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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,

Inc.).doi:10.2204/iodp.proc.314315316.113.2009

4. Raimbourg,H.,Hamano,Y.,Saito,S.,Kinoshita,M.,Kopf,A.,2011.AcousticandmechanicalpropertiesofN

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

,66:107,http://www.earth-planets-space.com/content/66/1/107

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)