Seismic Stratigraphy

Seismic Stratigraphy is basically a geologicapproach to the stratigraphic interpretationof seismic data.

Seismic reflections allow the directapplication of geologic concepts based onphysicl stratigraphy.

Primary seismic reflections are generated byphysicl surface in the rocks, consistingmainly of stratal surface and unconformitieswith volocities-density contrasts.

Therefore, primary seismic reflectionsparallel stratal surface and unconformities.

A seismic section is a record ofchronostratigraphic (time-stratigraphic)depositional and structural patterns and nota record of the time-transgressivelithostratigraphy (rock-stratigraphy).

Because seismic reflections follow chronostratigraphiccorrelations, it is not only possible to interpretpostdepositional deformation, but also it is possible tomake the following types of stratigraphic interpretationfrom the geometry of seismic reflections correlationpatterns: Geologic time correlations Definition of genetic depositional units Thickness and depositional environment of genetic units Paleobathymetry Burial history Relief and topography on unconformities Paleogeography and geologic history

However, lithofacies and rocktype cannot be determined directly from thegeometry of reflection patterns.

To accomplish these geologic objectivesyou follow three step interpretationalprocedure: 1 seismic sequence analysis 2 seismic facies analysis 3 analysis of relative changes of sea-level

Seismic sequence analysis is based on theidentification of stratigraphic unitscomposed of a relatively conformablesucession of genetically related startatermed depositional sequence.

The upper and lower boundaries ofdepositional sequences are unconformitiesor their correlative conformities.

Depositional sequence boundries arerecognized on seismic data by identifyingrefelctions caused by lateral terminations ofstrata termed:

Onlap Downalp Toplap truncation

Sequence stratigraphy arrived on the geologic stage in 1977 whenVail and his co-workers published on techniques they had developedat Esso Production Research to interpret seismic cross-sections. They assumed that continuous seismic reflectors on acousticgeophysical cross-sections are close matches to thechronostratigraphic surfaces, or time boundaries like bedding planesand unconformities.They had established that unconformities were clearly recognizableon marine seismic sections and assumed that like the unconformitiesof the Paleozoic identified by Wheeler (1958), Sloss (1963, 1972) andSloss and Speed (1974) were the products of worldwide changes insea level or eustasy. They noted that the unconformities enveloped packages of reflectorsand called these seismic sequences. They demarked these with seismic reflectors onlapping andterminating either against the lower unconformity surface or againsteach other.

Eustasy and Peter Vails revolution in stratigraphic analysis From USC Sequence Startigraphy Web(http://strata.geol.sc.edu/history/vial.html)

Using techniques developed by Schuchert (1916),Umbgrove (1939) and Wheeler (1958), they assumed thatthe position of onlapping seismic reflectors was controlledby the base level of the mean high water mark. Thus asediment (or seismic) encroachment chart could be drawnthat shows how far the sediment wedge of submarine,coastal and alluvial sediment (represented by the seismic)has onlapped a basin margin (Vail et al., 1977).

A sediment (or seismic) aggradation chart was alsoconstructed that shows the vertical component by whichonlapped seismic reflectors had climbed or fallen (Fig. 4c)(Vail et al., 1977).

They then correlated the cycles of relative changes of sealevel at multiple locations and construct charts thatincorporated the occurrence of global sediment (or seismic)onlap cycles.

Using the aggradational measurements from the seismic,Vail et al, estimated the magnitude of relative sea-levelexcursions. However it was recognized the position of aneustatic event had on the continent is complicated by thelocal effects of tectonic subsidence (Bally, 1981; Watts,1982; Thorne and Watts, 1984). This may explain why sea-level curves for the Jurassic compiled by Hallam (1981) andVail and Todd (1981) from different data sources recorddifferent positions for the same sea-level stands. Posamentier and Allen (1999) confirm this effect of localtectonism with a block diagram of a margin that hasdifferent tectonic signals along its length and consequentlydifferent relative sea level positions.