Seismic Velocity Anomaly and interpretation .pptx

By: Sharad Kumar Mishra, ONGC Ltd
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Elevation
Distance
Seismic Velocities : Understanding through Rock physics

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Seismic waves:
Seismic waves can be classified in to two classes:
Body waves & surface waves
 Body waves which propagate through the rock matrix and further can be subdivided in to two classes as P wave & S wave) as per
their particle motion in the rock matrix during its propagation.
 Surface waves which travel along the surface of the medium.
 Velocities of P- and S-waves (Vp and Vs) are determined by several aspects of a material called elastic constants (or moduli).
 Velocities of surface waves are governed mainly by the shear modulus of materials. S waves are transverse waves which involve
movement of the ground perpendicular to the velocity of propagation. They travel only through solids, and the absence of
detected S waves at large distances from earthquakes was the first indication that the Earth has a liquid core. S waves travel
typically 60% of the speed of P waves. They are typically more damaging than the P waves because they are several times higher
in amplitude.
 The waves which move the surface up and down are called Rayleigh waves and are sometimes described as "ground roll". Waves
whose amplitude of motion is parallel to the surface are called Love waves. Rayleigh waves travel at roughly 90% of the speed of
the S waves.
 Love waves involve the motion of the ground side-to-side, perpendicular to the propagation velocity. They usually travel slightly
faster than the Rayleigh waves.
 Love waves cannot exist in a uniform solid, and can only occur when there is a general increase of S- wave velocity with depth.

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Physical laws that applied on the seismic velocities:
Body waves are reflected and transmitted at interfaces where seismic velocity and/or
density change, and they obey Snell's law.
The velocities of P- and S-waves are given below in terms of the density (ρ) and elastic coefficients of a
material:
Vp = √((K+4/3G)/ρ)
Here K (bulk mod.) & G (Mod. of Rigidity) both are always positive,
Therefore Vp > Vs
Vs =√(G/ρ)

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Character of seismic waves:
Body waves are reflected and transmitted at
interfaces where seismic velocity and/or density
change, and they obey Snell's law.

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Regional velocity & density function in the subsurface
Where ρ: density of rock
Vp: P wave velocity
Vs: Shear wave velocity
μ: lame constant
Acoustic waves ( P wave & S wave) in subsurface are affected by
density of rock matrix in following ways:
There are a few more general rules to the velocity ranges of common materials:
o Unsaturated sediments have lower values than saturated sediments.
o Unconsolidated sediments have lower values than consolidated sediments.
o Velocities are very similar in saturated, unconsolidated sediments.
o Weathered rocks have lower values than similar rocks that are unweathered.
o Fractured rocks have lower values than similar rocks that are unfractured.

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Regional velocity & density function in the subsurface
K & G are correlated with density ρ in such a way that elastic wave velocity increases with depth in
subsurface. How ever when brine replaces gas in a rock , the density increases without any increase in
shear modulus, and shear velocity drops. Thus when other factors are similar, velocity varies inversely
with density.
Vp = √((K+4/3G)/ρ)

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Common pit falls during seismic data interpretations
Snell’s law or ray trace bending is
the change of the direction of the
ray at the interface between two
mediums or rock layers.
Hyugen’s principle describes the generation of diffraction hyperbolas we see in the seismic data from discontinuous
reflectors. Each point acts as a point source which generates waves that travel spherically in all directions. With
continuous reflectors the diffractions cancel, but when we have a discontinuity in the reflectors such as the edge of a
geological fault the diffractions do not cancel out and we see them within the seismic data. The migration algorithms will
collapse the diffractions to a point.

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As the from the source passes through the fast layer we
have pull up underneath it.
Due to the lateral changes in velocity as we pass across a
fault plane we will have distortion in the gathers causing
mis-stacking and distortion of the reflectors, creating a
shadow area around the fault.

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(A) with flat lying events the CMP is midway between
the shot and the receiver. (B) if the reflector is dipping
the CMP does not lie half way between the shot and
receiver, but is smeared across the dipping reflector. This
causes the velocities between flat lying events and
dipping events to be different (taken from Liner, 2002).
The lens effects caused by some rock layers such as salt
which focuses some raypaths and diffuses others. This is
why with subsalt we may have only small incident angles
and it can be difficult to do subsalt AVO.

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On the left are the seismic events before migration with
the diffractions. The termination of the seismic reflectors
creates a point source and diffractions come off of it.
After the migration the diffractions are collapsed and the
fault is apparent.
One of the classical pitfalls of seismic data with is what is
called “bow-tie” features, because they look like “bow-
ties”. Before we collapse the “bow-tie” it appears at first
that it will be two anticlines but in fact it becomes a
syncline.

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Actually, when the tilting is precompaction, there is an
increasing of the velocity interval with depth, and so one can
say that in time seismic lines (by far the more used) a constant
time-thickness emphasizes an increasing depth-thickness, since
the velocity interval is greater in the more buried sediments.
In the mathematical model, three isopach interval are considered.
However, in each interval, due to differential burial the compressional
wave velocity increases down-dip. In the uppermost interval (yellow), the
velocity ranges from 1804 to 2136 m/s. In the second interval (rose), the
velocity is higher. It ranges from 2130 m/s in the less buried sediments, to
2520 m/s in the more deeply buried sediments. In the green interval,
overlying the basement, the velocities change from 2400 to 2840 m/s. In
spite of the fact that in the model, the intervals are isopachous, they are
thinning down-dip on the seismic response.

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a geological model of a normal fault and its seismic response are
illustrated. Theoretically, due to the downward relative movement of the
hangingwall, intervals with quite different interval-velocities are
juxtaposed, which has important consequences on the seismic response.
The footwall reflectors below the fault plane (area of a lateral velocity
changing) will be pulled down, since, at same level, the velocity interval
in the hanging wall is smaller.
On the mathematical model, three sedimentary intervals are considered
above the basement. All intervals are affected by a normal fault. Hence,
the sediments of the hangingwall (downthrown faulted block) are denser.
They reached higher depths. Therefore, their compressional wave
velocities are higher than the sediments of the footwall. Subsequently, on
the seismic response, the reflectors of the footwall are pulled-down
below the fault plane, where there is a lateral change of interval-velocity.

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On this seismic line from offshore Angola, the pull-down of the yellow
marker (bottom of the evaporitic interval) is induced by the lateral
change of the interval-velocity created by the normal fault which limits a
Upper Tertiary depocenter. Indeed, such a fault put limestones
(upthrown block) and shales (downthrown block in juxtaposition.
The geological model of a reverse fault and its likely seismic response is
depicted. As illustrated, the reflectors of the footwall are pulled-up due to
a lateral change of the interval-velocities.

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The seismic response of a mathematical model of a reverse fault, in
which the sediments of the hangingwall are denser than those of the
footwall, corroborates the hypothesis that the reflectors below the fault
plane are pulled-up creating the common illusion of an anticline
structure
This seismic line from onshore France illustrates a seismic artifact
associated with a thrust fault, that is to say, an apparent anticline
structure under the reverse fault plane. In spite of the evidence of the
seismic pull-up, “explorationists” drilled a wildcat on such an artifact
thinking that they were testing a large under-thrust structural trap.
Actually, in certain basins, as we will see later, there are prolific petroleum
traps under reverse and thrust faults, hence explorationists must always
test their interpretations by time-depth conversions.

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Note all time-depth conversions corroborate anticline structures below the thrust
faults. In this particular example, coming, as the previous line, from the Colombia
foothills, a nice antiform structure (that is to say a potential structural trap) is
recognized on the pre-stack section. The same potential structure is also
recognized on the pre-stack migrated version, just under the fault plane, which
should make the interpretation questionable. Finally, the pre-stack migrated
depth versions strongly falsify the hypothesis of a sub-thrust structure. Actually,
the sub-thrust sediments are undeformed and not shortened.
This seismic line through Cuisiana #2A (discovery well), in the
Colombia foothills, was drilled by an international consortium
composed of BP, Total and Triton, in order to test the anticline
structure under upper thrust-faults. However, before drilling, several
time-depth conversions corroborated the hypothesis advanced by
certain explorationists that the sub-thrust antiform was a real
compressional structure and not a seismic artifact induced by the
hanging wall.

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On this reef geological model, above a planar limestone sole (light blue), a reef
with a compressional wave velocity of 5490 m/s, is laterally bounded by shaly
sediments (yellow) with a much lower velocity (3660 m/s), which are overlain
by even slower sediments (brown interval, 3050 m/s). The seismic answer of
such a model is roughly depicted on the right. The horizon associated with the
bottom of the reef shows a significant pull-up.
Notice that in the geological model the compressional wave velocity,
in the blue interval (limestone with a local reefal development)
changes significantly. It is much higher (around 5500 m/s) in the reef
than in the surrounding sediments. Such a reef is supposed to be
tight. The seismic response of such a model, on the right part of the
figure shows that not only the bottom of the reef, but all others
markers below are pulled-up.

Seismic Velocity Anomaly and interpretation .pptx

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