EPS – 435 Geophysical Applications Seismic Reflection Processing

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EPS – 435 Geophysical Applications Seismic Reflection Processing
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Seismic reflection data are routinely acquired for multiple purposes such as
exploration, mining, or engineering problems.
The seismic data are generally acquired in shot-gathers, i.e. the data is
sorted (stored) by grouping data from all receivers for the same shot.
However, each receiver detects seismic energy emitted at the source from
different subsurface points:
From Reynolds, 1997
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-01
EPS – 435
Geophysical Applications
Seismic Reflection Processing
In order to image the same depth point
(referred to as Common-Depth-Point, or
CDP), sources and receivers are moved
across the area to be imaged. Each CDP now
receives energy from different angles of
incidence.
The originally shot-sorted seismic data now
need to be re-sorted into CDP-gathers
representing the same point in the
subsurface:
From Reynolds, 1997
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-02
EPS – 435
Geophysical Applications
Seismic Reflection Processing
The same idea seen before
holds for 3-D seismic data
acquisition, where a grid of
source- and receiver lines is
laid out across the survey
area, and traces representing
the same ‘bin’, or subsurface
rectangle area are grouped
together.
From Reynolds, 1997
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
3-D seismic data acquisition
allows to fully represent
complex geologic structures
and rock-properties.
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EPS – 435
Geophysical Applications
Seismic Reflection Processing
In the following, several screen-shots from the demonstration in the class are
shown for several steps in the seismic data processing flow.
The data used are marine, multi-channel streamer data from offshore South
Korea, acquired in the Ulleung Basin in the East Sea (Sea of Japan).
The institute who shots the seismic data is the Korea Institute of Geosciences
and Mining and Materials (KIGAM).
The commercial processing software is GLOBE Claritas, from the Institute of
Nuclear Science in New Zealand.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-04
EPS – 435
Geophysical Applications
Seismic Reflection Processing
A typical marine seismic data processing sequence consists of the following steps:
(1) Geometry definition (location of shot/receiver, CDP)
(2) Quality control, definition of frequency content of data
(3) Bandpass-filter (time-domain)
(4) Deconvolution (shaping of source-wavelet) [Æ not shown]
(5) Velocity analyses and normal move-out (NMO) correction
(6) Stack
(7) Migration
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-05
EPS – 435
Geophysical Applications
Seismic Reflection Processing
D
R
M
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
Display of first shot-gather in
seismic line. The direct
arrival (D) is clearly seen in
the upper left corner.
Reflections (R) arrive at ~2.9
s two-way time (TWT). A
multiple (M) is seen at ~ 6 s
TWT.
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Geophysical Applications
Seismic Reflection Processing
Example shotgather (splitspread) from the
Rimouski-field trip
Platformreflection
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-07
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Definition of frequency content of
data (first shot-gather, zoom).
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-08
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Frequency-wave-number representation of same data set Æ used for
special design-filters (directionality)
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-09
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Application of band-pass filter to remove high-frequency noise.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-10
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Velocity-analyses are carried out on an initial stack or brute-stack (left) with
constant-veloctiy gathers (middle) and semblance plots (right).
In this example a velocity of 1480 m/s flattens perfectly the top few layers near the seafloor, but deeper arrivals are still bending
upwards – velocity is higher. The semblance is a representation of the stack-power of an NMO-corrected gather. It is maximum if
the reflection hyperbola has been completely flattened.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
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EPS – 435
Geophysical Applications
Seismic Reflection Processing
Velocity-analyses are carried out on an initial stack or brute-stack (left) with
constant-veloctiy gathers (middle) and semblance plots (right).
In this example a velocity of 1520 m/s flattens perfectly a layers at 3.5 s TWT. The velocity is too high for the seafloor reflections,
thus they bend downward.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-12
EPS – 435
Geophysical Applications
Seismic Reflection Processing
The seismic interpreter now defines a velocity-time function through the
points of maximum semblance, but avoiding spurious events.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-13
EPS – 435
Geophysical Applications
Seismic Reflection Processing
With these velocities defined along
the entire profile at several
locations (in this example a set of
10 velocities would suffice), the
seismic data will all get NMOcorrected (undo the hyperbola) and
the stacked to get a single trace
per CDP only. This forms then a
complete section showing an
acoustic image of the subsurface
lithology.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-14
EPS – 435
Geophysical Applications
Seismic Reflection Processing
The last step in the processing is
MIGRATION. This step ensures
that the seismic energy recorded
in time, gets properly imaged into
depth by using the determined
velocities!
Note that compared to the stack,
all diffraction hyperbolas have
been removed by migrating the
energy to the correct subsurface
points from where they
originated.
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-15
EPS – 435
Geophysical Applications
Seismic Reflection Processing
3-D seismic data offer the
possibility to explore
subsurface in all dimensions
and define even complex
structures, such as the saltdome and basin to the right.
Slicing of data cube in inline –
crossline and time-slices.
From Yilmaz, 2001
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
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EPS – 435
Geophysical Applications
Seismic Reflection Processing
3-D seismic data reveal new features, invisible by using 2D data only. A common attribute to
show seismic discontinuities (faults, channels) is coherency:
From Brown, 1999
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-17
EPS – 435
Geophysical Applications
Seismic Reflection Processing
From Brown, 1999
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-18
EPS – 435
Geophysical Applications
Seismic Reflection Processing
From Brown, 1999
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
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EPS – 435
Geophysical Applications
Seismic Reflection Processing
From Brown, 1999
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-20
EPS – 435
Geophysical Applications
From Brown, 1999
Seismic Reflection Processing
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-21
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Horizon-slice analyses
Offer 3D perspective of topography of
lithologic units and their relation to
structure features
From Yilmaz, 2001
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-22
EPS – 435
Geophysical Applications
Seismic Reflection Processing
4-D seismic data: repeated 3-D over same area.
Used to demonstrate production changes in subsurface (migration of oil, steaminjection, CO2-sequestration).
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-23
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Time-lapse seismic
data from (a) preand (b)
postproduction times.
[MacLeod et al., 1999]
From Yilmaz, 2001
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-24
EPS – 435
Geophysical Applications
Seismic Reflection Processing
Time-lapse 4-D seismic data (time-slices) from 6
individual 3-D surveys taken at various times around a
steam-injection hole [from Lumley, 1995].
The red, near circular feature
corresponds to the spatial extent
of the injected steam.
From Yilmaz, 2001
EPS435 – Fall 2008 Dr. Michael Riedel [email protected]
EPSC-435-12-25

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