ORIGINAL RESEARCH PAPER

Estimating the amount of gas-hydrate using effective mediumtheory: a case study in the Blake Ridge

Ranjana Ghosh • Kalachand Sain • Maheswar Ojha

Received: 28 September 2009 / Accepted: 20 April 2010 / Published online: 12 May 2010

� Springer Science+Business Media B.V. 2010

Abstract Estimating the amount of gas-hydrate and free-

gas is difficult in deep seas even with scientific coring and

downhole measurements. Well data may be incompatible

between holes of a site as well as with depth in the same

hole. In this paper, we demonstrate an approach to estimate

saturation of gas-hydrate from seismic velocities at any site

where data set is limited. The study is carried out in the

outer Blake Ridge, which is one of the most intensively

studied regions of natural gas-hydrate occurrences and a

very distinctive example of studying geophysical signa-

tures of gas-hydrate and free-gas in deep marine sediments.

Although, downhole measurements from both vertical

seismic profiles (VSPs) and sonic logs provide the most

accurate and direct measurements of velocity, only VSP

velocities at Ocean Drilling Program (ODP) Sites 994, 995,

and 997 on the Blake Ridge are used to estimate the sat-

uration of gas-hydrate and free-gas as sonic logs at ODP

sites are not reliable. Here we derive a general trend of the

background velocity with depth using the porosity and

mineralogy from coring at discrete depth intervals. Satu-

rations of gas-hydrate and free-gas are then estimated from

this background velocity using the effective medium

modeling. The porosity and mineralogical compositions are

taken from four different depths at Site 995, as data quality

is the best in this hole. Average saturations of gas-hydrate

and free-gas at three holes are estimated as 10–14 and

2–3%, respectively.

Keywords Gas-hydrate � Free-gas � VSP �Effective medium theory � Blake Ridge

Introduction

Natural gas-hydrate is an ice-like non-stoichiometric

clathrate of gas molecules (mainly methane) and water,

which forms when the constituents come into contact at

low temperature and high pressure (Sloan 1998, 2003).

Below the gas-hydrate-saturated sediments, methane may

be present as dissolved or free-gas, but not as crystalline

hydrate (MacKay et al. 1994). Methane stored within and

trapped as free-gas below hydrate-bearing sediments is

almost double the known other fossil fuel (crude oil, nat-

ural gas and coal) reserves. The global reserves of gas

trapped in gas-hydrate are speculated as 1–120 9 1015 m3

(Makogon et al. 2007; Klauda and Sandler 2005; Milkov

2004; Ahlbrandt 2002; Kvenvolden 1998a, b). However

reliable estimates are yet to be made. Dissociation of gas-

hydrate releases methane (green-house gas) into the

atmosphere, which may cause global warming and reduces

the rigidity of the sediment leading to slope failure or

seafloor instability (Sloan 1998; Kvenvolden 1998a, b).

Hence, quantification of natural gas-hydrate and free-gas

are essential to evaluate their resource potential and

impacts on the environment.

Seismic velocities provide important estimates of aver-

age saturations of hydrate and gas that, in general, agree

with estimates from other downhole logs (Paull et al. 1996;

Collett and Ladd 2000). Well data such as the porosity and

mineralogy are essential parameters required for rock

physics modeling. However, drilling itself is costly and

sometimes becomes difficult in the deep seas. Moreover,

well data may not be consistent between holes at the same

site as well as with depth in the same hole depending on the

conditions of hole and quality of log data. Therefore, use of

some empirical relations formulated for already explored

areas is the conventional way to determine a porosity-depth

R. Ghosh � K. Sain (&) � M. Ojha

National Geophysical Research Institute (Council of Scientific

and Industrial Research), Uppal Road, Hyderabad 500 606, India

e-mail: kalachandsain@yahoo.com

123

Mar Geophys Res (2010) 31:29–37

DOI 10.1007/s11001-010-9084-y

profile for an unexplored region or for a region having poor

data from coring. However, they may not be applicable in

areas with rock properties different from where they were

formulated. For overcoming these difficulties, we show that

a general background velocity-depth profile and a velocity-

porosity relation can be derived for a region with limited

data set. Saturations of gas-hydrate and free-gas are then

estimated at any location of the region by translating the

velocity anomaly with respect to the background velocity

trend using a rock physics modeling. The method can be

applied potentially to obtain a continuous saturation-depth

profile throughout the prospective region of gas-hydrate-

bearing sediment.

The study is carried out in the outer Blake Ridge

(Fig. 1a) as it provides the unique opportunities to under-

stand the geophysical signature of gas-hydrate and free-gas

in deep marine sediments (Holbrook 2001). Ocean Drilling

program (ODP) Leg 164 was the first dedicated academic

effort to investigate naturally occurring marine gas-hydrate

beneath the Blake-Bahama Ridge, a sediment drift deposit,

located off the coast of the southeastern United States at a

water depth of about 3,000 m. Most of the sediments

recovered in Leg 164 (Shipboard Scientific Party 1996)

were accumulated during the Miocene and Pliocene at very

rapid rates (up to 350 m/my). The stratigraphic sequence is

composed of lithologically rather homogeneous, nanno-

fossil-rich clays and claystones and variable amounts of

opaline silica. The sediments typically contain 0.5–1.5%

organic carbon, enough to generate significant quantities of

microbial methane (Shipboard Scientific Party 1996).

Gas-hydrate and free-gas saturations are estimated on the

Blake Ridge from several sources of velocity: (a) downhole

sonic log in the Leg 164 (Collett and Ladd 2000; Paull et al.

1996), (b) vertical seismic profiling (VSP) data in the Leg

164 (Holbrook et al. 1996; Paull et al. 1996), (c) wide-angle

reflection data recorded on ocean-bottom hydrophones

(OBH) or seismometers (Korenaga et al. 1997), and (d)

high-resolution 3-D seismic data (Hornbach et al. 2008).

Downhole measurements from sonic logs and VSPs provide

the accurate and direct measurement of P-wave velocity on

the Blake Ridge (Holbrook 2001). Although, sonic logs

provide a higher-resolution record and are superior for

identifying small-scale variations in velocity, we prefer to

interpret VSP velocities because VSP is conducted at sur-

face-seismic frequencies and thus provides more practical

links to the surface-seismic reflection study. Besides, their

long wavelengths make them less inclined to degradation

due to near-hole conditions which is especially important in

the low velocity zone (Holbrook 2001). Another reason for

choosing VSP velocity is because the sonic log at Site 997

did not detect the free-gas zone properly (Holbrook 2001).

Figure 1a (Shipboard Scientific Party 1996) shows the

location of Blake Ridge and ODP Sites 994, 995, and 997

drilled to depths of 700–750 m below sea floor (mbsf) along

a *10-km-long transect across the crest of the Blake Ridge.

The single channel seismic data acquired through three sites

(Fig. 1b) show a clear bottom-simulating reflector (BSR),

prime marker for gas hydrates at Sites 995 and 997, but no

BSR at Site 994.

Seismic velocities acquired in the Blake Ridge are

considered to estimate saturations of gas-hydrate and free-

gas across the BSR using different models of rock physics

based on empirical relations and seismic tools (Guerin

et al. 1999; Helgerud et al. 1999; Holbrook et al. 1996;

Jakobsen et al. 2000; Lee 2000; Tinivella and Lodolo 2000;

Holbrook 2001; Hornbach et al. 2008). The advantage of

an empirical relation is that it is based upon real data and is

simple to apply but it may not be valid in all geological

circumstances. In this paper, we have used the effective

medium theory (EMT) proposed by Jakobsen et al. (2000)

for estimating saturations of gas-hydrate and underlying

free-gas in the gas-hydrate-bearing sediment.

Fig. 1 a Map showing the location of drilled holes in the outer Blake

Ridge on ODP Leg 164 off the southeastern coast of the United

States. Gray region in offshore is the distribution of gas-hydrate

inferred from seismic and other data (Shipboard Scientific Party

1996), b single-channel seismic data on Blake Ridge (Holbrook et al.

1996)

30 Mar Geophys Res (2010) 31:29–37

123

Effective medium theory

The effective medium theory (EMT) is based on the first

principle of physics and can incorporate the physics-based

input parameters (Helgerud et al. 1999; Jakobsen et al.

2000). The EMT proposed by Jakobsen et al. (2000) is a

combination of self-consistent approximation (SCA)

(Willis 1977) and differential effective medium theory

(DEM) (Nishizawa 1982) coupled with smoothing

approximation for crystalline aggregate (Bonilla and Keller

1985). The EMT is capable of computing the effective

elastic properties due to infinitesimal change in any of the

constituents in the sediment, complexity in sediment

microstructure associated with the grain geometry, and

inherent anisotropy caused by the alignment of clay

platelets. The combined SCA-DEM theory was developed

to determine the elastic and electrical properties of sand-

stone (Sheng 1990, 1991) and elastic properties of intrin-

sically anisotropic shale (Hornby et al. 1994). The effective

medium is created by implanting inclusions of one material

within another. Both SCA and DEM methods determine

the effective elastic moduli of a medium from the

individual components and their geometry. In marine

sediments, grains and pore-fluids are fully connected

(bi-connected medium) at all realistic porosities. However,

SCA and DEM individually cannot model such a medium

which is bi-connected at all porosities. SCA constructs a bi-

connected medium within 40–60% porosity and it is not

valid above 60% porosity (Sheng 1990, 1991; Hornby et al.

1994).

The DEM model can include micro-structure of a

medium but it depends absolutely on the status of the

starting medium. If the starting medium is bi-connected,

the DEM determines the effective elastic properties pre-

serving the bi-connectivity of the medium at all porosities.

Jakobsen et al. (2000) extended the theory to determine the

elastic property of gas-hydrate-bearing sediment. Accord-

ing to this theory, two end-member distributions of gas-

hydrate are considered: (1) non-load bearing gas-hydrate in

which gas-hydrate is a part of the pore fluid, and remains

without appreciable grain contact (non-contact model,

Fig. 2a) and (2) load-bearing gas-hydrate where gas-

hydrate cements the grains and forms a continuous matrix

(contact model, Fig. 2b).

A large fraction of the offshore sedimentary basins are

rich in clay minerals in which plate-like clay minerals

commonly prefer horizontal alignments. Such type of

sediments exhibit aniosotropic elastic behavior in the limits

of the long wavelength (Hornby et al. 1994; Sayers 1994;

Cholach and Schmitt 2006). Clay platelets form the con-

nected skeleton of the sediment and silt minerals remain as

the isolated phase. Abundance of phyllosilicate minerals

such as clays in sediments contribute to the overall

anisotropy of many rocks because of large anisotropy of

the phyllosilicate single crystals and their texture, quanti-

tatively described by an orientation distribution function

(ODF)(Cholach and Schmitt 2006). Groups of clay plate-

lets are fully aligned locally, but the orientation of these

groups of clay platelets may vary depending on the depo-

sitional and stress history of the sediment (Jakobsen et al.

2000; Hornby et al. 1994; Sayers 1994). The orientation of

these crystalline aggregates varies from completely aligned

(transversely isotropic) to completely disorder (isotropic).

The degree of anisotropy depends on the precise orienta-

tion-distribution pattern of the clay platelets. The average

elastic properties of partially aligned clay platelets are

Fig. 2 a Gas-hydrate replaces

pore-fluid—non-contact model,

b gas-hydrate cements and coats

grain—contact-cementing

model. Bottom panel shows the

concept of effective medium

theory in which we calculate the

elastic properties of a

homogeneous medium (right)having equivalent elastic

properties of a heterogeneous

medium (left) with constituents

of different elastic properties

Mar Geophys Res (2010) 31:29–37 31

123

computed by the method of smoothing (Bonilla and Keller

1985). We have assumed the same ODF of clay platelets as

has been used by Jakobsen et al. (2000).

In modeling procedure, the starting phases are either

clay particles and pore water (required for the non-load

bearing/non-contact model) or clay particles and gas-

hydrate (required for load bearing/contact model), whose

effective elastic moduli are calculated by the SCA-DEM

method. In case of non-contact model, gas-hydrate is

included by replacing water, leaving the clay concen-

tration unchanged. Here, the effective microstructure is

connected clay (load-bearing), connected water and

unconnected gas-hydrate (non-load-bearing). Gas-hydrate

has the same aspect ratio (1/20) and orientation as the

clay platelets (Hornby et al. 1994; Jakobsen et al. 2000).

Up to this, the clay-water-hydrate composites are fully

aligned. But clay platelets are distributed according to

the depositional history of the sediment and generally

partially aligned. Water and gas-hydrate are also oriented

according to the ODF (taken from Jakobsen et al. 2000)

of clay platelets. After incorporating the anisotropy due

to distribution pattern of clay platelets by the method of

smoothing, quartz is included in the aggregated clay-

water-gas-hydrate composite using the DEM method.

The concentration of quartz is increased successively

from zero to the desired volume replacing all other

constituents in proportion to the volume fraction of the

inclusion (quartz) and other constituents. A spherical

inclusion model (aspect ratio of 1) is used for quartz

(Hornby et al. 1994). The only difference in case of

contact modeling of gas-hydrate is that the roles of water

and gas-hydrate are reversed.

Free-gas is included in the clay-water composite using

the DEM theory by replacing water, and leaving clay

unchanged. Then quartz is included in the clay-water-gas

composite. The velocity of the effective medium is com-

puted from the effective stiffness coefficients (Hornby et al.

1994; Jakobsen et al. 2000) of the sediment.

Procedure

First we describe the procedure in the form of a flow chart

(Fig. 3) and then provide details of each step separately.

Determination of background velocity-depth trend

The velocity-depth functions at all three sites (994, 995,

and 997) derived from the VSP data are shown in Fig. 4a.

The saturations of gas-hydrate and free-gas are estimated

using the rock physics modeling by translating the

deviation of velocity-depth profiles of gas-hydrate- and/or

free-gas-bearing sediment from the trend of the back-

ground (water-saturated) velocity. Results of ODP Leg 164

suggest that the lithology of the Blake Ridge is relatively

homogeneous and therefore all anomalies (e.g., reflectance,

velocity or resistivity) can be confidently attributed to gas-

hydrate or free-gas (Paull et al. 1996; Holbrook 2001). We,

therefore, assume a linear increase in velocity (VP) with

depth (Z) for the reference velocity as

VPðZÞ ¼ V0 þ GZ ð1Þ

Core porosity and mineralogy at four different depths of

Site-995A (Table 1), where data quality is the best, are

used for modeling. The elastic properties of different

constituents of the sediment are given in Table 2. Miner-

alogical components given as percentages (wt %) of total

sediment volume (Table 1) are to be expressed as a fraction

of total solid grains (Table 3), by approximating all the silt

minerals as quartz grains.

For water-saturated sediment, the bi-connected, par-

tially-aligned starting effective medium is constructed from

clay and water using the combined SCA-DEM theory

coupled with the smoothing approximation. Following the

DEM method, quartz is included in the partially aligned

clay-water composite. Water-saturated/background P-wave

velocities at four different depths are calculated as shown

in Table 3. Seafloor velocity (V0) and gradient (G) are then

determined as per Eq. 1 using the calculated velocities at

four depths (Table 3), the average values of which are

calculated as V0 = 1.52 km/s and G = 0.3/s. The average

seafloor velocity differs by *1% from individual values

calculated with respect to different depth intervals. We

Fig. 3 Flow chart of the procedure

32 Mar Geophys Res (2010) 31:29–37

123

therefore consider ±1% error in the calculation of seafloor

velocity. The trend of the reference velocity with error

bounds is shown in Fig. 4a.

Derivation of velocity-porosity relation

Due to various obstacles below deep seas (Lee 2000), the

continuous porosity-depth profile could not be made at all

sites even with coring and downhole measurements. Also,

the existing empirical velocity-porosity relations formu-

lated based on real data at a place may not be applicable to

all places. We, therefore, attempt to deduce a velocity-

porosity relation using the core porosity and mineralogy

from one representative hole. Porosity (/) is inversely

proportional to the velocity (V), and existing porosity-

velocity relations are expressed as polynomials of degree

two, four, etc. (Hamilton 1974; Hyndman et al. 1993;

Erickson and Jarrard 1998). We assume the porosity-

velocity relation as a polynomial of degree four

Fig. 4 a VSP velocity-depth profiles at Sites 994, 995 and 997 digitized from Holbrook et al. (1996) and computed background velocity-depth

profile (blue) with error bounds (red and pink), b computed porosity-depth function (blue) with error bounds (red and pink)

Table 1 Approximate composition of samples from Site 995A (Jakobsen et al. 2000; Shipboard Scientific Party 1996)

Depth (mbsf) Porosity (%) Clay wt (%) Quartz wt (%) Calcite wt (%) Other wt (%)

100 68 45 19 26 10

200 65 56 25 11 08

300 57 63 25 12 00

400 54 58 20 17 05

Table 2 Densities (q), P-wave (VP) and S-wave (VS) velocities of

different constituents used for modeling (Jakobsen et al. 2000)

Constituents q (g/cm3) VP (m/s) VS (m/s)

Clay 2.60 3,400 1,600

Water 1.00 1,500 0000

Hydrate 0.92 3,800 2,000

Quartz 2.70 5,980 4,040

Methane 0.23 692 0000

The data for methane is taken from Helgerud et al. (1999)

Mar Geophys Res (2010) 31:29–37 33

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/ ¼ aþ b=V þ c=V2 þ d=V3 ð2Þ

We solve Eq. 2 using the porosity and velocity (Table 3)

and find the coefficients a, b, c, and d of the polynomial.

The velocity-porosity relation derived for this region can

be expressed as

/ ¼ 0:32þ 0:1634=V � 3:12=V2 þ 5:729=V3 ð3Þ

Determination of porosity-depth function

The background velocity-depth function in Eq. 1 and the

velocity-porosity relation in Eq. 3 are combined to obtain the

porosity-depth profile (Fig. 4b). The error bound in porosity

is obtained from the error bounds in velocity-depth function.

Determination of volume of clay and quartz with depth

The background velocity and porosity are then used to

determine approximately the fractions of mineralogical

constituents in the sediment with depth. The amount of

clay and quartz are determined by varying the volume

fractions of clay and quartz at different depths and

matching the velocity calculated using the EMT with the

background velocity. From the contour plot of clay,

velocity and porosity (Fig. 5), the fraction of clay (or

quartz) can be determined at any depth by matching the

corresponding velocity and porosity at that depth.

Estimation about saturations of gas-hydrate

and free-gas

We have estimated the gas-hydrate saturation with depth and

associated error bounds (Fig. 6) throughout the gas-hydrate-

bearing sediments at all Sites of 994, 995 and 997 using the

EMT from the VSP velocity (Fig. 4a). The base of gas-

hydrate stability zone (BGHSZ) generally coincides with the

BSR which is deepest at Site 994 (Fig. 6). We model the gas-

hydrate-bearing sediment by considering gas-hydrate as a

part of pore fluid (non-contact model) in one case and as a

part of matrix (contact model) in other case.

Results and discussion

In this paper, we estimate gas-hydrate and free-gas satu-

rations throughout the depth range of their occurrences at

Sites 994, 995, and 997 from VSP velocity (Fig. 4a) in the

outer Blake ridge. The trend of the background velocity is

determined from the core data at Site 995. The accuracy of

the derived background velocity trend (Fig. 4a) and the

velocity-porosity relation in Eq. 3 are confirmed through a

reasonable comparison between the calculated porosity-

depth profile (Fig. 4b) and the shipboard-measured poros-

ity-depth function (Holbrook et al. 1996; Shipboard Sci-

entific Party 1996).

The calculated velocity for contact model is always

higher than that from the observed velocity and therefore,

does not provide any estimation of gas-hydrate at all sites.

The average gas-hydrate saturations, estimated for non-

contact model of gas-hydrate, are 10–11, 10–13, and 11–

14% of pore space within *250 m thick sedimentary

column at Sites 994, 995 and 997, respectively. The satu-

ration of free-gas with error bounds at three sites, calcu-

lated from the VSP velocities, is shown in Fig. 6. At Site

997, where the free-gas saturation is maximum, average

free-gas is estimated as 2–2.6% of pore volume within

*200-m thick sediment below the BGHSZ. The average

Table 3 Volume concentrations used in the effective medium modeling to compute the background P-wave velocities at corresponding depths

Depth (mbsf) Porosity (%) Clay (fraction) Quartz (fraction) Velocity km/s

100 68 0.14 0.18 1.54

200 65 0.20 0.15 1.56

300 57 0.27 0.16 1.63

400 54 0.27 0.19 1.66

Fig. 5 Contour plot between velocity and volume fraction of clay

minerals at different porosities

34 Mar Geophys Res (2010) 31:29–37

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saturation of free-gas is estimated as *2% of total porosity

at both Sites 994 and 995.

Results from previous studies show wide range of

variations in estimations. Using the weighted equation of

Lee et al. (1996), Holbrook et al. (1996) estimated gas-

hydrate as 2% at Site 994 and 5–7% at Sites 995 and 997,

respectively, which are less by 6–8% from our estimations

(Fig. 6). Based on the three-phase weighted equation with

a weighting factor of 1.1, Lee (2000) estimated the gas-

hydrate varying between 3.8% and 12.1% of pore volume

depending on the porosity, derived from the density log or

core analysis, from acoustic logs at Sites 994, 995 and 997.

Weighted equation is basically the weighted mean of

Wyllie et al.’s time average (1958) and Wood’s relation

(1941), which provides little physical insight (Helgerud

et al. 1999; Jakobsen et al. 2000). Guerin et al. (1999)

showed that the estimated gas-hydrate (5–10% of pore

space) using the cementation theory of Dvorkin and Nur

(1996) from sonic velocity agrees well with the estimations

derived from the pressure core sampler (PCS), and is close

to our estimations (average *10%). But the model, which

was actually developed and tested on sands with lower

porosity (36%), assumes spherical grains that do not rep-

resent the clay-rich marine sediments (Dvorkin and Nur

1996; Guerin et al. 1999). Free-gas saturation estimated by

Guerin et al. (1999) is\5% using the Gassmann/Hamilton

model from sonic velocity, comparable to our estimations

(Fig. 6). The maximum gas-hydrate saturations estimated

by Tinivella and Lodolo (2000) are about 10, 16 and 20%

at Sites 994, 995 and 997, which are 3–6% higher than our

estimations at Sites 995 and 997. Tinivella and Lodolo

(2000) predict free-gas is absent at Site 994 and only 1 and

4% at Sites 995 and 997, respectively. But we have esti-

mated *2% free-gas at Site 994 (Fig. 6). Maximum gas-

hydrate saturation estimated by Jakobsen et al. (2000) and

Helgerud et al. (1999) at Site 995 is *9% of sediment

volume, equivalent to *17% of pore space, which is quite

similar to our estimation (Fig. 6).

Though the maximum gas-hydrate saturation at Site 995

using the EMT proposed by Jakobsen et al. (2000) and the

EMT proposed by of Helgerud et al. (1999) are almost

same, the assumptions of two theories are widely different.

The concept of non-load-bearing gas-hydrate in both the

EMTs assumes gas-hydrate as a part of pore fluid. But, the

non-load-bearing model of Helgerud et al. (1999) provides

very high gas-hydrate saturation compared to other reliable

estimates, whereas, gas-hydrate saturation using non-load-

bearing (non-contact) model of Jakobsen et al. (2000)

matches quite accurately with other estimates. The load-

bearing model of Helgerud et al. (1999) assumes gas-

hydrate as a part of matrix (grain) and yields gas-hydrate

saturation comparable with other estimates, whereas, the

load-bearing model (contact model) of Jakobsen et al.

(2000) yields very low gas-hydrate saturation as in this

Fig. 6 Saturations of gas-hydrate and free-gas (of pore volume) with error limits as a function of depths at Sites a 994, b 995 and c 997

Mar Geophys Res (2010) 31:29–37 35

123

model gas-hydrate cements the grain and therefore velocity

of the effective medium is very high. Moreover, Helgerud

et al. (1999) assumes all solid grains including the clay

particles are spherical which are unlikely based on the

observations of seismic anisotropy for clay-rich sediments

(Pecher et al. 2003). Intrinsic elasticity of clay-rich sedi-

ments is significantly influenced by the distribution pattern

of anisotropic clay platelets. Other factors such as oriented

microcraks and in-situ stress distribution may significantly

influence anisotropy and the models that include these

factors would benefit by starting from the intrinsically

anisotropic matrix (Cholach and Schmitt 2006). The criti-

cal porosity (porosity at which rigidity of a material van-

ishes) also strongly depends on the clay content, thus the

EMT used by Helgerud et al. (1999) is limited if the critical

porosity is assumed to be 0.35 (Chand et al. 2004; Ecker

et al. 1998, 2000; Helgerud et al. 1999). Since the SCA-

DEM theory, capable of computing the effective elastic

properties of sediments at any finite porosity, we prefer the

EMT proposed by Jakobsen et al. (2000).

Recent coring and drilling in the eastern Indian offshore

show veins/fractures, filled with gas-hydrates (grain-dis-

placing morphology of gas-hydrate), in the shallow clay-

rich sediments (Cook and Goldberg 2008, Holland et al.

2008; Kastner et al. 2008; Lee and Collett 2008; Ghosh

et al. 2010). As the anisotropy in the Blake Ridge is

interpreted to be caused by the alignment of clay platelets

rather than the high-velocity gas-hydrate veins (Pecher

et al. 2003), we assume only the pore-filling morphology of

gas-hydrate.

Conclusions

We find that the distribution of gas-hydrate in pore space is

non-contact type (non-load-bearing model). Saturation of

gas-hydrate is zero for load-bearing model (contact model).

Jakobsen et al. (2000) also concluded the nature of gas-

hydrate distribution is non-contact type in this region. Our

estimation shows that the saturation of gas-hydrate (max-

imum of 20–23% of pore space) and free-gas (maximum of

3% of pore space) are higher at Site 997 than the corre-

sponding values at two other sites. Both gas-hydrate and

free-gas saturations estimated from our methodology

match quite well with those from other methods and data

sets (Paull et al. 1996; Collett and Ladd 2000; Ecker et al.

2000; Helgerud et al. 1999; Jakobsen et al. 2000).

We show that, from a limited data set, we can approx-

imate the reference velocity-depth profile, a velocity-

porosity relation, and mineralogical constituents as well as

gas-hydrate and free-gas saturations. We demonstrate this

approach at the outer Blake Ridge; however, it can be

employed potentially to use seismic velocities to estimate

saturation of gas-hydrate at any other region. The SCA-

DEM theory in combination with the smoothing approxi-

mation, though suitable for clay-rich sediment, can be used

for predicting gas-hydrate saturation for sand-rich lithology

also (Ghosh and Sain 2008). Since the theory underesti-

mates the effect of quartz, which has higher stiffness than

clay, by considering the clay platelets connected and quartz

grains unconnected even at much higher concentration, the

present approach provides higher estimates than other rock

physics models.

Acknowledgments We are grateful to the Director, NGRI for his

kind consent to publish this work. The Ministry of Earth Sciences,

Delhi and the Department of Science & Technology, Delhi are

acknowledged for financial support. We are grateful to the Editor and

two anonymous reviewers for their comments and suggestions.

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