reflection
Reflectors reflect contrasts of acoustic impedance:
Polarity of reflected wave depends on sign of reflection coefficient
pv
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’)
11v
22v
geophonesSource
11 v
BC
v
ABt
A
B
C
- 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.
Seismic Reflection
x
z
Two-way travel time is:
t0
Or
220 2
1
4z
tv
21
2202
1
2
21
22 4
v
xt
v
x
v
zt
4
2 22
1
xz
vt
11v
22v
geophonesSource
11 v
BC
v
ABt
A
B
C
Seismic Reflection
4
2 22
1
xz
vt
x
z
21
2202
1
2
21
22 4
v
xt
v
x
v
zt
Two-way travel time is:
t0
- 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.
220 2
1
4z
tv
Or
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 :
021
2200 tv
xtttt
0t𝛥𝑡
• 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
Common Mid- Point Stacking
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
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).
Distortions
Distortions
Buried focus
An example with Synthetic seismograms
Distortions
Diffractions
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)
Multiples
11v
22v
geophonesSource
Primary reflection
Tim
et1
2.t1
First multiple
0
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
Still not directly an image of the subsurface.
Unmigrated Seismic Reflection Profile Migrated Seismic Reflection Profile
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.
Deformation since Sueyi Time