i. basic techniques in structural geology field measurements and mapping terminology on folds and...

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