Marine and Petroleum Geology

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1 Marine and Petroleum Geology xxx (2010) 1e14 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift Carl Jörg Petersen a, Stefan Bünz a, *, Steinar Hustoft a, Jürgen Mienert a, Dirk Klaeschen b a University of Tromsø, Department of Geology, Dramsveien 201, N-9037 Tromsø, Norway b Leibniz Institute of Marine Sciences, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, Germany article info abstract Article history: Received 27 February 2009 Received in revised form 12 May 2010 Accepted 9 June 2010 Available online xxx Keywords: 3D seismic Fluid flow Acoustic chimneys P-Cable Gas hydrates Pockmarks The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to w500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at w160e170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Pockmarks represent common seafloor manifestations of fluid flow on continental margins around the world (Judd and Hovland, 2007). They were first described in the Scotia Shelf offshore Canada (King and MacLean, 1970). Pockmarks are defined as seafloor depressions, which are related to the escape of fluids and gases through the seabed. They can show a great variety of shapes and sizes with diameters ranging from 1 m to over 700 m (e.g. Hovland et al., 2002). Because of their association with seepage of methanerich fluids and gases, active pockmarks are usually characterized by precipitated authigenic carbonates (e.g. Greinert et al., 2001; Ritger et al., 1987) and chemosynthetic communities (Kennicutt et al., 1985). As a result of focused fluid flow, near-vertical wipe-out zones with little reflected seismic energy, so-called gas chimneys, may form beneath pockmarks representing feeding channels for * Corresponding author. Tel.: þ address: stefan.buenz@uit.no (S. Bünz). the upward migrating fluids and gases (Riedel et al., 2002; Zuhlsdorff and Spiess, 2004). On seismic data, gas chimneys are commonly characterized by low coherency, low amplitude, variable dip and pull-up or push-down effects of seismic reflections (e.g. Berndt et al., 2003; Ligtenberg, 2005; Westbrook et al., 2008b). Previous studies on the Norwegian continental margin document that bottom-simulating reflectors (BSR), which mark the base of gas hydrate stability, coexist with pockmarks and chimneys (Brown et al., 2006; Bünz et al., 2003; Mienert et al., 2001). Geochemical studies using sulfate gradients as flux indicator showed smaller fluxes inside chimneys located north of the Storegga Slide than in undisturbed sediment around them suggesting that these chimneys are not hydrate-lined fluid flow conduits (Paull et al., 2008). However, in the same region evidence for hydrate occurrence and active seepage in the pockmarks was found based on seismic data and core samples (Ivanov et al., 2007; Westbrook et al., 2008b). Thus, a broad range of structural styles and levels of fluid flow activity seems to occur on a local scale on the Norwegian margin. Located west of Svalbard at 80 N, the Vestnesa Ridge represents one of the northernmost gas hydrate /$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.marpetgeo

2 2 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 provinces that exist along Arctic continental margins (e.g. Eiken and Hinz, 1993; Posewang and Mienert, 1999). The pockmarks at the Vestnesa Ridge were first discovered and described by Vogt et al. (1994, 1999) who speculated that the pockmarks were formed by active or recent upward rising methane flow collecting in the ridge-crest trap. The occurrence of gas hydrate-bearing sediments, the evidence for active fluid flow and the geological setting on a young and sedimented ocean ridge make the Vestnesa Ridge a key location to study the interaction of gas hydrate formation and focused fluid flow as well as the possible impact of methane seepage on Arctic environments. In the recent past, 3D seismic data became an increasingly important tool for studies of geofluids (Bünz et al., 2005a; Gay et al., 2007, 2006; Hornbach et al., 2008; Hustoft et al., 2007; Purdy et al., 2008; Riedel, 2007; Sultan et al., 2007). However, 3D seismic research studies are mostly limited to areas of hydrocarbon exploration. Moreover, conventional 3D seismic technology is designed to image targets at greater depths focusing on hydrocarbon reservoirs, and processing of this type of data does not account for shallow subsurface structures. Many areas of scientific interest in the field of marine geology and geophysics are outside any conventional 3D seismic coverage, e.g. the Arctic regions, and the poor resolution of conventional 3D seismic data leaves many scientific questions unresolved. Therefore, the University of Tromsø (Petersen et al., 2008) in cooperation with Volcanic Basin Petroleum Research (VBPR, Oslo), National Oceanographic Centre Southampton (Southampton University) and IFM-GEOMAR (Kiel University) developed a lightweight high-resolution 3D seismic system, the P-Cable system (Planke and Berndt, 2004). In contrast to several 3D seismic studies, which used a set of parallel, closely spaced high-resolution 2D lines to construct a 3D seismic data volume (also called 2.5D seismic) (e.g., Hornbach et al., 2008; Wagner-Friedrichs et al., 2008; Zuhlsdorff and Spiess, 2004), the P-Cable system offers full 3D seismic data acquisition using a set of parallel streamers with high-resolution seismic sources, which make this system unique in academic research. Our study presents one of the first high-resolution 3D seismic studies using the P-Cable system to characterize the internal structure of chimneys to improve our understanding of the plumbing system in gas hydrate environments. The main purpose of this paper is to present high-resolution 3D seismic data of the P- Cable system, its acquisition and processing as well as its value in understanding focused fluid flow features piercing through the gas hydrate stability zone (GHSZ). Fig. 1. Simrad EM300 multi-beam bathymetry (grid size 50 m) of the W-Svalbard margin showing the Vestnesa Ridge with the location of the 3D seismic survey and the OBS; inlay figure: High-resolution EM300 multi-beam bathymetry (grid size 15 m, due to multiple coverage during 3D survey) shows abundance of pockmarks at ridge-crest.

3 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 3 Fig. 2. (a) Schematic diagram of P-Cable 3D seismic system; (b) and (c) Comparison between P-Cable 3D seismic data (bin size 10 m) and high-resolution 3D seismic exploration data from the Lower Congo Basin(bin size 12.5 m) (Gay et al., 2007), identical horizontal and vertical scales. 2. Regional setting The Vestnesa Ridge is a sediment drift located on hot and young oceanic crust at the eastern spreading segments of the Molloy Ridge in the Fram Strait west of Svalbard (Fig. 1)(Engen et al., 2008; Hustoft et al., 2009; Ritzmann et al., 2004). Seafloor spreading at the Molloy Ridge probably initiated at 19.6 Ma and was well established at 9.8 Ma (Engen et al., 2008). The Vestnesa Ridge is covered by more than 2 km thick sediments deposited as contourites by prevailing northward directed contour currents during the late Miocene and Pliocene (Eiken and Hinz, 1993). Deposition was probably stimulated by a rough underlying oceanic basement that reduced the speed of the northward directed West Spitsbergen Current (Vogt, 1986). The crest of the Vestnesa Ridge consists of silty turbidites and muddy-silty contourites of mid-weichselian and Holocene age, and the post 19 ka sedimentation rate was estimated to 9.6 cm/ka on average (Howe et al., 2008). Several seismic studies revealed the occurrence of a prominent bottom-simulating reflector (BSR) along the W-Svalbard margin (Eiken and Hinz, 1993; Mienert et al., 1998; Posewang and Mienert, 1999; Vanneste et al., 2005; Westbrook et al., 2008a). Hydrate concentrations derived from seismic velocity analyses show up to 11% (Hustoft et al., 2009) and 12%(Westbrook et al., 2008a) in the pore space of sediments above the BSR. The inferred gas-hydrate province on the NW- Svalbard margin covers an area of w3000km 2 (Bünz et al., 2008; Hustoft et al., 2009; Vanneste et al., 2005). Fluid fluxes of maximum 0.15 mm a 1 were estimated for the W-Svalbard margin based on numerical modeling assuming a steady-state 150-m-thick free gas zone beneath the BSR at w200 m below the seafloor (m bsf) (Haacke et al., 2008). The 3D seismic data volume is located at the western crest of the Vestnesa Ridge between 1200 and 1300 m of water depth (Fig. 1). Numerous pockmarks concentrate on the crest of the ridge (Fig. 1) (Vogt et al., 1994, 1999) and gas flares indicate ongoing focused fluid flow activity (Hustoft et al., 2009).

4 4 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 Fig. 3. Flowchart of 3D seismic data processing. 3. The P-Cable 3D seismic system The P-Cable system represents a high-resolution 3D seismic imaging tool, which consists of a cable towed perpendicular to the ship s steaming direction, a so-called cross cable, that is spread behind the vessel by two large trawl doors (Planke and Berndt, 2004; Fig. 2a). Up to 24 multi-channel streamers with a length of 25 m are attached to the cross cable. The array of single-channel streamers acquire 24 seismic lines simultaneously, thus covering an approx. 240 m wide swath with close inline spacing in a cost efficient way. GPS antennas are mounted on both the gun float and the trawl doors to ensure accurate navigation with uncertainties of less than a meter. The spatial resolution of such a system is at least one order of magnitude higher than conventional 3D seismic, whereas the temporal resolution is improved 3e5 times. The increases in resolution facilitate a much better target identification and achieve a much more accurate imaging of, for example, shallow subsurface structures and fluid flow systems. The advantages and benefits of the P-Cable seismic system with respect to data quality are illustrated in Fig. 2b and c. It shows a comparison of high-resolution 3D seismic exploration data from the Lower Congo Basin (Gay et al., 2007) with P-Cable data from a comparable target. Both images have the same horizontal and vertical scale and the P-Cable data shows superior quality in both horizontal and vertical resolution. The 3D seismic data presented in this paper were acquired in July 2007 (Mienert et al., 2007 submitted for publication) using the P-Cable 3D seismic system of the University of Tromsø. In total, an area of 23 km 2 was surveyed with 32 profiles using 8 or 12 parallel streamers (Fig. 1). Two GI guns with a total volume of 240 cubic inch provided seismic energy with frequencies from 20 to 250 Hz. A Fig. 4. Acquisition geometry calculation: (a) Triangular cross cable geometry assumed as starting geometry; (b) Streamer-channel and shot point positions after source and receiver relocation; (c) final CMP coverage.

5 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 5 Fig. 5. Illustration of residual static correction: (a) Seafloor bathymetry derived from 3D seismic data before residual static correction, lateral inconsistencies are due to variations in streamer depth; (b) Comparison of seafloor arrival times picked in 3D seismic data (black line) and calculated from multi-beam bathymetry (red line), differences are plotted as black dots, residual static correction is done according to smoothed residuals (green line); (c) Result of residual static correction: stripy patterns were successfully removed. shot rate of 10 s and ship speeds of about 4 knots yield shot point distances of about 20 m D seismic data processing The flowchart in Fig. 3 illustrates the data processing consisting of two major parts, the navigation processing and the seismic data processing. This was accomplished by using shell scripts, Seismic UNIX (SU) and Landmark s ProMAX software. First, the navigation data from the GPS antennas were processed to ensure reliable and accurate streamer-channel and shot coordinates. Interpolation of data gaps and smoothing of scattering data were applied depending on data quality. A triangular shape was assumed as a starting geometry of the cross cable and streamer-channel positions were calculated (Fig. 4a). Picking of the direct wave arrival allowed the calculation of the true source-receiver offsets for all channels. Subsequently, the triangular geometry was modified according to the source-receiver offsets derived from the direct wave arrival by using shift parameters (X-Shift and Y-Shift in Fig. 4a) derived from a leastsquares best-fitofthefirst channels of the outer streamers. Then, all streamer-channel positions were relocated using source position and the picked direct wave arrival. Each streamer was shifted in the direction of the previous shot until the source-receiver offset of the first channel matched the value derived from the direct wave arrival. The result is shown in Fig. 4b. The triangular geometry changed to a rather parabolic, more realistic shape of the cross cable. The uncertainty for each streamer-channel position is estimated to be below 1 m, much smaller than the bin size that was used for stacking. The common midpoint (CMP) distribution shown in Fig. 4c documents the dense 3D coverage and the high spatial resolution achieved. Although the tidal variation in the survey area was rather small (60 cm corresponding to shifts of 0.8 ms), it was significant enough to be taken into account. In order to derive water depths at the CMP locations from the 3D seismic data and to compare those with multibeam bathymetry data, a normal moveout (NMO) correction was applied. However, after tidal and NMO corrections lateral inconsistencies still remained in the data. They appear as stripy patterns in Fig. 5a, which shows the bathymetry derived from the NMO corrected seafloor reflection times of the 3D seismic data. Variations in streamer depths are likely to represent the main cause. The streamer depth is mainly controlled by the slack of the cross cable, which changed during the survey due to varying ship speed and currents that affected the acquisition geometry. In addition, waves in varying sea conditions produce jittering along profile due to variations of the air gun depth from shot to shot, which was corrected with a swell filter. The seafloor reflection travel times for the last channel of each streamer were picked automatically for both the swell filter and the residual static shift to correct for changes in streamer depth. The obtained travel times were converted to water depths and compared to high-resolution multi-beam bathymetry at the CMP locations (Fig. 5b). Due to the different frequencies of the systems, small-scale bathymetric features like the pockmarks appear differently in the depth profiles, e.g. the pockmarks in Fig. 5b show differences of up to 2 m in-depth and up to 100 m in width. Therefore, only the long-wave component of the residuals represents an estimate of the streamer depth variations, which were assumed to have occurred on a minute-to-hour scale. Consequently, the residuals were smoothed (green line in Fig. 5b) before the static was applied to avoid the incorporation of large residuals at the pockmark locations. Prior to smoothing, residuals larger than 3 ms were discarded, since they represent erroneous measurements. Fig. 5c shows the final result of the residual static correction. The processing successfully removed the stripe pattern that distorted the bathymetry derived directly from the 3D seismic data. The 3D seismic data were then processed with the ProMAX software (Fig. 3). A bin size of 10 m was used for the data presented herein. Afrequencyfilter with a pass band of 30e40e190e220 Hz was applied and the data were resampled to 1 ms before stacking. Finally, amplitude processing of the final stacked 3D seismic data volume removed amplitude artefacts across inlines. A spatial deconvolution noticeably reduced noise and increased lateral coherency. The stacked seismic data were not post-stack time-migrated at this point because of some gaps in the data that were due to missing shots, winding navigation and streamer malfunctions. Because the areas with data gaps were too large to be successfully interpolated, a post-stack time migration degraded the image quality of the internal chimney structures and introduced significant amounts of artefacts. We present the unmigrated data for the interpretation keeping in mind that dipping events are misplaced, diffraction hyperbolas have not been collapsed and

6 6 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 Fig. 6. (a) Hydrophone component of OBS, direct wave is flattened for display reasons, 19 phases were used for inversion (plotted as colored lines); (b) V P -depth model obtained from travel time inversion (solid line), reference curve for hydrate and gas-free sediment derived from ODP Site 986 (dashed line) located 200 km to the south (Hustoft et al., 2009); (c) Tie in of OBS data with closest inline of 3D seismic data with possible top of hydrate, BSR and base of free gas indicated. amplitudes are shifted slightly from their real locations. Thisdecision is based on the fact, that, without detailed knowledge of the velocity distribution outside and inside the chimneys, even a migration would not result in a truesubsurface image, but rather distort the image when wrong velocities are used. The moderate dips in the data (<1ms/10m) and the importance of lateral coherency preservation justify this approach. Better interpolation methods for seismic data and migration procedures for post-stack seismic data with gaps are currently being evaluated for future application. Also successful strategies for a migration of P-Cable data with lateral velocity anomalies (e.g. chimney structures) need to be developed and implemented. 4. Ocean bottom seismometer (OBS) OBS wide-angle seismic data provides constraints on the seismic velocities of the underlying sediment. Therefore, we deployed an OBS on the seafloor at the study site (see Fig. 1 for location) to obtain a 1D V P -model. The OBS profile (Fig. 1) was shot prior to the 3D seismic survey. Unfortunately, a malfunction of the P-Cable system meant that the 3D seismic survey was not carried out across the OBS location. However, the distance from the 3D seismic area to the OBS is only 200 m allowing the velocity model to be extrapolated into the 3D seismic data volume.

7 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 7 Fig. 7. Perspective view of depth-converted 3D seismic data showing high amplitudes associated with free gas accumulations below the BSR (green surface) and acoustic blanking associated with gas chimneys, which pierce through sediments from depth of at least 500 m bsf. Possible top of gas hydrate is indicated as red surface.

8 8 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 Fig. 8. Seafloor reflection amplitude map derived from 3D seismic data with locations of areas and profiles (dotted lines represent seismic lines shown in Fig. 9). Pockmarks appear as positive amplitude anomalies, but also show decreased amplitudes at their rims. Due to the limited streamer length of 25 m, no velocity information can be derived from the 3D P-Cable system. However, the OBS data enable an estimation of the sediment velocities. We used the hydrophone component (Fig. 6a) to model P-wave velocities (V P ) applying standard methods (e.g. Petersen et al., 2007; Bünz et al., 2005b; Plaza-Faverola et al., 2010) based on the rayinvr travel time inversion (Zelt and Smith, 1992). A total of 3051 data points were picked along 19 reflection phases. The model quality is assessed with the normalized chi-squared value, which should have a value of 1 or less. Assuming a pick uncertainty of 1 ms, we obtained a chi-squared value of that underlines the robustness of the velocity model shown hereinafter. 5. Distribution of gas hydrate and free gas Fig. 7a shows a 3D perspective view of a selected inline and two crosslines of the seismic data volume. The image shows the generally well-stratified sedimentary structure of the Vestnesa Ridge. However, the data also show vertical zones of acoustic transparency or disrupted reflections that pierce through the strata. These structures are similar to structures observed on the mid- Norwegian margin (Bünz et al., 2003) and can be classified as pipe or chimney systems that are the consequence of focused fluid flow through sedimentary strata. The pipe structures are most commonly connected to seafloor depressions (Fig. 7a) interpreted as pockmarks a widespread expression of fluid flow at the seafloor (Judd and Hovland, 2007). A bottom-simulating reflector (BSR) at 160e170 m bsf occurs at the predicted depth of the gas-hydrate stability zone on the seismic data (Vanneste et al., 2005) and provides evidence for the presence of gas hydrates (Fig. 7a, b). The BSR can be identified along the termination of enhanced reflections (Fig. 7a). A strong BSR and highamplitude sub-bsr reflections also indicate the occurrence of significant amount of free gas (Fig. 7a, b). The base of the GHSZ indicated by the BSR surface marks the lower boundary of a transparent layer (shown in green) commonly associated with an acoustic blanking zone due to the presence of gas hydrates (Figs. 6c and 7a). In the area of the fluid flow structures, the BSR is noticeably disrupted. Here, the base of the GHSZ might be significantly deformed by the vigorous flow of fluids, some of which might result from the hydrothermal circulation in this setting near the mid-oceanic ridge. The presence and concentration of gas hydrates in marine sediments is commonly inferred from the measurement of increased P-wave velocities within the GHSZ (e.g. Bünz and Mienert, 2004; Holbrook et al., 1996; Petersen et al., 2007). Here, velocities from the OBS located just outside the area of focused fluid flow along the crest of the Vestnesa Ridge (Fig. 1) indicate the presence of gas hydrates and free gas within sediments of the Ridge. All pipes structures pierce through the hydrate-bearing sediments. Travel time inversion of the OBS data yields the 1D V P edepth model shown in Fig. 6b. V P increases with depth from the seafloor reaching 1800 m/s at the BSR, where velocity drops abruptly down to 1250 m/s due to the presence of free gas. The obtained velocity profile can be compared to a reference curve for hydrate-free sediments (dashed solid line in Fig. 6b), which was derived from porosity measurements at ODP Leg 162 Site 986 located about 200 km away (Forsberg et al., 1999; Hustoft et al., 2009; Jansen et al., 1996). At the OBS location, we obtain higher velocity in a 44 m-thick interval above the BSR relative to the reference curve. In a 20 m interval directly above the BSR, V P increases to a maximum value of 1800 m/s. At the BSR, P-wave velocity abruptly decreases to 1250 m/s within a 7 m-thick interval. Further below, it increases again to 1400 m/s in a 15 m-thick layer, and further increases to 1600 m/in a 13 m thick interval. These lowvelocity layers indicate the presence of free gas within sediments. Overall, the sub-bsr low-velocity layer has a thickness of 35 m relative to the jreference curve. The long distance to the ODP reference site adds a degree of uncertainty to the interpretation of the velocity curve. However, previous seismic studies at the Vestnesa Ridge showed very similar velocity distribution with depth (Westbrook et al., 2008a; Hustoft et al., 2009). Thus, the positive V P anomaly above the BSR is associated with the occurrence of gas hydrate within the sediments. According to Hustoft et al. (2009), who estimated hydrate and gas concentration a few km further south on the Vestnesa Ridge, we can therefore expect similar gas hydrate concentrations of up to about 11% of pore space directly above the BSR and average concentrations of approximately 6% for the gas hydrate-existence zone. Also, we may assume average free gas concentrations of about 2% for the sub-bsr gas layer. If we tie in the OBS data with the closest inline of the 3D seismic data volume, we see that the layer with increased V P above the BSR corresponds to a distinct stratigraphic unit (Fig. 6c). This unit also appears acoustically more transparent than other units, which could be a result of the acoustic blanking effect commonly associated with the presence of hydrates in marine sediments (Holbrook et al., 2002). Presuming that hydrate occurrence is restricted to this

9 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 9 Fig. 9. Close-up perspective view of the seafloor reflection amplitude (same color scale as in Fig. 8) and a selected inline and crossline (area is marked as white box in Fig. 8) showing the internal structure of a gas chimney in detail. interval, the top of gas hydrate zone (TGHZ) can be traced along the sedimentary reflection through the entire 3D data volume (marked red surface in Fig. 7b). The part of the GHSZ, which extends from the seafloor down to the BSR, has a total volume of 3.86 km 3 inside the 3D study area of 23 km 2. The BSR surface and the TGHZ surface enclose a volume of 0.68 km 3, which is w18% of the sub-seafloor GHSZ volume. Since the single OBS station does not allow for a solid and reliable quantification of the entire 3D volume, in the future an array of OBS stations is to be planned jointly with the 3D survey. Nevertheless, the P-Cable 3D seismic data enable an efficient and accurate reservoir mapping of shallow gas/gas hydrate reservoirs. 6. Seabed and sub-seabed expressions of focused fluid flow The seafloor map of the 3D seismic data shows circular to elliptically shaped depressions that are interpreted as pockmarks, seabed expressions of focused fluid flow (Judd and Hovland, 2007). The pockmarks have a diameter of up to 700 m and depressions of up to 10 m, which are very typical values for these structures on the Norwegian Margin (Hustoftet al., 2009; Hjelstuen et al., 2010). We have selected two areas (A and B in Fig. 8) for the 3D seismic characterization of the focused fluid flow system on the Vestnesa Ridge. These two areas show characteristics that are representative for the total of 18 structures that were identified in the 3D seismic data volume. Fig. 8 shows a map of seafloor reflection amplitudes derived from the 3D seismic data with selected locations of seabed anomalies shown in detail hereinafter. In general, the seafloor shows rather uniform reflection amplitudes outside of the pockmarks. Inside the pockmarks, very heterogeneous amplitude patterns exist. The majority of structures exhibit positive amplitudes in or close to the centre, which are in some cases partly or fully encircled by a rim of decreased amplitudes relative to the surrounding seafloor (Figs. 8, 9 and 10a and 11a). Some pockmarks are characterized by a homogeneous positive amplitude anomaly, whereas others seem to intersect each other and exhibit complex amplitude pattern. The observed high amplitudes at and close to the seafloor

10 10 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 Fig. 10. (a) Seafloor amplitude map of area A (same color scale as in Fig. 8); (b) Seismic inlines A-G shown in (a); (c) Time slices 1e4 showing the internal structure of the chimney at different times, locations are marked in (b). (Figs. 8, 9 and 10a and 11a) might be indicative of a hard surface, e.g. precipitated carbonates and/or hydrate formation at the seafloor (e. g. Greinert et al., 2001) but only groundtruthing can confirm this assertion. In contrast, areas of low seafloor reflectivity could be associated with soft, muddy sediment, which has been recently extruded and frequently contains bubble-phase gas (Ritger et al., 1987; Roberts et al., 2006). The variability and heterogeneity of seabed pockmarks seen on the high-resolution 3D seismic data might be a sign of a relatively recent level of activity of these fluid flow structures. Beneath the seafloor pockmarks the seismic data shows vertical zones with a large heterogeneity and variability in terms of internal acoustic character. These vertical structures are partly characterized by zones of acoustic transparency but also show irregularly distributed high-amplitude patches within the GHSZ zone (Figs. 7 and 9). The seismic characteristics of these structures are very typical of focused fluid flow features on the mid-norwegian margin (Berndt et al., 2003) and of those that lie a few km to the southeast on the Vestnesa Ridge (Hustoft et al., 2009). These structures are commonly called pipes or chimneys (Cartwright et al., 2007). Fig. 9 shows a perspective view of a selected inline and crossline, which intersect with the centre of a large pockmark located in area A (for location see Fig. 8). Seismic data of area A is also displayed in Fig. 10b, which shows one of the larger chimneys in greater detail. We observe high-amplitude reflections within the chimney in the upper 50 ms TWT, i.e. about 40e50 m bsf (Figs. 9 and 10b). These reflections may be caused, like the seafloor features, by the presence of massive hydrate layers and/or authigenic carbonates or the presence of shallow gas within these structures (e.g. Holbrook et al., 2002). However, since the precipitation of carbonates is restricted to the sulfate reducing zone (few meters below the seafloor), carbonates must have been buried beneath sediments after their formation to occur at greater depth. In this case, precipitation of the deepest carbonates must have started at least 61e70 ka assuming maximum sedimentation rates of 9.6 cm/ka for the last 25 ka and 105 cm/ka for older periods (Howe et al., 2008). Given the fact that these structures appear to have formed recently (Hustoft et al., 2009) it appears doubtful that carbonates occur at depth beyond the sulfate-reducing zone. Hence, the origin of the high amplitudes most likely is due to gas hydrates or free gas.

11 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 11 Fig. 11. (a) Seafloor amplitude map of area B (same color scale as in Fig. 8); (b) Seismic inlines A-G shown in (a); (c) Time slices 1e4 showing the internal structure of the chimney at different times, locations are marked in (b). Significant disruption of sedimentary strata at the chimney outer limits can be identified. Close to the seafloor the sediment layers on the seismic profiles are clearly pushed down with respect to the surrounding strata. Approaching the base of gas hydrate stability, which is marked by the BSR, the internal reflections are shifted to earlier times, representing velocity pull-up features (Fig. 9). If these observations are due to the presence of gas causing a low-velocity anomaly or gas hydrates causing a high-velocity anomaly, or if they are the consequence of the mechanical deformation of the sediments is still unclear and requires further detailed analysis of all chimney structures in the 3D seismic volume. However, we suggest that it is most likely a combination of these effects. Push-down features could be due to free gas close to the seafloor within these apparently very active fluid flow features (Hustoft et al., 2009). Pull-up features close to the base of the GHSZ could be due to gas hydrates which occur in higher saturations directly above this surface. The flow of fluids within the chimneys probably results in higher concentrations of gas hydrates within these structures. When entering the free gas zone beneath the BSR, push-down effects are observed again that can be related to the presence of gas underneath the hydrates. The complex nature of the chimneys is also demonstrated by looking at time slice representations of the 3D seismic data that enable the visualization of the internal distribution of amplitude anomalies within the chimneys. Four selected time slices of area A are shown in Fig. 10c. Time slice 1 images the near-seafloor sediments at 1790 ms TWT (two-way time) bsl (below sea level) and reveals a heterogeneous, composite pattern of positive and negative amplitude anomalies, which is confined to the inner part of the pockmark. The acoustic chimney structure can be easily followed downwards with the time slices 2, 3 and 4. They visualize changes in both the chimney dimension and its internal structure. The fringe of the chimney can be traced down well below the BSR based on disrupted or distorted amplitudes. Apparently, the chimney diameter increases significantly with depth, e.g. from a short axis of about 400 m at the seafloor to about 600 m at 2020 ms TWT, though, this observation could be hampered by the fact that the data have not been migrated. A slight counter-clockwise rotation of

12 12 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 Fig. 12. Perspective view of a selected time slice at 1795 ms (close to the seafloor), an inline and a crossline (area is marked as white box in Fig. 8 and similar to Fig. 9) showing multi-fracturing processes and non-vertical chimney geometry, emphasizing the high-resolution of the P-Cable 3D seismic system. the chimney with depth can be observed. Whereas the shallow amplitude anomalies were confined to the inner parts of the chimney, the deeper time slices show that amplitude anomalies occur rather in the chimney rim, where they appear to follow the oval shape of the chimney geometry. The amplitudes in the inner part show significant amplitude blanking (time slices 3 and 4). Fig. 11 shows chimney structures of area B in detail (see Fig. 8 for location). The 3D seismic data show a complex chimney appearance. Also here, irregularly distributed high-amplitude reflections occur in the GHSZ, which are strongest in the sediment layer at 1800 ms TWT bsl (just above time slice 2, line B and G) and close to the seafloor (line B) (Fig. 11b). One high-amplitude patch seems to represent the top of a buried chimney structure (line D). Other amplitude anomalies appear as laterally continuous patterns inside the chimneys (line B). However, the chimney structures above and below the high amplitudes are discontinuous and show areas with amplitude blanking as well as undisturbed sediments. The four time slices shown in Fig. 11c reveal the internal structure of the chimneys. In slice 1 at 1763 ms TWT, circular bands of increased reflectivity are observed beneath some of the pockmarks, but increased seafloor amplitude patterns do not automatically imply increased sub-seafloor reflectivity. Similar to area A, the deeper time slices indicate that the chimney diameters increase with depth and amplitude anomalies seem to shift to oval bands at the chimney rims. However, the observation of the downward widening of the chimney structures does not warrant a further indepth interpretation given that the data have not been migrated. Fig. 12 shows a time slice, an inline and a crossline of the area marked by the white box in Fig. 8, which is the same area that is shown in Fig. 9. The inline cuts through a chimney structure and reveals a complex, heterogeneous distribution of high amplitudes just above the BSR. The time slice at 1795 ms TWT close to the seafloor shows also increased amplitudes. Furthermore, the inline cross-section of the chimney indicates that the position of the chimney centre shifts laterally with depth. As an alternative to the formation of hydrate and carbonate, the occurrence of free gas zones within the GHSZ might contribute to the observed high amplitudes in the sediment. A possible mechanism for preserving free gas within the GHSZ suggests that it is surrounded by hydrated, impermeable sediment, which prevents water getting into contact with the free gas (Suess et al., 1999). In principal, the polarity of the high-amplitude reflections indicates whether the cause is an impedance increase due to hydrates or carbonates or an impedance decrease due to free gas. However, in stratified sediments it is often difficult to determine the type of impedance contrast visually from the data because of thin-bed tuning effects which overprint the phase character that would be observed at a first order discontinuity. For a reliable polarity analysis a waveform inversion should be applied (Holbrook et al., 2002). Thus, we cannot interpret with confidence the polarities of the high-amplitude patches observed in our data without having first applied such an inversion. However, following Holbrook et al. (2002) the patches might be related to local velocity increases due to hydrate or carbonates than to free gas accumulations.

13 C.J. Petersen et al. / Marine and Petroleum Geology xxx (2010) 1e14 13 The inline section displayed in Fig. 12 intersects a gas chimney and shows that the chimney centre is not stationary with depth. The chimney structure is not straight vertical, but rather contorted and laterally shifted, i.e. the chimney s centre conduit at the BSR depth is shifted by about 200 m compared to its location close to the seafloor pockmark (Fig. 12). The discontinuous strata reflections of the chimney shown in Fig. 12 may indicate a sub-vertical conduit consisting of heterogeneously fractured sediments created under high pore fluid pressure. These hydrofractures might represent the currently open fluid pathway that is created according to the principle of least resistance when fluid pressure exceeds a specific threshold (Hubbert and Willis, 1957). Episodically active, vigorous and focused fluid flow could explain the indicated multi-fracturing processes that control the plumbing system and lead to extensive pockmark formation at the crest of the Vestnesa Ridge. This kind of internal chimney structure has not been previously documented by 3D seismic imagery, and it emphasizes the capability of the P-Cable 3D seismic system to identify subtle details of geo-objects such as gas chimneys and associated seafloor anomalies. 7. Conclusions The P-Cable 3D seismic system provides a high-resolution acoustic imaging tool for the shallow geosphere. Integrated with velocity information from ocean-bottom seismic measurements it allows a detailed characterization of gas hydrate and fluid flow dominated geological settings such as the sediment drift at the Vestnesa Ridge. The exceptional detail of the P-Cable 3D seismic system allows for an accurate imaging of gas hydrate reservoirs and the dynamic free gas system. High-resolution imaging of focused fluid flow structures provides insights into processes related to fluid expulsion of former or recent activity. The internal structure of these chimneys appears very complex as demonstrated by a great variety of amplitude anomalies that characterize the conduits for fluids and gases ascending from greater depth to the seafloor. The internal structure shows both low (amplitude blanking) and high amplitudes. The heterogeneous distribution of high-amplitude reflections above the BSR and at shallower depth in the upper 40e50 m bsf suggests formation of massive hydrate and/or authigenic carbonate in the vicinity of multi-fractured fluid pathways. Since carbonate precipitation is restricted to the upper meters below the seafloor, we associate the deeper high-amplitude reflections above the BSR with the occurrence of hydrate layers. An alternative explanation may be that free gas causes the high reflectivity if it is enclosed by gas hydrates. 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