Seismic interpretation Principles of seismic stratigraphic interpretation
Seismic interpretation Seismic stratigraphy is a technical for interpreting stratigraphic information from seismic data. The resolution of the seismic reflection follow gross bedding and as such they approximate time lines.
The key is that the contrast represented by seismic lines come from bedding surface and not lateral variations (facies changes).
Resolution of seismic data Understanding the resolution is important for the seismic method. A) a single cycle sine 30Hz in medium of velocity 2000 m/s B) Big Ben, 380 ft C) a gamma ray log.
Vertical resolution Can be defined as the minimum vertical distance between two interface needed to give rise to a single reflection that can be observed on a seismic section. In a single noise-free seismic trace this is governed by the wavelength of the seismic signal. The shorter the wavelength (and hence the higher the frequency) the greater the vertical resolution.
In addition to bed thickness constrains there are three other factors that limit final resolution of 1- the Earth the acts seismic as a filter data. that progressively attenuates the high-frequency components of the seismic data. 2- Acoustic velocity increases with depth due to compaction and increased cementation. This increases the wavelength of the signal with detrimental effect on the resolution. 3- If there is high ambient noise on the raw data, the processing stream may include a high-cut filter which has the effect of removing the high frequency necessary for finer resolution.
Seismic reflection termination patterns The first step in the stratigraphic interpretation is to determine the vertical and horizontal scale of the section. To find out on the header or the seismic data if the section has been migrated, and weather it is marine or land data.
Seismic data from the Outer Moray Firth, North Sea Water-bottom multiple caused by the sound waves bouncing twice between the sea-surface and the seabed, and being recorded at a two-way time(twt).
Seismic data from the Outer Moray Firth, North Sea The next step is to divide the seismic data into the discrete natural stratigraphic packages that make up the section. Identify and mark reflection terminations. It is a good idea to ignored zones of broken or chaotic reflections and to concentrate on better data. They can be interpreted later.
Seismic data from the Outer Moray Firth, North Sea Where reflection terminate in a consistent manner they define a line on the section (a seismic surface).
Reflection termination Lapout vs truncation Baselap Downlap Onlap Downlap- commonly seen at the base of prograding clinoforms It usually represent progradation of the basin margin. Onlap- termination of low-angle reflections against a steeper seismic surface. Two types: marine and coastal. Toplap- is the termination of inclined reflections against an Overlying lower angle surface.
Truncation Erosional truncation - The termination of strata against an overlying erosional surface. Fault truncation- termination of reflections against a syn- or postdepositional fault, slump, or intrusion plane.
Seismic facies Figure shows type of clinoforms. Once the seismic data has been divided into its component depositional packages further geological interpretation may be attempt. Geometry of the reflections. Prograding basinmargin are usually seen on seismic data to consist of topsets and clinoforms.
Offlap break Well-developed topsets and clinoforms Shelf and slope. Clinoforms with minor or absent topsets.
These are the nature of the reflection termination Against the upper boundary, the nature of the Reflection against the lower boundary and the Internal configuration of the reflection. Seimic facies classification Ramasayer (1979). Methodology for twodimensional seismic facies mapping, known as the A,B,C, technique. Three characteristic of each seismic package is recorded,
Proximal: C-On/P Distal: C-Dwn/Ob
These code are marked to a map, and distributions of the various seismic facies can be constructed using the entire seismic grid. Together with log data it is possible to make a geological facies map from seismic lines. For example the eocene line presented here has not been drill but it probably represent a basin margin slope assemblage.
Seismic facies map; the map is delineate d by SB s
Recognition of stratigraphic surface The key surface that divide stratigraphy into component systems tracts are sequence boundaries, transgressive surface, maximum flooding surface and marine onlap/downlap surfaces between the lowstand fans and the lowstand wedge.
A sequence boundary can be recognized on seismic data on two ways: From the development of high relief truncation surface, particularly one that erodes the topsets of older units; and By a downward shift of coastal onlap across the boundary.
High-relief erosion surface These are sequences boundaries, associated with glacial lowstand and fluvial erosion
Stratigraphic surface Coastal onlaps is the proximal onlap of topset reflections. They formed at or near sea-level within shallow marine processes. A downward shift in coastal onlap implies a fall in relative sea-level, accompanied by subaerial exposure and erosion over the topset area. In type 1 sequence boundaries the topset reflections onlap an older clinoform. In type 2 SB the topset reflections onlap an older topset boundary.
Sequence boundary, Three topset reflection against an older clinoform. Type 1. A fall in relative sea-level of around 100 m =0.1 s TWT.
Stratigraphic surface Transgressive surface- marks the end of lowstand progradation, and the onset of transgression. It need not be associated with any reflection terminations, but will mark the boundary between a topset-clinoform interval and an interval of only topsets.
Stratigraphic surface Maximum flooding surface- is recognized on seismic data as a surface where clinoforms downlap on to underlying topsets, which may display backstepping and apparent truncation. Not every downlap surface is a maximum flooding surface.
Seismic surface within a sequence