Seismic Stratigraphy I - February 8 Assignment: * Stoker et al. (1997) * Boulton (1990) Seismic Stratigraphy I - February 8 Basic concepts § Seismic data § Seismic stratigraphic concepts Assignment: Review: * Text Chapter 5 * Cooper et al. (1991) * Bartek et al. (1991) Seismic Stratigraphy II - February 13 Seismic stratigraphy of non-glacial and glacial margins § Overview and examples Regional seismic stratigraphy and core data § Antarctic Peninsula § Weddell Sea § Prydz Bay § Wilkes Land § Ross Sea Assignment: Review: * Ross Sea Atlas & the paper with the explanatory text Seismic Stratigraphy III - February 20 Student presentations Class discussion: Ross Sea seismic stratigraphy

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

There are many types of seismic systems – to address different geologic questions. Seismic reflections follow geologic boundaries --- that generally follow geologic-time lines (or geologic-time gaps). 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

Seismic resolution differs for most all seismic systems Single channel seismic data & Intermdiate resolution Multichannel seismic data & Low resolution Anderson and Bartek (1992) Cooper et al. (1991) Why is resolution important? 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? Why don’t they look the same in both profiles? A B A 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.

As a geologic interpreter, here is what you CAN and CANNOT do with seismic reflection data: And another thing that you cannot do is…..

How seismic data are useful on polar margins O’Brien et al. (2001) How seismic data are useful on polar margins 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…..

Glacial depositional environments 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 Also includes: Glossary of Glacimarine and Acoustic Terminology Look at some seismic examples going from hi-resolution to low-resolution systems

Right half of image Entire image Nadir (directly below the ‘fish’)

Map view of the seafloor Fader – in Davies et al (1997, p.303) Scotian Shelf (Canada) Map view of the seafloor Profile view of the subsurface

Wilkes Land margin Map view of the seafloor Profile view of the subsurface 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.

Buried Ice Sours: 2D vs 3D Seismic Data Norwegian continental shelf 2D 3D Using specialized 3-D seismic techniques, iceberg ploughmarks on paleo-seafloors (now buried) can be imaged and mapped. Iceberg ploughmarks 2-D seismic data

high-resolution seismic profiles Axial profile of a fiord. Outer edge of the shelf. Examples of high-resolution seismic profiles (Labrador shelf) The profiles illustrate * marine facies (commonly layered) * subglacial facies (mostly chaotic) Bell and Josenhase (1997) Moran and Fader (1997)

The Ross Sea continental shelf has large glacial features that include: broad and deeply-incised glacial troughs and large morainal banks.

Norwegian Margin Overlapping debris flows on the continental slope

~500m In near- surface rock Arctic Similar reflection geometries are seen on Arctic and Antarctic margins. ~500m In near- surface rock Antarctica The apparent difference in dip of the continental slope is because the profiles are not at the same horizontal scale.

Ocean Seafloor

Fundamental seismic concepts

Z2>Z1 Z2<Z1 Seismic reflections occur where there is a distinct change in acoustic impedance. Z = p * V The amplitude of seismic reflections is determined by the “reflection coefficient (R)”: R = (Z2-Z1)/(Z2+Z1)

“Ring the geologic bell” 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.

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

and the best resolution is about 1 Fresnel zone Side reflection How wide can a geologic feature be and still be distinctly resolved on seismic data? (“depth” to feature) As features get narrower, the reflections begin to look like hyperbolas… and the best resolution is about 1 Fresnel zone Best horizontal resolution Some reflections are caused by features that are off to the side, and not directly below the seismic source.

Effect of Velocity on Seismic Reflections Geometries shown on a seismic section commonly ARE NOT the real geometry of the feature because ??…. A B Geologic horizon “B” is flat but the seismic reflection from it is not. Why isn’t reflector “B” flat? …a correction for rock velocities must be made to convert reflection times to depths. Rule of thumb: 1 sec of water = 750 m 1 sec of shallow rock = ~1000 m A B Do you see the distorted reflections that may be due to velocity variations in overlying strata?

There are several types of artifacts in seismic data. BEWARE…. There are several types of artifacts in seismic data. Sea floor Multiple reflections are probably the most common.

Using seismic data to estimate rock properties? 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.

From acoustic impedance, rock density can be estimated. And, then from density, grain size can be estimated.

Seismic stratigraphic concepts

Basic depositional and stratal concepts In the ocean, sediments are distributed principally by currents. A basic tenet of seismic stratigraphy is that seismic reflections follow lithologic boundaries. These boundaries commonly follow geologic-time surfaces (or geologic-time gaps). Lithologic section (in meters) 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. Arrows denote increasing geologic time gaps Time / Lithologic section (in geologic time units)

Unconformities are important features to identify and map – but why? Unconformities denote times when paleoenvironmental conditions changed. Changes can be due to:

Unconformities have different geometries. The geometries of unconformities and strata help identify the type of paleoenvironmental changes and relative timing of the changes. * erosional * non depositional * other. What are some of the key geometries? Badley (1985)

Unconformities and Onlap Onlap is an important geometric characteristic of seismic sections BECAUSE….? Onlap points to the location of important unconformities (time gaps) that bound geologic sequences. There are other important geometries to look for also...

These geometries also help us to identify unconformities: Bally (1987)

These are reflection geometries from non-polar continental shelves.

downslope by density flows and along slope by deep-ocean 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).

Common depositional features found on the continental margin (and imaged by seismic data) Unconformities help outline the geometries of these depositional features. The internal reflection patterns give information on depositional environments. Mitchum et al. (1977)

Seismic reflection patterns reveal information on depositional environments Mitchum et al. (1977)

Seismic stratigraphy of non-glacial and glacial margins

Geologic and seismic facies of the non-glacial and glacial continental margin Non-glacial margin (e.g., Antarctica before ice) 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. Glacial margin (e.g., Antarctica with ice) Antarctica’s history includes non-glacial and glacial periods,…. so all of these features are possible in Antarctic seismic profiles.

Summary Primary reflections originate from acoustic-impedance boundaries: Z = p * V (density times velocity) 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. From reflection geometries, we can infer structural and depositional processes. From seismic properties, we can infer sediment types and depositional environments. Unconformities are important to identify because they signal times at which paleoenvironmental conditions have changed.

END OF LECTURE