pe geosci geophysics printable

8/13/2019 PE Geosci Geophysics Printable

1/21

Reflection Seismic

Fundamentals

Outline

Principles Acquisition and processing

Interpretation methods

Attributes

Some current applications

Goal of Seismic

To make an image of the subsurface

rock distribution

structure

stratigraphy

To make estimates of rock and fluid

properties velocity (linked to porosity, fluid content)

anisotropy, other attributes

A Seismic Section

Two-W

ayTime

Distance

Many individual tracesplotted adjacent toone another

Looks Like GeologyDiapirs

Slope deposits

Deep water systems

Detachment faults

Interpretations of SeismicProcessed seismic Interpretation overlay

Note: well-tie forcalibration


2/21

Principles Basic Ideabat

insect

Single source

Two receivers (ears)

Processor (bats brain) to create spatial

perception

Additional spatial resolution from flying

to new location

Single

reflector

(but it also

moves)

Nothing in-between!

Sound going out Sound coming back

Acoustic Waves

P-waves (Primary waves, Pressure

waves)

S-waves (Secondary waves, Shear

waves)

Infinitesimal oscillations of particles within a mediumCaused by a shock disturbance (external source)

Disturbance passes a point, and the particles return to rest

Sound waves passing through the air are P-waves

P-WavesPush this studentto the left

and the disturbance propagates to the left

P-Waves

close together far apart

Plot of the closeness (inverse of distance) as a function ofposition (an analog of amplitude). This plot is a snapshot intime. It will change at the next instant of time as the wavemoves along.

P-WavesRepresentation of particles (nodes) in

a material, showing movements

(exaggerated) as a P-wave passes


3/21

P-Wave Animation

Particles vibrate (oscillate) this way

Wave front propagates this way

Note: continuing

excitation of motion

S-Waves (1)

Wave propagationStudents standing on springs,and springs above them, too

Vertical polarisation

S-Waves (2)Horizontal polarisation

Wave propagation Students standing on aplatform with castors(no friction)

S-Wave Animation

Particles vibrate (oscillate) this way

Wave front propagates this way

Note: continuingexcitation of motion

Point Source In reflection seismology, the source is

usually at a point:

This could be an explosion (dynamite)

(typically, a few metres underground)

Or an air-gun (in marine surveys) (a few

metres under the water surface)

Or a vibrator truck (on land, at the surface)

Spherical Radiation

t = t0t = t1

t = t2

t = t3

Wavefront propagatesaway from the source point


4/21

Point Source

In this animation, the sourceis continuously pulsing

Downwards Propagation

We usually think of the seismic

energy propagating downwards

(sub-vertical), so here is a

previous image rotated to show

the way that the

compressional/dilational

waveform looks in that view

Wave Relationships Changes at Interfaces

Frequency is conserved

So, if rock velocity changes,

the wavelength changes for

every frequency

Velocity Change: Fast>Slow

Change in velocity

Also note reflection of waveformfrom interface (negative reflection)

Velocity Change: Slow>Fast

Change in velocity

Also note reflection of waveformfrom interface (positive reflection)


5/21

Typical Rock Velocities Incidence Angle

It is standard practice to represent thewave motion as a vector (a ray) whichwe can easily imagine as showing themovement of the wavefront

Conversions at Interfaces

Same as Snells Law (optics)

Total reflection

The usual case

Frequency / Power

Imagine an explosion (or other sharp

sound)

The noise is composed of a range of

frequencies, each with its own power

frequency

power

Reflection of a Sharp Sound Imagine that we hear an echo of the

explosion (from previous slide)

What does the echo sound like?

Well, pretty much the same but less

loud (lower amplitude), and deeper in

pitch (higher-frequency components are

attenuated)

A Simple Illustration Lets sum together a bunch of signals of

differing frequencies

What does the resulting signal look like?

All are in-phase at the centre of the plot


6/21

Harmonics (1 to 11)

First harmonic

Second harmonic

Third harmonic

Fourth harmonic

Fifth harmonic

Sixth harmonic

Seventh harmonic

Eighth harmonic

Ninth harmonic

Tenth harmonic

Eleventh harmonic

Time

Amplitude

Summed Waveforms

Sum first & second

Sum first, second & third

Sum first through fourth

Sum first through fifth

Sum first through sixth

Sum first through seventh

Sum first through eighth

Sum first through ninth

Sum first through tenth

Sum first through eleventh

Summed harmonics from:

1st = fundamental frequency

to 11th.

Note progressive reduction

in:

- side lobe amplitude

- peak event width

with increasing frequency

bandwidth.

Black line (sum of 1st to 11th harmonics)

will appear again in next slide

Single Event (Wavelet)This waveform is created from

the previous sum by reducing

the amplitude of each

component frequency away

from the central spike to

represent the single reflection

event

Wavelet length (time) is a function of the frequency contentof the signal for typical seismic data (~20-80 Hz), the widthis about 10-15 milliseconds

~10 ms

Visual Display of Wavelet

Often, the wavelet isdepicted with the positivepart filled in with colour

This helps the eye/brainto see the peaks

Peak

Trough

Normal-Incidence ReflectionA wavelet is created atthe appropriate TWT

distance / velocity = time

x 2 for travel both ways

Acoustic Impedance


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Energy of Reflections

The Reflection Coefficient tells us howmuch of the incident energy is reflected

back typically much smaller than 1

Also remember the loss of

energy due to spherical

dispersion, which depends

on distance (time)

R12 = I2 I1

I1 + I2

Multiple Layers (Interfaces)

Reflections in Series

=

=

=

=

Interval TWTlayer 1

Interval TWTlayer 2

Interval TWTlayer 3

Interval TWTlayer 4

Synthetic Seismogram

Top and Bottom of Bed Adding Waveforms Together


8/21

Interference: Top and Bottom

b is small compared towavelength (which is determinedby velocity and frequency)

Plotted so that the onset of thereflections are at 0 time

What happens when weadd these together?

Issues to consider as beds get thinner

Signal: Top + Bottom

Thickness

(expressed as ratio ofseismic wavelength)

Note that at thicknesses less than about wavelength, itis not possible to clearly resolve top and bottom of unit

from Widness (1973)

Wedge Example

The wedge is a way of

seeing the effects of variable

(unknown) thickness of the

middle layer

Note how the seismic

wavelets interfere with each

other, even when the layerthickness is greater than the

wavelength of the sonic wave

Note that this applies to real wedges of rocks:unconformities, depositional thinning, etc

Very Thin Beds

Destructive

interference of

two wavelets

This example looks at what mightbe the perfect reflector but isnt

Reflections are CompositesI p(Kg/m 2*s) Synt het ic t race

Initial Final

InitialFinal

100

200

300

TVD

(ft)

Rock column shows lithology (alsohas a GR log)

Centre column shows density ofrocks lumped into 5 m intervals. Thefinal curve is the bulk density afterwater replaces the oil.

The synthetic trace shows thewaveform(s) calculated for this rocksequence

Note that a single peak representsa heterogeneous sequence of rocks

Note also how the changes in

saturation affect the seismic signal

Recall: Wavelength & Velocity

InitialFinal

100

200

300

=216ft=65.4metres

x f = V

If f = 34 Hz

V = 2223.6 m/sec

How much geology ishidden in this singlewaveform??


9/21

Reflections are Composites

Note how difficult it may be toinfer the causative rock(impedance) distribution thatcaused the observed seismic signal

Add signals

Wavelet has the

characteristicsof the sourcesignal, AND thereceiver system

Tuning

Internal reflection of waveleads to a duplicate signal(slightly) delayed in time

If the bed thickness and velocity are just right, theduplicate signal is offset by exactly one wavelength,and thus can reinforce the weak signal from theprimary reflection (here, from the base of the bed)

R2

R3add together

Frequency (Wavelength)

Piper Sand Unit

Scott Sand Unit

Mid-Shale Unit

Saltire Unit

KP Transition

17 Hz Ricker 25 Hz Ricker 35 Hz Ricker Vertical depth fromsurface (m)

Time (ms)Well A1 Synthetic traces

20 m thick

intra-reservoir

shale layer

from Valerie Biran (REM 2001)

Note how higher frequency

data resolves shale top, and

also reveals intra-sand details

Multiple Layers

Need a table like this...

...and the sign of the

reflection coefficients...

to produce a synthetic trace

like this at key locations

Precision We ALWAYS operate with time (TWT)

specified to the nearest 1 msec (0.001

sec)

For a rock velocity of 2000 m/sec, this

precision equates to 1 metre of distance

0.0005 sec OWT x 2000 m

0.001 sec TWT

2= 0.0005 sec OWT

sec= 1.0 metre

Many Traces: Side by Side

Here, the same trace is repeated side-by-side, but with minor verticalshifts. Note how the coloured-in peaks (and the intervening troughs)almost merge together to give the appearance of continuous layers.

Time

Distance


10/21

Imaging Faults

Note: fault surface is not directly imaged. Instead,the fault effect is recognised in our mind when wesee the discontinuity of the reflectors

See also later commenton diffractions

Small Faults

Note how it becomes difficult to recognise thefault effect when the fault offset is small

Is the reflector offset??

We operate on the assumption that faultthrows of about the seismicwavelength can be resolved

Some of the Nitty-Gritty Acquisition

Acquisition Activities Acquisition Equipment


11/21

Marine Acquisition Realistic Survey Methods

Multiple shots fired into receiver array, sorting within

the computer to add together images of the same

reflection point

Raypaths From One Shot

Note how each successive receiver (away

from shotpoint) has a longer path for the

seismic energy

GathersIndividual traces

Hyperbola shape

Called Normal Moveout (NMO)

Estimation of Velocity

The three hyperbolae assume a differentvelocity. The red curve is related to thecorrect velocity.

Dipping Relector

Note how ray-paths are not

symmetric around shotpoint


12/21

Dipping Reflector Real Example of Gather

Lots of raypaths anddifferent types of

waves.

You can see why the

interpretation of shot

gathers is a

specialist task!

Processing

Need to put energy (data) into correct

locations

Have to correct for irregular acquisition

geometries and distortions caused by

non-uniform velocities (especially near-

surface, weathered layer)

A lot like the bats brain..

Raypaths from Reflection Points

Note that the subsurface configurationmay prevent signals reaching, orreturning from, certain locations

Impacts of Velocity Anomaly If there is a shallow body of slow

material, the underlying reflections are

late this is a push-down

If there is a high-velocity anomaly at

shallow depth, you get a pull-up

Every reflection event below (later) than

the anomaly is affected

Velocity Anomaly

The body with the anomalous velocity is replacingmaterial whose velocity is 2440 m/sec


13/21

Seismic Profile

Seismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 15 25 35

Distance

TWT(sec

Layer 1

Layer 2

Anomalous Body

Layer 3

Layer 4

Layer 5

The reflections belowthe anomaly all havethe same push-down

Velocity Push-DownSeismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

1820 m/s Seismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

2300 m/s

Seismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

2400 m/s Seismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

2440 m/s

12 msec push-down

3 msec push-down

70 msec push-down

no push-down

Velocity Pull-UpSeismic Hor izons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

2440 m/s

no pull-up

Seismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

2500 m/s

5 msec pull-up

Seismic Hor izons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

2600 m/s

13 msec pull-up

Seismic Horizons

-4.000

-3.000

-2.000

-1.000

0.000

5 10 15 20 25 30 35

Distance (km)

TWT(sec)

3200 m/s

49 msec pull-up

A trap???

Velocity Anomalies

Case illustrated here with a simple

geometry (flat top)

More-complex shapes are possible

Velocity anomaly might not be different

rock, but fluid content (gas?)

Gas Cloud

Gas canaccumulate insmall stringers ofsand/silt, causingsignificantscattering ofseismic energy

Large Scale Composed of

Small Scale Heterogeneities

ShaleQuant

5 6 0 0

5 6 1 0

5 6 2 0

5 6 3 0

5 6 4 0

5 6 5 0

0 5 0 1 0 0

C l a y %

Depth

Sha leQuan t

2650

2700

ShaleQuant

5 6 0 0

5 6 1 0

5 6 2 0

5 6 3 0

5 6 4 0

5 6 5 0

0 5 0 1 0 0

C l a y %

Depth

Sha leQuan t

2650

2700

2650

2700


14/21

Mudstones from West WalesClose-up

Most mudstones are not uniform;they have small stringers of silts

Oil stain (fluorescence) in mudstoneswith stringers of sand/silt

Micro-Reservoirs

Cross-laminated sand/silt layer

capped by an erosion surface

Wavy and discontinuous sand and silt

rich lamina alternating with clay rich

layers

HC can chargethe silty laminae

Diffractions

End of body withdifferent acousticimpedance The traces near

the end receivereflections thatform a hyperbola

Ray paths toreceivers


15/21

Gas Cloud

Thin, dis-continuous stringers of silt maybecome charged with gas

The ends of these silts may act as

diffractors

Many of them acting together can cause

serious disruption of the seismic signals

But Some Anomalies are Real

Pipes indicating extreme fluid flow events

Interpretation Mapping Events

Compressional and rarefactional energy

is the product of many interactions

Peak or trough is an event if it

continues for a significant distance

By hand: draw a (coloured) line along

the event Then transfer the TWT of the event, at

each shotpoint, to a map, and draw

contours

Picking Events

Here, the green event has been picked. We interpret thisto indicate that the rock layers are in the shape of ananticline, with each trace indicating the depth at that point.

Example


16/21

Example

Picking the Brent event

(tied to well control)

3D Seismic

Southern North Sea

Time slice through

3D dataset

Central North Sea

Time slice through

3D dataset

Attributes AVO Amplitude versus Offset

Some due to raypath lengths (in theory,

can be removed by data processing)

Some due to incidence angles

sensitive to changes in rock properties

(e.g. porosity, rigidity, etc)

also sensitive to fluid content (saturations)


17/21

Other Methods

Time-lapse seismic (4D) Wellbore seismic (VSP)

Cross-well seismic

Full-Field Reservoir Simulation

Saturation changes

Pore pressure changes

bar

Gas injection Gas out of solution

Gas production

WaterOilGas

Water flood

+Vp

eff

+Vp

-Vp -Vp

Vp

Stress change effectFluid change effect

Vp changes during common hydrocarbon production processes

After

Nur, 1995

FLOW

SIMULATOR

GEOMECHANICAL

SIMULATOR

SEISMIC

MODEL

PETRO-PHYSICAL

MODEL

Fluidchangeeffect

Stresschange effect

Porepressurechange

Permeabilitychange

Schematic of the elements of the modelling method

Forward

modelling

Geometry

change due to

deformation

Mean effective stress distribution at the end of the simulation

Localized effects

at faultsPerturbed stress field

above and below reservoir

Unperturbed stress field

(constant gradient)Apparent deepening of reservoir

due to decreasing pore pressure


18/21

Reflector at

top ofcaprock

Reservoir

base

Reservoir

top

Time-lapsed seismic trace model

Pull-up in reflector event

due to stress change effects

Perturbations at reflector eventdue to fluid change effects

Wellbore Seismic Survey

Shallow 3-componentreference geophone

Movable 3-componentwall lock geophone

Seismic Waves

1. Downgoing multiple

2. Direct arrival

3. Upgoing reflection4. Upgoing multiple

WESTERN

ATLAS

WESTERNATLAS

WESTERN

ATLAS

WESTERNATLAS

SourceRecording

Survey Well

Wireline

4

1

3

3

2

Baker Atlas

VSP Record

Baker Atlas

Types of Surveys


19/21

Frequencies of Survey Tools

10-4 10-2 10-1 100 101 102 103 104 105 106 10810-3

free

oscillation

VSP

naturalearthquake

explorationgeophysics well

logging

UltraSonic

Frequency (cycles/sec)

It Isnt all THAT Mysterious!

Interpretation Examples

Brasil, Campos Basin

Eastern Mediterranean Nicaragua


20/21

Norway Ormen Lange

Tunisia

Gulf of Mexico

Uncertainty / Errors Picking an event (in time) involves an

error

So calculated velocities are uncertain

And predictions based on those

numbers must compound the errors

What is the size of the errors?

Wavelength & Velocity

InitialFinal

100

200

300

=216ft=65.4metres

x f = V

If f = 34 Hz

V = 2223.6 m/sec

How much geology is hiddenin this single waveform??

Time between peaks is 1/34 sec =0.0294 sec = 29.4 msec


21/21

Wavelength & Velocity

InitialFinal

100

200

300

=216ft=65.4metres

x f = V

If f = 34 HzV = 2223.6 m/sec

Time between peaks is 1/34 sec =0.0294 sec = 29.4 msec

Location (time)uncertainty

How much geology is hiddenin this single waveform??

Error is about one part in 6.5parts, or about 16%. So, we willbe VERY conservative if we saythe error is +/- 5%.

Propagating Errors

So, 10% error (+/- 5%) in time of peaktranslates to 10% error in calculated

velocity.

If we use that velocity to predict the

depth of another observed peak, we

compound the errors.

Rules for Seismic

ALWAYS work to the nearest

millisecond (msec)

THINK about the potential for expensive

errors if you are sloppy!

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