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Seismic interpretationSeismic interpretation Principles of seismicPrinciples of seismic stratigraphic stratigraphic interpretationinterpretation Seismic interpretationSeismic interpretation Seismic Seismic stratigraphy stratigraphy is ais a technic technic forfor interpretinginterpreting stratigraphic stratigraphic information frominformation from seismic data..seismic data.. The resolution of the seismic reflectionThe resolution of the seismic reflection follow gross bedding and as such theyfollow gross bedding and as such they approximate time lines.approximate time lines. The key is that the contrast represented by seismicThe key is that the contrast represented by seismic lines come from bedding surface and not laterallines come from bedding surface and not lateral variations (variations (facies facies changes).changes). Resolution of seismic dataResolution of seismic data Understanding theUnderstanding the resolution isresolution is importantfor importantfor thethe seismic method.seismic method. –– A) a single cycleA) a single cycle sine 30Hz insine 30Hz in medium of velocitymedium of velocity 2000 m/s2000 m/s –– B) Big Ben, 380 ftB) Big Ben, 380 ft –– C) aC) a gama gama ray log.ray log. Vertical resolutionVertical resolution Can beCan be difined difined as the minimum vertical distanceas the minimum vertical distance between two interface needed to give rise to abetween two interface needed to give rise to a single reflection that can be observed on a seismicsingle reflection that can be observed on a seismic section.section. In a single noise-free seismic trace this isIn a single noise-free seismic trace this is governed by the wavelength of the seismic signal.governed by the wavelength of the seismic signal. –– The shorter the wavelength (and hence the higher theThe shorter the wavelength (and hence the higher the frequency) the greater the vertical resolution.frequency) the greater the vertical resolution. InIn adition adition to bed thickness constrainsto bed thickness constrains there are three other factors that limitthere are three other factors that limit final resolution of the seismic data.final resolution of the seismic data. 1- the Earth acts as a filter that progressively1- the Earth acts as a filter that progressively attenuates the high-frequency components of theattenuates the high-frequency components of the seismic data.seismic data. 2- Acoustic velocity increases with depth due to2- Acoustic velocity increases with depth due to compaction and increased cementation. Thiscompaction and increased cementation. This increases the wavelength of the signal withincreases the wavelength of the signal with detrimental effect on the resolution.detrimental effect on the resolution. 3- If there is high ambient noise on the raw data,3- If there is high ambient noise on the raw data, the processing stream may include a high-cut filterthe processing stream may include a high-cut filter which has the effect of removing the highwhich has the effect of removing the high frequency necessary for finer resolution.frequency necessary for finer resolution. Seismic reflection terminationSeismic reflection termination patternspatterns The first step in the The first step in the stratigraphicstratigraphic interpretation is to determine the verticalinterpretation is to determine the vertical and horizontal scale of the section.and horizontal scale of the section. To find out on the header or the seismicTo find out on the header or the seismic data if the section has been migrated, anddata if the section has been migrated, and weather it is marine or land data.weather it is marine or land data. Seismic data from the OuterSeismic data from the Outer Moray Firth, North SeaMoray Firth, North Sea Water-bottomWater-bottom multiplemultiple caused by thecaused by the sound wavessound waves bouncing twicebouncing twice between thebetween the sea-surface andsea-surface and the sea-bed,the sea-bed, and beingand being recorded at arecorded at a two-waytwo-way time(TWT).time(TWT). Seismic data from the OuterSeismic data from the Outer Moray Firth, North SeaMoray Firth, North Sea The next step is to divideThe next step is to divide the seismic data into thethe seismic data into the discrete naturaldiscrete natural stratigraphic stratigraphic packages thatpackages that make up the section.make up the section. Identify and markIdentify and mark reflection terminations.reflection terminations. It is a good idea to ignoredIt is a good idea to ignored zones of broken or chaoticzones of broken or chaotic reflections and toreflections and to concentrate on better data.concentrate on better data. They can be interpretedThey can be interpreted later.later. Seismic data from the OuterSeismic data from the Outer Moray Firth, North SeaMoray Firth, North Sea Where reflectionWhere reflection terminate in aterminate in a consistent mannerconsistent manner they define a linethey define a line on the section (aon the section (a seismic surface).seismic surface). Reflection terminationReflection termination Lapout Lapout vsvs truncationtruncation BaselapBaselap –– DownlapDownlap –– OnlapOnlap 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. TruncationTruncation Erosional Erosional truncation - The termination oftruncation - The termination of strata against an overlyingstrata against an overlying erosional erosional surface.surface. Fault truncation- termination of reflectionsFault truncation- termination of reflections agains agains aa syn syn- or post-depositional fault,- or post-depositional fault, slump, or intrusion plane.slump, or intrusion plane. SeismicSeismic facies facies Figure shows type ofFigure shows type of clinoforms clinoforms.. Once the seismic data has been divided into itsOnce the seismic data has been divided into its component depositional packages further geologicalcomponent depositional packages further geological interpretation may be attempt.interpretation may be attempt. Geometry of the reflections.Geometry of the reflections. Prograding Prograding basin-margin arebasin-margin are usually seen on seismic data to consist of usually seen on seismic data to consist of topsets topsets andand clinoformsclinoforms.. OfflapOfflap break break Well-developedWell-developed topsetstopsets and and clinoformsclinoforms Shelf Shelf and slope.and slope. ClinoformsClinoforms with with minor or absentminor or absent topsetstopsets.. Seimic faciesSeimic facies classificationclassification RamasayerRamasayer (1979).(1979). Methodology forMethodology for two-dimensionaltwo-dimensional seismicseismic facies facies mapping, knownmapping, known as the A,B,C,as the A,B,C, technique. Threetechnique. Three characteristic ofcharacteristic of eacheach seimic seimic package ispackage is recorded, givenrecorded, given code letters.code letters. These are the nature of the reflection termination Against the upper boundary, the nature of the Reflection agianst the lower boundary and the Internal configuration of the reflection. Proximal: C-On/P Distal: C-Dwn/Ob These code are marked to a map, andThese code are marked to a map, and distributions of the various seismic distributions of the various seismic faciesfacies can be constructed using the entire seismiccan be constructed using the entire seismic grid.grid. Together with log data it is possible to makeTogetherwith log data it is possible to make a geologicala geological facies facies map from seismic lines. map from seismic lines. For example the eocene line presented hereFor example the eocene line presented here has not been drill but it probably represent ahas not been drill but it probably represent a basin margin slope assemblage.basin margin slope assemblage. SeismicSeismic faiesfaies map; map; the map isthe map is deliniateddeliniated by SBby SB’’ss Recognition ofRecognition of stratigraphic stratigraphic surfacesurface The key surface that divideThe key surface that divide stratigraphy stratigraphy into into component systems tracts component systems tracts areare sequence sequence boundaries, boundaries, transgressivetransgressive surface, surface, maximunmaximun flooding surface and marineflooding surface and marine onlap onlap//downlapdownlap surfaces between the surfaces between the lowstandlowstand fans and the fans and the lowstandlowstand wedge. wedge. A sequence boundary can beA sequence boundary can be recognized on seismic data on tworecognized on seismic data on two ways:ways: From theFrom the develoment develoment of high relief of high relief truncation surface, particularly one thattruncation surface, particularly one that erodes the erodes the topsetstopsets of older units; and of older units; and By a downward shift of coastal By a downward shift of coastal onlaponlap across across the boundary.the boundary. High-reliefHigh-relief erosion surfaceerosion surface These areThese are sequencessequences boundaries,boundaries, associated withassociated with glacial glacial lowstandlowstand and fluvialand fluvial erosionerosion StratigraphicStratigraphic surface surface Coatal onlapsCoatal onlaps is the proximal is the proximal onlap onlap of of topsettopset reflections. They formed at or near sea-levelreflections. They formed at or near sea-level within shallow marine processes.within shallow marine processes. AdownwardAdownward shift in coastal shift in coastal onlap imlies onlap imlies a fall in a fall in relative sea-level, accompanied byrelative sea-level, accompanied by subaerial subaerial exposure and erosion over the exposure and erosion over the topsettopset area. area. In type 1 sequence boundaries the In type 1 sequence boundaries the topsettopset reflectionsreflections onlap onlap an older an older clinoformclinoform. In type 2 SB. In type 2 SB thethe topset topset reflections reflections onlap onlap an older an older topset topset boundary.boundary. Sequence boundary,Sequence boundary, ThreeThree topset topset reflectionreflection agains agains an an older older clinoformclinoform.. Type 1. A fall inType 1. A fall in relative sea-level ofrelative sea-level of around 100 m =0.1 saround 100 m =0.1 s TWT.TWT. TransgressiveTransgressive surface- marks the end of surface- marks the end of lowstand progradationlowstand progradation, and the onset of, and the onset of transgression. It need not be associated withtransgression. It need not be associated with any reflection terminations, but will markany reflection terminations, but will mark the boundary between a the boundary between a topsettopset--clinoformclinoform interval and an interval of onlyinterval and an interval of only topsets topsets.. StratigraphicStratigraphic surface surface StratigraphicStratigraphic surface surface Maximun Maximun flooding surface- is recognized onflooding surface- is recognized on seismic data as a surface whereseismic data as a surface where clinoforms clinoforms downlapdownlap on to underlying on to underlying topsets topsets, which, which may displaymay display backstepping backstepping and apparent and apparent truncation. Not everytruncation. Not every downlap downlap surface is a surface is a maximunmaximun flooding surface. flooding surface. Seimic surface within a sequence
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