Presentation on theme: "First Reflection seismic experiment in Oklahoma in 1921"— Presentation transcript:
1 First Reflection seismic experiment in Oklahoma in 1921
2 Reflection Seismology Seismic Reflection is the most important tool we have to image subsurface structure.Provides detailed imaging of approximately horizontal layering in the earthReflection seismology is echo or depth sounding
3 Reflection Seismology A ship is moving and fires air-guns about every 10 s.The pulses travel downward and are partially reflected back up from the reflectorsWaves are then recorded by seismic receivers
4 Seismic Section The result is a seismic section Provides a picture of the subsurface structureThe resulting structure can be interpreted as stratigraphy
5 Seismic Section Yet, there are reasons that the section is NOT true. Vertical scale is not depth, but timeActual time is two-way travel-time (down and back).Can be converted to depth, but must know velocity structure.
6 Seismic SectionIf layering is not horizontal, reflections will not come from directly below the source.Problem solved using a migration algorithm.
7 Seismic SectionThere may be multiple reflections in addition to the primary reflectionsThis makes artifacts. Algorithms exits to minimize.
8 Normal Moveout (NMO)The time to travel to a receiver a distance x away from the source isx: src—rec offset (m)T0: vertical travel-time∆t(x): moveout wrt offsethi : ith layer thickness (m)V: layer velocityS: source coordinateR: receiver coordinateA: reflection point (mid-point of ray for flat layers).
9 Normal moveout derivation hvS M Rx/ xLt: travel-timeL: right triangle hypotenusev: velocity of layer (km/s)t0: two-way timex: source-receiver offseth: layer thickness (km)
10 Normal Moveout (NMO)Rearrange NMO equation for the velocity of the layerto : two way travel-time (s)x: offset from src-rec (km)Δt(x): moveout at x-offset (s)
11 Multiple Layers What velocity should be used for two layers? Answer: Root Mean Square (RMS) velocityτi : one wave interval time (top to bottom time)Can solve for v2Can iterate this procedure for deeper layers
12 Multiple Layers: Dix formula Another way to calculate velocity of any layerB index: layer bottomT index: layer topvlayer : layer velocitytB : 2-way tt to layer bottomtT : 2-way tt to layer topvrms,B RMS velocity to bottomvrms,T RMS velocity to top
13 StackingAlmost always the reflections are weak and are hard to recognize because of inevitable noiseTo increase the signal-to-noise ratio, stacking is usedTake repeated measurements and averageSignal (reflections) add constructively and noise cancels
14 Stacking Finding layer velocities and stacking done simultaneously Try range of vrmsNMO correct the times of the seismogramsIf the vrms is correctstrong reflections are foundIf vrms is incorrectReflections are not found.
15 Dipping ReflectorsIf a reflector is dipping, its apparent position and dip are wrong in an unmigrated section.Wave raypaths are least-time paths and hence reflect from up dip points.Unmigrated reflector is shallower and with less dip.Travel-time hyperbola offset: h (thickness), alpha (dip)
16 Curved Reflectors: Syncline If a interface is sufficiently curved, there can be more than one reflection returned from the interface: multi-pathing.As the shot point is shifted from 1 to 10, three arrivals are produced that make a ‘bow tie’ travel-time pattern.Migration can ‘unwrap’ the bow-tie to improve the quality of the migrated image.
17 Curved Reflectors: Anticline Anticline has simpler response wrt Syncline.An anticline seismic image is broadenedAt edges of anticline two arrivals exist.
18 MigrationCorrecting for the position and shape of the reflector is called migrationComplicated and requires large amounts of computer timeBe aware of possible distortions in un-migrated sections
19 Faulted Reflectors Consider a point source (reflector). The distance that a source-receiver measures is on the arcs shownMultiple source-receivers produce the reflection hyperbola shownA stepped reflector behaves normally except near the stepProduces a reflection hyperbolaDifficult to tell position of faultMigration removes diffraction effects and reveals features more clearly
20 Faulted ReflectorsMigration removes diffraction effects and reveals features more clearly
21 Multiple ReflectionsThe positions of multiples can be anticipated from the position of the primary reflectorsHowever, sometimes it is difficult to recognize a primary reflector that comes in with the same TWT as the multiplesCan be distinguishedMoveout for the primary is less than for the multiple, so it stacks using a higher velocity
22 Marine Surveys Most common source is an Air Gun Produces P-waves (no S-waves)Receiver is a hydrophoneMicrophone that responds to change in pressureMounted at regular intervals and towed behind the ship in a streamer
23 Land Surveys Most common source is an explosion Buried small charge fired by detonatorReceiver is a geophoneOften times in clusters to improve s/n ratioMoving the system on land is much harder resulting in much more expense
24 Data RecordingThe output of each receiver is connected to an amplifierData is recorded digitallysampled at regular intervals, often only 1 msec
25 Common Depth Point Stacking Common Depth Point Stacking (CDP) uses rays that have all reflected from the same part of the interfaceUses pairs of shot points and receivers that are symmetrical about the reflector pointCDP stacking gives better data for computing velocities and stackingNumber of channels that areadded are the fold240-fold stacking is common
26 Static CorrectionsIn Land Surveys, significant topography has to be corrected forAdditionally, must take into account the effect of the topsoil and other weathered layersCalled Static Correctionsputs data on convenienthorizontal plane
27 Data DisplayDeep reflections are weaker due to energy loss and spreadingSometimes amplified to compensate- equalization
28 Vibroseis Vibroseis produces a continuous wave with changing frequency To find the travel time, the recorded signal is shifted in time until the entire waveform matches the sourceThe energy required is smallAn advantage where non-intrusive is preferred
29 VibroseisThe waves are generated by a vehicle with a plate that presses rhythmically against the groundinstantaneous force of 15 tonnes1 metric tonne =1000 kg or 2205 lbFrequency is swept from 10 to 100 Hz over 30 secSometimes several vehicles operate in unisonincreases energyhas reachedMoho (30 km)
30 What is a Reflector? Rays are reflected when they meet an interface Abrupt change in seismic velocityHow abrupt?We can define the acoustic impedance
31 Reflection Coefficient Transmission coefficientOnly valid for normal rays
32 Reflection Coefficient Because the S-wave velocity and the P-wave velocities are different, their coefficients of reflection can also differ.Sometimes S-waves are reflected more strongly than P-waves.Consider the ratio of reflected energyCheck on this and compare with problem 10Ken says (R+T)2 = 1Check on this
33 Reflection Coefficient Example Calculate the reflection coefficient of sandstone overlying limestone.Suppose that the properties of sandstone are at the bottom of their ranges while those of limestone are at the top
34 Reflection Coefficient Example Suppose instead we have materials with different properties:Lithological boundary, but no seismic boundaryIn reality, may produce a weak reflector
35 Bright SpotThe interface in a hydrocarbon reservoir between a gas layer and underlying oil or water produces a strong reflectionCalled a ‘bright spot’A strong, horizontal ‘bright spot’Evidence for presence of gas
36 Vertical ResolutionSuppose two interfaces could be brought progressively closer togetherReflected pulses would overlap more and moreAt some point, the pulses cannot be resolvedThe shape of the combined pulses also changes
37 Vertical ResolutionOften the two interfaces are two sides of a thin layer sandwiched within anotherShale layer within SandstoneOne interface has positive R and other negativeNegative R means that the reflected pulse is invertedInterferenceCan produce no reflected waveMay want to add some discussion of a pulse on a rope here
38 Vertical ResolutionSince the pulse reflected from the lower interface has to travel further by twice the separation of the interfacesDifficult to resolve when they are less than half a wavelength apartVertical resolution can be improved by using a shorter pulseShorter pulses are more rapidly attenuatedMust compromise between resolution and depth penetration
39 Vertical ResolutionAnother situation where there is no reflection is when an interface is a gradual change of velocities and densities extending over more than about half a wavelength, rather than being abruptCan be thought of as many thin, sandwiched layersInterference causes cancelationLithological boundary with no seismic reflector
40 Synthetic Reflection Seismograms Can be used to improve interpretationFirst, choose a pulse that corresponds to the sourceNext, calculate the reflection and transmission coefficientsBased on borehole dataPropagate the pulse‘down throughthe layers’
41 Synthetic Reflection Seismograms If there are several interfaces closer together than the length of the average wavelength of the pulse, the reflected pulses often combine to give peaks that do no coincide with any of the interfaces
42 3D SurveyingWe are often interested in the form of structures perpendicular to the seismic sections we have been studyingThere can be reflections from dipping interfaces outside the plane of the sectionSideswipeThis can be addressed by using a regular grid on the surface (3D survey)Often times the grid is on 50 m spacing
43 3D SurveyingA CDP gather comprises pairs of shot points and receivers from all around the CDPStacking , migration follow the same principlesObviously more difficult and computer intensive
44 3D SurveyingWe can think of the reflectors revealed as being embedded in a blockCan take ‘slices’ in any directionMore sophisticated processing can reveal properties of the rock such as porosityDetermines the amountof hydrocarbon that arock can hold
45 3D SurveyingMay want to add the image from plate 6 of the book here and comment on the use of porosity
46 Time-Lapse ModelingBy repeating surveys at intervals, the extraction of hydrocarbons can be followed and remaining pockets of oil detectedThough 3D surveying is much more expensive, both for data acquisition and reduction, it can pay for itself in the increased understanding of the structure of hydrocarbon reservoirs
47 Forming Hydrocarbon Reservoirs Organic material (minute plants & animals) is buried in a source rock that protects it from oxidation (often clays in a sedimentary basin)Bacterial action operating at temperatures of 100 to 200°C changes the organic matter into droplets of oilThe droplets are squeezed out of the source rockBeing lighter than water, usually move upwardDeformation can cause them to move sidewaysImpervious cap rock (shale) prevents leakageTo be extractable, reservoir rock must be porous and permeableTo be commercially viable, must be concentrated
48 Hydrocarbon Traps Structural Traps result from tectonic processes Folds, domes, faults, etc
49 Hydrocarbon TrapsStratigraphic Traps are formed by lithological variation in the strata at the time of depositionLens of permeable and porous sandstone or a carbonate reef, surrounded by impermeable rocks
50 Hydrocarbon TrapsCombination Traps have both structural and stratigraphic featuresWhere low density salt is squeezed upwards to form a salt dome, both tilting strata and causing hydrocarbons to concentrate as well as blocking off their escape
51 Hydrocarbon TrapsHow can hydrocarbon traps be located with seismic reflection?Easiest to recognize are structural ones.Stratigraphic traps which have tilted reservoir rcoks terminating upwards in an unconformity are also fairly easy to spotBright spots show presence of gas-oil or gas-water interfaceSmaller and harder to recognize traps are being exploitedHigh quality surveys necessaryClosely spaced stations, high resolution sourcesPrecise processing of data
52 Sequence Stratigraphy Sequence stratigraphy is the building up of a stratigraphy using seismic sectionsCan provide constraints on global sea-level changeUsed as a dating toolPossible because chronostratigraphic boundaries (surfaces formed within a negligible interval of time) are also often reflectorsStratigraphic sequenceSequence of strata of common genesis bounded by unconformities
53 Sequence Stratigraphy How a sequence is built up and terminated depends on the interplay of deposition and changes in sea level (eustatic changes)Complex, but all we are concerned with is the relative sea level changes while the sediments are being deposited
54 Sequence Stratigraphy Consider deposition along a coastline where sediments are being supplied by a river, and sea level is constantPrograding successionThinning at topTop lap
55 Sequence Stratigraphy Consider deposition along a coastline where sediments are being supplied by a river, and sea level is rising steadilySuccessive near horizontal layersOnlap where they butt up to the coastline
56 Sequence Stratigraphy Slow rise in sea level (and constant sea level) results in progradation (lateral buildup of stratigraphy)Rapid rise of sea level results in aggradation (vertical buildup of stratigraphy
57 Sequence Stratigraphy An unconformitydefines the boundary of a sequenceoccurs when sea level falls fast enough for erosion rather than deposition to occurModerate sea level retreatTruncates tops of bedsRapid sea level retreatErosion cuts laterally
58 Sequence Stratigraphy Beds that end are termed discordantAppear in seismic sections as reflectors that cease laterallyCan be used for working out the history of deposition and erosion
59 Sequence Stratigraphy and Eustasy Sequence stratigraphy provides a record of the changes in local relative sea levelTypes and volumes of various facies within the sequence provides constraints on the amounts of rise and fall of sea levelFossils taken from boreholes can be used to estimate their timesCorrelations between widely separated basins can be used to determine global sea level changes.