opening the black box of anisotropic seismic processing · zero offset seismic experiment surface...
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Opening the Black Box of Anisotropic Seismic Processing
Morgan Brown
Tenax Geoscience, LLC
PBGS Annual Exploration Meeting
May 7, 2015
Seismic Velocity Anisotropy
• Seismic Velocity Anisotropy ”Anisotropy”
• Seismic waves travel at different speeds through a rock volume, depending on direction
• This presentation: P-waves only
Formative Anisotropy Experience
Gonna
be a
long 7
years!
“Jon, which student will work on anisotropy?”…
“What about Morgan?”…
…“I don’t know, Tariq, anisotropy is hard.”
…”No, it has to be one of the smart ones.”
Tariq Alkhalifah
Jon Claerbout
The Need For This Talk
Even after you master the mathematics of
anisotropy, can you answer these questions?
• What rocks have it? How much?
• How does it affect real data?
• Has processing handled it correctly?
• Will it change an interpretation?
http://en.wikipedia.org/wiki/Seismic_anisotropy
Anisotropy Pitfall: Lateral Shifts
Surface Location
Imaged reflector
with Anisotropic
Data
Imaged reflector
with Isotropic Data
Surface Location
Tim
e
11o dip error
Anisotropy Pitfall: Well/PSDM Mistie
Z
X
Pie
rre S
ha
le
Top Niobara from seismic
Top Niobrara from well tops
Anisotropy Pitfall: Hockey Sticks
Z
Angle
(deg)
These events
should be flat
But this event is
“unflattenable”,
due to anisotropy
Hockey sticks impair:
• Image quality
• Interpretation
• Velocity analysis
• Fracture analysis
• AVO analysis
Anisotropy Pitfall: AVO Errors
14% difference
in offset
50o
Surface LocationSurface Location
Depth
Ray traced in
anisotropic earth
Ray traced in
isotropic earth
If you convert offset to
angle for AVO analysis
without correctly handling
anisotropy, you’ll screw up
the AVO gradient.
Anisotropy Pitfall: Natural Fractures?
E
N
Azimuthal Anisotropy is measured to infer potential
natural fractures. Maps are produced and
interpreted without full appreciation of the meaning
of the measurements and their ambiguity.
Azimuthal Anisotropy isn’t simple!
Anisotropy Causes
1) http://www.fredonia.edu/department/geosciences/lash/fractures/capseal/capseal.htm
2) http://www.earth.ox.ac.uk/~oesis/micro/
3) http://www.rupestreweb.info/tacitas.html
Rock FabricShale Sandstone
Slow
Fast
~0-5% anisotropy~10-20% anisotropy
Weak layer
Thin Layering of isotropic materials
Weak layer
The aggregate is “squishy” when squeezed
vertically, but stiff when squeezed horizontally.
Thus, vertical velocity < horizontal velocity
Slow
FastWood
Wood
Sponge
Sponge1) 2)
Open fracturesx
y
Similarly, when rocks have a dominant open fracture set,
they are stiffer (faster velocity) in the fracture direction
3)
Vertical or Bedding
Plane-oriented Anisotropy
Horizontal or Azimuthal Anisotropy
Isotropic Wavefront
X
Z
Simple imaging experiment highlights basic anisotropy concepts
Fire a source in an isotropic, constant velocity medium.
The wavefront is semicircular
Elliptical VTI Wavefront
Slo
w
Fast
Elliptical Vertical Transverse Isotropy (VTI)
Summary: The simplest type of vertical anisotropy
Effect: Wavefront is stretched horizontally.
Math: One parameter, Thomsen d, describes the stretch.
Causes
Anelliptic VTI Wavefront
Slo
w
Fast
Anelliptic Vertical Transverse Isotropy (VTI)
Summary: The most common type of vertical anisotropy
Math: Two parameters, d and h, describe the stretch.
Causes
L=1
Anelliptic TTI Wavefront
Anelliptic Tilted Transverse Isotropy (TTI)
Summary: Tilted VTI (here: 30o)
Real rocks: Parameter ambiguity; Tilt/azimuth may not follow dip!
Math: 4 parameters, d, h, tilt, azimuth, describe the stretch
Causes
Anelliptic HTI Wavefront
X
Y
Anelliptic Horizontal Transverse Isotropy (HTI)
Summary: Horizontal VTI, aka “Azimuthal anisotropy”
Real rocks: 5-10x weaker than VTI; fractures must be vertical
Cause
Zero Offset Seismic ExperimentD
epth
Surface Location
Seismic energy is sent out from the surface to the buried targets. Some of the energy is reflected and
recorded by the geophone at the surface.
Zero Offset Seismic Experiment
Surface Location
Tim
eThe recorded signal is called a “seismic trace”. Notice how the
apparent location of the reflection on the zero offset seismic
panel is displaced from the actual reflection point.
Depth
Zero Offset Seismic Experiment
Surface Location
Tim
e
By taking zero offset seismic experiments across the spread, we record a multitude of seismic reflections.
The events are “stacked” on top of each other, inconsistent with the real “geology”. Migration is an
automated procedure to correctly position events which are mis-positioned due to geologic dip.
Zero Offset Migration
Surface Location
The reflection happened at an unknown location along the
semicircle. “Spray” the energy to all possible locations.
Tim
e
Zero Offset Migration
Surface Location
Tim
e
Repeat the process for many adjacent traces. I’ve made the lines transparent. When multiple lines
overlay one another, the color saturation increases. A geophysicist would interpret a reflection as
occurring at this location.
Zero Offset Migration
Surface Location
Tim
e
Migration accurately recovers the known target geology. Ah, the magic!
Zero Offset Migration (elliptical VTI data)
Surface Location
Tim
e
4o dip error In the following slides, we repeat the “migration”
exercise, using data modeled in an anisotropic
medium.
For elliptical VTI (d=0.05), we note time shifts and
mild dip changes, which worsen as dip increases
Zero Offset Migration (anelliptic VTI data)
Surface Location
Tim
e
11o dip error
3o dip error
1o dip error
Switching from elliptical VTI to anelliptic VTI
(d=0.05, h=0.1), the time shifts and dip errors
worsen.
Zero Offset Migration (TTI data #1)
Surface Location
Tim
e
19o dip error
8o dip error
3o dip error
When the medium is TTI (d=0.05, h=0.1, tilt=-30o),
the time shifts and dip errors are severe. Large
lateral reflector shifts are apparent, and even the
flat reflector is mis-positioned!
This could change an interpretation…
Zero Offset Migration (TTI data #2)
Surface Location
Tim
e
2o dip error When the tilt is rotated (d=0.05, h=0.1, tilt=30o), the
dip errors and shifts are less on the steep dips than
even for elliptical VTI! But the flat reflector is still
mispositioned
This could change an interpretation…
Isotropic versus TTI RTM
De
pth
X TTI RTMIsotropic RTM
Try as I could to beat TTI, it simply made the imaging better in
every test. This geology is tailor-made for a TTI test – how do
we build TTI models in “real” examples?
TTI – it seems to work, but what’s the workflow?
2D line shot near Rocky Flats, Colorado. Trivial h=0.05, d=0.0, tilt=41o (where beds dip, 0o elsewhere)
Anisotropy Pitfall: Well/Seismic Mistie
• VTI Vfocusing > Vvertical
• PSDM events positioned too deep
• Depth mistie controlled almost completely by d
Z
X
Pie
rre S
ha
le
Top Niobara from seismic
Top Niobrara from well tops
Anisotropy Pitfall: False Anisotropy
Time processor 1 Time processor 2
Refraction statics has a long wavelength and “datum” component. If not handled
consistently, the data may suffer a large “bulk shift”.
Here, two processed versions of the same data have a 40-60 ms time shift
Static shifts masquerade as large, shallow Thomsen d!
How to Cheat with Thomsen d
Well
Mis
tie
(%)
-10
12
3D
ep
th
X
d volume
Z
X
After isotropic PSDM with a sloppy velocity
How to Cheat with Thomsen d
Well
Mis
tie
(%)
-10
12
3D
ep
th
X
d volume
Z
X
After isotropic PSDM with a sensible velocity
Pie
rre S
hale
Thomsen d is a “magic” PSDM well tie fudge factor – it shifts PSDM events vertically. An
image may appear to tie wells perfectly, but clients should ask how the image got there.
With anisotropy, apply Occam’s razor – it is likely “smooth”, so misties should be spatially
consistent. Scott MacKay nicely describes this issue.
If Thomsen d isn’t “geologic”, then the dips between the wells are immediately suspect!
Always QC Thomsen d! Peer into the black box!
PSDM Velocity Update
X
Z
PSDM Gathers with different velocity
Offset
Z Z Z
Too Fast Correct Too Slow
Angle Gathers: PSTM Isotropic Velocity
X
Z
Y Angle
(deg)
Angle Gathers: Updated Isotropic Velocity
X
Z
Y Angle
(deg)
These (isotropic) angle gathers look pretty flat,
but let’s zoom in to the top Niobrara reflection…
Anisotropy Pitfall: Hockey Sticks
• Events should be flat with angle
• Two velocities apparently needed to flatten this gather
• The VTI h parameter controls these two velocities in a systematic way
Z
Angle
(deg)
V(q)
dominated
by d
V(q) affected
by d and e
Anelliptic VTI Velocity
qhqd
qdhqd
qdeqd
qeqqd
qeqqdq
422
0
422
0
422
0
4222
0
4222
0
2
sinsin1
sin21sin1
sinsin1
sinsin1sin1
sincossin1)(
P
P
P
P
P
V
V
V
V
VV
Thomsen parameters
In real rocks, h > 0
V(q) dominated
by d
V(q) affected
by d and e
V(q
) (f
t/se
c)
Incidence Angle, q (deg)
Isotropic
d=0.05,
h=0.1
Anellipticity parameter, hed/12d
Refraction Velocities for PSDM?
300 ft
8,700 ft
Here, we show a long offset head wave arrival.
d210 PNMO VV e 10PREFR VV
Velocities derived from reflections and refractions are contaminated by
anisotropy…in an inconsistent way!
Near-surface models must be corrected for anisotropy before being
merged with PSDM velocities
A Robust VTI PSDM Workflow
• Isotropic PSDM velocity updates (0-30o only)
• Build Thomsen d model from well/seismic misties• Ensure statistical “goodness” of misties (MacKay)
• Estimate h from large angles (30-50o)• Either multi-parameter analysis or brute-force scans
• Fix d and h, run VTI PSDM velocity updates (0-50o)
• Final mistie correction
A Right Way to do VTI PSDM
Z
Angle (deg)
Isotropic PSDM angle
gather, muted to 30 deg
incidence angle. Well top
shown in purple
Isotropic PSDM angle
gather, no mute applied.
Hockey sticks and vertical
misties readily apparent
A competent, for-purpose
VTI workflow can flatten
hockey sticks and correct
for well/seismic misties
0 15 30 45
A Wrong Way to do VTI PSDM
• The best approach is to first estimate the “near angle” isotropic velocity, measure dfrom misties, h from hockey sticks, and finally perform VTI PSDMs
• If we use all the angles in the isotropic updates, we obtain a d which is too large. We can estimate an h which helps flatten the gathers, but we are unlikely to ever get gathers as flat as in the first approach
Well top Well toph
d
Angle gather from
isotropic velocity obtained
by flattening 0-30o.
Thomsen d causes
seismic to mistie well top.
h causes hockey sticking.
Angle gather from
isotropic velocity obtained
by flattening 0-50o.
This imaging velocity is
faster than the 0-30o
velocity. It reduces the
apparent h and increases
the apparent d
Good Bad
VTI versus HTI
• The VTI effect causes:
• Depth misties between well tops and seismic
• “Hockey sticks” on migrated gathers
• Idealized HTI causes “sinusoid” vs azimuth angle
• From the best-fitting sinusoid, we can infer:
• Fracture/stress orientation
• Fracture/stress magnitude
Azimuth
Angle (deg)
Fast Slow
Incidence
Angle (deg)
NIOB
VTI HTI
OVT or Azimuth/Incidence Angle GathersD
ep
th (
ft)
East
Incidence
Angle
(deg)
Azimuth
Angle
(deg)
North
q=10o
f=
90
o
f=
-90
o
f=0o
q=10o
q=10o
q=10o
q=50o
HTI Analysis Workflow
• Stack WEM or RTM incidence/azimuth angle gathers from ~25-50 deg incidence angle to compute azimuth gathers
• For each event on each angle gather, compute best fitting sinusoid (in azimuth)
• For each sinusoid, tabulate “amplitude” of sinusoid (fracture intensity), “phase” of sinusoid (fracture orientation), and quality of fit
• Generate 3D volumes for fracture intensity, fracture orientation, and quality
• Pick horizon(s) of interest
• Flatten fracture volumes to horizon(s)
• Map fracture attributes on horizon slices
amplitude
phase
What Does HTI Really Mean?
• Perform “raytracing” in a simple 3-layer HTI model
• Background velocity = 12K fps
• Vary HTI d and orientation in each layer, analyze azimuthal traveltime variations at base of layer 3
• Shoot 45o ray up from base of layer 3
Layer 1
Layer 2
Layer 3
5000 ft
1000 ft
1000 ftAnalysis
Depth
What Does HTI Really Mean?
• ~1000 ft thick fractured layer produces measurable HTI
• Is d=0.05 too strong?
d=0.0f=N/A
d=0.0; f=N/A
d=0.05; f=45o
Layer 3
What Does HTI Really Mean?
• Detectability threshold test
• d=0.005 over the entire section is barely detectable
• Yet, typical real data measurements indicate this magnitude of d is common
d=0.005f=45o
d=0.005; f=45o
d=0.005; f=45o
Layer 1/2/3
What Does HTI Really Mean?
• Opposing orientations in layers 2/3
• HTI (Nearly) cancels out
• But notice a nonzero deviation from NMO time – will look like a pull-up!
d=0.0f=N/A
d=0.05; f=135o
d=0.05; f=45o
Layer 3
Layer 2
What Does HTI Really Mean?
• “Fracture unconformity”
• Net HTI effect relatively weak
• Apparent orientation differs from either interval orientation
d=0.01f=90o
d=0.03; f=45o
d=0.03; f=45o
Layer 2/3 Layer 1
Apparent f=65o
HTI “Fracture” Map
E
N~0.1%
~0.3%
FMI
This is a typical “unconventional resources” HTI response (DJ
Basin, Niobrara) – weak apparent fracture magnitude, in spite of
observed fractures in FMI and azimuthal anisotropy in sonic
scanners.
From the above slides, it is clear that layer stripped HTI solutions
are key. However, the odds are small that “cancelling” interval HTI
responses consistently produce the observed (lack of response).
More likely, the in situ P-wave HTI effect is small in most quiet
basins. We saw that a fairly strong 0.5%, top-to-bottom, HTI
effect barely produces an observable sinusoidal response.
HTI technology seems more applicable in more tectonically
complex geologies. Unfortunately, a dipping HTI effect becomes a
TTI effect, making parameter estimation ambiguous. Triclinic?!?
HTI – Caveat Emptor!
Conclusions
• Goals for the Audience
• Basic concepts
• Real data effects
• Contract with confidence (e.g., VTI PSDM)
• Anisotropy Pitfalls
• Positioning errors
• False anisotropy / d cheating
• AVO errors
• HTI is tricky at best
Acknowledgements
• Fidelity E&P
• Dave List, Chris Lang, Patrick Rutty
• Wave Imaging Technology/GeoCenter
• Randy Ray, Ned Sterne, Jim Applegate