3.12 Seismostratigraphy (Seismisk stratigrafi)
Seismostratigraphy is the study of stratigraphy and depositional facies as they can be interpreted using seismic data.
Seismic reflectors form where significant, rapid changes of acoustic impedance (density x sound velocity) take place in the bedrock. Seismic reflectors will therefore chiefly be linked to bedding planes and unconformity surfaces.
Seismostratigraphy relies upon the assumption that persistent primary seismic reflectors are time-stratigraphical horizons rather than lithological boundaries. A seismic section is therefore assumed to give a chronostratigraphical representation of the succession, not a lithological one.
Seismostratigraphy comprises seismic sequence analysis, seismic facies analysis and analysis of changes in relative sea level. In the seismic section (seismisk snitt), various seismic successions or sequences are identified. Each sequence is characterized by a succession of conformable seismic reflectors. The sequences are bounded at the base and top by seismic unconformity surfaces or correlatable conformity surfaces. A unit of this nature is looked upon as a chronostratigraphical sequence.
Seismic facies analysis comprises analysis of the acoustical properties and patterns of the seismic reflectors to thereby enable the lithology to be predicted. The sequence analysis forms the basis for the construction of chronostratigraphical diagrams and diagrams depicting sea level changes. These may be correlated locally with wells and regionally with a global diagram for sea level changes, to establish the age of the sequence boundaries.
Seismic data are remote-sensing recordings of certain physical properties in the bedrock. Subdivision into seismic sequences and prediction of lithology involve a great deal of interpretation. Seismic sequences will therefore not be true geological units and will be unable to fulfil the demands set for defining such features (cf. Section 2.4). NSK, therefore, does not consider it necessary to propose special rules for defining and naming units on the basis of analyses of seismostratigraphy.