Seismic Velocity, Attenuation (Q), and Anisotropy

Chapter 6
Seismic Velocity, Attenuation (Q),

and Anisotropy
S eismic Acquisition and Processing

Types of Velocity
• Interval Velocity
• Average Velocity
• RMS Velocity
• NMO Velocity
• Stacking Velocity
• Migration Velocity
S eismic Acquisition and Proces6s-2 ing

Interval Velocity
• The speed at which a seismic wave propagates in a single layer

• It is a physical property of an earth layer
• It is measured by a sonic log (2 sensors a distance d apart
measure the time t required to travel between them; Vint = d/t)
• It is determined by lithology, compaction, porosity, fluid content,
etc.

Interval Velocity
Interval Velocity generated from advanced Seismic Processing

i.e. Tomographic Inversion. Note the variation of velocity in each
layer that may correlate with layer’s physical properties.

Factors affect Interval Velocity

Average Velocity
Average Velocity(Vavg)
The depth D of a formation divided by the time T for a seismic wave to
travel vertically from the surface to that formation. D = sum of
thicknesses of all layers = Σdi, T = sum of traveltimes in all layers = Σ(di/
Vi).
Vavg= (V1τ1 + V2τ2 + V3τ3) / T3

T3= traveltime to base of layer 3 = τ1+ τ2+ τ3

Average Velocity: Usage
In areas with little structure or lateral velocity variation

• At well locations, check-shot data provide a direct
means of converting from T obtained from seismic data
to subsurface depth D.
• In the absence of check-shot data, Vavg is estimated
from seismic data : D = Vavg T/2
In areas with structure and/or lateral velocity variation

• Migration approaches are required

Root Mean Squared (RMS) Velocity
VRMS2= (V1 τ1+ V2 τ2+ V3 τ3) / T3

2 2
2
T3= traveltime to base of

layer 3 = τ1+ τ2+ τ3

DIX Formula and Comparison
DIX Formula is used for

estimating Vint from Vnmo or Vrms.
For Time to Depth Conversion, we

need Vave. So that if we have Vrms or
Vnmo from Velocity Analysis, we can
convert it into Vint using DIX
equation then calculate Vave using
equation of Vave explain in the
previous slide.
S eismic Acquisition and Proces9sing

NMO Velocity
• NMO velocity is the velocity used to correct for normal moveout (NMO), i.e. to
make primary reflections on CMP gather records occur at the same time on all
traces.
• For horizontal or gently dipping layers, NMO velocity = RMS velocity
S eismic Acquisition and Proces6-s10ing

Stacking Velocity
Stacking Velocity is the velocity chosen by the analyst and should give the
optimum CMP stack output when used for NMO corrections. Stacking velocity is
whatever the analyst chooses!
S eismic Acquisition and Proces1s1 ing

Migration Velocity
Migration Velocity commonly refer to velocity that is analyzed for NMO-ing

PSTM gathers. Velocity obtained from NMO process is smoothed so that it
allows rays to propagate smoothly without complexity e.g. no over-turned
path. Migration velocity can be represented as Vrms or Vint.

Velocity from Borehole Measurements

Seismic versus Borehole Velocity
Seismic Vrms values are usually faster than sonic or
check shot Vrms, primarily because of anisotropy.
• The difference is zero near the surface.
Vrms • The difference may increase to 10%-12% at depth
x • Differences greater than about 10% suggest data
x
problems.
x
x For Time-Depth conversion it is important to scale
Seismic Vrms
seismic Vrms values to match well Vrms values so well
x markers will tie seismic interpretation
Depth
x
x
x
C
h
ec
k
s
h
ot
Vr
ms
Variation of velocity with lithology

Attenuation (Q)
When seismic wave travels through any medium, its mechanical energy
is progressively converted to heat (through friction and viscosity).
• On grain boundaries, pores, cracks, water, gas, etc.
• This conversion causes the amplitude to decrease and the pulse to
broaden

Attenuation (Q)
Attenuation is measured in terms of rock quality factor, Q:
Typical values:
Q ≈ 30 for weathered sedimentary rocks;
Q ≈ 80-120 for compacted sedimentary rocks;
Q ≈ 1000 for granite.

Q Estimation: Spectral Ratio Method
Q can be estimated using above

equation. t is time difference between
shallow (green) and deep (blue)
window. β is slope of the ratio between
deep and shallow .

Q in use (Q PSDM)
Seismic stack section. (a) Stack after conventional PSDM. (b) Stack after Q tomography and Q
PSDM. (c)Wiggle display of conventional PSDM stack (filled with black colour) overlaid on Q
PSDM stack. (d) Amplitude spectrum plots of (a) and (b) around the gas pocket and reservoirs.
(Joe Zhou, et.el., SEG San Antonio 2011 Annual Meeting)

Anisotropy
Seismic anisotropy is a term used in seismology to describe the
directional
dependence of seismic wavespeed in a medium (rocks) within the Earth.
There are 3 main types of anisotropy:

• HTI: Horizontal Transverse Isotropy
• VTI: Vertical Transverse Isotropy
• TTI: Tilted Transverse Isotropy

Anisotropy Parameters
• Anisotropy parameters: ε, γ, δ, σ and η.
• ε, γ and δ are known as Thomsen’s Parameters (1986)
• σ and η are obtained from seismic measurements (Alkhalifah
and
Tsvankin,1995).
Vp : P wave
Vs : S
wave
ψ : parallel
τ:
S eismic Acquisition and Proces2s1 ing perpendicul
Anisotropy in Hydrocarbon Exploration
Anisotropy is useful for:
• High Order NMO
• Anisotropic PSTM and PSDM
• Anisotropic AVO
• Fracture Analysis

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What are the different types of seismic velocities discussed in the document?

What are the different types of seismic velocities discussed in the document?

What factors affect interval velocity?

What factors affect interval velocity?

Chapter 6

Seismic Velocity, Attenuation (Q),

S eismic Acquisition and Processing

S eismic Acquisition and Proces6s-2 ing

• The speed at which a seismic wave propagates in a single layer

S eismic Acquisition and Proces6s-3 ing

Interval Velocity generated from advanced Seismic Processing

S eismic Acquisition and Proces6s-4 ing

S eismic Acquisition and Proces6s-5 ing

Vavg= (V1τ1 + V2τ2 + V3τ3) / T3

S eismic Acquisition and Proces6s-6 ing

In areas with little structure or lateral velocity variation

In areas with structure and/or lateral velocity variation

S eismic Acquisition and Proces6s-7 ing

VRMS2= (V1 τ1+ V2 τ2+ V3 τ3) / T3

T3= traveltime to base of

S eismic Acquisition and Proces6s-8 ing

DIX Formula is used for

For Time to Depth Conversion, we

S eismic Acquisition and Proces9sing

S eismic Acquisition and Proces6-s10ing

S eismic Acquisition and Proces1s1 ing

Migration Velocity commonly refer to velocity that is analyzed for NMO-ing

S eismic Acquisition and Proces1s2 ing

S eismic Acquisition and Proces1s3 ing

S eismic Acquisition and Proces1s5 ing

S eismic Acquisition and Proces1s6 ing

Attenuation is measured in terms of rock quality factor, Q:

S eismic Acquisition and Proces1s7 ing

Q can be estimated using above

S eismic Acquisition and Proces1s8 ing

S eismic Acquisition and Proces1s9 ing

There are 3 main types of anisotropy:

S eismic Acquisition and Proces2s0 ing

S eismic Acquisition and Proces2s2 ing

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