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Geophysical stratigraphy:

Due to the lack of surface outcrop in many areas geophysical methods of correlation have been developed that

exploit the methods of physics to mapg the physical properties of rocks, including:

Density

PermeabilityPorosity

Character of pore fluids

Variations in those properties with depth reveal the presence of different rock types and are used to create

vertical and lateral sections of rocks that can't directly be examined. There are two general approaches:

Well logs: that record information provided by probes that are placed down boreholes

Seismic studies: in which physical features of subsurface rocks are approximated based on seismic

wave propagation

Well logging

Direct sampling: Not all well information is remote. Bentonite muds are continually circulated through the drill

pipe as a coolant and lubricant for the drill bit. Rock chips are brought to surface with the mud, captured,

identified and logged, creating a direct lithologic record.

Aditional information comes from devices lowered into the borehole:

Caliper: measures the width of the drill hole. This indicates the presence of mudrocks, which are prone

to caving and sagging, hence constricting the borehole slightly.

Sonde: A probe lowered into hole to measure various electrical and physical properties of the rocks.

Gas detectors and gas chromatographs: measure gases in the well.

Gamma-ray log: measures natural radioactivity of the strata

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What the sonde records:

Spontaneous potential (SP) log: measures

difference in electrical potential between an

electrode on the sonde and one at the surface.

An electrical potential exists between the

natural pore fluids of the rock and the drilling

mud that invades those pores. Therefore, SP

logs are a measure of permeability:

Shales and limestone nearlyimpermeable and have a 0 reading

Negative deflection for sandstones (highpermeability) or fluids with higher

conductivity than the drilling mud (likesaltwater)Positive deflection for fluids with lower

conductivity than mud (like freshwater)

Resistivity (R) log: measures resistivity of

fluids in the surrounding rock to an applied

electrical current. Resistivity indicates amount

of fluid in the pore spaces, therefore R logs are

measures of porosity. Resistivity increases

with decreasing pore space.

High resistivity: dense rocks with no pores (quartzite, limestone.), rocks with non-conducting fluid intheir pores (like petroleum)

Low resistivity: rocks with significant porosity (sandstone), rocks with conducting fluid in their pores(like salt water), rocks containing significant amounts of water in their crystal structure (clay-rich rocks).

Examples of Spontaneous Potential logs:

Fluvial deposit with point bar sequence and overbank mud shows fining-upward sequence.

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Deltaic deposit grading into shoreline Coarsening upward

sequence with thick sands.

Deltaic deposit grading into distributary channel andinterdistributary bay Coarsening upward sequence.

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Deltaic deposit grading into delta plain Coarsening

upward sequence with thick sands.

Regressive shoreline Coarsening upward

sequence.

Dipmeter: measures resistivity in four directions. By this means, it locates contacts and identifies their dip

direction, allowing identification of folds, faults, and other structure.

Gamma-ray log: measures natural radioactivity of the rock. Most gamma radiation comes from decay of

40K. Therefore, the gamma-ray log is sensitive to rocks high in K-bearing minerals (feldspars, micas,

clays) including:

shales

feldspathic and lithic sandstones

In contrast, limestones and quartz rich sandstones produce low gamma ray values.

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Well-logging techniques all have one significant drawback: They require you to drill a borehole. There is a less

expensive alternative:

Seismic stratigraphy

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Shots, pulses of sound are generated:

by explosives or a mechanical thumper

on Vibraseis trucks on land

by a shipboard air gun at sea.

Those waves that are propagated nearly straight

downward can be reflected off subsurface

interfaces of materials of different densities, such

as contacts between rock units.

Travel time is recorded by an array of

geophones on land or hydrophones at sea.

Reflections from each shot are recorded

as individual seismic profiles by the

geophones.

Information from each geophone in the

array is correlated, processed to removenoise, and summed up across the array,

yielding a vertical line in which reflectors

as shown as wave-shaped deviations. This

is a one-dimensional plot of reflectors

beneath the shot

The array and thumper are then moved

slightly along a transect and a new seismicshot made, which yields a separate trace.

Ultimately individual traces are displayed

together as seismic profiles,

approximating two dimensional images of

reflectors below the transect, each vertical line of which represents one shot.

More ambitious seismic techniques involve the deployment of two-dimensional geophone arrays to develop

three-dimensional seismic profiles.

Definitions:

Reflector: boundary that creates a seismic reflection

Reflection: acoustic waves created by sounds bouncing off of a reflectorImpedance: physical rock property of sound propagating through rock. A function of average sound

velocity and rock density

Impedance contrast: physical boundary within rocks producing a reflection

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Seismic stratigraphy can be

used for both deep and

shallow structural analysis.

Layered reflectors appear as

distinct horizons, whereas

structureless units or those of

uniform density show

random reflections. (E.G.:

the contrast between marine

sediments and a rising salt

structure - right.) But what,

exactly are these reflectors?

Simply, they are density

contrasts. These may be

caused by:

contacts between rock units

interface of different pore fluids (E.G.: petroleum and water)unconformities

diagenetic features

The traces of seismic reflections have numerous aspects that can be measured:

amplitude

duration (2-way travel time)

area, etc.

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This is pleasingly quantifiable, however a large element of inference

goes into the interpretation of seismic profiles, because:

Seismic profiles are NOT cross sections because the vertical

scale is two-way travel time, not thickness.

Reflector horizons needn't be lithologic boundaries. Layers with

high concentrations of chert nodules make nice reflectors, for

instance.The resolution of seismic stratigraphy is low. A single seismic

pulse on a seismic profile may be up to 150 m. thick. (right)

As with so much else in stratigraphy, the ability to amass large quantities of information compensates for the

uncertainty inherent in the information. In this case, patterns that are likely to be connected to stratigraphy can be

observed at great depths in unexposed rock on land or beneath the sea, into which no well has been bored.

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Of course, if well-log or

outcrop information is also

available, seismic reflectors

can reliably be connected to

known lithologies. By this

means we have learned that

marine sediments tend to

contrast strongly with

continental ones. (right)

The presence of petroleum

can be revealed by

anomalous horizontal

reflectors indicating the

interface of petroleum and

water, or by a brightening of

the profile caused by the

presence of gas.

Cornell University, through the Consortium on Continental Reflection Profiling (COCORP) has used seismic

methods to profile major orogenies. Among the interesting results: Whereas the traditional view was that the

Piedmont and Blue Ridge had deep crustal roots, it develops that they are underlain by extensive thrust faults

and have actually been thrust onto Paleozoic sediments.

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Seismic sequences: The geometry of

unconformities that truncate beds is

sufficiently distinctive to be identifiable in

seismic profiles, allowing identification of

seismic sequences - unconformity

bounded "packages" whose presence is

revealed by seismic reflections. Indeed,

the development of sequence stratigraphy

has gone hand-in-glove with that of

seismic stratigraphy, because only

seismic methods can identify sequence

boundaries on a large scale.

When connected to lithologic

information, these can be correlated with

age to identify sea level cycles. (right)

For many, the hope has been that these

would be caused by global eustatic sea

level change, enabling their use in global

sequence stratigraphy. In 1977, Vail,

Mitchum, and Thompson published a

summation of first and second order sea

level curves based on major

unconformities.

Second-order cycles appeared to be

markedly asymmetric, because of the

depositional asymmetry of transgression

and regressions in which transgressions are erosional, but sediment can continue to aggrade up during

regressions. Identifying onlap and offlap of sediments onto continents, enabled Haq and colleagues to develop

an adjusted curve showing sea level over the last 200 my.

As your text makes clear, the reliability and usefulness of the Vail curve is controversial.