HighResolutionStudiesHydrateBlakeRidge--Gettrust,2002

7/30/2019 HighResolutionStudiesHydrateBlakeRidge--Gettrust,2002

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High-Resolution Studies of Hydrates on Blake Ridge

J. F. Gettrust Code 7432, Naval research Laboratory

Deep-tow multichannel seismic data are used to obtain significantly improvedimages and physical property estimates of marine sediments within the upper 1km of the sediment column. The technology that supports this effort is based on

a Helmholtz resonator source (220Hz 1kHz frequency band, 200 dB // 1 Pa @1 m source level) that operates at any ocean depth. The source and 48independent hydrophones are deployed approximately 300 meters above the

seafloor. This system has proven to be optimal for studies of marine hydrates asit provides high-resolution data through the hydrate stability zone (HSZ),including the strong seismic signal from the bottom simulating reflector (BSR)that is related to the temperature-pressure driven phase change from solidhydrate to gas and water.

Data taken with this system (the Deep Towed Acoustics/Geophysics System, orDTAGS) on the Blake Ridge revealed that numerous faults, separated laterally

by tens to hundreds of meters and extending from the base of the HSZ to theseafloor are ubiquitous on both the flank and crest of the Blake Ridge. Thesefaults provide more permeable pathways for the flow of water and methane thatmay concentrate hydrates along vertically oriented paths on the Blake Ridge.Compressional velocity estimates from DTAGS data, suggest that zones of highcompressional velocity (consistent with hydrated sediments) are laterallydiscontinuous and may be consistent with hydrate concentration near thesefaults.

These seismic data have been the impetus for the development of lattice gasnumerical simulations of fluid/gas flow through complex media as a means tobetter understand hydrate generation, dissociation and movement withinsediments. Examples of flow through a geologic fault and flow with multi-component media are presented.

Improvements in methods for determining the location and concentration ofhydrated marine sediments may rest with the development of bottom-mountedseismic sources and ocean bottom cables. This combination will allowinvestigators to observe both compressional and shear seismic data. Aslaboratory studies indicate that the shear properties of sediments are quitesensitive to being hydrated, observation of shear waves could significantlyimprove our ability to study the HSZ An additional benefit results from the fact


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High-Resolution Studies ofHydrates on Blake Ridge

Methane Hydrates Interagency R&D Conference

March 20-22, 2002

J. F. Gettrust, NRL


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Blake Ridge

After Taylor et al., USGS OFR 99-72, 1999


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Conventional ApproachThe Existence of gas hydrates inferred from:

Bottom Simulating Reflector (BSR) Blanking

Seafloor Reflection

Bottom Simulating Reflector (BSR)

Blanking

Conventional (surface-tow) seismic data from the Blake Ridge


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How Well Do The BSR & Blanking Predict

Hydrate Content?

Does the BSR accurately represent the

geographical extent of gas hydrates?

Does Blanking accurately predict theconcentration of hydrates?

How useful would these diagnostic toolsbe in more complex geologic regimes?


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ODP Leg 164 Drill Sites and Seismic Reflection Data


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ODP Leg 164: VSPs, Chlorinity [CL-], CaCO3

No BSR BSR BSR


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ODP Data & Conventional Seismic

Reflection Observations

An observed BSR is still the most reliableindicator that Hydrates exist in an area.

However, the absence of a BSR does not

preclude the existence of Hydrates. Apparently, neither the existence of a BSR or

the degree of blanking is a good predictor of

the concentration of Hydrates.


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Alternative technique for determining the

distribution and concentration of hydrates

High-resolution deep-tow seismic

source combined with:

Deep-tow Multichannel hydrophones

Ocean bottom cables (OBC)


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Seafloor

Geological

Faults

BSR


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Advantage of deep-tow geometry in resolving rough 2-D morphology

After W. T. Wood, 2001


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Note: Discontinuous BSR

Conventional data show BSR from

same region to be continuous


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SUBBOTTOMD

EPTH(m)

200

400

600

0

SUBBOTTO

M

DEPTH(m)

200

400

600

0

RANGE (km)0.0 1.0 2.0

SEAFLOOR

1.0 1.2 1.3 1.4

P-velocity relative to

baseline (no hydrates)

D T A G S d a ta fr o m t h e s o u th e r n f la n k o f th e B la k e R id g e ( u p p er) s h o wg r o w t h fa u lt s p e n e t r a t in g th e B S R a s w e l l a s m o s t o f th e h y d r ate st a b il ityz o n e . O ff s e t i n th e B S R i n d i c a te s f lu id f l o w . A n o m a lo u s ly h i g h s ed im e n tv e l o c i t y is r el a te d to v o lu m e p e rc e n t o f h y d ra te in th e s e d im e n t c o lu m n .(From Rowe and Gettrust, 1994, International Conf. on Nat. Gas Hydrates.)

Velocity analysis of Blake Ridge data show laterally discontinuous regions of

high P-velocity consistent with concentration of hydrates along growth faults

that are separated by 10s to 100s of meters and cut through the hydratestability zone.

Note: Vp Estimates

Consistent with

Hydrate ConcentrationHigher in Lower Part

of HSZ


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Implications from High-Resolution Seismic

Studies of Gas Hydrates

In deep water, deep-tow sources & receivers are

required for detailed sampling of the upper 1 km ofsediments i.e, the hydrate stability zone.

High-resolution Vp and images from deep-tow

seismic consistent with models that suggest thathydrates tend to concentrate within more permeablelayers & faults.

The ability to resolve detailed structure andproperties is required to monitor extraction of gashydrates and to quantify environmental issues relatedto gas hydrates


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Numerical SimulationsR. Pandey, W. Wood, J. Gettrust

Lattice Gas Approach

Single component flow studies through faults

Calibrated fluid flow (testing lattice gas for Darcys

Law diffusion) Response of linear & non-linear flow (i.e., Darcy &

non-Darcy regimes in porous media)

Multi-component flow & phase separation (uniformmedia)


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Fluid Flow Through Geologic Fault

M lti t I i ibl M ( d) 3*M ( hit ) L 30 G it


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Multi-component, Immiscible, Mass(red) = 3*Mass(white), L=30, Gravity

T=1 T=100

Upward Bias, small

T=500T=200

Concentration at Bottom

T=100T=1

Upward Bias, large

T=500T=200

Even Distribution

i i ibl ( d) 3* ( hi ) 30 G i


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Multi-component, Miscible, Mass(red) = 3*Mass(white), L=30, Gravity

Upward Bias, small

Upward Bias, large

Concentration at Bottom

Even Distribution

T=1 T=100

T=500T=200

T=1 T=100

T=500T=200


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Future High-Resolution

Gas Hydrate Studies

Bottom source bottom receiverseismic systems to obtain shear velocity

information.

Changes in Vs may be a better

discriminator of the concentration of gas

hydrates.

Vs not sensitive to free-gas (allows

discrimination of potential BSR signals).


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Data from USGS GHASTLI (Gas Hydrate and Sediment Test Laboratory Instrument)

A i l d l t d t t th l f


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A simple model to demonstrate the value of

Bottom-source and receivers.

shot 100 m above bottom


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Using Shear Wave Information to Identify Gas Content (identification of BSR)


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Using Shear Wave Information to Identify Gas Content (identification of BSR)

After K. Andreassen et al., EAGE, 2001


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Summary

High-resolution, deep-tow seismic can provide azeroth-order estimate of the spatial distribution ofhydrates a more holistic approach is required toestimate concentration.

Bottom-source coupled with bottom-mounted 4-

component receiving arrays will provide much betterestimates of hydrate distribution and concentration.

Laboratory studies using devices such as GHASTLI

are required to constrain interpretation of remotesensing data.

All of these facets are applicable to the study of gashydrates as a resource or as a geohazard.

Documents

HighResolutionStudiesHydrateBlakeRidge--Gettrust,2002