Phase, freq and spectra in seismic interpretation.pptx

SHARAD KUMAR MISHRA, GEOPHYSICIST 1
PHASE, POLARITY, AMPLITUDE AND SEISMIC
FACIES
Courtesy: https://www.pngwing.com/en/free-png-mvssa

 A wavelet is a wave-like oscillation with an amplitude that begins at zero, increases or
decreases, and then returns to zero one or more times.
 The seismic wavelet is described by three components: amplitude, frequency, and phase.
 Amplitude is the maximum displacement from the origin, while frequency is described as
the number of complete cycles per second.
 Phase is defined as the relative shift of the sine wave at each frequency, measured in
terms of phase angle at a fixed point.
 The seismic trace is composed of a series of reflection coefficients, which result from
differences in acoustic impedance between the earth's layers, that are convolved with a
proper wavelet.
 Understanding the parameters of the seismic wavelet greatly affects the outcome of
seismic interpretation and reservoir characterization processes, especially the phase of the
wavelet.
 As the phase of the wavelet varies, the expected reservoir signature also varies.
Seismic wavelet:

 Wavelets are representations of short wavelike oscillations with different
frequency ranges and shapes. Because they can take on many forms—
nearly any frequency, wavelength, and specific shape is possible—
researchers can use them to identify and match specific wave patterns in
almost any continuous signal.
Types of Seismic wavelet:

A wavelet is a transient waveform of finite length. Based on the phase specifications or their maximum energy distributions,
there may be four types of wavelets:
1: minimum Phase,
2: mixed Phase
3: zero phase
4:maximum phase
 Fig. 1 shows their schematic displays with corresponding amplitude
and phase spectra. The wavelet energy is front-loaded, middle-
loaded, and back-loaded for minimum phase, mixed phase, and
maximum phase wavelets, respectively.
 A zero phase wavelet is symmetrical with a maximum at time zero.
 Zero phase wavelets have energy before time zero, which makes
them non-causal, and therefore they are not physically realizable.
 A zero phase wavelet has a shorter duration than its minimum
phase equivalent, which makes it a wavelet with higher resolving
power.
 All of these wavelets in the given example have the same amplitude
spectrum, and the only difference in frequency domain is their
phase spectra.
Seismic wavelet Character:

 The seismic wavelet is assumed to be always minimum phase, which is a causal signal and its energy is zero before time zero.
 If the source wavelet in the seismic trace is not minimum phase, this may create problems during deconvolution process. To
avoid such situation, it is necessary to convert first the source wavelet into its minimum phase equivalent.
 In land surveys using vibroseis as a source , the seismic data is always zero phase data.
 In the offshore, air gun data is maximum or mixed phase.
 The impulsive sources generally produce minimum phase source signals.
 The convolution of two minimum phase wavelets is minimum phase.
 The convolution of a zero-phase and minimum phase wavelet is mixed phase.
 The zero-phase wavelet is of shorter duration than the minimum phase equivalent. The wavelet is symmetrical with a
maximum at time zero (non-causal). This wavelet is useful for increased resolving power and ease of picking reflection events
(peak or trough).
 A special type of wavelet often used for modelling purposes is the Ricker wavelet which is defined by it's dominant frequency.
The Ricker wavelet is by definition zero-phase, but a minimum phase equivalent can be constructed. The Ricker wavelet is
used because it is simple to understand and often seems to represent a typical earth response.
Seismic wavelet:

Seismic forward model:
Convolution model of seismic trace generation: Seismic trace is considered to be a primary only reflectivity model convolved with
the seismic wavelet which is summed with some uncorrelated noise and can be represented by the equation:
S(t) = R(t) * W(t) + N(t)

 An increase in acoustic impedance (AI) across a reflecting boundary corresponds to a positive RC and should,
theoretically, produce a perfect zero-phase reflection. Globally, we have two polarity conventions:
 Normal polarity for zero-phased sections plots a positive RC as positive peak (rightward deflection of wiggle by
convention). This corresponds to the SEG convention.
Zero-phase wavelet
 For a positive RC (increase in impedance), the number recorded on the tape should be positive, and the first motion
centered on the RC should be displayed as a peak (Fig. 11.4). If a zero-phase dataset is said to be SEG reverse polarity,
that would mean for a positive RC the motion centered on the RC would be displayed as a trough
Seismic forward model: Polarity

Seismic forward model: Polarity convention
Diagram showing zero-phase processing.
a. An impedance curve. b. The minimum-phase
wavelet; c. The first derivative waveform of
minimum-phase wavelet; d. Zero-phase wavelet
with symmetrical side lobes; e. The reflection of
seafloor surface.
American and European polarity schemes (modified
form Wang et al., 2015). Abbreviations: Z— acoustic
impedance; R— reflection coefficient; 0◦— zero-phase
wavelet.
European vs American conventions
Different companies own conventions

Seismic forward model: Seismic Resolution
 The resolution of zero-phase data is one-quarter of the
dominant wavelength (λ) for an interlayer with opposite
polarity at the top and bottom (dipolar interlayer)
 It is λ/4.6 for an interlayer with the same polarity at the
top and bottom (homopolar interlayer, (Badley, 1985).
 The limit of discernibility is approximately one sixtieth
(the amplitude of the composite wave is approximately
25% of a single reflection,) in the most favorable instance
for a dipolar interlayer (Brown, 2011), and it is only λ/4.6
for a homopolar interlayer .
 Seismic response of carbonate weathered crust
(compiled from Badley, 1985; Wang et al., 2015).
Reflections from top and bottom interfaces are positive,
and the limit of separability is λ/4.6. If the thickness is
between 0.28λ and 0.18λ, a low-frequency waveform
will be generated. When the thickness is less than ~λ/4,
the two events corresponding to the top and bottom of
the reservoir will disappear.

Seismic forward model: Seismic Resolution
 The low impedance sand encased in between high impedance
shale produces a negative reflection on top and a positive reflection
on the bottom .
 The top and bottom reflections from sandstone can be completely
separated when the thickness of sand is greater than λ.
 If the thickness is less than λ/2, the two reflection pulses overlap
and interfere to form a skew-symmetrical complex waveform.
 If the thickness is equal to or greater than λ/4, the amplitude of
composite wave reaches the maximum (tuning amplitude), and the
time thickness can be directly read from the composite wave.
 When the thickness is less than λ/4, the period from trough
(corresponding to the top interface) to peak (corresponding to the
bottom interface) is greater than the interlayer thickness, the shape
of the complex reflection does not change, and the thickness is
proportional to the amplitude.
 When the thickness is λ/64, the composite wave is still visible
because the amplitude of composite wave is about 22% of a single
reflection.

Seismic forward model: Response of different layers and trace apearance
Zero-phase wavelet in American polarity are
displayed.
 a. Zero-phase symmetrical wave generated by a
single interface;
 b. Right downward (red arrow) skew-symmetrical
waveform generated by a low-impedance gas
sands of λ/4 thickness.
 c. Left downward (red arrow) skew-symmetrical
waveform generated by a high-impedance
limestone of λ/4 thickness.
 d. Diagram showing the composition and
decomposition of seismic wave. The amplitude of
composite wave at each sampling point is the sum
of amplitude of reflections at five interfaces. This
type of complex wave is called a low frequency
waveform, which is manifested by the broadening
of the wave crest (or the reducing of the
frequency) corresponding to the transition layer.

Seismic forward model: Response of different layers and trace apearance
Diagram showing phase shifting (a) and polarity (b) in right side figure.
Seismic waveform after phase shifting (modified form Zeng, 2011):
• The wave generated by a single positive reflection interface is symmetric,
where the center lobe is a peak.
• 90◦ phase wave of this single reflection shows as a “differential” or left
downward symmetrical wave. The composite wave generated by a thin
compact sandstone shows as a left downward symmetrical wave, in which the
center lobe is a trough after the 90◦ phase shifting.
• Phase shifting and polarity, the arrow colour is consistent with the digital
colour (modified from Brown, 2011): A composite wave of thin gas sand
• may be shown as left downward skew-symmetrical wave (European polarity)
or right downward skew-symmetrical wave (American polarity).
• After a 90◦phase shifting (clockwise), they become a zero-phase wave with
trough-center-lobe or a zero-phase with peak-center-lobe wave.
• Conversely, after a − 90◦ phase shifting (counterclockwise), they become a
zero-phase wave with peak-center-lobe wave (European polarity) or a zero-
phase wave with trough-center-lobe (American polarity).

 Fig.below describes the effect of phase rotation on the expected reflection event from the reservoir.
 At t = 0, the reservoir signature has a peak from zero phase to zero crossing at ±90°, a total phase flip at 180°, zero crossing at ±270°, and a full return to peak at 360°.
 seismic waves that propagate through a highly attenuating layer lose energy, and this energy loss affect high frequencies more significantly than low frequency components. The quality
factor Q may approach values as low as 10 in some rocks, the shifts in frequency content could be very large. The rocks with fractures filled with various fluids also causes attenuation
during wave propagation and loss of high frequencies. The phase of seismic data rotates with an increase in depth due to physical principles (loss of energy with depth).
 Essentially low values of Q ( like shale) will give greater phase rotation for a given sediment thickness than higher values of Q (e.i Sandstone and lime stone) .
 Phase in seismic data is simply known as the lateral time delay in the start of a reflection recording, and because it is amplitude-independent, phase can be used as a good continuity
indicator in poor reflectivity areas in the seismic data with a higher sensitivity to reflection discontinuity caused by pinch outs, faults, fractures, and other structural and stratigraphic
seismic features.
 Furthermore, polarity is compatible to reflection coefficient of the seismic data. In other words, if the beddings’ boundary gave a positive acoustic impedance, it corresponds to a
positive polarity and vice versa.
 Processing steps, from simple rotation to sophisticated time-varying Q compensation, can help produce seismic data that are zero phase.
Seismic wavelet: Reasons of phase rotation

 The main cause of Phase rotation in seismic data is Q compensation. Easiest way of determination of Q factor is VSP data. Q cannot be derived from stacked traces owing to mixing of traces
with different path length, the spectral distortion due to moveout and the incorporation of AVO effects. The presence of gas in a sandstone can give rise to anomalously low Q (high
absorption), particularly at intermediate saturations. This has driven interest in the use of Q in direct hydrocarbon detection.
 The wavelet processing like zero phasing of the seismic, is the correction associated to compensation for absorption effects in the seismic bandwidth.
 Inverse Q filtering is a way of correcting for the exponential loss of high-frequency energy and increasing wavelet distortion with increasing two-way time.
 When correctly applied the amplitude corrections give rise to data that have true relative amplitude and the phase correction enhances vertical resolution, particularly with increasing
depth.
Zero phasing
 The process of zero phasing seismic is conceptually straightforward. If the wavelet is known then a convolution of inverse operator can be applied to transform it to zero phase .
Seismic wavelet: How phase processing can be done

Seismic interpretation is fundamentally based on interpreting
changes in amplitude.
The changing amplitude values that define the seismic trace are
typically explained using the convolutional model. This model
states that trace amplitudes have three controlling factors:
The reflection coefficient (RC) series (geology).
The seismic wavelet.
The wavelet's interactions through convolution.
Large impedance (velocity x density) contrasts at geologic
boundaries will generally have higher amplitudes on the seismic
trace.
Interpreters associate changes in seismic amplitudes with
changes in the geology; this is a good assumption only if all of
the factors that affect trace amplitudes have been considered.
Seismic wavelet: Amplitude

In seismic facies analysis the following
parameters should be considered:
1. Position (regional setting),
2. External form,
3. Internal configuration,
4. Reflection amplitude
5. Dominant frequency
6. Reflection continuity,
7. Abundance of reflections
8. Line direction (dip or strike section)
9. Reflection termination
10. Size/ formation thickness
11. Slope break
12. Reflection smoothness
13. Special waveform pattern
14. Appearance & reflection polarity
15. AVO anomaly
Seismic wavelet: seismic facies

Reference: https://www.e3s-conferences.org/articles/e3sconf/pdf/2020/54/e3sconf_icaeer2020_01038.pdf

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Phase, freq and spectra in seismic interpretation.pptx

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