Seismic Stratigraphy I - February 8

Basic concepts Seismic data Seismic stratigraphic concepts

Seismic stratigraphy of non-glacial and glacial margins Overview and examples

Seismic Stratigraphy II - February 13

Regional seismic stratigraphy and core data Antarctic Peninsula

Weddell Sea Prydz Bay Wilkes Land Ross Sea

Seismic Stratigraphy III - February 20

• Student presentations• Class discussion: Ross Sea seismic stratigraphy

Assignment:Review:* Text Chapter 5 * Cooper et al. (1991)* Bartek et al. (1991)

Assignment:Review:* Ross Sea Atlas & the paper with the explanatory text

Assignment:* Stoker et al. (1997) * Boulton (1990)

What are seismic data and why are they useful?

What can (and can’t) we do with seismic data?

What are the special characteristics of seismic data from polar margins, and how are these characteristics manifested on the different segments of the Antarctic margin?

How do we “ground truth” seismic data?

How have seismic data helped us to understand the evolution of Cenozoic paleoenvironments in Antarctica?

Some questions that we will try to answer in the next three class sessions:

Seismic data overview

Seismic reflections follow geologic boundaries --- that generally follow geologic-time lines (or geologic-time gaps).

There are many types of seismic systems – to address different geologic questions.

Davies et al (1997)

Seismic sections

* show geometry and seismic character; and

* commonly give reflection time (not depth); and

* are not true geologic depth- cross-sections.

Anderson and Bartek (1992)

Seismic systems have different penetration depths and different resolutions.

In general, the larger the energy source:• the deeper the penetration • the poorer the resolution

These two seismic lines were recorded over the same location.

The vertical scales are the same but the horizontal scales differ.

Can you Delta Fan Complex in both seismic profiles?

Important Concepts:

1. For correct comparisons, seismic records must be at the same scales AND

2. For correct identifications, seismic-system resolution must match the size of the feature being studied.

Seismic resolution differs for most all seismic systems

Why is resolution important?

Why don’t they look the same in both profiles?

A

B

A

Single channel seismic data & Intermdiate resolution

Multichannel seismic data & Low resolutionAnderson and Bartek (1992)

Cooper et al. (1991)

Seismic-Reflection Data: Utility and Limitations

Seismic-reflection data provide --- Reflection geometries Reflection amplitudes and times

that can be used to infer (?) ---- From Geometries:

Faulting Uplift/subsidence Intrusion Erosion Etc.

From amplitudes and times: Sediment densities Sediment velocities

and further infer ---- Sea-level changes Sediment types and physical properties Sediment compaction Etc.

but, cannot be used to derive Isotopic composition Geologic time and age Paleoclimates Milankovich cycles Etc.

As a geologic interpreter, here is what you CAN and CANNOT do with seismic reflection data:

And another thing that you cannot do is…..

In studying glacial history,

seismic reflection data help decipher:• subsurface geometries: ice features• depositional paleoenvironments.

Drilling data show that Antarctica’s Cenozoic paleoenvironments have ranged from temperate to polar.

Seismic-reflection data help to extend the drilling information.

AND…..

How seismic data are useful on polar margins

O’Brien et al. (2001)

Glacial depositionalenvironments are highly varied, ….

...and they commonly have different geologic- and seismic-facies.

Next …. show some seismic examples:

* from highest resolution (and least penetration)

* to lowest resolution (and most penetration).

Some seismic examples

Also includes: Glossary of Glacimarine and Acoustic Terminology

1997

Look at some seismic examples going from hi-resolution to low-resolution systems

Right half of image

Nadir (directly below the ‘fish’)

Entire image

Nadir

Map view of the seafloor

Profile view of the subsurface

Scotian Shelf (Canada)

Fader – in Davies et al (1997, p.303)

Iceberg gouges are found here in about 470 m water depth, but elsewhere in Antarctica they occur in water depths of up to 700-800 m.

Map view of the seafloor

Profile view of the subsurface

Wilkes Landmargin

2-D seismic data

2D

3D

Using specialized 3-D seismic techniques, iceberg ploughmarks on paleo-seafloors (now buried) can be imaged and mapped.

Iceberg ploughmarks

Norwegian continental shelf

Buried Ice Sours: 2D vs 3D Seismic Data

The profiles illustrate

* marine facies (commonly layered)

* subglacial facies (mostly chaotic)

Examples of high-resolution seismic profiles(Labrador shelf)

Bell and Josenhase (1997)

Moran and Fader (1997)

Axial profile of a fiord.

Outer edge of the shelf.

The Ross Sea continental shelf has large glacial features that include:

• broad and deeply-incised glacial troughs and

• large morainal banks.

NorwegianMargin

Overlapping debris flows on the continental slope

~500mIn near- surface rock

~500m In near- surface rock

Arctic

Antarctica

Similar reflection geometries are seen on Arctic and Antarctic margins.

The apparent difference in dip of the continental slope is because the profiles are not at the same horizontal scale.

SeafloorOcean

Fundamental seismic concepts

Fundamental Seismic Concepts

Sedimentary units are characterized by their “acoustic impedance (Z)” whichis the product of “density times velocity”: Z = p * V

Practically all primary reflections originate from acoustic-impedanceboundaries between sedimentary units. The boundaries are caused bylithologic changes, such as due to: change in depositional conditions or mechanics of deposition (e.g., transport,

precipitation, currents). change in sediment lithification. variation in sediment type and/or supply.

Z2>Z1 Z2<Z1

The amplitude of seismic reflections is determined by the “reflection coefficient (R)”: R = (Z2-Z1)/(Z2+Z1)

Seismic reflections occur where there is a distinct change in acoustic impedance. Z = p * V

Different seismic sources

Where there are many geologic units, there are many changes in rock velocity, density and acoustic impedance.

The composite seismic trace is the sum of all the waveforms from each of the geologic boundaries.

“Ring the geologic bell”

If different seismic sources are used over the same geologic section….

…then a different seismic traces are observed…

Hence…seismic sections recorded over the same location may not look the same.

Smallestresolution

How thin can a geologic bed be and still be distinctly resolved on seismic data?

As beds get thinner, the chance for waveform interference increases.

A bed thickness of about 1/4 the wavelength of the seismic pulse is the smallest (or “best”) resolution.

Side reflection

How wide can a geologic feature be and still be distinctly resolved on seismic data?

As features get narrower, the reflections begin to look like hyperbolas…

Some reflections are caused by features that are off to the side, and not directly below the seismic source.

(“depth” to feature)

and the best resolution is about 1 Fresnel zone

Best horizontal resolution

Geometries shown on a seismic section commonly ARE NOT the real geometry of the feature because ??….

…a correction for rock velocities must be made to convert reflection times to depths.

Rule of thumb:1 sec of water = 750 m1 sec of shallow rock = ~1000 m

Do you see the distorted reflections that may be due to velocity variations in overlying strata?

Effect of Velocity on Seismic Reflections

A

A

B

B

Geologic horizon “B” is flat but the seismic reflection from it is not.

Why isn’t reflector “B” flat?

BEWARE….There are several types of artifacts in seismic data.

Sea floor

Multiple reflections are probably the most common.

The amplitudes of seismic reflections can be used to estimate physical properties ofrock..

This is most commonly done using qualitative criteria to define seismic facies.

With careful calibration, quantitative estimates of rock properties are also possible.

Using seismic data to estimate rock properties?

From acoustic impedance, rock density can be estimated.

And, then from density, grain size can be estimated.

Seismic stratigraphic concepts

Lithologic section (in meters)

Time / Lithologic section (in geologic time units)

A basic tenet of seismic stratigraphy is that seismic reflections follow lithologic boundaries. These boundaries commonly follow geologic-time surfaces (or geologic-time gaps).

Sequence: A relatively conformable succession of genetically related strata bounded by unconformities and their correlative conformities (Van Wagon et al, 1988).

Stratigraphic sections (and their seismic representations) contain only a small part (~5%) of the geologic record --- time gaps (i.e., hiatuses) mostly prevail.

In the ocean, sediments are distributed principally by currents.

Basic depositional and stratal concepts

Arrows denote increasing geologic time gaps

Key Factors Affecting Unconformity Geometries

Changes in:

Sea Level

Sedimentation Rate

Intraplate Stress Thermal subsidence Transtension/Transpression Crustal Flexure

Sediment loading Ice loading

Regional plate-boundary changes

Glacial Erosion

Ocean Currents

Items 1-4: Greatest effect on continental shelf

Item 5: Greatest effect on continental slope and rise

Unconformities denote times when paleoenvironmental conditions changed.

Changes can be due to:

Unconformities are important features to identify and map – but why?

The geometries of unconformities and strata help identify the type of paleoenvironmental changes and relative timing of the changes. * erosional * non depositional * other.

Unconformities have different geometries.

What are some of the key geometries?

Badley (1985)

Onlap is an important geometric characteristic of seismic sections

BECAUSE….?

There are other important geometries to look for also...

Onlap points to the location of important unconformities (time gaps) that bound geologic sequences.

Unconformities and Onlap

These geometries also help us to identify unconformities:

Bally (1987)

These are reflection geometries from non-polar continental shelves.

Channel-levy

These deep-water features from the continental rise result from the movement of sediment

• downslope by density flows and

• along slope by deep-ocean currents (e.g., ACC).

Unconformities help outline the geometries of these depositional features.

The internal reflection patterns give information on depositional environments.

Common depositional features found on the continental margin

(and imaged by seismic data)

Mitchum et al. (1977)

Generalized Interpretations for Reflection Patterns

Reflection Configuration Interpretation

Stratified – parallel and draped Uniform suspension sedimentation under tranquil conditions.

Stratified – ponded Current-controlled deposition.

Divergent Varying rates of deposition or differential

erosion and deposition.

Clinoformal -- oblique High-energy conditions and common high sediment supply and small basin subsidence.

Clinoformal -- oblique Lower-energy regime with low sediment

supply and rapid basin subsidence.

Chaotic ‘Agitated’ deposition such as mass flows, diamictons, rapid infill of channels.

Reflection free Uniform lithology (e.g., massive muds) or

highly reworked (e.g., homogenized mass flow deposits).

Seismic reflection patterns reveal information on depositional environments

Mitchum et al. (1977)

Seismic stratigraphy of non-glacial and glacial margins

Badley (1985)

Geologic and seismic facies have some similarities and differences on non-glacial and glacial margins…

…..depending on proximity of glaciers and depth of water on the continental shelf.

Geologic and seismic facies of the non-glacial and glacial continental margin

Antarctica’s history includes non-glacial and glacial periods,…. so all of these features are possible in Antarctic seismic profiles.

Glacial margin (e.g., Antarctica with ice)

Non-glacial margin (e.g., Antarctica before ice)

Primary reflections originate from acoustic-impedance boundaries: Z = p * V (density times velocity)

Summary

From reflection geometries, we can infer structural and depositional processes.

Unconformities are important to identify because they signal times at which paleoenvironmental conditions have changed.

From seismic properties, we can infer sediment types and depositional environments.

To convert a seismic profile into a geologic cross section, you must multiply rock velocity by reflection time to get depth. AND

To properly compare sections, you must ensure they have the same vertical and horizontal scales.

END OF LECTURE