Post on 25-Jun-2020
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3D- POROSITY MAPPING IN THE NANKAI TROUGH SEISMOGENIC ZONE
Velocity –Porosity Relationship in hanging wall of Nankai Trough
Gurkirat Singh Nahar,
Indian Institute of Technology, Roorkee
Masataka Kinoshita
University of Tokyo
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Abstract
We used the Seismic velocity data to compute the porosity in the seismogenic zone of Nankai Trough in the south
west of Japan. We mapped the porosity by integrating seismic velocity information, core sample and the logging
while drilling (LWD) data from four site holes in the area and finally deriving out a relation between porosity and
velocity by fitting the data to Wylie’s Time average equation. Predicted high porosity near the mega splay fault
in the older accretionary prism indicate an overpressure in the anomalous zone .We measured the overpressure at
high porosity region along a particular cross-section to get an insight about the properties of the region.
Keywords: Mega-Splay Fault, Pore pressure, Overpressure, Nankai Trough, Wylie’s Time-average Equation
1. Introduction
Most of the world’s largest Earthquakes and Tsunamis occur along the global belt of the subduction zones.
At the subduction zones, Megasplay faults rise from the plate boundary megathrust propagating upwards towards
the seaward side and finally breaks off into various splay faults which intersect the sea floor. The energy and
displacement is very efficiently transferred to the sea floor which has the potential to cause huge tsunamis due to
large water column depth in the seaward side.
Nankai trough is a narrow basin formed due the subduction of the Philippine Sea plate below the Over lying
Eurasian plate at a rate of 4-6.5 cm/year (Fig.1; Seno et al.,1993;Miyazaki and Heki,2001).Thick package of
underthrust sediments have been mapped under accretionary prism and Kumano sediments by 3D seismic studies
[Moore et al., 2007,2009] . A low velocity anomaly in this underthrust sediments indicate the possibility of some
external fluid injection or some overpressure in the underthrust rock sediments, which resist the increase in the
velocity contrary to the normal trend. It is well established that the overpressure can tremendously lead to
degradation in the fault strength due to increased pore pressure [e.g., Hubbert and Rubey, 1959; Tobin and Saffer,
2009; Saffer and Tobin, 2011]. So to compute the overpressure in the underthrust sediments, porosity has to be
estimated in the region.
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Figure 1(c)
Fig1. (a)Seafloor topography in the Nankai Trough off Kumano. (b)Enlarged seafloor topography around the survey area
off Kumano. White and yellow circles on the solid black line indicate OBS positions. The red star and circles indicate the
locations of seafloor outcrop for rock sampling and boreholes (sites used for present study: C0001, C0002, C0004,
C0006) (c) Seismic reflection profile extracted from 3Dseismic volume.
2. Data and In situ Measurements
Porosity mapping is very crucial for Overpressure estimation. Various physical and elastic property data
has been obtained from core samples and bore well logging conducted by IODP expeditions. We had the
Porosity and sonic velocity data from the core as well as log data, but the core data could not be obtained
for very greater depths. Thus log Porosity (Resistivity derived) and sonic compressional velocity data
were used to formulate a site specific porosity-velocity relationship by fitting the data to Wylie’s Time
Average Equation.
2.1 Velocity Measurements
Shipboard laboratory measurements were made on Core samples obtained from Site C0001, C0002,
C0004, and C0006 in IODP Expedition 314.But due to lack of data at greater depths the log data [Fig.8]
from above sites was plotted with depth ranging from about 400 mbsf for site C0004 to max of 3000 mbsf
for site C0002 .
2.2 Porosity measurements
Porosity from obtained core samples was measured in the laboratory. Porosity from Site C0001, C0002,
C0004, and C0006 was obtained Neutron Porosity logs. But due to highly scattered data, we used the
Porosity derived from Resistivity logs using the Archie’s formula (eq.1) taking tortuosity factor ‘a’=1,
cementation exponent ‘m’=2.4, and assuming saturation of fluid (Sw) in the pores as unity.
𝑅𝑡 = 𝑎 φ−𝑚Sw−𝑛𝑅𝑤 (1)
It should be noted that the resistivity-derived porosity estimate is not intended to provide the
“True” porosity but remains a very useful estimate, especially below 500 m where no other data exist.
Resistivity of the formation water (eq.2), in this case dominantly seawater, changes with
Temperature .Shipley, Ogawa, Blum, et al. (1995) defined the relationship between the fluid
Resistivity (Rw) and borehole temperature ‘T’ in degree Celsius as:
Rw = 1/ (2.8 + 0.1T), (2)
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3. Velocity-Porosity Relation and parameter fitting
3.1 Empirical Velocity-Porosity relation
The global empirical relation between Porosity (φ) and Velocity (Vp) which are inversely related to each
other are as follows (Wylie, 1959)
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𝑉=
φ
𝑉𝑓+
1−φ
𝑉𝑚 (3)
With Vf as sonic velocity in fluid and Vm as sonic velocity in grain matrix. The cross plots between
Velocity and Resistivity-derived Porosity from the log data were fitted for Wylie’s equation (eq.3) for
All the four sites. Core and log data derived Porosity-velocity cross plot (Fig.2) were made for Site
C0001 to check the consistency of the data.
Core obtained data showing higher
Porosities due to right-shifted curve
indicate the error due to overestimated
porosity from the smectite clay
samples which release the bound water
on sample heating during porosity
measurement and thus give higher
values of porosity .Even core samples
show vertically shifted curve which is
result of highly consolidated core
samples, thus resulting in higher
velocity. Anyhow the trend of both
curves is similar, thus log data was
incorporated for better in-situ
measurements even at greater depths.
3.2 Site Specific Porosity-Velocity Relationship
In order to map the Porosity in the region under study, Porosity-velocity cross plots from all the four sites i.e.
Site C0001, C0002, C0004, and C0006 were compiled all together to get a fitting curve (Fig3.)according to
(eq.3) .The Nonlinear Bisquare fitted curve (Fig 3.) gives the relationship between velocity and porosity (eq.4)
which could be used to convert the velocity obtained from seismic reflection survey to porosity in the 3
dimensional volume.
φ = 1.209
𝑉 − 0.1377 (4)
Figure 2. Velocity porosity cross plots for log (blue) and core (orange) obtained data
from Site C0001
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Figure 3.Velocity-Porosity cross plot for compiled data from sites (C0001,C0002,C0004,C0006) and nonlinear Bisquare fitted curve according to
wylie's eq.(3) with an RMS of 0.1299
4. Porosity Mapping in the Seismogenic Zone and Results
Obtained relationship between porosity and velocity was used to map the 3D porosity in volume in the
Seismogenic zone of Nankai Tough .The seismic Reflection survey provided the 3D Velocity data which
was used to have an insight into the trend of porosity. The Porosity (𝜑 ) with depth (z) decreases
exponentially (Athy, 1930) (eq.5) with minimum porosity of about 10-12 % at subduction plate boundary.
𝜑 = φo ∗ 𝑒−𝑍/𝑍𝑜 (5)
The low velocity underthrust sediments which are primarily the subducted sediments originating from
Shikoku basin in the south of Nankai Trough axis show higher porosity ranging 17-20% at depths of
greater than 6000mbsf(Fig.4). The maximum porosity anomaly mapped at about 19.85% at depths of
7450-8450mbsf is found only in the western side of area under study. The slightly lower porosity values
at 17-19% extends down (towards north) to depth of 9750mbsf (Fig 5.) and were mapped in western as
well as eastern zone.
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N
Figure 4.Porosity distribution in an inline section superimposed on seismic section with three cross line intersects depicting the trend of porosity
anomaly (17-20% in shades of green) in the underthrust sediments for depths greater than 6000 mbsf .
Fig .5a Fig. 5b
Figure 5 a) high porosity anomaly of 19.85% in the western block of underthrust sediments about 1000m in vertical dimensions. b) Porosity trend
ranging from approximately 17-19 % porosity extending deeper in the north direction from 7300 m to the depth of 9750 m
5. Overpressure estimation
The underthrust sediments which are hypothesized as clay sediments or Shikoku basin sediments have
retained higher porosities even at such greater depths of more than 6000 meters below sea floor (mbsf) and
could be the critical sites for rupture to begin and propagate into weaker zones. In the very initial phase of
Porosity
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study, we calculated overpressure at five locations along a particular cross-section consisting of all the porosity
anomaly ranges (Fig.6).
The confining pressure ‘PL’ at depths of maximum porosity anomalies in a cross section (Fig.6) has been
calculated by integrating the bulk density ‘𝜌𝑏’ (eq. 6) over the depth ‘z’ (eq. 7) where PL* in (eq. 8)
Represents the confining pressure or overburden after removing the hydrostatic pressure and it incorporates
the porosity terms (Athy, 1930) (eq.5).
PL=∫ (𝜌𝑏) ∗ 𝑔 ∗ 𝑑𝑧 + (𝜌𝑤) ∗ 𝑔 ∗ 𝑙𝑧
0 (7)
PL*=PL-Phyd= (𝜌𝑔 − 𝜌𝑤)*g*[z+φ𝑜* 𝑍𝑜 *(𝑒−𝑧/𝑍𝑜-1)] (8)
Effective stress ‘σ’ (Terzaghi, 1936) is given as follows (eq. 9) with Pf being the pore pressure which is
resultant sum of hydrostatic pressure and overpressure (if present). Porosity decreases with increasing
consolidation or effective stress exponentially for overpressure considered zero. But higher porosity at depths
indicate the presence of overpressure. Thus overpressure is calculated as ΔPL that is the difference between
confining pressures at depth of maximum porosity anomaly ‘Z1’ and at shallower depths ‘Z2’ for equivalent
porosity (eq. 10)
σ = PL-Pf (9)
ΔPL= PL*(Z1) - PL*(Z2) (10)
Table 1. Overpressure for probed wells
Figure 6. Five well probes constructed to extract the porosity data with depth in
Cross section .Western most well(right most) has highest porosity of 19.85% and
other porosities of wells are tabulted in table(1)
𝜌𝑏 = 𝜑𝜌𝑓 + (1 − 𝜑)𝜌𝑔 (6)
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Figure 7 Well probe porosity data extracted from 5 wells as in (fig. 6) plotted versus depth .Depth z2 (max. porosity for well 1)is point where
overpressure was calculated using depths z2 and z1 with equivalent porosities of 19.85 % using (eq.7 to eq. 10 ).
6. Conclusions
We mapped the porosity in the seismogenic zone of Nankai Trough by converting the seismic velocity
data into 3D porosity using the logging data obtained at 4 sites and fitting the parameters into the Wylie’s
empirical relation. The main findings and conclusions are:
1) The anomalous low velocity underthrust sediments seem to have maximum porosity of 19.85% at a
depth of 7450mbsf extending up to 8450mbsf in the western block just below the Megasplay fault as
evident from (fig.4). Another patches of 18.52%-19% porosity of volume extends from 7340-9000m
depth in eastern as well as western block along with 17-18% porosity western volume going up to
9750mbsf depth
2) There is a gap of approximately 3-5 km between the western and eastern porosity anomalies.
3) Eastern block does not have 20% porosity anomaly yet the overpressure studies shows
It has maximum overpressure approximately 36.586 MPa, which is even higher than the 19.85%
western Block anomaly overpressure of 30.394MPa (Table. 1), reason primarily being the porosity
gradient (Fig.7).
4) Overpressure might be due to external fluid injection at high pressures in the rock body or the high
temperatures of about 100-150 °C could have led to dehydration in the smectite clay which helped the
underthrust sediments to retain such high porosities even at depths greater than 7000mbsf.
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Acknowledgement
I would like to express my gratitude to Prof. Masataka Kinoshita for providing me with this valuable
opportunity to take an internship in the lab of Earthquake Research Institute (ERI). We used the open source data
provided by IODP and JAMSTEC. I would also like to appreciate all the support provided by International Liaison
Office, University of Tokyo as well as all the members of ERI during the stay in Tokyo. The internship was funded
by GSS-UTRIP scholarship.
7. References
1. Hashimoto, Y., H. J. Tobin, and M. Knuth (2010), Velocity‐porosity relationships for slope apron
and accreted sediments in the Nankai Trough Seismogenic Zone Experiment, Integrated Ocean
Drilling Program Expedition 315 Site C0001, Geochem. Geophys. Geosyst., 11, Q0AD05, doi:
10/29/2010GC003217.
2. Hoffman, N.W., and Tobin, H.J., 2004. An empirical relationship between velocity and porosity for
underthrust sediments in the Nankai Trough accretionary prism. In Mikada, H., Moore, G.F., Taira,
A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196, 1–23
3. Kinoshita, M., Tobin, H., Ashi, J., Kimura, G., Lallement, S., Screaton, E.J., Curewitz, D., Masago,
H., Moe, K.T., and the Expedition 314/315/316 Scientists, Proc. IODP, 314/315/316:Washington,
DC (Integrated Ocean Drilling Program Management International,
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ankaiaccretionaryprismcoresamples.Geochem.Geophys.Geosyst.12,Q0AD10.http://dx.doi.org/10.10
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5. Sugihara et al.,(2014) Re-evaluation of temperature at the updip limit of locked portion of Nankai
megasplay inferred from IODP Site C0002 temperature observatory Earth, Planets and Space 2014
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6. Tsuji, T., R. Kamei, and R. Pratt (2014), Pore pressure distribution of a mega-splay fault system in
the Nankai Trough subduction zone: Insight into up-dip extent of the seismogenic zone, Earth
Planet. Sci. Lett., 396, 165–178.
7. Tudge, J., and H. J. Tobin (2013), Velocity-porosity relationships in smectite-rich sediments:
Shikoku Basin, Japan, Geochem. Geophys. Geosyst., 14, 5194–5207, doi: 10.1002/2013GC004974.
8. Wyllie, M.R.J., Gregory, A.R., and Gardner, G.H.F., 1958. An experimental investigation of factors
affecting elastic wave velocities in porous media. Geophysics, 23:400.
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Figure 8: Log data from sites C0001, C0002, C0004, and C0006 plotted for different parameters that is Resistivity, Sonic Velocity and Resistivity
derived Porosity versus Depth (m)
Additional Figures (Log data)