As offshore petroleum exploration anddevelopment move into deeper water,industry must contend increasingly withgas hydrate, a solid compound thatbinds water and a low-molecular-weightgas (usually methane). Gas hydrate hasbeen long studied in industry from anengineering viewpoint, due to its ten-dency to clog gas pipelines.

However, hydrate also occurs natu-rally wherever there are high pressures,low temperatures, and sufficient con-centrations of gas and water. These con-ditions prevail in two naturalenvironments, both of which are sitesof active exploration: permafrostregions and marine sediments on con-tinental slopes. In this article we discussseismic detection of gas hydrate inmarine sediments.

Gas hydrate in deepwater sedimentsposes both new opportunities and newhazards. An enormous quantity of nat-ural gas, likely far exceeding the globalinventory of conventional fossil fuels, islocked up worldwide in hydrates. Ex-traction of this unconventional resourcepresents unique exploration, engineer-ing, and economic challenges, and sev-eral countries, including the UnitedStates, Japan, Canada, India, and Korea,have initiated joint industry-academic-governmental programs to begin study-ing those challenges. Hydrates alsoconstitute a potential drilling hazard.Because hydrates are only stable in arestricted range of pressure and tem-perature, any activity that sufficientlyraises temperature or lowers pressurecould destabilize them, releasing poten-tially large volumes of gas and decreas-ing the shear strength of the hostsediments. Assessment of the opportu-nities and hazards associated withhydrates requires reliable methods ofdetecting hydrate and accurate maps oftheir distribution and concentration.

Hydrate may occur only within theupper few hundred meters of deepwa-ter sediment, at any depth between theseafloor and the base of the stabilityzone, which is controlled by local pres-sure and temperature. Hydrate is occa-sionally exposed at the seafloor, whereit can be detected either visually oracoustically by strong seismic reflectionamplitudes or high backscatter on sides-can sonar records (although this signa-ture is often complicated by associated

authigenic carbonate hardgrounds).However, much hydrate exists withinthe pore spaces of sediments at depthsdown to the base of the hydrate stabil-ity zone, and it need not be associatedwith seafloor hydrate outcrops. There-fore, accurate mapping of hydrate occur-rences requires methods of detecting andquantifying hydrate at depth, not justhydrate exposed at the seafloor.

The standard method for determin-ing where hydrate occurs at depth is byidentifying a bottom-simulating reflec-tor (BSR) on seismic reflection sections.The BSR represents a reflection from thehydrate-gas phase boundary, whichgenerates an impedance contrast be-cause hydrate-bearing sediments have ahigher P-wave velocity than gas-bearingsediments. The essential characteristicof the BSR is its cross-cutting relationshipto strata, which identifies it as a chemi-cal phase boundary rather than a strati-graphic reflection. Using the BSR as thesole indicator of hydrate occurrence hassevere limitations, however. While BSRsare common in hydrate-bearing sedi-ments, they are not ubiquitous; indeed,in environments where fluid flow ishighly focused, such as the Gulf ofMexico, they are rare or absent. Resultsfrom Ocean Drilling Program Leg 164showed that hydrates can be presenteven where BSRs are lacking. Moreover,the amplitude of the BSR is strongly sen-sitive to small concentrations of gasbeneath the HSZ. Determination ofhydrate concentrations from BSR ampli-tudes requires careful AVO modeling,preferably by prestack full waveform

inversion, and even then, the BSR con-fers information only about the concen-tration of hydrate within a few metersof the phase boundary. The BSR, then,reliably indicates the presence of hydratebut says very little about its vertical orlateral distribution.

How can hydrate be detected atdepth within sediments? Some recentseismic reflection results from the BlakeRidge provide an instructive case study.The Blake Ridge, one of the best-stud-ied hydrate provinces in the world,could be described as the type sectionof hydrate deposits. The first hydrateBSR was discovered on the Blake Ridge,and the first samples of marine gashydrates were recovered there. The ridgeis a sediment drift deposit formed bycontour-hugging currents of the WesternBoundary Undercurrent, which flowssouth along the western margin of theNorth Atlantic Ocean (Figure 1). Theridge juts southeastward into the deeperocean basin; the hydrate-bearing por-tion of the ridge occurs in water depthsof about 2000-4000 m. Because of its rel-ative sedimentological and tectonic sim-plicity, the Blake Ridge is an excellentlocale to study the hydrate/gas system;in particular, the relatively uniformlithology (muds and silts) provides a vir-tual tabula rasa against which stronganomalies in physical properties (e.g.,velocity, density, and reflectance) can beconfidently interpreted in terms ofhydrate or free gas. In Fall 2000, weacquired seismic reflection data on theBlake Ridge aboard the R/V MauriceEwing, using a 2-GI gun source (105/105in3) and a 6000-m, 480-channel digitalstreamer. The resulting seismic data areof excellent quality and resolution andcontain three different examples of directseismic detection of gas hydrate:enhanced reflectors (hydrate brightspots), cross-stratal reflections in thehydrate stability zone (paleo-BSRs),and zones of reduced reflectance(amplitude blanking).

Depending on its concentration,hydrate can either enhance or suppressseismic reflectance. Recent studies of per-mafrost hydrates in the Mallik well ofArctic Canada show that hydrate maypreferentially form in more porous (andthus lower-velocity) layers, raising theirvelocity relative to the less porous(higher-velocity) layers. At low satura-

686 THE LEADING EDGE JULY 2002

Seismic detection of marine methane hydrateW. S. HOLBROOK, A. R. GORMAN, M. HORNBACH, K. L. HACKWITH, AND J. NEALON, University of Wyoming, Laramie, U.S.D. LIZARRALDE, Georgia Institute of Technology, Atlanta, U.S.I. A. PECHER, Institute of Geological and Nuclear Science, Lower Hutt, New Zealand

Figure 1. Location of Blake Ridge, offshore SEUnited States. The white region on the BlakeRidge shows the mapped extent of gas hydratedeposits.

tions (below ~25% of pore space),hydrate may thereby reduce the imped-ance contrast between more- and less-porous strata, suppressing seismicreflectancea phenomenon calledblanking. At high hydrate saturation,however, hydrate-bearing layers canhave velocities significantly greater thanthe surrounding sediment, thus gener-

ating enhanced reflectance. The BlakeRidge data show examples of bothenhanced and suppressed reflectancedue to the effects of hydrate.

Numerous anomalously brightreflections in the data set likely corre-spond to layers of concentrated hydrate.A particularly clear example occurs online 3D-03 (Figure 2), where several

short, strong reflections occur in an oth-erwise simple section of stratified,faulted sediments. Waveform inversionof the prestack data clearly shows that apositive velocity anomaly at a sub-seafloor depth of 250 m is responsible forthe bright reflection at 3.87 s. (Interest-ingly, visual inspection of the reflectionfalsely suggests a reversed polaritycompared to the seafloora cautionarytale of the dangers of casually interpret-ing waveform polarity of thin-bed reflec-tions.) The vertical association of thesebright reflections with disruptions in theunderlying gas zone makes a clear casethat these events represent concentratedhydrate formed by upward migration ofmethane gas along faults or fractures.The high-velocity anomaly reaches apeak of 2.1 km/s against a backgroundof 1.9 km/s, consistent with a hydratesaturation of 60-80% of the pore volume.These observations indicate that, even ina relatively low-methane-flux environ-ment such as the Blake Ridge, free gascan penetrate upward hundreds ofmeters through the hydrate stabilityzone before forming hydrate. The prin-cipal barrier to upward migration of gasis not formation of hydrate, but naturaltraps, such as low-permeability cappingsediment.

Upward migration of free gas inhydrate systems often creates zones ofvertically reduced reflection amplitudes,or chimneys. An example is given inFigure 3, which shows two adjacentchimneys of about 100 m radius, each ofwhich overlies a disruption in the BSR.Immediately beneath each chimney,reflection amplitudes in the free gas zone(at and beneath the BSR) are anom-alously weak, suggesting that free gashas escaped from beneath the BSR,migrated upward along faults or hydro-fractures, and created the chimneys. Gasescape features, such as chimneys anddisruptions in the BSR, are often associ-ated with bright amplitudes in thehydrate stability zone (e.g., Figure 2).

The example in Figure 4 showsbright, laterally restricted reflections attraveltimes of 100-250 ms beneath theseafloor, which are particularly pro-nounced within a 500-m radius of a cleardisruption in the BSR and sub-BSR gas-charged zone. The association of the dis-rupted BSR with enhanced amplitudesin the hydrate stability zone suggeststhat the bright, shallow reflections arelayers of concentrated hydrate formedby the rapid migration of free gas out ofthe hydrate stability zone. Although suchfeatures are relatively common inhydrate-bearing regions, to our knowl-edge, a gas hydrate chimney has neverbeen drilled, nor has a chimney been

JULY 2002 THE LEADING EDGE 687

Figure 2. Seismic data from line 3D-03, showing prominent, high-amplitude reflections (arrows) inthe hydrate stability zone. BSR is the bottom-simulating reflection, which marks the phase bound-ary between the hydrate stability zone and the underlying free gas zone. Right inset shows detailedvelocity-depth function at site of inverted triangle, derived by waveform inversion of prestack data.The strong reflection at 3.76 s two-way traveltime is caused by a high-velocity layer at 2.96 kmdepth, likely a zone of concentrated gas hydrate.

Figure 3. Seismic data from line R37, showing two adjacent chimneys of low reflectance, imme-diately overlying disruptions in the BSR. The chimneys are thought to represent gas-migrationfeatures.

Seafloor

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definitively determined to correspondto a high-velocity anomaly (the narrowradius of these features makes velocityanalysis challenging). For that reason, itremains unclear whether chimneys arezones of locally enhanced gas hydrateconcentration (with accordinglyblanked amplitudes) or merely dis-turbed zones due to fluid migration

(with amplitudes reduced by scattering).Another example of a reflection from

gas hydrate comes from the erodingflank of the Blake Ridge, where a clearreflection cross-cuts dipping strata butlies well within the hydrate stabilityzone, ~80 m above the present BSR(Figure 5). We interpret this reflection asthe top of a zone of concentrated hydrate

that formed after one or more episodesof seafloor erosion. Following erosion,the subseafloor temperature gradient re-equilibrates, causing the hydrate/gasphase boundary to move downward(analogously, after sedimentation thephase boundary moves upward). Thezone of concentrated hydrate lies instrata that are currently highly gas-charged beneath the present-day BSR.The crosscutting reflection thus repre-sents a paleo-BSR, marking the posi-tion of the phase boundary prior toerosion.

The interpretation of the paleo-BSRas a reflection from the top of a zone ofconcentrated hydrate is bolstered by twocharacteristics of the underlying lens ofmaterial: high P-velocities and reducedreflectance (amplitude blanking).Reflectance is clearly anomalously lowin a lens of ~80 m thickness immediatelybetween the present-day BSR and thepaleo-BSR described above: Strata thatpass downward through the paleo-BSRhave distinctly lower reflectance withinthe lens than above it. Detailed velocityanalysis shows that the lens has a sig-nificantly higher P-velocity (1910 m/s)than adjacent strata at the same sub-seafloor burial depth (1820 m/s), con-sistent with a hydrate saturation of 40%.The combined evidence of a low-reflectance, high-velocity lens capped bya cross-stratal reflector convincingly sup-ports an interpretation of enhancedhydrate concentrations.

Unfortunately, amplitude blankingdue to hydrate can be difficult to iden-tify unequivocally, because reflectancecan change laterally for reasons that havenothing to do with hydrate. Of particu-lar importance (and difficulty) is the def-inition of a reference reflectanceidentifying areas of low reflectancebegs the question of low relative towhat? In particular, it is invalid to com-pare reflectances of strata at differentstratigraphic levels (which have no a pri-ori reason to be similar), and it is espe-cially important to disregard thereflectances of strata beneath the BSR,which are often enhanced by free gas.(The overall contrast between highreflection amplitudes beneath the BSRand low amplitudes above the BSR is notdue to amplitude blanking, but rather toamplitude enhancement by gas beneaththe BSR.) Carefully calibrated amplitudeblanking can be useful as an indicator ofpossible hydrate accumulations, butquantitative estimates of hydrate con-centration are very difficult to obtainsolely from reflectance.

The best-case scenario for interpret-ing hydrate occurrence from amplitudeblanking is when, as in Figure 5, ampli-

688 THE LEADING EDGE JULY 2002

Figure 4. Seismic data from line R38, showing short, high-amplitude reflections in the hydratestability zone in association with disruptions in the BSR. These features are interpreted as con-centrated hydrate resulting from vertical migration of free gas from below the BSR.

tudes of individual strata are reduced ina zone that also has an anomalously highP-wave velocity. The hydrate concen-tration is then best inferred from themagnitude of the P-wave velocity, notthe degree of blanking. Blanking aloneshould be considered a tenuous indica-tor of hydrate unless, at a minimum,blanked zones are also independentlyconfirmed to have locally enhanced P-wave velocity. Indeed, any direct indi-cation of hydrate, including bright spotswithin the hydrate stability zone, mustbe confirmed as a high-VP anomaly byseismic velocity analyses in order to beconfidently associated with methanehydrate.

It is important to recognize thathydrate occurrences are still relativelypoorly known, and their distributionand detection are likely to vary consid-erably from place to place. The examplesshown here from the Blake Ridge arerelatively simple and straightforwardbut might easily be masked in more com-plex sedimentological or tectonic envi-ronments.

A key challenge, then, is to developreliable techniques for detecting andquantifying gas hydrate occurrences incomplex geologic environments. Surfaceseismic techniques are likely to remaina linchpin of hydrate detection, but it isparticularly important to obtain broad-band, high-resolution data that also havesufficient source-receiver offsets to accu-rately determine seismic velocities andamplitude-variation-with-offset behav-

ior. Multicomponent, ocean-bottomcables are likely to be an important tech-nology to characterize hydrate-bearingsediments, because the addition ofhydrate to sediments can increase theirshear-wave velocity as well as their P-wave velocity. Some nonseismic toolsshow promise for detecting and quanti-fying hydrates, especially geoelectricalsounding, which responds to the rela-tively high resistivity of hydrate-bearingsediment. Ultimately, what is needed arefocused studies of methane hydrate sys-tems using an array of complementarytechniques, applied in the full range ofnatural environments in which hydratesoccur. These studies will have wide-ranging implications: Methane hydratesare of interest not just as a potential fos-sil fuel reserve, but also due to their pos-sible role in climate change and the

global carbon cycle. Due in part to ongo-ing efforts of the Ocean DrillingProgram, and to collaborations amongindustry, government, and academiaforged by the Department of Energysrecently initiated National MethaneHydrate R&D Program (http://www.netl.doe.gov/scng/hydrate/maincontent.htm), thenext few years promise to be a time ofquantum increase in knowledge ofhydrate systems and their geologic andgeophysical signatures.

Suggested reading. Migration of methanegas through the hydrate stability zone ina low-flux hydrate province by Gormanet al. (Geology, 2002). Elastic-wave veloc-ity in marine sediments with gas hydrates:Effective medium modeling by Helgerudet al. (Geophysical Research Letters, 1999).Methane hydrate and free gas on theBlake Ridge from vertical seismic profil-ing by Holbrook et al. (Science, 1996).Direct seismic detection of methanehydrate on the Blake Ridge by Hornbachet al. (GEOPHYSICS, 2002). Gas hydratesGeological perspective and global changeby Kvenvolden (Reviews of Geophysics,1993). Amplitude blanking related to thepore-filling of gas hydrate in sedimentsby Lee and Dillon (Marine GeophysicalResearches, 2001). Scientific results fromJAPEX/JNOC/GSC Mallik 2L-38 gashydrate research well, Mackenzie Delta,Northwest Territories, Canada byDallimore et al. (Geological Survey ofCanada, 1999). Geophysical studies ofmarine gas hydrate in northern Cascadiaby Hyndman et al. (Geophysical Monograph124, American Geophysical Union, 2000).TLE

Acknowledgments: We thank the captain and crewof the R/V Maurice Ewing for a successfulcruise. This work was funded by the NationalScience Foundation and the U.S. Department ofEnergy.

Corresponding author: steveh@uwyo.edu

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Figure 5. Seismic data from line 3D-82x, showing a zone of reduced amplitudes (blanking),capped by a top hydrate reflection that cross-cuts dipping strata. The top-hydrate reflection isa paleo-BSR produced when seafloor erosion caused the hydrate/gas phase boundary to migratedownward, freezing free gas into hydrate.