Geophysical stratigraphy II: Seismic Stratigraphy
Shots, pulses of sound are generated:
- by explosives or a mechanical thumper on Vibroseis 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 remove noise, 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 seismic shot 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.
- Reflector: boundary that creates a seismic reflection
- Reflection: acoustic waves created by sounds bouncing off of a reflector
- Impedance: 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
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)
- diagenetic features
The traces of seismic reflections have numerous aspects that can be measured:
- duration (2-way travel time)
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 rock 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.
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.
Nevertheless, bedding in sediments tends to show up in seismic profiles whereas structureless rocks such as salt diapirs, reefs, and crystalline basement rock lacks coherent reflectors. (above)
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.
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. (A - right)
When connected to lithologic information, these can be correlated with age to identify sea level cycles. (B - 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, Exxon stratigraphers Vail, Mitchum, and Thompson published a summation of first and second order sea level curves based on major global unconformities identified in this way - the Exxon-Vail curve.
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 or prograde up during regressions, causing regressions to appear more abrupt thatn they are in the sequence strat record. Identifying patterns of onlap and offlap caused by retrogradation and aggradation/progradation) 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 Exxon-Vail curve and its derivatives are regarded by many as an industry standard, however their reliability and usefulness are controversial, with some studies advocating caution.