Introduction to seismic interpretation

Introduction
to Seismic
interpretation
By: Amir I. Abdelaziz
Assistant Lecturer , Geology Department, Faculty of Science
Helwan University, Egypt.
2017

Seismic method
Use acoustic waves (sound) to image the
subsurface
• Measure
• time for sound to get from surface to subsurface
reflectors and back - Two-way travel time (twt)
• Amplitude of reflection
• Wanted:
• Depth - Need to know subsurface velocities
• Rock properties (porosity, saturation, etc.)

Seismic acquisition
• Some energy will be reflected, some will be transmitted where there is a change in AI
• Amount reflected (amplitude of reflection) will depend on the relative difference in physical
properties across the interface
V11
V11
V22
V - V
V + V
2 2
2 2 11
11
– Define reflection coefficient (RC)
RC = AI2 – AI1
AI2 + AI1
– If AI2 > AI1 – positive RC
– If AI2 < AI1 – negative RC

Seismic resolution
• Convolutional theorem just described has
interesting implications for vertical resolution
– Each interface produces a distinct reflection.
– If reflections are widely enough spaced, each will
be recognizable.
– Once reflections start to get closer, they start to
interfere with each other.
– At some point adjacent reflections could be so
close that they completely cancel each other out.

Seismic acquisition
• Example 1:
V = 7,000 m/s
F = 50 Hz
l= V/F
= 7,000/50
[(m/s)/(cycles/s)]
= …….m
• Example 2:
V = 3,000 m/s
F = 50 Hz
l= V/F
= 3,000/50 [(m/s)/(cycles/s)]
= ……….m

Seismic acquisition
From the following Tuning Curve:
The following seismic section / Tuning curve showing
zone between TWT (0 to 200 ms) with dominant
frequency = 45 HZ. If the velocity of this interval is
5000m/sec.
-Compute the tuning thickness.
- Can you see/pick this interval in the seismic
section if its thickness is 15 meter? Why?

Understanding the Data
• Common Depth Points (CDPs)
• Floating datum
• Two way time (TWT)
• Time versus depth

Common Depth Points (CDPs)
CDPs are defined as ‘the common
reflecting point at depth on a
reflector or the halfway point when
a wave travels from a source to a
reflector to a receiver’.

Floating datum
The floating datum line represents travel time between the recording surface and the zero line
(generally sea level). This travel time depends on rock type, how weathered the rock is, and
other factors.
The topographic elevation is the height above sea level of the surface along which the
seismic data were acquired.

Time versus depth
• Two way time (TWT) does not
equate directly to depth.
• Depth of a specific reflector
can be determined using
boreholes.
• For example, 926 m depth =
0.58 sec. TWT

Seismic Processing
Energy within a seismic trace consists of –
Signal:
that carries desired information
Noise:
that contains unwanted information

Random Noise
• energy which does not exhibit correlation from trace to trace
• not generally source generated (the noise would be present whether we were shooting or not)
Examples:
Swell Noise
Instrument Noise
Shark bites
Powerline Noise
Traffic (vehicles, people, animals)
Wind
Falling Debris
Earthquakes

Coherent Noise
• predictable from trace to trace across a group of traces i.e. have a phase relationship between
adjacent traces
• often source generated ( if we weren’t shooting the noise would not occur)
Examples:
Multiples
Direct arrivals
Ground roll
Refractions
Airwave
Cable Jerk
Autofire
Ship’s propeller noise

Sources of noise in marine surface seismic

Basic steps in Seismic Processing

FK filter
• FK filter: Is a dip or velocity filter. Where filtering is performed on data after it has undergone a
2D fourier transform into the FK domain.
•Produce a diagram that shows how each of the following events in the XT domain are
represented on an FK spectrum: shallow dip, steep dip, flat event.
•Define each of the following terms : pass zone, reject zone, taper zone.
The Fourier Transform
Time domain Frequency Domain

Know Your Events
Frequency ( Hz )
Wavenumber, K = ---------------------------- X Dip ( ms / tr )
Trace Interval ( m )
We can now separate events on the basis of their dips and/or frequencies

FK-PLOT of a shot record with cable jerk noise

FK-PLOT of a shot record with no cable jerk noise

XT-PLOT of a shot record with cable jerk noise

Deconvolutional Model

Statistical Deconvolution
We can now use Statistical deconvolution techniques to remove the system
wavelet, by compressing it to a spike, and hence derive the reflectivity.

Sea Bottom Multiple

Positioning Problems

Energy
Source

The seismic ray hits an inclined
surface at 90º and reflects back
0.4 s -
The reflection is
displayed beneath the
source-receiver midpoint
Bounce
Point
Migration

Time for an Exercise
1
     
2 3 4 65
Where would the reflection lie?
90º
Migration

1
     
2 3 4 65
Compass
Migration

1
     
2 3 4 65
Migration

Exercise Answer
1
     
2 3 4 65
The reflection is downdip and its
dip is less than the interface
Migration

Pre Stack migration (2D marine)
Field data in
Mute
NMO correction
Migration
Stack
Amplitude scaling
Filtering
Final products
Energy spreading corrections Amplitude recovery
Low-cut / velocity filtering Noise rejection
Wavelet compression / demultiple Deconvolution
Velocity Analysis
Near/far offset noise removal
Summing traces within a cmp.
Lowcut / highcut / signal enhance
Time variant amplitude balancing
Cosmetic plotting test:bias/gain
Final velocity field
Initial velocity field
Migration
Data re-ordering
Possible additional velocity Analysis
CMP gather
Remove source wavelet from data Signature decon.
Define positions and relationships Geometry
DemultipleRemoving multiples
SHOT SORTED
CMP SORTED
OFFSET SORTED
STACK SORTED

Processing Sequence (3D marine)
Field data QC & edit
Signature Decon.
Amplitude recovery
Noise rejection
Deconvolution
NMO correction
Apply automatic field edits
QC navigation data and select
binning strategy
2:1 data reduction
Temporal resample
DMO on velocity lines
Nav./seismic merge
Mute
3D DMO & Stack
Migration
Migration vel. field
Demultiple
Time variant filtering
Time variant scaling
final products
field data
QC shot data
As early as possible!
QC stack displaysPre Stack Migration

Processing Sequence (2D land)
Basic Processes
Field data in
Geometry
Amplitude recovery
Noise rejection
Deconvolution
CMP gather
Mute
NMO correction
2D DMO & Stack
Migration
Amplitude scaling
Filtering
Final products
Residual statics
Field statics
loop
2D DMO

Virtually all processing projects will require the following:
• Accurate locating of shots and receivers and their relationship
• Time-correction to a known datum
• Amplitude recovery
• Noise attenuation
• Signal enhancement
• Cmp gathering
• Demultiple
• Normal move-out correction (including muting any noise introduced)
• Residual time-corrections (land data)
• Correction for smearing within a cmp - dip-moveout correction
• Data reduction - stacking
• Repositioning / imaging - Migration

Seismic interpretation
• Check line scale and orientation.
• Work from the top of the section,
where clarity is usually best,
towards the bottom.
• Distinguish the major reflectors
and geometries of seismic
sequences.

Understanding the data (1)
CDPs are typically marked at intervals along the top of seismic lines and they are
regularly spaced to form a horizontal scale. Here, 80 CDPs represent about 1
kilometre (km).

Signals from farther away
will provide information for
deeper horizons
Gaps in land seismic data are
due to omissions where data
could not be acquired
For example, it is not always
possible to transmit the
signal above pipes, in
sensitive areas and above
buildings

• Two way time (TWT) is recorded on
the vertical axis of the seismic line in
fractions of a second. Sometimes it is
more convenient to express time as
milliseconds.
• TWT is the time required for the
seismic wave to travel from the
source to some point below the
surface and back up to the receiver.

Objectives of Well-Seismic Ties
• Well-seismic ties allow well data, measured in units of depth, to be
compared to seismic data, measured in units of time.
• This allows us to relate horizon tops identified in a well with specific
reflections on the seismic section.
• We use sonic and density well logs to generate a synthetic seismic
trace.
• The synthetic trace is compared to the real seismic data collected
near the well location.

Measurements In Time and In Depth

Check Shot Data
• Check shots: measure the vertical one-way
time from surface to various depths
(geophone positions) within the well.
– Used to determine start time of top of well-
log curves
– Used to calibrate the relationship between
well depths and times calculated from a
sonic log

Pulses Types
• Two options for defining the pulse:
A. Use software that estimates the pulse
based on a ‘window’ of the real seismic
data at the well (recommended)
B. Use a standard pulse shape specifying
polarity, peak frequency and phase:
• Minimum phase
• Zero phase
• Quadrature

 We ‘block’ the velocity (sonic) and density logs and compute an impedance ‘log’
Velocity Density Impedance
=x
Shale
Sand
Shale
Sand
Shale
Lithology
Reflection
Coefficients

• We calculate the reflection coefficients at the step-changes in impedance
* 
Wavelet
• We convolve our pulse with the RC series to get individual wavelets
• Each RC generates a wavelet whose amplitude is proportional to the RC

Synthetic
• We sum the individual wavelets to get the synthetic seismic trace
The Modeling Process

Forward Process
Earth Impedance

Forward Process
Earth Reflection
Coefficients
Impedance

Forward Process
Earth Reflection
Coefficients
WaveletImpedance

Forward Process
Earth Reflection
Coefficients
Wavelet Wavelet
Superposition
Impedance

Forward Process
Earth Reflection
Coefficients
Wavelet Wavelet
Superposition
Recorded
Trace
Impedance

Forward Process
Earth Reflection
Coefficients
Wavelet Wavelet
Superposition
Recorded
Trace
Seismic
Section
Impedance

Tying Synthetic to Seismic Data
Tying Synthetic to Seismic
Data

Tying Synthetic to Seismic Data

2D Seismic data
X1 X2
Y1
Y2
2 D Seismic acquisition

Bad
data
Good
data
Very
Good
data
3 D Seismic acquisition

Top down approach - Picking intersted Horizon
• Start at the top of the section, where definition is usually best
• Work down the section toward the zone where the signal to
noise ratio is reduced and the reflector definition is less clear

Seismic Stratigraphic analysis

Interpret Faults
Inline 500
Inline 400
Inline 300
Inline 200
Inline 100

Interpret Horizon
Inline 500
Inline 400
Inline 300
Inline 200
Inline 100

Create and Associate Fault Polygon Set

velocity from check-shot survey
• The velocity required for the map is Average velocity
• In case of determining velocity from check-shot survey, the result velocity will multiplying by 2 (to convert
it to one way time).

Depth contour map
• We extract the depth map values from the velocity & one way time map.

Depth Conversion-Simple Average Velocity
• Convert depth to depth below seismic reference datum, and convert TWT to a one-way
time.

Depth Conversion-Simple Average Velocity

Types of Petroleum Traps
❖ Several geologic structures may act as petroleum traps,
but all have two basic conditions in common:
1) Porous, permeable reservoir rock that will contain
quantities of oil and gas that make it worth drilling.
2) Impermeable cap rock that traps oil and gas preventing
it from escaping to the surface.
❖ Types of Petroleum traps include:
1) Anticline Trap 3) Fault Trap
2) Salt Dome Trap 4) Stratigraphic Trap

1) Anticline Trap:
• If a permeable rock like sandstone or limestone is located between
impermeable rock layers like shale and the rocks are folded into an
anticline, oil and gas can move upward in the permeable reservoir rocks,
and accumulate in the upper region of the anticline.

How to find oil: Source rock, reservoir rock, traps

2) Fault Trap :
➢ If faulting can shift permeable and impermeable rocks so
that the permeable rocks always have impermeable
rocks above them, then an oil trap can form.
➢ Note that both normal faults and reverse faults can form
this type of oil trap.

3) Salt Dome Trap :
➢ Here we see salt that has moved up through the Earth,
punching through and bending rock along the way.
➢ Oil can come to rest right up against the impermeable
salt, which makes salt an effective trap rock.

4) Limestone Reef Trap :
➢ Limestone reef trap is a type of stratigraphic trap.
➢ When coral reefs become buried by other
impermeable sediments they can form excellent oil
sources and reservoirs.

Example 1
Which petroleum trap would be formed by a simple fold?
(A) anticline (B) fault
(C) salt dome (D) stratigraphic
The limestone reef trap belongs to which type of petroleum
trap?
(A) anticline (B) fault
(C) salt dome (D) stratigraphic

Example 2
Based on the diagram below, which is the correct match
between the force and petroleum trap produced?

Introduction to seismic interpretation

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Introduction to seismic interpretation