AAPG-CUSCAAPG-CUSC

Geophysics for GeologistsGeophysics for Geologists

K. M. ShemisK. M. Shemis

April 2009April 2009

Geophysics is a part of remote sensing Geophysics is a part of remote sensing science as it measures the underneath science as it measures the underneath earth layers using surface tools.earth layers using surface tools.

Geophysics is used in many science Geophysics is used in many science branches like petroleum industry, branches like petroleum industry, archaeology, hydrology,…. etc.archaeology, hydrology,…. etc.

Petroleum ExplorationGeophysics

Magnetism

(Magnetic Susceptibility)

Gravity

(Density)

Seismology(Acoustic Impedance)

(ρV)

Seismic WaveletSeismic Wavelet

Mechanical waveMechanical wave Seismic wave Seismic wave Sonic wave Sonic wave Acoustic waveAcoustic wave Sound waveSound wave

Types of waves

Electromagnetic

e.g. Light waves

(Don’t need a media to propagate)

Mechanical waves

(need a media to propagate)

Surface waves

Seismic waves

P-Waves

S-Waves

Rayleigh waves

Love waves

P-Waves = Primary waves, the particle motion in the P-Waves = Primary waves, the particle motion in the direction of wave propagation, it’s faster than S-direction of wave propagation, it’s faster than S-waves and propagate through fluids.waves and propagate through fluids.

S-Waves = Secondary waves, the particle motion is S-Waves = Secondary waves, the particle motion is perpendicular to the direction of wave propagation, perpendicular to the direction of wave propagation, it’s slower than P-waves and faster than Surface it’s slower than P-waves and faster than Surface waves and can’t propagate through fluids.waves and can’t propagate through fluids.

Surface waves are propagating through surface Surface waves are propagating through surface planes, it’s slower than S-waves and it’s considered planes, it’s slower than S-waves and it’s considered as seismic noise.as seismic noise.

Snell’s law: Snell’s law:

Reflection Coefficient (R)Reflection Coefficient (R) Acoustic Impedance (Z) = Acoustic Impedance (Z) =

Density (Density (ρρ) x Velocity (V)) x Velocity (V)

R = Z2-Z1/Z2+Z1R = Z2-Z1/Z2+Z1 1 ≥ R ≥ -11 ≥ R ≥ -1 Polarity: The sign of R Polarity: The sign of R

+ve or –ve.+ve or –ve. Magnitude: The absolute Magnitude: The absolute

value of R and its refer to value of R and its refer to the amplitude value (e.g. the amplitude value (e.g. +0.5 = -0.5 as magnitude)+0.5 = -0.5 as magnitude)

If the A.I of the lower If the A.I of the lower formation is higher than formation is higher than the upper one , the the upper one , the reflection polarity will be reflection polarity will be +ve and vice versa.+ve and vice versa.

If the difference in A.I If the difference in A.I between the two between the two formations is high, the formations is high, the reflection magnitude reflection magnitude (Amplitude) will be high.(Amplitude) will be high.

Seismic MethodsSeismic Methods

Surface seismic Borehole seismic

Reflection RefractionVSP Sonic Logging

To build a model using the surface To build a model using the surface seismic, we have to go through the seismic, we have to go through the following steps:following steps:

1- Seismic Data 1- Seismic Data AcquisitionAcquisition..

2- Seismic Data 2- Seismic Data ProcessingProcessing

3- Seismic Data 3- Seismic Data InterpretationInterpretation

1- Seismic Data Acquisition1- Seismic Data Acquisition

The Seismic Data Acquisition is categorized The Seismic Data Acquisition is categorized to:to:

Land or MarineLand or Marine

2D or 3D2D or 3D

Procedures of Land Procedures of Land Seismic Data AcquisitionSeismic Data Acquisition

1) Mine Clearance (removal of small arms) if needed

2) The area is surveyed to determine shot and receiver locations using integrated handheld survey equipment which uses GPS or Laser ranging to determine distances, elevations, and locations.

3) Flags are planted at shot and receiver-station locations. Accuracy for measuring distances in land surveying is less than 1cm.

4) Shot holes are drilled (only if dynamite is the source of energy)

5) Seismic cables are laid-out, geophones planted at correct locations.

9) Data acquisition in the vicinity of cultural locations should be coordinated to increase safety and decrease noise.

10) After the shot-holes or vibrating points are recorded, cables are moved to the adjoining area to continue the survey.

11) Moving cables should be properly coordinated with data acquisition and movement of equipment and personnel should not interfere with data recording

Energy Source (Dynamite)

Detonator.

Explosive charge.

Heli-portable drilling operations.

Lowering of charge into shothole.

Conventional drilling rig.

Energy Source (VIBRATORS)

Source Array (Vibrators)

coil hanging inside cylindrical magnet by springMagnet moves with ground.Coil stays fixed by spring inertia.electric current producedmeasure ground velocityimportant to plant them firmly and vertically

Receivers (Geophones)

Geophones

Receiver Array (Geophones)Receiver Array (Geophones)

Procedures of Marine Procedures of Marine Seismic Data AcquisitionSeismic Data Acquisition

1- 1- Streamers are lowered. All sections and connected equipment are checked.

2- Gun arrays are lowered.

3- After deploying the streamers and the air gun arrays, the vessel proceeds to the data area.

4- The guns refill time is 8 seconds and the vessel speed is about 4.65 knots, and divided by 8 seconds gives a shot interval of 18.75 meters. The shot interval is varied by changing the speed or changing the firing interval.

5) Recording 2 ms sampling interval, 5 second record length, 148 channels for each streamer, 6 streamers, and 1000 shots give 2.22 billion samples of data. Seismic data is recorded on cartridges.

6) Vessel must be steered close to the lines being surveyed.

7) Navigation and steering must be coordinated at all times.

8) Time sharing in the field with other seismic data acquisition vessels in the area is coordinated to avoid interference between crews.

Energy Source (Air Gun)

Air GunAir Gun

Source Array (Air Gun)Source Array (Air Gun)

generates voltage when stressedmeasure fluid acceleration independent of wave directionDepth is 10-15 m under water.mounted in a streamer (~ 6 km long)

Marine Receivers (Hydrophones)

OCEAN BOTTOM CABLEOBC

Advantages of OBC Acquisition Image

There are several advantages in acquiring seismic data using an ocean-bottom cable.

The sea floor is generally a quieter environment to record seismic data than the surface, with better coupling, and shorter travel times in deepwater surveys. The result is often seismic data with improved signal content and frequency bandwidth compared to conventional streamers.

Shear waves do not travel through water, and so conventional marine sources do not generate them and hydrophones will not record them. However, part of the downgoing p-wave energy produced by an airgun will be converted to upgoing shear wave energy at a reflecting horizon. Shear wave receivers placed on the seafloor will be able to record this mode-converted shear wave energy.

2-D Acquisition

CDP Fold

Spread refers to the relative locations of source and receivers.

The main difference between 2D and 3D data Acquisition is the way of arranging sources and receivers relative to each others.

Types Split: source at center of receivers line end-on: source at end of receivers line Broadside: source is offset perpendicular to receivers line Cross: two crossing receiver lines

2-D Acquisition Spreads

2-D Field methodsSpreads

Split End-on

3-D Acquisition

What is 3-D seismic? It is a group of closely spaced source and

receiver lines forming a grid that covers an area.Receiver and source lines are perpendicular to

each other.

Why 3-D seismic?3-D migration provide better positioning over

dipping reflectors.presents a more detailed image of the subsurface

IntroductionIntroduction

Inline: direction parallel to receiver lines. Crossline: direction parallel to shot lines. CMP bin: a small rectangle (1/2 RI x 1/2 SI)

that contains all the traces which belong to the same CMP.

Box (unit cell): area bounded by two adjacent receiver lines and two adjacent source lines.

Patch (template): area of all live receivers recording from the same source.

Swath: length over which sources are recorded without crossline rollover.

3-D Terminology3-D Terminology

Fold: The number of midpoints that are stacked within a CMP bin.

Fold Taper: The width of the additional fringe area that needs to be added to the 3-D surface area to build up full fold .

Migration Apron: The width of the fringe area that needs to be added to the 3-D survey to allow proper migration of any dipping event.

Shot Density (SD): The number of source points/km2 or source points/mi2. Together with the number of channels, NC, and the size of the CMP bin, SD determines the fold.

Super Bin: This term (and others like macro bin or maxi bin) applies to a group of neighboring CMP bins

3-D Terminology3-D Terminology

1. Receiver lines are laid in parallel lines.2. Source lines are laid in parallel lines in a direction

perpendicular to receiver lines.3. An area of receivers (patch) is activated.4. Source at patch center is shot and recorded.5. Patch is moved crossline one source interval.6. Source at new patch center is shot and recorded.7. Repeat this until source line is finished.8. This is one swath.9. Roll over one source-line interval and begin recording

the next swath.10. Keep doing this until the survey is finished.

3-D Swath Shooting Method3-D Swath Shooting Method

1. Shallowest reflector of interest2. Deepest reflector of interest3. Target Size4. Target dip5. Multiples6. Cost

Factors Controlling 3-D Survey DesignFactors Controlling 3-D Survey Design

- RI and SI- RLI and SLI- CMP Fold- Maximum Offset- Minimum Offset- Record length- Frequency- Migration Aperture

Parameters of 3-D Survey DesignParameters of 3-D Survey Design

Seismic Data Recording Seismic Data Recording ForFor

2D or 3D, Land or Marine2D or 3D, Land or Marine

1- Signal from detectors is digitized and amplified.

2- Filters may be applied to remove unwanted frequencies.

3- Traces are displayed for quality control.

4- Data is recorded and stored in tapes for later retrieval and processing.

Seismic data Recording

Earth’s surfaceEarth’s surface

Subsurface reflectorSubsurface reflector

SSRR

A/D A/D ConverterConverter

AmplifierAmplifier

FilterFilter

Trace displayTrace display

RecordingRecording

Tape Tape storagestorage

Seismic data Recording

Recorder

Camera unit and monitor record.

2- Seismic Data Processing2- Seismic Data Processing

A very simplified, “traditional” processing flow.

Multiplexing and Multiplexing and DemultiplexingDemultiplexing

Conversion of scan sequential mode to Conversion of scan sequential mode to trace sequential mode.trace sequential mode.

Essentially a Matrix transposition (rows to Essentially a Matrix transposition (rows to columns and vice versa)columns and vice versa)

Ch-1Ch-2

Ch-4Ch-5

Ch-12

Ch-20

Ch-48

Amp

Demultiplexing

SCAN-1,ch-1 SCAN-1,ch-24 SCAN-1,ch-36 SCAN-1,ch-48

SC-1200,ch-1 SC-1200,ch-24 SC-1200,ch-36 SC-1200,ch-48

SCAN-2,ch-1 SCAN-2,ch-24 SCAN-2,ch-36 SCAN-2,ch-48

SCAN-3,ch-1 SCAN-3,ch-24 SCAN-3,ch-36 SCAN-3,ch-48

SCAN-4,ch-1 SCAN-4,ch-24 SCAN-4,ch-36 SCAN-4,ch-48

SC-2500,ch-48SC-2500,ch-36SC-2500,ch-24SC-2500,ch-48

Gain RecoveryGain Recovery

The gain loss (Amplitude loss) is due to:The gain loss (Amplitude loss) is due to:

1- Geometric Spreading (Spherical Divergence)1- Geometric Spreading (Spherical Divergence)

2- Attenuation Loss2- Attenuation Loss

3- Reflectivity3- Reflectivity

Spherical DivergenceSpherical Divergence

Spherical DivergenceSpherical Divergence

Attenuation LossAttenuation Loss

The energy loss is due to the inelastic propagation of waves, the collision of particles to each others produces heat which is considered as an energy loss

ReflectivityReflectivity

Time

Amplitude

Decay curve

Recovery function

Amplitude Recovery:Amplitude Recovery: Geometric spreading Geometric spreading

(Spherical divergence), (Spherical divergence), attenuation loss and attenuation loss and reflectivity combine to reflectivity combine to cause a rapid decay of cause a rapid decay of the seismic signal with the seismic signal with time (depth). If these time (depth). If these losses are left losses are left uncorrected, only the uncorrected, only the uppermost reflectors uppermost reflectors would be visible on the would be visible on the final section.final section.

AfterBefore

Amplitude Recovery is Amplitude Recovery is becoming a very becoming a very important topic for important topic for amplitude variation with amplitude variation with offset (AVO) studiesoffset (AVO) studies

Static CorrectionStatic Correction

Reflector

Surface

LVL

Data collected on land need to have the effects of changes in ground Data collected on land need to have the effects of changes in ground surface elevation removed and, potentially, the effects of changes in surface elevation removed and, potentially, the effects of changes in thickness (and velocity) of the “Low Velocity layer”.thickness (and velocity) of the “Low Velocity layer”.

Datum

DeconvolutionDeconvolution

Geological reflection (Desired) +Geological reflection (Desired) + Source wavelet (Undesired)Source wavelet (Undesired)

Convolvution

Output Trace

Output Trace

Deconvolvution

Geological reflection (Desired)Geological reflection (Desired)

Deconvolution:-Deconvolution:-Suppresses multiples.Suppresses multiples.Removes effect of source signature.Removes effect of source signature.Reduces duration of wavelet (increase Reduces duration of wavelet (increase

wavelet frequency).wavelet frequency).

Before After

Normal Moveout (NMO) Normal Moveout (NMO) & Stacking& Stacking

After NMO Cor.Before NMO Cor.

Offset

Time

StackingStacking

Each common mid point gather after normal move out correction is summed together to yield a stacked trace.

Stacking enhances the S\N ratio and reduces the random noise.

CDP Stacking

MigrationMigration

source receiver receiversource

Assumed

Actual

For a dipping horizon, the reflection point is not at the Mid Point, but offset up dip from it.

Unmigrated Section

Multiple

Time Migrated Section

What Migration and When?

•Post-Stack

•Pre-stack

•Time

•Depth

Classes

PoststackDepthMigration

PrestackDepthMigration

PoststackTimeMigration

PrestackTimeMigration

Lateral velocity variation

Stru

ctur

al c

ompl

exity

Migration – What Kind?

Unmigrated Section

Time Migrated Section

Depth Migrated Section

Seismic DisplaySeismic Display

2D View2D View

3D View (3D Cube)3D View (3D Cube)

3D View (Inline, Crossline &Time Slice)3D View (Inline, Crossline &Time Slice)

Vertical Seismic Profile Vertical Seismic Profile

VSPVSP

The VSP idea is depending on recording The VSP idea is depending on recording the seismic data directly in the bore hole to the seismic data directly in the bore hole to minimize the negative effects of the earth minimize the negative effects of the earth overburden.overburden.

The main use of VSP data is the The main use of VSP data is the correlation with the surface seismic to correlation with the surface seismic to define the actual formation boundaries on define the actual formation boundaries on the surface seismicthe surface seismic

Common Types of the VSPCommon Types of the VSP

Offset VSPOffset VSP

Zero Offset VSPZero Offset VSP

Walkabove VSPWalkabove VSP

Walkaway VSPWalkaway VSP

Depth

Time

Upgoing Reflection Downgoing

Reflections

Output of different types of VSPOutput of different types of VSP

Synthetic SeismogramSynthetic Seismogram

If we don’t have VSP data in the well, we If we don’t have VSP data in the well, we can use Sonic and Density Logs to can use Sonic and Density Logs to synthesize a seismogram (Synthetic synthesize a seismogram (Synthetic Seismogram).Seismogram).

We synthesize wavelets from Reflection We synthesize wavelets from Reflection Coefficient (r.c)Coefficient (r.c)

R.C = R.C = ρρ.v (Density*Velocity).v (Density*Velocity) Density is from Density LogDensity is from Density Log Velocity is from Sonic Log Velocity is from Sonic Log

3- Seismic Data Interpretation

Petroleum TrapPetroleum Trap

The four basic steps in seismic data The four basic steps in seismic data interpretation are:interpretation are:

1- Picking.1- Picking.2- Timing.2- Timing.3- Posting to the map.3- Posting to the map.4- Contouring4- Contouring

PickingPicking

TimingTiming

Posting to the mapPosting to the map

ContouringContouring

- How to define the formation top (depth) on - How to define the formation top (depth) on the seismic section (time) ???the seismic section (time) ???

We are tying a VSP of a specific well to We are tying a VSP of a specific well to the Seismic section to define the the Seismic section to define the Formation Top on the seismic section, and Formation Top on the seismic section, and then extend the interpretation by tying the then extend the interpretation by tying the seismic sections with each others.seismic sections with each others.

Tie between Seismic Sections

Seismic Interpretation of Reflection Seismic Interpretation of Reflection Seismic Data:Seismic Data:

1- Structural Interpretation1- Structural Interpretation2- Stratigraphic Interpretation2- Stratigraphic Interpretation3- Seismic Attributes Analysis3- Seismic Attributes Analysis

1- Structural Interpretation1- Structural Interpretation

Fault DefinitionFault Definition

The reflections change amplitude across the The reflections change amplitude across the center line. The reflections on the right have center line. The reflections on the right have higher amplitudes than those on lefthigher amplitudes than those on left

Fault DefinitionFault Definition

There is a slight change in phase from the left to the right There is a slight change in phase from the left to the right side of the figure. The left side shows a symmetrical side of the figure. The left side shows a symmetrical doublet, where as the right doublet is asymmetricaldoublet, where as the right doublet is asymmetrical

Fault DefinitionFault Definition

Reflections on the left side are dipping where as the Reflections on the left side are dipping where as the reflections on the right are horizontal. There is change in reflections on the right are horizontal. There is change in dip from one side of the section to the other.dip from one side of the section to the other.

Fault DefinitionFault Definition

There is change in nose level. The left side has There is change in nose level. The left side has clean reflections whereas the traces are noisy clean reflections whereas the traces are noisy on the right.on the right.

Fault DefinitionFault Definition

There is an offset from the left side to the right There is an offset from the left side to the right side.side.

Coherency Cube

2- Stratigraphic Interpretation2- Stratigraphic Interpretation

Sequence Stratigraphy FeaturesSequence Stratigraphy Features

ToplapToplap termination of strata against an overlying termination of strata against an overlying

surface, representing the result of non-surface, representing the result of non-deposition and/or minor erosion deposition and/or minor erosion

TruncationTruncation this implies the deposition of strata and their subsequent this implies the deposition of strata and their subsequent

tilting and removal along an unconformity surface. This tilting and removal along an unconformity surface. This termination is the most reliable top-discordant criterion of termination is the most reliable top-discordant criterion of a sequence boundary a sequence boundary

OnlapOnlap The initially horizontal strata progressively terminate The initially horizontal strata progressively terminate

against an initially inclined surface, or in which initially against an initially inclined surface, or in which initially inclined strata terminate progressively updip against a inclined strata terminate progressively updip against a surface of greater initial inclination surface of greater initial inclination

DownlapDownlap relationship in which seismic reflections of relationship in which seismic reflections of

inclined strata terminate downdip against an inclined strata terminate downdip against an inclined or horizontal surface inclined or horizontal surface

OfflapOfflap termination of strata against an overlying termination of strata against an overlying

surface, normally without non-deposition or surface, normally without non-deposition or erosionerosion

Offlap

3- Seismic Attribute Analysis3- Seismic Attribute Analysis

A seismic attribute is any measure of seismic data that helps us better visualize or quantify features of interpretation interest. Seismic attributes fall into two broad categories – those that help us quantify the morphological component of seismic data (e.g. Coherency Cube) and those that help us quantify the reflectivity component of seismic data (e.g. AVO). The morphological attributes help us extract information on reflector dip, azimuth, and terminations, which can in turn be related to faults, channels, fractures, diapirs, and carbonate buildups. The reflectivity attributes help us extract information on reflector amplitude, waveform, and variation with illumination angle, which can in turn be related to lithology, reservoir thickness, and the presence of hydrocarbons.

What is Seismic attribute

Amplitude Variation with OffsetAmplitude Variation with Offset(AVO)(AVO)

After NMO Cor.Before NMO Cor.

Offset

Time

When we throw a stone into water, the When we throw a stone into water, the amplitude of the wave will be different at amplitude of the wave will be different at different incident angle. When we send a different incident angle. When we send a wave (instead of stone) into earth (which is wave (instead of stone) into earth (which is filled with fluids), the amplitude of the filled with fluids), the amplitude of the wave will vary with incidence angle (or wave will vary with incidence angle (or offset, as the offset increase the incidence offset, as the offset increase the incidence angle increase). This variation is called angle increase). This variation is called “Amplitude Variation with Offset” or “AVO”“Amplitude Variation with Offset” or “AVO”

The AVO response depend on the The AVO response depend on the elasticity of the fluid (expressed by elasticity of the fluid (expressed by Poisson’s ratio Vp/Vs). As water is very Poisson’s ratio Vp/Vs). As water is very inelastic and gas is very elastic, the AVO inelastic and gas is very elastic, the AVO response in the case of Gas-wet sand is response in the case of Gas-wet sand is opposite of the AVO response in the case opposite of the AVO response in the case of Water-wet sand.of Water-wet sand.

The determination of the type of amplitude The determination of the type of amplitude variation with offset is depending on the variation with offset is depending on the acoustic impedance of reservoir and its acoustic impedance of reservoir and its surrounding formationssurrounding formations

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