[Developments in Sedimentology] Contourites Volume 60 || Chapter 16 Seismic Expression of Contourite Depositional Systems

C H A P T E R 1 6

SEISMIC EXPRESSION OF CONTOURITE

DEPOSITIONAL SYSTEMS

T. Nielsen, P.C. Knutz and A. Kuijpers

Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade, Copenhagen, Denmark

Contents

16.1. Introduction 301

16.2. Seismic Identification and Characteristics of Contourites 30216.2.1. Large scale – first-order seismic elements 30216.2.2. Medium scale – second-order seismic elements 304

16.2.3. Small scale – third-order seismic elements 30516.3. Seismic Methods and Interpretation Concepts in Contourite Studies 305

16.3.1. Seismic scale 305

16.3.2. Reflection-seismic methods 30816.3.3. Seismic mapping 31416.3.4. Seismic interpretation 315

16.4. Summary 320Acknowledgements 321

16.1. INTRODUCTION

Since the early days of recognition of contourites in the marine sedimentaryrecord, reflection seismics has been used to identify and map these deposits, and thisis now considered as a standard method in most contourite studies. In fact, theinitial recognition of a contourite deposit in the marine setting is most often bymeans of seismics because the geometry of these deposits is an important diagnosticcriterion. Throughout the last decades, reflection-seismic investigations of con-tourite deposits have benefited greatly from the increased interest in deep-waterareas by the petroleum industry, which has lead to improved quality of the seismicdata and to sophisticated interpretation techniques.

A full reflection-seismic study of contourite deposits encompasses both seismicidentification based on individual seismic profiles and more accurate mapping of thegeometry of the deposits using either a 2-D grid or a 3-D volume of seismic data.

Developments in Sedimentology, Volume 60 � 2008 Published by Elsevier B.V.

ISSN 0070-4571, DOI: 10.1016/S0070-4571(08)00216-1

301

Further objectives of seismic studies of contourites are the prediction of thelithology and the reconstruction of the geological and (palaeo)oceanographichistory. Clearly, this does not only pose demands to the interpreter, but also tothe quality of the seismic data and its tie to borehole information.

In this chapter, we give an overview of reflection-seismic characteristics usefulfor the identification of contourite deposits (see also Howe, 2008), followed by areview of some general acoustic and physical properties relevant to seismic studiesof contourites. In addition, the chapter treats the significance of the seismic source,acquisition and processing parameters and the seismic-to-geology conversion.Interpretation concepts and terms used in seismic contourite studies are alsodiscussed. From this, some guidelines and a summary of the seismic expression ofcontourites are presented.

16.2. SEISMIC IDENTIFICATION AND CHARACTERISTICS

OF CONTOURITES

The seismic appearance of large, current-controlled sediment deposits inrelation to bottom-current regime and slope and basin morphology has previouslybeen described by Jones et al. (1970), McCave and Tucholke (1986), Kidd and Hill(1986), Eiken and Hinz (1993), Howe et al. (1994) and Stoker et al. (1998a). In1999, Faugeres et al. introduced a triple-scale approach that involves a set of keyattributes to be used in the seismic definition of contourite drifts. This approach haslater been expanded by other authors (e.g. Rebesco and Stow, 2001; Stow et al.,2002c). These seismic criteria, at times slightly modified, are summarised anddiscussed below. To place the criteria into a seismic-scale context, we furthercorrelate the key attributes to a hierarchic range of elements that we call ‘‘seismicelements’’. We suggest dividing the seismic characterisation of contourites into‘‘orders of seismic elements’’, where increasing order numbers refer to an increasinglevel of seismic interpretation details. Accordingly, the ‘‘first-order seismicelement’’ refers to the overall drift geometry, while, for instance, ‘‘third-orderseismic elements’’ refer to internal drift stratigraphic details. In Figure 16.1, wepropose a conceptual model based on the type (1) and (2) drift systems of Faugeresand Stow (2008) (see below) illustrating the case of increasing bottom-currentactivity along a continental margin. The model considers the whole sediment-drift accumulation, but is independent of the spatial dimension and thus valid forscales of tens to thousands of metres of thickness and length.

16.2.1. Large scale – first-order seismic elements

The seismic identification of a contourite deposit at the large scale is based on theoverall architecture of the drift depositional system, i.e. the external geometry ofthe drift, the lower and upper boundaries confining the drift system, and theconfiguration of larger internal seismic units.

302 Seismic Expression of Contourite Depositional Systems

16.2.1.1. External geometryBased on their overall morphology, contourite drifts are either sheeted ormounded. They can be further classified based on different geological and oceano-graphic settings, particularly the mounded ones. Today, drift systems are groupedinto five main types (Faugeres et al., 1999): (1) sheeted drifts, (2) giant elongateddrifts, (3) channel-related drifts, (4) confined drifts and (5) mixed drift systems. Thecharacteristics of the different drift systems are dealt with in detail in Faugeres andStow (2008).

It can be difficult to distinguish a contourite drift system from other deep-seadeposits, but a guiding characteristic is that contourite deposits form elongatedalong-slope geometries that follow the direction of geostrophic bottom currents. Incontrast, turbidite–fan systems are driven by gravity-induced mass transport,favouring elongation in a down-slope direction. However, the two end-memberprocesses may interact to form more complex sediment patterns on continentalslopes (Mulder et al., 2008), which in some cases cannot be separated solely on thebasis of the geometry of the deposits.

16.2.1.2. Bounding reflectorsThe proximal part of a contourite drift deposit normally records a major change inthe depositional style from a non-current-dominated to a current-dominatedregime, and vice versa when the contourite deposition ceases. These changes resultin regional unconformities that confine the upper and lower boundaries of the driftas a whole. The basal unconformity is commonly revealed by a continuous high-amplitude reflector of semi-regional to regional extent, that may extend beyond thelimits of the drift system, and which represents non-deposition or erosion producedby strong along-slope currents with average current velocities well above the

Sheeted frift(first-order seismic element)

Mounded elongate drift(first-order seismic element)

Pre-drift sediments

Moat

Post-driftsediments

Large-scale depositional units(first-order seismic element)

Depositional sub-units(second-order seismic element)

Seismic facies(third-order seismic element)

Largeunconformities(first-order seismic element)

Figure 16.1 Conceptual model of principal seismic characteristics of contourite deposits.(Modified from Stow et al., 2002c, reproduced with permission from the Geological SocietyPublishing House.)

T. Nielsen et al. 303

threshold for deposition (unconformities that have developed more locally anddisplay deep incision into the substratum are more likely the product of slide scarsor erosion by turbidite channels). Internally, reflectors often display low-angledownlap onto the basal unconformity. If the drift is buried, the upper boundingunconformity is also characterised by a continuous high-amplitude reflector ofsemi-regional to regional extent. If the contourite drift is still active, the sea-bedreflector forms the upper boundary.

16.2.1.3. Gross internal characterThe overall internal seismic character of a drift as a whole is that of a uniformpattern of continuous, low- to medium-amplitude reflectors that tend to follow thegross drift morphology. This pattern reflects the long-lasting, stable conditions thatare a prerequisite for building up a large contourite drift. However, large temporarychanges in current strength and sediment supply may occur during the lifetime ofthe contourite, causing shifts between erosional and depositional environments.Such major shifts are revealed by continuous, high- to moderate-amplitude internalreflectors that are either unconformable or conformable, and which bound large-scale, first-order seismic units within the drift (Figure 16.1). The shape of thesefirst-order units tends, however, to follow the overall geometry of the drift,indicating the temporary character of the changes in the depositional regime.

16.2.2. Medium scale – second-order seismic elements

In terms of seismic interpretation, the ‘‘medium scale’’ deals with the internal driftarchitecture, i.e. the internal character of the large-scale units recognised as first-order seismic elements. These are composed of second-order seismic sub-units,commonly displaying:

• a lens-shaped, upward-convex geometry;• a more or less uniform stacking pattern (reflects long intervals of relatively stable

conditions typical for most contourites);• a progradational stacking pattern that shows migration in a down-current

direction or an aggrading stacking pattern (the latter being most common forsheeted drifts);

• downlapping reflector terminations (toplapping might occur in connection tointernal erosional unconformities).

While first-order seismic units reflect larger temporary changes in the depositionalenvironment, the presence of the second-order seismic sub-units results fromsmaller fluctuations causing variations in sediment characteristics like composition,homogeneity, compaction, bedding, biotubation and similar aspects, as describedby, among others, Stow and Faugeres (2008).

Second-order seismic sub-units can be observed independent of the resolutionof the seismic profile (see below) (Laberg et al., 2001, 2002), showing that minorfluctuations in the depositional regime in many cases are more frequent thanrevealed from conventional multi-channel seismic data only.


16.2.3. Small scale – third-order seismic elements

Once the second-order seismic sub-units have been established at the medium-scale level, examination of their internal acoustic character can be performed on thebasis of seismic facies and seismic attribute analysis. Using seismic facies analyses,information can be gained on the gross lithology and depositional environment ofthe individual sub-units.

As for the contourite sedimentary facies (Stow and Faugeres, 2008), noseismic facies or attribute is unique for contourites. Furthermore, the seismicfacies and attributes depend strongly on the seismic acquisition and processingparameters. Hence they do not provide ‘‘stand-alone’’ diagnostic tools, but maybe useful in combination with geometry of seismic units and correlation withcore information. The most common seismic facies configurations in contouritedrifts are:

• continuous, (sub)parallel reflection configurations;• wavy reflection configurations;• structureless or reflection-free configurations.

Seismic attributes can be used to map current-induced bedforms (sediment waves andripples, erosional furrows, moats and channels, etc.), which in turn can be related todifferent current regimes and sediment types. Attribute analysis can also help toextract information on lithology by providing information on amplitude, phaseand dip-azimuth of a seismic reflector.

In general, seismic facies analysis are used to interpret variations in thecontourite depositional environment, but a few attempts have been made tocorrelate the seismic facies to specific sediment facies observed in cored sections(e.g. Knutz et al., 2002a; Stow et al., 2002c). More recently, seismic attributeanalysis has been introduced also in contourite studies to interpret the depositionalenvironment and sedimentary facies (e.g. Knutz and Cartwright, 2004; Hohbeinand Cartwright, 2006; Viana et al., 2007). However, an unequivocal correlationbetween contourite sediment facies and seismic facies or attributes does yet not existand more work on this topic is needed.

16.3. SEISMIC METHODS AND INTERPRETATION CONCEPTS

IN CONTOURITE STUDIES

16.3.1. Seismic scale

For correct interpretation of seismic data, it is important to acknowledge thedifference between a seismic profile and a geological profile. Hence the seismicreflectors do not uniquely correspond to actual bed interfaces. Moreover, thehorizontal scale on a conventional seismic profile is displayed in the metric system,while the vertical scale is displayed as two-way travel times (twtt). Thus, seismicprofiles tend to be highly exaggerated on the vertical scale, leading to distortion ofthickness and dip of layers.


16.3.1.1. Seismic reflectorsIn sedimentological terms, a layer is defined by the lithology, whereas seismically aunit is defined by its bulk density and the velocity of sound propagation. Changes inthe acoustic impedance (velocity multiplied by density; Sheriff, 1984) will cause theseismic signal to be reflected, and the strength of the acoustic impedance contrastbetween two units determines the amplitude of the reflected signal. Thus, theappearance and character of a seismic reflector is the result of changes in the physicalproperties of the strata. However, changes in these properties usually, but notalways, correspond to lithological contrasts, which are often determined by changesin the depositional conditions. Possible causes of such chances in a contouriteenvironment are numerous, but they are generally related to bottom-currentstrength (controlling sedimentation rate and flux, non-deposition or erosion) andsediment sources (Stow and Faugeres, 2008). The most confident way to determinethe origin of a reflector evidently is by sediment sampling, or ground-truthing.

16.3.1.2. Resolution and penetrationSeismic resolution is the key to extraction of stratigraphic details from the seismicdata. Seismic resolution comprises two aspects: vertical and horizontal resolution.Vertical resolution refers to the minimum thickness of a layer that can be distin-guished on the seismic profile, whereas the horizontal resolution is the ability torecognise two laterally separated features as two distinct reflections. The seismicresolution is proportional to the wavelength of the seismic signal, and thus to thefrequency component of the seismic source (wavelength equals velocity divided byfrequency; Sheriff, 1984). The seismic resolution is therefore tied to the type ofseismic system used to attain the data.

As a rough estimate, both the vertical and horizontal resolutions correspond to1/4 of the dominant wavelength of the seismic signal. Thus, the higher thefrequency of the seismic source, the higher the resulting seismic resolution, andvice versa. Accordingly, the resolution will decrease with increasing penetrationdepth because the earth filters the highest frequencies, while the seismic velocityincreases (Telford et al., 1976). The seismic resolution is therefore not fixed, butcorresponds to different depth levels (Figure 16.2). This is particular relevant inseismic studies of very thick or deeply buried contourite deposits.

The seismic penetration is expressed as the maximum depth from which seismicreflections can be picked with reasonable certainty (Sheriff, 1984). The penetrationdepends on the size of the seismic source (the bigger the source volume, the deeperthe penetration) but also on the type of seismic source as well as on sedimentolo-gical factors such as grain-size distribution, degree of compaction and diagenesis.Because the earth filters the higher frequencies, there is an inverse relationshipbetween penetration and frequency, i.e. high-frequency seismic sources give lowpenetration, whereas low-frequency sources provide high penetration.

16.3.1.3. Seismic processingThe basic objective of seismic processing is to improve the quality of the seismicprofile by enhancing what are regarded as useful seismic signals, and removing or


attenuating what is seen as noise. Seismic processing involves complicated mathe-matic algorithms (Yilmaz, 1987), which is beyond the scope of this chapter. It is,however, important to bear in mind that data processing involves subjectivechoices, meaning that a seismic profile processed by one person candiffer substantially from that processed by another person. For instance, areflector described as ‘‘high-amplitude’’ on one seismic profile might appear as‘‘medium-amplitude’’ on another seismic profile across the same drift deposit(Figures 16.3 and 16.4). A summary of seismic expressions of contourites, asgiven in the previous section, is therefore a guideline rather than a rule.

Pre-drift sediments

Migrating moat Large-scaledepositional units Bounding reflectors

Post-drift sediments

SW ne3 km

(a)

(b)

3

4

5

3

4

5

twtt

(s)

twtt

(s)

Figure 16.2 Buried contourite deposits off NEGreenland.The drift is only crossed by a singleseismic line and the designation as a contourite is based solely on the seismic characteristics.(a) Multi-channel, low-frequency seismic profile. Acquisition frequency range of the air-gunarray was 5^70Hz. To improve the vertical resolution of the resulting seismic profile, a post-cruise deconvolution operation designed to compensate for the effects of the Earth’s naturalfiltering of the high frequencies was applied, keeping the dominant frequency range at10^60 Hz. Assuming a likely velocity increase from 1600m s�1 to 2300m s�1, the nominalseismic resolution varies from about 6.5m in the upper part to about 10m at the base of thedisplayed section. This indicates an average maximum vertical resolution for the contouritedeposits of 7^8m. (b) Interpretation of the seismic profile. The designation as a contouritedrift is primary based on recognition of first-order seismic elements (see Figure 16.1), i.e. themounded cross-sectional shape, the uniform internal pattern of continuous low- to medium-amplitude reflectors which bound large-scale upward-convex depositional units that tend tofollow the overall shape, and the palaeomoat development to the left.The base of the drift ismarked by a high-amplitude, continuous reflector and the top by a medium-amplitudecontinuous reflector onto which the overlying post-drift unit is onlapping. (Data courtesy ofGEUS, Denmark.)


16.3.2. Reflection-seismic methods

The characteristics of the seismic system are as critical to the resulting seismicprofile as is the subsurface geology. Thus, the seismic expression and level ofinterpretation, i.e. the level of detail of the stratigraphic and sedimentologicalinformation obtainable from seismics, is dependent on the technique used toacquire the data.

In general, there are two types of recording arrays in reflection-seismicsystems: single-channel seismics (SCS) and multi-channel seismics (MCS). Stillsome ultrahigh-frequency seismic data (see below) are recorded as analogue data,but digital recording of both SCS and MCS has become standard nowadays.This allows processing of the data to enhance the quality of the resulting seismicprofile.

4

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twtt

(s)

4

3

2

twtt

(s)

(a)

(b)

a

bc

de

a b c d e

10 km

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Large-scale depositional units(first-order seismic elements)

Internal erosional unconformities

Figure 16.3 Seismic loop across the Eirik Drift off South Greenland composed of seismics ofvarious vintage and different acquisition and processing settings, illustrating how the seismicdatabase influences the seismic characteristics of the drift. (a) Multi-channel low-resolutionprofiles. Profiles a,b and d-e are post-2000 data, profile c is from the 1970s. Note the betterresolution of the younger data sets and the mismatches at the extremities of profile c, due to theinaccurate navigation of the older data. Profiles a and b have similar acquisition settings butdifferent processing settings causing minor differences in the seismic appearance, whileprofiles d and e are part of the same data set and have similar seismic looks. Profiles a and e areacquired the same year, but have different acquisition and processing settings resulting indifferent seismic appearances. (b) Interpretation of the seismic loop. The first-order seismicelements (see Figure16.1) can be followed around in the loopwith confidence. Also some of thesecond-order seismic elements can be followed around, but with less certainty and clearvariations in appearances.The largest variation in the seismic expression is of the third-orderseismic elements, where the seismic facies appearance of the sub-units clearly differs with thedifferent seismic settings.


Because most seismic sources generate sound with a distinct frequency content,there is a trade-off in all reflection-seismic systems between high penetration,which requires low frequencies, and high resolution, which demands highfrequencies (Figure 16.5). Reflection-seismic systems can be grouped intocategories (see below), corresponding to the relationship of the source frequency,level of resolution and depth of penetration.

16.3.2.1. Low-resolution seismicsLow-resolution (LR) seismics applies a source frequency of 5–50 Hz. Surveys usingLR seismics are usually devoted to investigating the deeper parts of the substratum.The seismic systems consist of a low-frequency, high-energy source (or array ofsources) and multi-channel hydrophone streamer(s). The signal is digitally recordedand require subsequent processing to produce a seismic profile for interpretation.This, in turn, facilitates computer-based interpretation allowing adaptation of thedisplay modes and analysis of various seismic attributes, which is a valuable tool inseismic studies of contourites, even in detail.

16.3.2.2. High-resolution seismicsHigh-resolution (HR) seismic sources have a frequency range of approximately50–2000 Hz, and encompass seismic sources like small water guns, air guns and sleeveguns in the lower end of the frequency range and sparker, boomer, chirp and parametricecho sounders in the higher end. These seismic techniques are relatively simple to handle,can be obtained from even small vessels, and are usually based on single-channel dataacquisition, and thus do not require extensive processing. HR seismic systems are widelyused for shallow to medium sub-bottom investigations, in both shallow- and deep-waterareas. In the latter case, the source may be built into a deep-towed vehicle to avoid the lossof energy over a thick water column.

Because the acquisition of HR data is relatively simple, concurrent acquisitionof ultrahigh-resolution (UHR) seismics is common, providing both good penetra-tion and resolution in one survey (Figures 16.6 and 16.7).

16.3.2.3. Ultrahigh-resolution seismicsFor detailed information of the sea floor and near-sea floor conditions, ultrahigh-frequency sources like sub-bottom profilers (3–10 kHz), multi and single-channelecho sounders (10–500 kHz) and side-scan sonars (10–500 kHz) are used. Due tothe high frequency of these systems, the penetration ranges from none (i.e. only thesea floor is mapped) to a few tens of metres below the sea floor. Thus, identifyingcontourite deposits exclusively using this type of seismic systems is difficult, con-sidering the triple-scale approach described above. Therefore, the use of UHRseismics in contourite investigations is mainly to identify and map sea-floor featuresrelated to bottom-current activity (Wynn and Masson, 2008). However, the UHRseismic systems lying at the lower end of the high-frequency range (�3–5 kHz) canbe useful for investigations of the shallower, most recent part of a contourite driftaccumulation (Figure 16.7) and for tying of high resolution sediment cores used inpalaeoceanographic studies (Knutz, 2008).



Figure 16.4 The effect of processing on the seismic character of the drift illustrated bydifferent versions of the same multi-channel LR seismic profile (air-gun source, maximumfrequency in data below 100Hz) across the Eirik Drift off South Greenland.The upper panelshows the impact of deconvolution parameters upon seismic resolution; the top-left panelshows the original data without deconvolution (no Decon), the top-central panel focuses ondeeper-lying structures using a 28/200 deconvolution parameter setting, and the top-rightpanel enhances the resolution in the shallow part using a 22/200 deconvolution parametersetting. Notice that first-order seismic elements (see Figure 16.1) can be followed on allthree versions, whereas marked dissimilarities occur at the second order and, in particular,third-order seismic-element level. The lower-left panels A1 and A2 are close-ups of the ‘‘noDecon’’ and ‘‘Decon 22/200’’ versions, respectively, illustrating how high-frequency wavyfeatures are enhanced on the latter (A2). Detailed examinations prove that these are realfeatures, probably caused by sediment waves.The lower-right panels B1 and B2 are close-upsof the ‘‘no Decon’’and ‘‘Decon 22/200’’ versions, respectively, illustrating how the seismic faciesof the subunits varies with different deconvolution parameters. The seemingly clear andcontinuous reflection pattern of the ‘‘no Decon’’ version (B1) is due to the low-frequencycontent, whereas the more crispy and less prominent reflection pattern of the ‘‘Decon 22/200’’version (B2) is the result of finer geological details being enhanced by the higher frequencycontent. A similar difference in the seismic-facies appearance is observable between A1 andA2. (Data courtesy of GEUS, Denmark. Data processed byR.Rasmussen, GEUS.)

We1400

1600

1800

twtt

(ms)

1 km

Figure 16.5 Single-channel high-resolution (HR) seismic profile off Southwest Portugal.The seismic system consisted of a Geo-spark 800 source and a 48-element streamer, resultingin a high vertical resolution (>30 cm) and a relative shallow penetration (<500m).The profileshows a series of up-slope migrating sediment deposits, interpreted as mega sediment wavesthat form part of a larger current-controlled accumulation deposited by the Mediterraneanoutflow water. Due to the high-frequency character of the seismic system, the base of thecurrent-controlled deposits cannot be mapped, and classification as a specific contourite typeis therefore not feasible on the basis of this type of seismics alone. In contrast, the HR of thedata permit detailed investigations of the relationship between the individual megawaves thatbuild up the youngest, near-surface part of the accumulation. Also this type of seismics is verysuitable for ground-truthing by shallowcoring. (Data courtesy of GEUS, Denmark.)


Using UHR seismics favours the resolution needed for detailed studies of theshallower part of the contourite, but lacks the penetration necessary to study thedeeper parts (Figure 16.8). Therefore, as mentioned earlier, a full-scale seismicstudy of thick contourite deposits requires the use of different types of seismicsystems in order to achieve both the high penetration needed to map the driftgeometry, and the high resolution necessary for sediment-core correlation.

(a)

(b)

WInternal unit? post-drift sediments?

Profile 1Profile 2

Profile 1Profile 2

? Top contourite ?

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se

Bedrock

7 km

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63°N

65°N

67°N

55°W 50°W

Davis Strait

100015001

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2000

2500

Labrador Sea

Green

land

Ice Sh

eet

Figure 16.6 Drift deposit off West Greenland, first recognised and mapped using a 2-D grid ofmulti-channel low-resolution (LR) seismic data, and subsequently studied in detail using high-resolution (HR) seismics (Nielsen et al., 2001, 2003). Inset map (lower right) shows the studyarea and the sea-floor bathymetry in metres. The bathymetric high located in the central partshows the elongated outline of the drift. Also shown is the position of seismic profiles 1 and 2.(a) Seismic profile 1. LR seismic data showing the seismic characteristics (see Figure 16.1)identifying the deposits as a contourite drift. (b) Seismic profile 2. HR seismic data revealing thedetails of the contourite drift (sedimentwaves confirming the influence of strongbottomcurrents).


(a)

(b)

(c)

2.8

2.3

1.8

twtt

(s)

sw ne( Lateral migration direction of the channel system)

Basal erosional unconformity

Basaltic basement

InternalunconformitiesLarge-scale units

Bottom-current direction away

Outcropping basaltsSand-wave fields

nw se

- Current direction

Lens-like sand body

Figure 16.7 Contourite depositional system at the outlet of the Faroe Bank Channel, theNorth Atlantic.The deposits were surveyed during theTTR 7 cruise (Nielsen et al., 1998a^b).Identification of the drift deposits was based on high-resolution seismic data, whereas arealmapping and reconstruction of the current pattern involved long-range and deep-towed side-scan sonar data. Further details of the current pattern also involved bottom-sampling results(Akhmetzhanov et al., 2007). (a) Map of the study area with outline and interpretation oflong-range side-scan mosaic. Position of profiles in figures b and c is also shown. (b) Single-channel HR air-gun seismic profile illustrating the seismic characteristics of the drift deposits(see Figure 16.1). (c) Deep-towed side-scan profile image (upper) showing bottom-currentsea-floor features and built-inUHR Pinger profile (lower) revealing details of the most recentdeposition at the surface of the contourite. (Modified from Kenyon et al., 2004 withpermission of Springer Science and BusinessMedia.)


16.3.3. Seismic mapping

In addition to the vertical and horizontal seismic resolution, the density of theseismic profiles, or the seismic grid, is of major importance for the resolution ofthe mapping. As the recognition of a contourite deposit is highly depending on theoverall drift morphology, a seismic study of contourites is ideally based on a grid ofdata (2-D) or a volume of data (3-D).

16.3.3.1. 2-D mappingThe conceptual profile shown in Figure 16.1 is constructed as a dip line, i.e. itillustrates a seismic profile perpendicular to the slope and bottom-currentdirection. If the model profile was directed parallel to the slope (i.e. a strikedirection), it would have appeared quite different. For instance, the

(a)

(b)1.1

1.2

1.3

2 km

twtt

(s)

DANA97/9-04

Figure 16.8 Comparison of two different seismic profiles across a contourite drift systemnortheast of the Faroe Island, illustrating how the use of different seismic systems is a trade-offbetween penetration and resolution. (a) Multi-channel HR sleve-gun seismic profile. Therelative low frequency allows penetration below the base of the drift deposit, making itpossible to map the overall drift morphology. However, the resolution is less than10m s�1 twtt,i.e. only layers more than �8^10m thick can be distinguished. Thus the internal reflectionpattern reveals only a rough picture of the interior of the drift. (b) Analogue UHR Pingerprofile across the same part of the drift system. The penetration is max. 60m s�1 twtt,preventing the base of the drift to be recognised, but the resolution is at least 1m s�1 twtt, i.e.internal layers less than 1m are revealed, allowing detailed investigation of the shallow part ofthe drift and tying to sediment cores.


characteristic moat in the up-slope direction would not be expressed on aslope-parallel profile. Moreover, the internal reflection pattern would bemore or less uniform and parallel, and therefore difficult to distinguish fromother types of marine deposits. Thus, when attempting to identify and mapsediment packages induced by bottom currents, not only the grid density, butalso the direction of the seismic profiles with respect to the overall driftgeometry should be considered. Examples of 2-D seismic mapping of contour-ites are shown in Figures 16.6 and 16.7.

16.3.3.2. 3-D mapping3-D seismic surveys provide the most accurate data for mapping contourite depositsin a process-oriented context. The ability to visualise geological surfaces fromseismic reflectors is particular relevant for identifying small-scale morphologicalfeatures such as moats, channels and migrating bedforms associated with contourites.3-D seismic studies are further enhanced by software tools that allow the extractionof different attributes from the seismic data (see below) and the ability to performhorizontal slicing of the 3-D cube. However, acquisition of 3-D seismic data is verycostly and mainly associated with hydrocarbon exploration. Thus, it is not readilyavailable for academic use and it provides only limited coverage of deep-sea areas.Another problem is that the seismic processing is often aimed at enhancing theresolution at depths typically varying between 1 and 4 s twtt at the expense of theacoustic clarity in the top 500 ms interval.

Despite the access limitations of commercial 3-D seismic data, the recent focus onenhanced imaging techniques in marine research means that such data will becomemore available for basic research in the near future. Also, a non-commercial, HR 3-Dseismic platform under development (Marsset et al., 2002; Praeg et al., 2006;Vanneste et al., 2007) should stimulate considerable progress in 3-D sub-sea-floorimaging of deep-sea contourites. The ability to map stratigraphic features in threedimensions at a detail otherwise unattainable has already provided new insightson processes and deposits related to contour currents (Figure 16.9) (Knutz andCartwright, 2003).

16.3.4. Seismic interpretation

A prime motive in the application of reflection-seismic methods in contouritestudies is to unravel the sediment-drift stratigraphy. Previously, this task waspursued simply by identifying the continuity, amplitude and stacking pattern ofthe reflections seen on the seismic profile. The recent advances in seismic technol-ogy, in particular the use of digital 3-D volumes, provide new opportunities forseismic facies and seismic attribute analysis, and thus extracting more detailedsubsurface information from contourite depositional systems.

Concurrent with the development of seismic methods, the terminology withinseismic interpretation has become increasingly confusing. Terms like ‘‘sequence’’,‘‘facies’’ and ‘‘attributes’’ appear in a multitude of contexts to the extent that the scientistmust clarify the meaning of these terms within the context of the specific study.


Figure 16.9 Detailed morphological reconstruction of a buried contourite-slide depositionalsystem based on 3-D seismic data from the continental slope west of Shetland (see multicolorversion of this figure on the enclosed CD-ROM). (a) Seismic cross-section with reflectorsrepresenting the Intra-Neogene Unconformity (INU) and two internal horizons demarcatingthe top of unit B1 (mounded drifts of Pliocene age) and of unit B2 (multiple slide events ofearly Pleistocene age) (Knutz and Cartwright, 2003). (b) Amplitude attribute extraction of theTop B2 surface, illustrating lobate slides and thin down-slope channels outlined by lightcontrasts presumably representing sand-rich features (Knutz and Cartwright, 2004). (c) 3-Dview of time-depth maps representing the INU and theTop B1 surface.The series of moundeddrifts separated by moat^channels have developed in response to southward-flowing bottomcurrents that were active during the Pliocene^early Pleistocene.The basal reflector representsthe regional erosional unconformity (INU) of late Miocene^early Pliocene age. Sea-floordepressions created by slumping are thought to have promoted the initial accumulation ofcontourites by acting as sediment traps for fine-grained clastic material supplied by along-slope currents.


16.3.4.1. Seismic unitsSince the concepts of ‘‘seismic stratigraphy’’ and ‘‘seismic sequence stratigraphy’’were introduced in the 1970–1980s (Vail et al., 1977a; van Wagoner et al., 1988),a whole set of terms has developed for seismic interpretation that are now widelyused. However, the conventional concept of seismic sequence stratigraphy – andthe terminology involved – is not fully applicable to contourite studies. While theconventional seismic sequence-stratigraphic concept implies that the sequences andtheir stratal components form in response to base-level changes and down-slopeshelf-to-basin sediment transport, contourites are primarily controlled by bottom-current regimes and sediment pathways orientated parallel to the continentalmargins. Also, in conventional seismic sequence stratigraphy the unconformitiesthat bound the sequences are inferred to be autocyclic and the results of predictablechanges in accommodation space, whereas unconformities in contourite depositstend to occur in a non-predictable pattern as a result of changes in current regime.Hence, the use of the conventional seismic sequence stratigraphic terminology indeep-water current-controlled depositional environments may lead tomisinterpretations.

In recent years, there has been an increasing demand for the developmentof a new, more comprehensive sequence-stratigraphic concept encompassingdeep-water current-controlled settings (Shannon et al., 2005). Although somedeep-water depositional processes have been included, this mainly concernsgravity-driven depositional settings (Posamentier and Kolla, 2003). At thepresent stage, a ‘‘seismic sequence’’ is still expected to have a depositionalaffinity closely tied to down-slope-directed sediment input and base-levelchanges. We thus suggest the use of the simple term ‘‘seismic unit’’ that isnot associated with any specific depositional environment to denote a strati-graphic subdivision of a sediment-drift succession. The breakdown into seismicunits, in contrast, may be based on analysis of reflection terminations andinternal reflector patterns using the traditional technique introduced byMitchum and Vail (1977) among others.

16.3.4.2. Seismic faciesAnalysis of the seismic facies is another method commonly used in the seismicinterpretation of contourites, and deals with investigation of vertical and lateralvariations of internal reflections. Synonymous terms are ‘‘seismic reflectionpattern’’, ‘‘acoustic facies’’ or ‘‘echo character’’ analysis, the latter mostly used inconnection with UHR seismic data.

Seismic-facies analysis in relation to contourite studies mostly deals with inves-tigation of the contourite sub-units, i.e. at the small-scale level. At the larger-scalelevel, one should refer to the ‘‘internal reflection pattern’’ rather than the ‘‘seismicfacies’’.

The seismic facies is relative to the type of seismic method employed. Thismeans that a seismic facies will display differently on different types of seismic data(Figure 16.3), and will also depend on processing parameters (Figure 16.4). Seismicfacies sometimes reflect sedimentary structures like waves and progradation.


However, because the reflections result from changes in the physical parametersthrough the sedimentary succession, there is no unequivocal correlation betweenseismic facies and sedimentary structures within the facies. A seismic facies char-acterised by a parallel reflection configuration, for instance, need not necessarilyindicate the existence of fine parallel banding or stratification of the sediments.

16.3.4.3. Seismic attributesThe advent of digital recording of seismic data and 3-D seismic technology hasfacilitated interpretation using seismic attribute analysis. A seismic attribute is ameasure of the character of the seismic data. As seismic reflections derive fromchanges in the physical parameters of the sub-surface, the character of the reflec-tions contains valuable clues of geological significance. An attribute can be ameasure of a single reflector, an interval (e.g. a sub-unit) or – in 3-D seismics – ahorizon.

Basically there are two categories of seismic attributes: those that quantify thegeometric aspects and those that quantify the reflectivity components of the seismicdata. The geometric attributes reveal information on dip, azimuth and terminationof a reflector or horizon, which can in turn be related to current-induced bed formslike waves, ripples, furrows, moats and channels. The reflectivity attributes revealinformation on reflector amplitude, frequency and phase, which in turn might berelated to lithology. As the types of attributes are numerous, so are the computa-tional methods and a variety of seismic attribute analysis techniques exist (Barnes,2001, 2006). In the context of contourite studies, the morphological attributes haveproven to be particular useful (Figure 16.9; see section ‘‘3-D mapping’’ above).

16.3.4.4. Seismic inversion (‘converting seismics into geology’)Seismic inversion, broadly defined, is the study of acoustic information like velo-city, impedance and amplitude to extract geological information of the subsurfacelayers like density, porosity and compaction. It is a difficult process, since theseismic measurements are limited and the earth extremely complex, and there aremany different inversion methods (Yilmaz, 1987).

In seismic depth conversion, the twtt is converted to depth. This processrequires estimation of the seismic velocity, which in fact is the most uncertainlink between seismics and geology. Therefore, contourite interpretation whichmakes use of core and borehole information in combination with seismics requiresinformation on the seismic velocity used for the correlation (Figure 16.10).


(a)

(b)

nw

se

MD95-2006

1 km

30 ms –1

twtt

IIIII

IB

IA

DF 3

25

40 90SI units

140

20

15

10

5

m

Magneticsusceptibility

MD95–2006

Lith

ol. u

nits

Lith

ofac

ies

ka B

P

1

2

3

4

5

10.89

14.86

17.66

24.31

25.81

33.48

I-A

I-B

II

III

~500 m

~23 m

Figure 16.10 Interpretation of muddy contourites from the Barra Fan on the UK Margin,using core information in combinationwith HR seismics.The correlation is based on a seismicvelocity of 1525 m s^1. (a) High-resolution seismic profile (DTBoomer) from the lower part ofthe fan. (b) Close-up of the profile and correlation with core lithostratigraphy (MD95-2006).The weakly layered upper unit IA, representing Late Glacial^Holocene deposition, isinterpreted as a glacial^marine hemipelagic facies with a gradual transition to a silty^muddycontourite in the upper 1.5 m. The parallel reflectors in unit II correspond to sandy turbiditelayers in the core record. Seismic unit III approaching the limit of acoustic penetration isinterpreted as a silty^muddy contourite in the lower core section. (Adapted from Knutz et al.,2002b, reproducedwith permission from Elsevier.)


16.4. SUMMARY

The seismic character of contourite features is to a large extent depending onthe methods used in acquisition and processing of the seismic data. Prerequisites foran adequate seismic study of contourites are therefore:

• identification of the type of seismic sources (LR, HR or UHR), acquisitionparameters (SCS or MCS; 2-D or 3-D), processing sequences (e.g. filtering,deconvolution) and display parameters (e.g. twtt versus depth, axes scale,polarity);

• recognition of how these factors affect the interpretation and level of detailresolved by the seismics in the contourite study;

• careful attention for the seismic-to-geology conversion when using core andborehole information (seismic resolution versus core data, velocity estimation);

• specification of the definitions and use of interpretational methods and terms(seismic sequence or unit, seismic facies) relevant for the current-controlleddepositional environment.

Large-scale First-order seismic elements(overall drift architecture)

Moat

Large-scale depositional units

Largeunconformities

Depositional sub-units

Seismic facies e.g. Reflection strength map

Second-order seismic elements(internal architecture)

Third-order seismic elements(seismic facies and attributes)

UHR

0–10 m 10–500 m 0.5–1.5 km >1.5 km

Pinger

Echo sounder

Side-scan sona

Sparker

Boom

erTopasC

hirp

Water gun

Air gun

Sleeve gun

arrays of:

Air gun

Water gun

Sleever gun

HR

LR

Medium-scale

Small-scale

Frequency

Resolution

Sources

Penetration

Sei

smic

Met

hods

Sei

smic

Cha

ract

eris

tics

Figure 16.11 Correlation between the seismic methods, frequency of the seismic source andthe seismic characteristics of contourite deposits.


Seismic identification and stratigraphic breakdown of a contourite depositionalsystem embodies three aspects, which reflect the increasing level of detail ofinterpretational seismic elements:

• first-order seismic elements (large-scale features):the overall architecture of the drift focusing on the gross geometry and large-scale depositional units;

• second-order seismic elements (medium-scale features):the internal architecture (drift structure) resolving the sub-units that build up thelarge-scale depositional units;

• third-order seismic elements (small-scale features):

the seismic attributes and seismic facies configuration of the depositional subunits.An overview of the correlation between the seismic methods, frequency of the

seismic source and the seismic characteristics is shown in Figure 16.11.

ACKNOWLEDGEMENTS

We acknowledge the good cooperation with the editors of this book and thereview by Dr J.S. Laberg. We thank The Geological Survey of Denmark andGreenland (GEUS) for allowing the reproduction of seismic data, and R. Rasmussen(GEUS) for providing good processing examples.


Documents

[Developments in Sedimentology] Contourites Volume 60 || Chapter 16 Seismic Expression of Contourite Depositional Systems