Seismic attribute analysis using complex trace analysis

1. -By
SOMAK HAJRA
M.Sc.-Tech APPLIED GEOPHYSICS
2012MC0093

2. SEISMIC ATTRIBUTES
-AN INTRODUCTION
 “Seismic Attributes are all the information obtained from
seismic data, either by direct measurements or by logical or
experience based reasoning”.
 Seismic attributes may also be defined as all of the measured,
computed or implied quantities obtained from the seismic
surveys which provide a link between rock properties and
seismic data.
 E.g. Reflection strength, apparent polarity, instantaneous
frequency, instantaneous phase etc.

3. CLASSIFICATION OF ATTRIBUTES
 PRE-STACK - AVO, velocity &
azimuthal variations.
 INSTANTANEOUS – Trace
envelope, instantaneous frequency
& phase.
 PHYSICAL - These relate to physical
qualities & quantities like lithology
& wave propagation.
 WINDOW- summarizing
information from vertical
window of data.
 FOURIER- obtained in freq.
domain through Fourier
analysis, like spectral
decomposition.
 POST-STACK – Attributes of CDP
stacked/ migrated data.
 WAVELET – Instantaneous
frequency at the peak of the
envelope equals the mean frequency
of the wavelet amplitude spectrum.
 GEOMETRICAL - Geometrical
attributes describe the spatial and
temporal relationship of all other
attributes & include lateral
information's like dip and azimuth.
 MULTI-TRACE- calculated using
more than one seismic traces to give
lateral variations in data.
E.g. Volumetric curvature, dip etc.

4. FLOWCHART SHOWING BRIEF CLASSIFICATION OF ATTRIBUTES

5. WHAT IS A COMPLEX TRACE?
 Complex trace analysis treats a
seismic trace as a real part of
analytical signal:
 F(t)=f(t)+jf*(t), where j= √-1
 The quadrature component f *(t)
is determined either by a linear
convolution (Hilbert Transform)
or by a phasor representation.
 If Y(t) be the time-varying signal
composed of real & imaginary
parts, we have:

6. COMPONENTS OF A COMPLEX TRACE
 If A(t) & θ(t) be the
Amplitude & time-dependent
phase
respectively, then the
real seismic trace can be
written as:
f(t)= A(t).cos θ(t)
 So, f*(t)=A(t).sin θ(t)
 Thus,
F(t)= A(t).e^(jθ(t))

7. ANALYSIS OF SEISMIC ATTRIBUTES
SINGLE TRACE TYPE
 Seismic attributes which are calculated
using single seismic trace as input.
 The classes of seismic attributes are:
Horizon (loop) Horizon A
•Peak amplitude
•Duration
•Symmetry
Sample (volume, instantaneous)
•Amplitude
•Time
•Frequency
Interval
•Average amplitude
•Maximum (Minimum) Duration
•Isochron

8. COLOUR CODING OF SEISMIC ATTRIBUTES

9. REFLECTION STRENGTH
 Reflection strength is the amplitude of the
envelope & is given by the equation:
 A(t)=|F(t)|= √{(f(t)^2)+(f *(t)^2)}
 The envelope represents the instantaneous
energy of the signal and is proportional in
its magnitude to the reflection coefficient.
 The envelope is useful in highlighting
discontinuities, changes in lithology, faults,
changes in deposition, tuning effect, and
sequence boundaries.
 Hydrocarbon accumulations, like gas,
shows high amplitude reflections or,
‘bright spots’.

10. INSTANTANEOUS PHASE
 Instantaneous phase is the angle of lag or
lead of the harmonic components of a
seismic pulse with respect to a reference.
 For example, a zero-phase wave would be
symmetric whereas a 90° phase wave
would be perfectly asymmetric.
 It is represented as:
θ(t)= arc tan [f*(t)/f(t)]
 Phase is independent of amplitude but is
related to instantaneous frequency and
hence makes weakly coherent events
clearer.
 It is also the best indicator of lateral
continuity.

11. INSTANTANEOUS FREQUENCY
 It is the time derivative of instantaneous phase &
is represented as: W(t)= d θ(t)/dt
 Reflection events are composite of individual
reflections from a number of closely spaced
reflectors, the superposition of which produces a
characteristic frequency pattern.
 Variations like pinch-outs or hydrocarbon-water
interfaces tend to change the instantaneous
frequency value more rapidly.
 A low frequency shift (“low frequency shadow”)
occurs due to reflections from reflectors below gas
sands, condensates or oil reservoirs.
 The adjacent seismic (a) & the corresponding
frequency(b) section shows low frequency anomaly
at shallow depths indicating presence of shallow
gas.

12. WEIGHED AVERAGE FREQUENCY APPARENT POLARITY
 Weighed average frequency
emphasizes the frequency of
stronger reflection events &
smoothes irregularities caused
by noise.
 It is given by the equation:
where, freq(t)=w(t-T) &
env (t)=A(t-T).L(T)
L(T) being the low pass filter.
 It is an excellent tool for
enhancing reflection continuity.
 Apparent polarity is the sign of
f(t) when A(t) is maximum and
are especially sensitive to data
quality.
 ‘Bright Spots’ associated with
gas accumulations show
negative polarity for reservoir
top reflections.
 ‘Flat Spots’ associated with
reflections from gas-oil or gas-water
interfaces show positive
polarity.
 The +ve or –ve sign is assigned
assuming a zero phase wavelet.

13. SOME OTHER DERIVED ATTRIBUTES
 Amplitude Derivative (RE)=dA(t)/dt which highlights the change in reflectivity and is
also related to the absorption of energy.
 Second Derivative of Envelope (DDE)=d2A(t)/dt2 which indicates all reflecting
interfaces visible within seismic band-width showing sharpness of events & changes in
lithology.
 Cosine of Instantaneous Phase C(t)=cos θ(t) which gives detailed visualization of
bedding configurations.
 Instantaneous Acceleration AC(t)=d(F(t))/dt which accentuates bedding differences.
 Thin Bed Indicator TB(t)=F(t)-F’(t) which highlights the location where the
instantaneous frequency jumps in the reverse direction due to very close reflectors, i.e.,
thin beds. Computed from large spikes of instantaneous frequency, indicating
overlapped events.
 Instantaneous Bandwidth B(t)={d(E(t))/dt}/2*pi*E(t). It shows overall effects of
absorption and seismic character changes.
 Instantaneous Q=pi*(instantaneous frequency) * (envelope)/derivative of
envelope. It May indicate liquid content by ratio of pressure versus shear wave section Q
factors.
 Relative Acoustic Impedance which calculates the running sum of the trace to which a
low cut filter is applied. It is an indicator of impedance changes, in a relative sense.

14. MULTI-TRACE TYPE
Seismic attributes which are calculated
using more than a single seismic trace as
input are known as multi-trace type.
It is based on the correlation of two or
more seismic traces.

15. SPECTRAL DECOMPOSITION
The spectral analysis is a procedure that decomposes a time
series into a spectrum of cycles of different lengths. It is also
known as frequency domain analysis. The spectral analysis
describes the distribution of the power at a specific
frequency of a signal, based on a finite set of data. It
replaces the single input trace with a gather of traces
corresponding to the spectral decomposition of the
input attribute. Creating spectral decomposition attributes
enables us to illuminate the structures with different
frequency bands to see if any of them gives us better
resolution.
In spectral decomposition we use complex traces.
Basically a single trace is convolved with the first real
wavelet to get the real trace for the first frequency, and
the input trace is convolved with the first wavelet of
the imaginary part to get the imaginary trace. Then a
complex trace attribute is constructed, such as
envelope, phase, etc. This step is repeated for each
filtered wavelet operating on the same trace and obtain
band limited traces.

16. CONCLUSION
Our increasing reliance on seismic data requires that we extract the most
information available from the seismic response. Seismic attributes are
important because they enable interpreters to extract more information from
seismic data. Applications of attributes include:
 Hydrocarbon play evaluation, prospect identification and risking, reservoir
characterization, well planning and field development.
 Description of shape or other characteristics of a seismic trace over specific
intervals or at specific times.
 They are used for qualitative analysis (e.g., data quality, seismic facies
mapping) and quantitative analysis (e.g., net sand, porosity prediction).
It may be noted that these attributes are very useful tools analysis of a single
attribute may not provide a conclusive definitive information. Instead, useful
conclusions can be drawn by using a combination of attributes together.

17. REFERENCES:
 D. Subrahmanyam & P. H. Rao, Seismic Attributes- A Review; International
Conference & Exposition on Petroleum Geophysics; Pg-398
 M. T. Taner, F. Koehler, and R. E. Sheriff, Geophysics; Complex Seismic Trace Analysis;
44 (6), 1041 (1979). doi:10.1190/1.1440994
 M. Turhan Taner, SEISMIC ATTRIBUTES
 S. M. Rahman; Constraint of Complex Trace Analysis for Seismic Data Processing;
J. Sci. Res. 3 (1), 65-73 (2011)
 Schroeder, Using Seismic Attributes ;AAPG
 http://en.wikipedia.org/wiki
THANK YOU