Transcript Seismic Methods Geoph 465/565 ERB 2104 Lecture 2 – Sept 6

Lee M. Liberty
Research Professor
Boise State University
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http://www.seismicunix.com/w/Making_Simple_2D_Velocity_Models
makevel  make a velocity function
triseis  generate synthetic seismograms
sufdmod2 (sufdmod1) – finite difference modelling
suea2df - (an)elastic anisotropic 2D finite difference
forward modeling, 4th order in space
suea2df > output \
nxl=1 xl=1 \
zl=0,2,4,6,8,10,12,14,16,18,20,22,100 \
rhol=1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0,1.0 \
vpl=400,400,1700,1720,1740,1760,1850,1700,1800,1750,1800,1750,1900 \
vsl=300,300,350,400,450,500,550,600,650,700,750,800,900 \
dt=.00002\
bc=2,10,10,10 bc_r=0. \
dx=1.0 sx=0 sz=0 xmin=-100 xmax=100 zmax=100 favg=60 \
lt=0.3 \
efile='test_in.bin' snfile='test_snp' \
tsw=1 verbose=1
suwind < ss.su j=10 | suresamp dt=0.001 nt=600 > outfile1.su
Seismic velocities – shear waves
P wave and S wave velocities depend on physical properties of
medium through which they travel:
VP 
4
K m
3

m
Vs 
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K = the bulk modulus, or the reciprocal of
compressibility
m = shear modulus or the second Lamé constant
 = density.
2m
l = k- 3
nE
= ( 1 + n ) ( 1 – 2n)
n  Poisson’s ratio, E = Young’s modulus
Seismic velocities
& polarization
Earthquake epicenter/hypocenter
calculations
B
Elastic DCoefficients
and
Seismic
Velocities
E
F
G
H
I
J
C
Rock Type
Shale (AZ)
Siltstone (CO)
Limestone (PA)
Limestone (AZ)
Quartzite (MT)
Sandstone (WY)
Slate (MA)
Schist (MA)
Schist (CO)
Gneiss (MA)
Marble (MD)
Marble (VT)
Granite (MA)
Granite (MA)
Gabbro (PA)
Diabase (ME)
Basalt (OR)
Andesite (ID)
Tuff (OR)
Density Young's Modulus Poisson's Ratio
r
E
m
2.00
2.00
2.00
2.00
3.00
3.00
3.00
3.00
2.70
2.64
2.87
2.71
2.66
2.65
3.05
2.96
2.74
2.57
1.45
0.120
0.120
1.100
1.100
0.636
0.140
0.487
0.544
0.680
0.255
0.717
0.343
0.416
0.354
0.727
1.020
0.630
0.540
0.014
0.040
0.040
0.156
0.180
0.115
0.060
0.115
0.181
0.200
0.146
0.270
0.141
0.055
0.096
0.162
0.271
0.220
0.180
0.110
Vp
(m/s)
2454
2454
7640
7728
4675
2169
4091
4440
5290
3189
5587
3643
3967
3693
5043
6569
5124
4776
996
Vs
(m/s)
1698
1698
4877
4828
3083
1484
2698
2771
3239
2053
3136
2355
2722
2469
3203
3682
3070
2984
659
Vp/Vs
Vs as %Vp
1.44
1.44
1.57
1.60
1.52
1.46
1.52
1.60
1.63
1.55
1.78
1.55
1.46
1.50
1.57
1.78
1.67
1.60
1.51
69.22%
69.22%
63.84%
62.47%
65.96%
68.42%
65.96%
62.41%
61.24%
64.38%
56.13%
64.65%
68.62%
66.85%
63.51%
56.05%
59.91%
62.47%
66.20%
K
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Increases with
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Decreases due to
◦ mafic mineral content (Nafe-Drake curve)
◦ with pressure (modulus change > density change)
◦ presence of fluid, e.g. porous sand or partial melt
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No S waves in
◦ fluids, e.g. water of
molten rock.
Velocity=0
m
Vs 
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Direct hydrocarbon and lithology indication
S-waves can provide insights into the nature of subsurface
lithologies and pore-saturating fluids, highlighting reservoirs
not previously visible using only P-waves.
Investigations into quantitative saturation and pressure changes
S-waves can help monitor time-lapse variations. During
production or injection, reservoir fluid saturation and pressure
can change dramatically.
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Identifying drilling hazards
Methods such as pore-pressure prediction
can highlight the presence of shallow gas.
Improved illumination: Subsurface imagine
is often improved through wide azimuth
illumination, multicomponent technology
offers a cost effective means of acquiring
such data in an offshore environment.
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Strong P-wave multiples
Combination of hydrophone and the water bottom geophone
can help to reduce water-borne multiple contamination.
Fracture density and orientation
As a result of S-wave anisotropy S-waves usually split into
two waves, a fast and a slow mode. These split S-waves are
sensitive to fractures and can provide information about
fracture density (fracture porosity) and orientation (directions
of preferred permeability).
Gas seepages
P-wave reflections may be disturbed by gas trapped in the
subsurface. S-waves can be used to help clarify the
subsurface image because they are unaffected by pore fluids.
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Improved fault definition and clearer imaging
through gas clouds from the S-wave results
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Gas seepages. A gas chimney degrades the pwave image (left). Shear waves (right), unaffected
by gas, provide a clearer image of the reservoir.
Shear-wave data (right) provide an improved
image of the reservoir sands compared to pwave results (left).
Large datasets - automated analyses
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Azimuthal variations in velocities
Non-hyperbolic moveout in seismic
reflections
Azimuthal variations in amplitudes
(AVOA)
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Shear-wave splitting
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Converted wave amplitude ratios
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Azimuthal variations in Q (attenuation)
(QVOA)
Frequency dependent shear-wave
splitting
Changes in shear-wave
anisotropy in time-lapse
data
 Shear wave velocity
before/after CO2 injection
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Shear wave velocities are one of the most important
parameters for geotechnical estimation of earthquake shaking
response at the ground surface.
Ground shaking is strongly affected by the upper few
hundred meters below the surface.
Soft soil conditions correlate with low near-surface shearwave velocities
Shear-wave velocity gradients as well as seismic impedance
boundaries (e.g., the overburden-bedrock interface) can
result in strong earthquake amplification effects
From Hunter et al., 2010
From Hunter et al., 2010
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Changing wavelengths and an increase in shear wave
amplitudes as seismic energy passes from a high-velocity
medium (e.g., rock) to lower-velocity medium (e.g., soil)
(Shearer and Orcutt, 1987)
Resonance amplification between the free surface (ground)
and underlying bedrock interface effect the fundamental
frequency and higher harmonics.
Focusing or defocusing effects (Bard and Bouchon, 1985)
Basin-edge effects, in which upcoming seismic waves
impinging on the edges of the buried bedrock valley generate
surface waves that interfere constructively within the buried
valley, resulting in anomalously large-amplitude horizontal
and vertical energy (Lomnitz et al., 1999)
From Hunter et al., 2010
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Seismic cone penetrometer (soft sediments)
Shear-wave vertical seismic profiling (where
boreholes are present)
Refraction methods (discrete boundaries)
Multichannel analysis of surface waves
Seismic-reflection profiling
Horizontal-to-vertical spectral analyses of
ambient noise.
From Hunter et al., 2010
Horizontal geophone(s) installed in a
penetrometer pushed into soft soil, surface
shear-wave source
Very good contact with soil, minimal signal to
noise, additional geotechnical data collected
 Usually limited to near surface soft soils only,
refusal in stiff soils
 Used in zones
containing thick soft
(Quaternary) soils
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From Hunter et al., 2010
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Well-locking horizontal geophone(s) in a PVCcased borehole, surface shear-wave source
Advantage: Good contact with soil depending on
quality of casing grout, repeat measurements in
preserved BH, ancillary geophysical logs,
geologic and geotechnical samples
Disadvantage: Cost of drilling and casing, poorquality grouting can result in tube-wave
interference
Application area: All zones containing soft or
stiff soils and rock
From Hunter et al., 2010
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Surface array of horizontal geophones,
surface shear-wave source
Advantage: Refraction (interval) velocities,
refection average velocities, for significant
impedance contrasts
Disadvantage: Refraction model requires VS
to increase with depth, “hidden” layers
missed
Application area: Most soils and rock where
significant seismic-impedance boundaries
occur
From Hunter et al., 2010
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Surface array of geophones, inversion of
dispersion curves
Advantage:
Economical, velocity inversions mapped
Disadvantage:
Depth limited
Application area: All zones containing soft or
stiff soils and rock
From Hunter et al., 2010
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Moving array of horizontal geophones,
surface shear-wave source
Advantage: Detailed subsurface shear-wave
stratigraphic depths and velocities, velocity
inversions mapped
Disadvantage: Operational costs are high,
significant impedance contrasts are required
Application area: Best results in soft soils
overlying bedrock
From Hunter et al., 2010
From Hunter et al., 2010
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Surface-mounted three-component lowfrequency seismometers
Advantage: Simple acquisition procedure,
minimal equipment
Disadvantage: Data quality strongly affected
by local noise
Application area: All zones where subsurface
seismic impedance boundaries occur; best
results in soft soils overlying bedrock
From Hunter et al., 2010