Seismic Interpretation

Seismic interpretationSeismic interpretation

Principles of seismicPrinciples of seismic stratigraphic stratigraphicinterpretationinterpretation

Seismic interpretationSeismic interpretation

Seismic Seismic stratigraphy stratigraphy is ais a technic technic forforinterpretinginterpreting stratigraphic stratigraphic information frominformation fromseismic data..seismic data..

The resolution of the seismic reflectionThe resolution of the seismic reflectionfollow gross bedding and as such theyfollow gross bedding and as such theyapproximate time lines.approximate time lines.

The key is that the contrast represented by seismicThe key is that the contrast represented by seismiclines come from bedding surface and not laterallines come from bedding surface and not lateralvariations (variations (facies facies changes).changes).

Resolution of seismic dataResolution of seismic data Understanding theUnderstanding the

resolution isresolution isimportantfor importantfor thetheseismic method.seismic method.–– A) a single cycleA) a single cycle

sine 30Hz insine 30Hz inmedium of velocitymedium of velocity2000 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 distancebetween two interface needed to give rise to abetween two interface needed to give rise to asingle reflection that can be observed on a seismicsingle reflection that can be observed on a seismicsection.section.

In a single noise-free seismic trace this isIn a single noise-free seismic trace this isgoverned 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 constrainsthere are three other factors that limitthere are three other factors that limitfinal 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 progressivelyattenuates the high-frequency components of theattenuates the high-frequency components of theseismic data.seismic data.

2- Acoustic velocity increases with depth due to2- Acoustic velocity increases with depth due tocompaction and increased cementation. Thiscompaction and increased cementation. Thisincreases the wavelength of the signal withincreases the wavelength of the signal withdetrimental 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 filterwhich has the effect of removing the highwhich has the effect of removing the highfrequency necessary for finer resolution.frequency necessary for finer resolution.

Seismic reflection terminationSeismic reflection terminationpatternspatterns

The first step in the The first step in the stratigraphicstratigraphicinterpretation is to determine the verticalinterpretation is to determine the verticaland 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 seismicdata if the section has been migrated, anddata if the section has been migrated, andweather it is marine or land data.weather it is marine or land data.

Seismic data from the OuterSeismic data from the OuterMoray Firth, North SeaMoray Firth, North Sea

Water-bottomWater-bottommultiplemultiplecaused by thecaused by thesound wavessound wavesbouncing twicebouncing twicebetween thebetween thesea-surface andsea-surface andthe sea-bed,the sea-bed,and beingand beingrecorded at arecorded at atwo-waytwo-waytime(TWT).time(TWT).


The next step is to divideThe next step is to dividethe seismic data into thethe seismic data into thediscrete naturaldiscrete naturalstratigraphic stratigraphic packages thatpackages thatmake up the section.make up the section.

Identify and markIdentify and markreflection terminations.reflection terminations.

It is a good idea to ignoredIt is a good idea to ignoredzones of broken or chaoticzones of broken or chaoticreflections and toreflections and toconcentrate on better data.concentrate on better data.They can be interpretedThey can be interpretedlater.later.


Where reflectionWhere reflectionterminate in aterminate in aconsistent mannerconsistent mannerthey define a linethey define a lineon the section (aon the section (aseismic surface).seismic surface).

Reflection terminationReflection termination Lapout Lapout vsvs

truncationtruncation

BaselapBaselap–– DownlapDownlap

–– OnlapOnlap

Downlap- commonly seen at the base of prograding clinoformsIt 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 ofstrata against an overlyingstrata against an overlying erosional erosional surface.surface.

Fault truncation- termination of reflectionsFault truncation- termination of reflectionsagains 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 geologicalinterpretation may be attempt.interpretation may be attempt.

Geometry of the reflections.Geometry of the reflections. Prograding Prograding basin-margin arebasin-margin areusually seen on seismic data to consist of usually seen on seismic data to consist of topsets topsets andandclinoformsclinoforms..

OfflapOfflap break break

Well-developedWell-developedtopsetstopsets and andclinoformsclinoforms Shelf Shelfand slope.and slope.

ClinoformsClinoforms with withminor or absentminor or absenttopsetstopsets..

Seimic faciesSeimic faciesclassificationclassificationRamasayerRamasayer(1979).(1979).

Methodology forMethodology fortwo-dimensionaltwo-dimensionalseismicseismic facies faciesmapping, knownmapping, knownas the A,B,C,as the A,B,C,technique. Threetechnique. Threecharacteristic ofcharacteristic ofeacheach seimic seimicpackage ispackage isrecorded, givenrecorded, givencode letters.code letters.

These are the nature of the reflection terminationAgainst 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, anddistributions of the various seismic distributions of the various seismic faciesfaciescan be constructed using the entire seismiccan be constructed using the entire seismicgrid.grid.

Together with log data it is possible to makeTogether with log data it is possible to makea geologicala geological facies facies map from seismic lines. map from seismic lines.

For example the eocene line presented hereFor example the eocene line presented herehas not been drill but it probably represent ahas not been drill but it probably represent abasin margin slope assemblage.basin margin slope assemblage.

SeismicSeismicfaiesfaies map; map;the map isthe map isdeliniateddeliniatedby SBby SB’’ss

Recognition ofRecognition of stratigraphic stratigraphicsurfacesurface

The key surface that divideThe key surface that divide stratigraphy stratigraphy into intocomponent systems tracts component systems tracts areare sequence sequenceboundaries, boundaries, transgressivetransgressive surface, surface, maximunmaximunflooding surface and marineflooding surface and marine onlap onlap//downlapdownlapsurfaces between the surfaces between the lowstandlowstand fans and the fans and thelowstandlowstand wedge. wedge.

A sequence boundary can beA sequence boundary can berecognized 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 thaterodes 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 acrossthe boundary.the boundary.

High-reliefHigh-relieferosion surfaceerosion surface

These areThese aresequencessequencesboundaries,boundaries,associated withassociated withglacial glacial lowstandlowstandand fluvialand fluvialerosionerosion

StratigraphicStratigraphic surface surface

Coatal onlapsCoatal onlaps is the proximal is the proximal onlap onlap of of topsettopsetreflections. They formed at or near sea-levelreflections. They formed at or near sea-levelwithin shallow marine processes.within shallow marine processes.

AdownwardAdownward shift in coastal shift in coastal onlap imlies onlap imlies a fall in a fall inrelative sea-level, accompanied byrelative sea-level, accompanied by subaerial subaerialexposure and erosion over the exposure and erosion over the topsettopset area. area.

In type 1 sequence boundaries the In type 1 sequence boundaries the topsettopsetreflectionsreflections onlap onlap an older an older clinoformclinoform. In type 2 SB. In type 2 SBthethe topset topset reflections reflections onlap onlap an older an older topset topsetboundary.boundary.

Sequence boundary,Sequence boundary,ThreeThree topset topsetreflectionreflection agains agains an anolder older clinoformclinoform..Type 1. A fall inType 1. A fall inrelative sea-level ofrelative sea-level ofaround 100 m =0.1 saround 100 m =0.1 sTWT.TWT.

TransgressiveTransgressive surface- marks the end of surface- marks the end oflowstand progradationlowstand progradation, and the onset of, and the onset oftransgression. It need not be associated withtransgression. It need not be associated withany reflection terminations, but will markany reflection terminations, but will markthe boundary between a the boundary between a topsettopset--clinoformclinoforminterval and an interval of onlyinterval and an interval of only topsets topsets..



Maximun Maximun flooding surface- is recognized onflooding surface- is recognized onseismic data as a surface whereseismic data as a surface where clinoforms clinoformsdownlapdownlap on to underlying on to underlying topsets topsets, which, whichmay displaymay display backstepping backstepping and apparent and apparenttruncation. Not everytruncation. Not every downlap downlap surface is a surface is amaximunmaximun flooding surface. flooding surface.

Seimic surface within a sequence

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Seismic Interpretation