Pore pressure near the plate boundary décollement
in the Nankai Trough off the Muroto peninsula:
Insight from seismic velocity, logging data, and laboratory data.
Takeshi Tsuji (ORI, Univ. Tokyo)
Patrizia Costa Pisani (Univ. Hawaii)
Gregory Moore (CDEX, JAMSTEC)
LDEO Logging Symposium, October 6th, 2006
Outline
• Background and Purpose
• Survey area
• Dataset: seismic data & borehole data
• Method: Pore pressure prediction from seismic data
• Interpretation
• Conclusions
LDEO Logging Symposium, October 6th, 2006
Background
• Since the pore pressure along the décollement influences the
frictional characteristics, it plays a key role in the earthquake
mechanism and deformation features of the accretionary prism.
• To avoid a drilling disaster, the pore pressure should be estimated.
estimated
Purpose
• We develop a theoretical based method to estimate pore pressure
from seismic velocities, laboratory data, and logging data, although
many studies of pore pressure prediction rely on empirical relations
(Eberhart-Phillips et al., 1989).
– We introduce an application of Differential Effective Medium
(DEM) theory and crack aspect ratio distribution.
LDEO Logging Symposium, October 6th, 2006
Why we use theoretical method?
• If the sedimentary sequence is stratified and many boreholes penetrate it,
we can estimate pore pressure by comparing abnormal compaction trend
with normal compaction trend (Eaton, 1972).
• However, it is difficult to obtain normal compaction trend in our study
area.
– A few boreholes penetrate the accretionary prism.
– The accretionary prism is gradually consolidated with accreted
process.
We predict pore pressure based on effective medium model by
integrating laboratory-derived P- and S-wave velocities under
confining pressure.
LDEO Logging Symposium, October 6th, 2006
Concept of pore-pressure estimation from seismic data
Terzaghi’s relation:
Pe : Effective pressure
Pc : Confining pressure
Pf : Pore fluid pressure
p f = pc ! pe
Confining pressure Pc can be obtained by
integrating the density:
z
pc ( z ) = g ! " ( z ' )dz '
0
Overpressure
Effective pressure Pe controls the deformation
of the solid. Therefore, velocity of rocks
generally depend on effective pressure.
Relation between confining-pressure,
effective-pressure, and pore-pressure.
When we obtain effective pressure from seismic velocity, we can estimate
pore pressure by subtracting the effective pressure from confining pressure.
LDEO Logging Symposium, October 6th, 2006
Survey area: Nankai accretionary prism off Muroto
Seismic data
3-D data （80 km×8 km×12 s）
was obtained by the R/V
Ewing in 1999.
Acquisition system
Streamer cable
Length : 6 km
Channel number : 240
Channel interval : 25 m
Airgun system
Bathymetric map of the Nankai Trough. The red rectangle shows
the 3-D seismic reflection survey area. The location of inline 284,
used for the pore pressure prediction, is shown as a black line.
Tuned airgun
Total volume
: 70 ℓ (4276 inch3)
LDEO Logging Symposium, October 6th, 2006
Survey area: Nankai accretionary prism off Muroto
Two-Way-Travel time [s]
2
１０ km
Study area
4
6
8
Philippine Sea Plate
Philippine Sea Plate
Moore et al., 2001
LDEO Logging Symposium, October 6th, 2006
Dataset: Seismic profile and the interval velocity
Seismic profile
Seismic profile across the
Nankai Trough. Vertical axis
shows the depth (4 ~ 7 km).
Seismic interval velocity
Seismic interval velocity obtained
by 3-D tomographic velocity
inversion. To delineate pressure
distribution in the subduction
direction, we use 2-D velocity
model which crosses ODP Sites
808 and 1174.
LDEO Logging Symposium, October 6th, 2006
Previous research: seismic attributes analysis via neural network
• We estimated the characteristics along the décollement qualitatively
from seismic attributes analysis via neural network.
Tsuji et al., 2005
Classification result along the
décollement via SOM, and
geometry of seafloor and top of the
oceanic crust. The regions with the
same color belong to the same class
and probably have similar
properties. The white dashed line
shows the classification boundary.
Properties along the décollement
may change at the classification
boundary.
By using boreholes data, we concluded that classification result indicates that
the plate boundary slip initiates mainly from the classification
boundary.
LDEO Logging Symposium, October 6th, 2006
Dataset: Ocean Drilling Program (ODP) data
• Site 1173 is a reference site (at hydrostatic conditions).
• Site 1174 is located in the proto-thrust zone.
• Site 808 is located in the frontal-thrust zone.
Grain1173
elastic
• Logging data were acquired at Sites
andmoduli
808. (Voigt average)
f K +f
+f
K
+f
K
• Discrete samples were obtainedKat=Sites
1173, K1174,
and 808.
g
clay
clay
quartz
quartz
plagioclase
plagioclase
calcite
calcite
µ g = f clay µ clay + f quartz µ quartz + f plagioclase µ plagioclase + f calcite µ calcite
Seismic profile and locations of ODP sites.
The mineral components of XRD analysis at Site 1174
(Moore et al., 2001)
LDEO Logging Symposium, October 6th, 2006
Dataset: Pore pressure at ODP sites
To predict pore pressure from
seismic velocity, we need to know
pore pressure at the boreholes.
•
By using Rubey and Hubbert relation
obtained at Site 1173 (hydrostatic),
pore pressures at Sites 1174 and 808
were estimated (Straub, 2002).
" = 0.66e !0.115 Pe
•
Indirect pore pressure estimation
derived from reconsolidation tests
(Karig, 1993).
– An overpressure increases with
depth from Upper Shikoku Facies.
– A rapid increase in overpressure is
observed at the décollement.
LDEO Logging Symposium, October 6th, 2006
Method: Processing flow of pore pressure
prediction from seismic velocity
Step 1
• From the aspect ratio distribution calibrated by
laboratory and logging data, we calculate
theoretical velocity parameterized by effective
pressure via DEM theory.
Step 2
• The theoretical velocity is fitted to the seismic
interval velocity by using effective pressure as a
fitting parameter, and the pressure distribution
within the accretionary prism is estimated.
Processing flow for the pore pressure
prediction. Light and dark gray boxes
represent the input data and iteratively
obtained results, respectively.
JFES 2006.10.04
Rock Physics theory
•
•
We use Differential Effective Medium (DEM) theory (Berryman, 1992)
to calculate theoretical velocity.
We introduce crack aspect ratio spectrum to change velocities with
effective pressure.
DEM theory models two-phase composites by incrementally adding
Toksoz et al. (1976) gave the expression for the crack thinning and
inclusions (oblate spheroidal pore) to a background matrix phase, and then
closure with
pressure:
recomputing
theeffective
new effective
background material at each increment.
dc("*) ! Pe
dK ( =
y ) * [E*1 !1 E 2 E 3 ( E 3 + E 4 )]
(1 ! y )c(" )
=
K(AK '! K ) Tiijj ( y )
dy
3
dµ * ( y )
1
1
(1 ! y )
= ( µ '! µ * ) (Tijij ( y ) ! Tiijj ( y ))
closure ofdycracks, the5 theoretical
velocity
3
By the
effective pressure.
y = ! c(" i )
increases with
i
We assume the pore shape
is oblate sphere.
Firstly we add large aspect ratio pore and then add smaller aspect ratio
cracks, since the velocities are much affected by the thinner cracks.
LDEO Logging Symposium, October 6th, 2006
Estimation of crack aspect ratio distribution
We estimated the aspect ratio distribution via Kuster-Toksoz theory and
inversion technique (Cheng et al., 1979) from the relation between velocities
and effective pressures of the Nobeoka outcrop samples.
Thinner crack
Laboratory-derived velocity – pressure relationship of
the Nobeoka outcrop samples (Tsuji et al., 2006).
Round pore
Normalized aspect ratio distributions of
Nobeoka outcrop samples.
LDEO Logging Symposium, October 6th, 2006
Modeling of aspect ratio distribution
•Below
the estimated
aspect
-1,, the
ratioa distribution
seems
to have
a linear
relation
theaspect
aspect
ratio1~10
10-1.5
changes
with
gradient
S.
For Because
large
ratio
we concentration
use
constant linearly
gradient
1/2
to construct
the
aspect
andratio
its concentration,
we and
modeled
thatthat
byinagradient
linear relation
S is between
the quantity
thatratio
determines
the
features
isbecause
obtained
the
relationship
between
aspect
andpore
its concentration
fit to
and 68 aspect
calibration
process.
the average
aspectratio.
ratio distribution.
•
The aspect ratio is divided into two parts:
(1) Round pore which does not close with
effective pressure.
c 0 (# i ) = 1 2 " # i
(10-1 ! # i ! 1)
(2) Crack which closes with increasing of
effective pressure.
c0 (# i ) = S " # i
(10-4.8 ! # i ! 10-1.5 )
Normalized aspect ratio distributions of
Nobeoka outcrop samples.
LDEO Logging Symposium, October 6th, 2006
Estimation of S at boreholes
• At the boreholes (Site 808 and 1173), we determined S by fitting the
theoretical velocity at effective pressure to the sonic velocity.
•
If we obtain S within the accretionary prism
by extrapolating and interpolating S of the
boreholes, we can calculate the theoretical
velocity from grain elastic moduli, density,
and aspect ratio distribution via DEM theory.
•
Then, we can estimate effective pressure by
iteratively fitting the theoretical velocity to
the seismic interval velocity.
•
We constrain S by porosity.
• in aspect
Crack
feature
S may
depend on clay content (e.g., Xu and White, 1995).
Change
ratio
of 68 cracks
for depth
•direction
However,
itCracks
is difficult
to with
extrapolate and interpolate S by using a few
Site 808.
are
thinning
• atClay
content
(gamma-ray
log & laboratory samples) has a linear
effective
pressure and
finally
closed.
boreholes
near
the
deformation front.
relationship with porosity in our study area (Steurer and Underwood, 2003).
LDEO Logging Symposium, October 6th, 2006
Relationship between crack feature S and porosity
•
We constructed the relationship between S and porosity at the boreholes.
•
Furthermore, we estimated S by fitting the pressure-dependent theoretical
velocities to the laboratory-derived velocities.
S = ~0.9 (φ=0~0.06)
Example of velocity – pressure relationship of the
Nobeoka outcrop samples.
S = 1~1.2 (φ=0.17~0.43)
Relation between S and porosity. Open rectangle,
close dots, and open triangle represent the relations
obtained by the Nobeoka outcrop samples, seafloor
outcrop samples, and borehole data, respectively.
Example of velocity – pressure relationship of the
seafloor outcrop samples.
JFES 04/10/2006
Estimation of effective pressure
•
Calculate the theoretical velocity via DEM theory from grain elastic
moduli, density, and aspect ratio distribution S.
•
Estimate effective pressure by iteratively fitting the theoretical velocity
to the seismic interval velocity.
LDEO Logging Symposium, October 6th, 2006
Pore pressure
Overpressure
LDEO Logging Symposium, October 6th, 2006
Interpretation
Abnormal high pore
pressure occurs within
the underthrust
sequence.
•
An increase in vertical
load due to the thickening
prism and low permeable
cap along the décollement
may raise pore pressure.
•
This overpressure can
account for the weak
coupling along the
décollement and lowtapered prism geometry
off Muroto peninsula.
LDEO Logging Symposium, October 6th, 2006
Interpretation
An overpressure within
the accreted sequence
initiates from the
deformation front.
• An increase in horizontal
compaction within the
accreted sequence and
low permeable marine
sediment raise the porepressure.
LDEO Logging Symposium, October 6th, 2006
Conclusions
• DEM theory and aspect ratio spectrum were used to delineate
pore pressure in the Nankai accretionary prism off the Muroto
peninsula.
• The overpressure occurs within the subducting sedimentary
sequence.
– The increase in overburden load and low permeable cap
along the décollement may increase pore-pressure of the
underthrust sequence.
• The overpressure within the accreted sequence initiates from the
deformation front.
– The increase in horizontal compaction within the accreted
sequence and low permeable marine sediment raise pore
pressure.
LDEO Logging Symposium, October 6th, 2006
Appendix: Theoretical relation between velocity and porosity
• If we assume constant grain
elastic moduli, we can obtain
theoretical relationship between
velocity and porosity
parameterized by effective
pressure.
• The relation is coincided well
with the relation between
seismic velocity and porosity.
Theoretical relationship between velocity and porosity
and the relations between seismic interval velocity and
the logging porosity.
LDEO Logging Symposium, October 6th, 2006