# Last Time GY 305: Geophysics UNIVERSITY OF SOUTH ALABAMA Seismic Stratigraphy

```Last Time
UNIVERSITY OF SOUTH ALABAMA
Seismic Stratigraphy
GY 305: Geophysics
Lecture 13:
Sequence Stratigraphy 1
•
•
•
•
•
More on Sonic Logs (AKA Acoustic logs)
More on Synthetic seismic profiles
More on Seismic sections (how they work)
More on Interpretations of seismic sections
Peter Vail
Side Scan Sonar
Sideways looking sonar developed in the
1960s. Has the ability to locate
interesting features on the sea floor
Side Scan Sonar
Iceberg scars of the
Newfoundland coast
http://www.abc.se/~m10354/mar/img/sture/system2.jpg
High Tech “Sonar” Techniques
•GLORIA: First deployed in
the 1980’s
•Capable of surveying 18,000
km2 per day
•50 m resolution
http://www.l-3klein.com/image_gallery/530_images/scours/scours.html
High Tech “Sonar” Techniques
•TOBI: First deployed in the
late 1980’s
•Capable of 70 m penetration
•2 m resolution
Sonic Logs
Sonic Logs
Ultimately, we are going to
have to interpret reflection
characteristics of rock units.
•Sonic (acoustic logs) display the travel
time of P-waves through a formation.
•The tool emits a sound wave that travels
from the source (T) to the formation and
back to receiver(s) (R).
Consider controls on
reflection strength and
polarity…
•It is used to establish baseline seismic
wave transmission data (e.g., to produce
synthetic seismic profiles)
http://pangea.stanford.edu/~sklemp/bering_chukchi/images/seismic.section.gif
http://www.spwla.org
Seismic Stratigraphy
Recall from an earlier lab…
•Acoustic impedance
of a formation (1) is
simply defined as:
a1 = v1 x d1
v1 = the velocity of a
seismic wave
d1 = rock density
http://homepage.ufp.pt/biblioteca/GlossarySaltTectonics/PlatesJPG/Fig.A001b-AcousticImpedance.jpg
http://openlearn.open.ac.uk/file.php/2292/S278_1_022i.jpg
Sequence Stratigraphy
Today
A Short History
&quot;Pete's ideas evolved naturally from his first
pioneering work on the importance of stratal
surfaces in rocks as geologic time lines,&quot; wrote
Robert Mitchum, a longtime friend and former
Exxon colleague.
&quot;He soon recognized the cyclic occurrence of
bundles of strata he called sequences in well logs,
seismic reflections and outcrops.
&quot;Observing that sequence boundaries appear
synchronous globally, he postulated that cyclic
eustatic sea level changes are major controls on
stratigraphy, along with basin tectonics and
sediment supply,&quot; Mitchum said.
&quot;This realization led to the development of
eustatic cycle charts”. (2003 AAPG Address)
Sequence Stratigraphy
•
•
•
The basic concept (systems tracts)
All sorts of STs (systems tracts)
sea level change
Sequence Stratigraphy
Sequence Stratigraphy
A Short History
A Short History
The “Vail Curve” is synonymous
with eustasy
And is used by
everyone in soft
rock geology.
•stratigraphers
•sedimentologists
•paleontologists
•petroleum geol.
Sequence Stratigraphy
Sequence Stratigraphy
A Short History
A Short History
Why?
Because sea level controls what sediment is produced and
where it is deposited.
http://strata.geol.sc.edu/terminology/system-tract.html
Accommodation Space: The thickness of sediment that can be
deposited (it’s sea level-controlled)
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy
Sequence Stratigraphy
A Short History
A Short History
Accommodation
Space:
Note: Vail’s surfaces
actually define
packages of
sedimentary rock.
(+) with sea level
highstands;
Those associated
with specific states
of sea level are
called systems tracts.
(-) with sea level
lowstands
http://strata.geol.sc.edu/terminology/system-tract.html
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
System Tracts come in numerous styles and are named
according to sea level position/state during deposition.
The 6 most important are
Sequence Stratigraphy (System Tracts)
LST: Low stand systems tract: includes deposits that accumulate
after the onset of relative a sea-level rise.
1) LST: Low stand systems tract
2) HST: High stand systems tract
3) TST: Transgressive systems tract
4) FSST: Falling stage systems tract
5) RST: Regressive systems tract
6) FRST: Forced regressive systems tract
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
Sequence Stratigraphy (System Tracts)
LST: Low stand systems tract: includes deposits that accumulate
after the onset of relative a sea-level rise.
LST: Low stand systems tract: includes deposits that
accumulate after the onset of relative a sea-level rise.
Lowstand Systems Tract sediments often fill or partially infill
incised valleys that were cut into the Highstand Systems Tract,
and other earlier deposits, during the FSST.
Lowstand Systems Tract sediments often fill or partially
infill incised valleys that were cut into the Highstand
Systems Tract, and other earlier deposits, during the FSST.
Stacking patterns exhibit [backstepping, onlapping,
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
Sequence Stratigraphy (System Tracts)
Remember that you are doing all this via seismic traces
Remember that you are doing all this via seismic traces. The
nature of the surfaces can tell you the stage(s) of sea level
during deposition.
Clinoforms are parallel but inclined surfaces
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
Sequence Stratigraphy (System Tracts)
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
LST: Low stand systems tract: includes deposits that accumulate
after the onset of relative a sea-level rise.
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
HST: High stand systems tract: the progradational deposits that
form when sediment accumulation rates exceed the rate of
increase in accommodation space. Stacking patterns exhibit
http://strata.geol.sc.edu/terminology/system-tract.html
http://strata.geol.sc.edu/terminology/system-tract.html
Sequence Stratigraphy (System Tracts)
HST: High stand systems tract: the progradational deposits that
form when sediment accumulation rates exceed the rate of
increase in accommodation space. Stacking patterns exhibit