# Techniques of Structural Geology

```The two appendices introduce
critical background knowledge
for studying structural geology.
Appendix 1 covers the basics of
orientation and representation
of structures, which includes
the Attitude of Planes and
Lines; Graphical Presentation of
Orientation
Data;
Geologic
Maps;
and
Portrayal
of
Structures in 3-D. Appendix 2
covers
basic
Geophysical
Techniques, including Seismic
Studies; Analysis of Gravity
Anomalies; and Geomagnetic
Studies.
Appendix 1: The Orientation
Representation of Structures
and
i)
Right hand rule: if one faces in
the
direction
of
azimuth
specifying the strike, the plane
must dip down to the right
For a graphical introduction to Trend &
Plunge:
http://serc.carleton.edu/images/NAGTWo
rkshops/structure/rake_diagram.jpg
Trend & Plunge are a measurement of
linear features. Similarly to Strike & Dip,
Trend & Plunge can be written as a
bearing or azimuth.
Reference & Resources
LAB Exercise #1
http://seismo.berkeley.edu/~burgmann/EP
S116/labs/lab1/lab1_2009.pdf


The Attitude of Planes and Lines
a) Attitude = orientation in space
(fundamental to the description
of structures)
b) Strike line: the line in the plane
that is horizontal
c) Strike: the angle measured from
geographic north to the strike
line
d) Dip line: the line that lies in the
planar structure perpendicular to
strike line.
e) Dip: angle measured in a vertical
plane from the horizontal down
to the dip line. Dip is always
perpendicular to the strike line.
For a graphical introduciton to Strike &
Dip:
http://drannabalog.com/internet/StrikeDip
.gif
Strike & Dip are measurements of planar
features.
Strike & Dip can be recorded as either a
bearing or an azimuth (Refer to Lab 1).
f) Trend line: the orthogonal
projection of the linear structure
onto the horizontal plane
g) Trend: angle between geographic
north and the trend line
h) Plunge: angle between horizontal
and the linear structure itself
Graphical
Presentation
of
Orientation Data
a) Histograms: plots of one part of
orientation data against the
frequency of orientations that are
found within particular azimuth
intervals
b) Rose
diagrams:
essentially
histograms for which the
orientation axis is transformed
into a circle to give a true
angular plot
c) Spherical projection: include
equal-angle projection and equalare projection.
For an introduction to Stereonets and
their applications. Review LAB
Exercise 2.
Reference & Resources
LAB Exercise #2
http://seismo.berkeley.edu/~burgmann/EP
S116/labs/lab2/lab2_2011.pdf

Map and Cross Section
A geological map is a representation
of geometric and structural features
based on observed and collected data.
a) Scale: the ratio of the distance on
the map to the equivalent distance on
the ground
a) Cross section: complements the
information on geologic maps by
showing the variation of
1
structure with depth, usually as it
would appear on a vertical plane
that cuts across the area of a
geologic map.
a. Understanding apparent
dip is crucial to
understanding
cross
sections (Review notes
from 2/11 lecture).
b) Vertical
exaggeration:
the
vertical scale is larger than the
horizontal scale ratio
Reference & Resources
Figure A1.5
Page 698 T&M Structural Geology
Appendix 2: Geophysical Techniques
 Background:
a) Wave velocities increase with
density and depth, paths within
earth tend to be concave up.
b) P waves: compressional, particle
motion parallel to wave motion.
c) S waves: secondary, particle
motion perpendicular to wave
motion.
d) P waves primarily used for
geophysical studies.

Seismic studies
a) Seismic refraction: used to infer
the structure of the Earth by
studying the arrival times of
synthetic and natural seismic
rays that propagate through
boundaries where the wave
velocity changes
Tong, Xinyue (Dennis), 2011
Edited by Jason Utas and Max Dieckmann, 2013
b) Seismic reflection: the structure
of the Earth is revealed by
reflections of P waves off
internal
boundaries
and
heterogeneities.
c) Record section: the resulting plot
of seismic studies
d) Stacking: method of enhancing
the signal-to-noise ratio by
occur at different angles from the
same subsurface point
e) Migration: a technique that
allows the true locations of
reflectors to be determined. All
reflections are plotted as though
they are vertical reflections,
eliminating artifacts and errors
un raw record.
f) First motion studies: First arrival
of a P wave, characterized by
compression or rarefaction,
determined by the orientation of
the fault and slip direction. Used
to determine the orientation of
and sense of slip on faults at
depth.
Reference & Resources
Box A2-2 and Figure A2.2
Page 705 T&M Structural Geology

Earthquake Geometry
a)
Gravity
Anomaly:
The
difference between a measured
value of the acceleration due to
gravity at a given location and a
reference value for the given
location.
b) Assumptions
for
reference
values: (1) gravity measured at
sea level and (2) all surrounding
topography
is
level/lacks
topographical variation.

Corrections that must be applied to
gravity measurements:
a) Free-air = difference due to
altitude above sea level
b) Bouguer = corrects for local
topography
(differences
in
mountains ,valleys, etc.) Often
referred
to
as
“terrain
correction.”
Note: Gravity anomalies are unspecific
and can often reflect a variety of
underlying structures such as faults or
intrusions.
Reference & Resources
Figure A2.4
Page 709 T&M Structural Geology
 Geomagnetic Studies
Magnetic anomalies: measurements of the
variation of the Earth’s magnetic field
relative to some locally defined reference.
Generally used to infer underlying rock
types and structures that may be covered
by other rocks, sediments, or structures.
Reference & Resources
Figure A2.3
Page 708 T&M Structural Geology
Analysis of Gravity Anomalies
Paleomagnetism: found by measuring the
orientation of magnetic fields preserved in
rocks. Can tell us (1) about the strength
of the field in the past, (2) the direction of
the magnetic field and times of pole
reversals, and (3) the latitude at which a
given sample was magnetized.
Reference & Resources
Figure A2.5
Page 712 T&M Structural Geology
2
Tong, Xinyue (Dennis), 2011
Edited by Jason Utas and Max Dieckmann, 2013
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