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Integrity of overburden formations to the underlying reservoir rocks plays a vital role in permanent storage of CO2 within geological traps. Focused fluid escape structures (also known as chimney and pipe structures) are highly permeable (sub-)vertical geological features within the sedimentary rocks that can represent a substantial risk to geological CO2 storage projects. Despite the importance of such structures to the subsurface hydraulic system, their internal structures and fluid flow properties are poorly constrained to date. In this study, we examined the internal structure of a representative gas chimney underlying the Scanner Pockmark Complex in the North Sea, where methane is currently leaking to the seabed. We recovered an acoustic velocity model of the chimney structure and the host formations, using 3-D full waveform inversion (FWI) technique. The inverted data consists of ~125000 seismic traces from 23 ocean bottom seismometers (OBS) covering an area of 66 km2. We set the FWI scheme to include only diving waves at offsets of 1300-4600 m, corresponding to incident angles of about 60º to 80º respectively at the maximum well-resolved depth of 700 mbsl. The inverted model, at the maximum frequency of 7 Hz, revealed a complex fluid escape structure with an indirect vertical gas migration pathway that manifested itself as a simple gas pipe structure on a coinciding seismic reflection image. In addition, integration of the recovered velocity structures with a calibrated Gassmann model suggested a patchy gas saturation within the area., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

European Geosciences Union (EGU) General Assembly, 7-12 April 2019, Vienna, Austria.-- 1 page, In 2013, we carried out the Galicia-3D controlled-source reflection-refraction seismic experiment at the Deep Galicia rifted margin in the northeast Atlantic Ocean, west of Spain. We acquired more than 3200 km of seismic profiles within a three-dimensional (3D) box measuring 64 by 20 km (1280 km2). The main features within this box are: the peridotite ridge (PR), composed of serpentinized peridotite; a series of fault bounded, rotated basement blocks; and the S reflector, which has been interpreted across most of the box as a low angle detachment fault forming the crust-mantle boundary. Shots from two 3300 cu.in airgun arrays fired alternately in a flip-flop configuration, with a shot spacing of 37.5 m, were recorded by four six-kilometer streamers of R/V Marcus Langseth, as well as by 72 Ocean Bottom Seismometers (OBS) of the Galicia-3D network. We present tomographic results obtained using three well known controlled-source seismic tomography codes: the three-dimensional (3D) first-arrival time seismic tomography code FAST, and the 3D joint refraction-reflection travel-time tomography codes TOMO3D and JIVE3D. Results obtained with three different codes were compared with the seismic reflection images of the area, and this comparison confirms that the main features of the area are accurately represented in all three results. The 3.5, 5 and 6.5 km/s contours match well the top of the acoustic and crystalline basements and the S-reflector, respectively.Largest differences between results ofdifferent codes occur, as expected, in poorly resolved areas. n addition, due to differences in parameterisations and inversion schemes, the same structure is expressed in three different ways in the results. Below the S-reflector, P-wave velocities indicate a degree of serpentinisation of 45 to 30 %. The distribution of the serpentinisation allows us to estimate the amount of water reaching the mantle, and suggests that the water reaches the mantle when faults are active. Our models allow us to explore the relationship between extension inferred from fault heaves and crustal thickness variations in 3D

Evaluation of seismic reflection data has revealed structures cross-cutting the overburden within many sedimentary basins worldwide, including those in the North Sea and Norwegian Sea. These seismically-imaged pipes and chimneys are considered to be possible pathways for fluid flow. Natural fluids from deeper strata have migrated through these structures at some point in geological time. We test the hypothesis that many chimney and pipe structures imaged on seismic reflection profiles worldwide are the consequence of (1) a fracture network that has been reactivated by pore fluid pressure which facilitates the migration of fluids upwards; and (2) shallow sub-seafloor lateral migration of fluids along stratigraphic interfaces and near-surface fractures. An experimental approach to determine the physical properties of these structures beneath the sub-seafloor is described, with particular reference to an investigation of the Scanner Pockmark complex in the North Sea. The study is relevant to storage operators, policy-makers and those keen to demonstrate that it is possible to constrain and fully understand the physical properties and possible fluid flow pathways in the sedimentary overburden above sub-seafloor CO2 storage reservoirsKeywords: seismic chimney; pipe; fluid flow; fracture network

An edited version of this paper was published by AGU. Copyright 2017 American Geophysical Union. Goswami, B.K., Weitemeyer, K.A., Bünz, S., Minshull, T.A., Westbrook, G.K., Ker, S. & Sinha, M.C. (2017). Variations in pockmark composition at the Vestnesa Ridge: Insights from marine controlled source electromagnetic and seismic data. Geochemistry Geophysics Geosystems, 18(3), 1111-1125. https://doi.org/10.1002/2016GC006700. To view the published open abstract, go to http://dx.doi.org/10.1002/2016GC006700. The Vestnesa Ridge marks the northern boundary of a known submarine gas hydrate province in the west Svalbard margin. Several seafloor pockmarks at the eastern segment of the ridge are sites of active methane venting. Until recently, seismic reflection data were the main tool for imaging beneath the ridge. Coincident controlled source electromagnetic (CSEM), high‐resolution two‐dimensional (2‐D) airgun, sweep frequency SYSIF, and three‐dimensional (3‐D) p‐cable seismic reflection data were acquired at the south‐eastern part of the ridge between 2011 and 2013. The CSEM and seismic data contain profiles across and along the ridge, passing several active and inactive pockmarks. Joint interpretation of resistivity models obtained from CSEM and seismic reflection data provides new information regarding the fluid composition beneath the pockmarks. There is considerable variation in transverse resistance and seismic reflection characteristics of the gas hydrate stability zone (GHSZ) between the ridge flanks and chimneys beneath pockmarks. Layered seismic reflectors on the flanks are associated with around 300 Ωm2 transverse resistance, whereas the seismic reflectors within the chimneys exhibit amplitude blanking and chaotic patterns. The transverse resistance of the GHSZ within the chimneys vary between 400 and 1200 Ωm2. Variance attributes obtained from the 3‐D p‐cable data also highlight faults and chimneys, which coincide with the resistivity anomalies. Based on the joint data interpretation, widespread gas hydrate presence is likely at the ridge, with both hydrates and free gas contained within the faults and chimneys. However, at the active chimneys the effect of gas likely dominates the resistive anomalies. Plain Language Summary: Active methane venting was observed at several seafloor locations at the south‐eastern segment of the Vestnesa Ridge in the west Svalbard margin. New geophysical data acquired in the area between 2011 and 2013 provide new constraints at understanding potential factors enabling the active venting in the area. High levels of free gas is inferred beneath the locations of known vents based on the joint analysis of multiple geophysical datasets.

American Geophysical Union Fall Meeting, 12-16 December 2016, San Francisco, The west Galicia margin (NW Spain) is a magma-poor margin with limited sedimentary cover, providing ideal conditions to study the processes of continental extension and break-up through seismic imaging. In 2013, we collected a 65km x 20km 3D multi-channel seismic dataset extending from the edge of the Galicia Bank over the feather edge of the continental crust to beyond the Peridotite Ridge to the west. The volume has been processed through to 3D prestack time migration and 3D depth conversion and provides 3D images of the hyper-extended continental crust, consisting of well-defined but internally intensely deformed rotated faults blocks with associated syn-kinematic sedimentary wedges. The rotated fault blocks contain possible low-angle normal faults that in places appear to define and elsewhere to offset top basement; we interpret them as early faults that predate the formation of the fault blocks. The early faults are in places cut and offset by the later block-bounding faults that appear to detach downwards onto a bright reflection, the S reflector, a detachment fault and locally the crust-mantle boundary. S is corrugated on both depth and time structure maps, with the corrugations sweeping in an arc from E-W proximally to ESE-WNW towards the ocean. We interpret the corrugations as forming during slip on S and representing the displacement (and hence extension) direction on the detachment leading to breakup. Early synrift sediment within the most recent faults blocks are intensely deformed, representing the internal deformation of the fault blocks. However, in places simple synrift wedges can be seen to thicken towards the block-bounding faults. The angular relationship of the top of these wedges to the block-bounding faults demonstrates that they were deposited during slip on the faults and low-angle slip on the underlying S detachment. Although beneath the continental crust S appears continuous and shows only minor distortion in the vicinity of the block-bounding faults, just east of the Peridotite Ridge, S appears to have been offset by landward-dipping faults associated with the development of the Ridge. We conclude that rifting to breakup was a complex, time variant, 3D process involving slip at low-angles, multiple generations of faults and the unroofing of mantle along later landward-dipping structures

The Arctic continental margin contains large amounts of methane in the form of methane hydrates. The west Svalbard continental slope is an area where active methane seeps have been reported near the landward limit of the hydrate stability zone. The presence of bottom simulating reflectors (BSRs) on seismic reflection data in water depths greater than 600 m suggests the presence of free gas beneath gas hydrates in the area. Resistivity obtained from marine controlled source electromagnetic (CSEM) data provides a useful complement to seismic methods for detecting shallow hydrate and gas as they are more resistive than surrounding water saturated sediments. We acquired two CSEM lines in the west Svalbard continental slope, extending from the edge of the continental shelf (250 m water depth) to water depths of around 800 m. High resistivities (5–12 Ωm) observed above the BSR support the presence of gas hydrate in water depths greater than 600 m. High resistivities (3–4 Ωm) at 390–600 m water depth also suggest possible hydrate occurrence within the gas hydrate stability zone (GHSZ) of the continental slope. In addition, high resistivities (4–8 Ωm) landward of the GHSZ are coincident with high-amplitude reflectors and low velocities reported in seismic data that indicate the likely presence of free gas. Pore space saturation estimates using a connectivity equation suggest 20–50 per cent hydrate within the lower slope sediments and less than 12 per cent within the upper slope sediments. A free gas zone beneath the GHSZ (10–20 per cent gas saturation) is connected to the high free gas saturated (10–45 per cent) area at the edge of the continental shelf, where most of the seeps are observed. This evidence supports the presence of lateral free gas migration beneath the GHSZ towards the continental shelf.

We acquired coincident marine controlled source electromagnetic (CSEM), high-resolution seismic reflection and ocean-bottom seismometer (OBS) data over an active pockmark in the crest of the southern part of the Vestnesa Ridge, to estimate fluid composition within an underlying fluid-migration chimney. Synthetic model studies suggest resistivity obtained from CSEM data can resolve gas or hydrate saturation greater than 5% within the chimney. Acoustic chimneys imaged by seismic reflection data beneath the pockmark and on the ridge flanks were found to be associated with high-resistivity anomalies (+2–4 ?m). High-velocity anomalies (+0.3 km/s), within the gas-hydrate stability zone (GHSZ) and low-velocity anomalies (?0.2 km/s) underlying the GHSZ, were also observed. Joint analysis of the resistivity and velocity anomaly indicates pore saturation of up to 52% hydrate with 28% free gas, or up to 73% hydrate with 4% free gas, within the chimney beneath the pockmark assuming a nonuniform and uniform fluid distribution, respectively. Similarly, we estimate up to 30% hydrate with 4% free gas or 30% hydrate with 2% free gas within the pore space of the GHSZ outside the central chimney assuming a nonuniform and uniform fluid distribution, respectively. High levels of free-gas saturation in the top part of the chimney are consistent with episodic gas venting from the pockmark.

Methane hydrate close to the hydrate stability limit in seafloor sediment could represent an important source of methane to the oceans and atmosphere as the oceans warm. We investigate the extent to which patterns of past and future ocean-temperature fluctuations influence hydrate stability in a region offshore West Svalbard where active gas venting has been observed. We model the transient behavior of the gas hydrate stability zone at 400–500 m water depth (mwd) in response to past temperature changes inferred from historical measurements and proxy data and we model future changes predicted by seven climate models and two climate-forcing scenarios (Representative Concentration Pathways RCPs 2.6 and 8.5). We show that over the past 2000 year, a combination of annual and decadal temperature fluctuations could have triggered multiple hydrate-sourced methane emissions from seabed shallower than 400 mwd during episodes when the multidecadal average temperature was similar to that over the last century (?2.6°C). These temperature fluctuations can explain current methane emissions at 400 mwd, but decades to centuries of ocean warming are required to generate emissions in water deeper than 420 m. In the venting area, future methane emissions are relatively insensitive to the choice of climate model and RCP scenario until 2050 year, but are more sensitive to the RCP scenario after 2050 year. By 2100 CE, we estimate an ocean uptake of 97–1050 TgC from marine Arctic hydrate-sourced methane emissions, which is 0.06–0.67% of the ocean uptake from anthropogenic CO2 emissions for the period 1750–2011.

16TH SEISMIX International Symposium on Multi-scale Seismic Imaging of the Earth's crust and Upper Mantle, 12-17 October 2014, Castelldefels, Barcelona.-- 1 page, Galicia 3D, a reflection-refraction and long offset seismic experiment was carried out in 2013, at the Galicia rifted margin, in the northeast Atlantic Ocean, west of Spain. The survey covered a 64 km by 20 km (1280 km2) zone where the main geological features are: the fault bounded, rotated, continental crustal blocks, the S reflector which has been interpreted to be a low angle detachment fault, and the Peridotite Ridge (PR) thought be upper mantle exhumed to the seafloor during rifting. In the 3D box, two airgun arrays of 3300 cu.in. were fired alternately (in flip-flop configuration) every 37.5 m. All shots are recorded by 44 short period four component ocean bottom seismometers and 26 ocean bottom hydrophones. We present the P-wave velocity volume derived by the 3D first arrival time tomography. Tomographic results are validated by synthetic tests and by the comparison with published multi-channel and wide-angle seismic lines. The main outcomes are as follows: 1) The top of the syntectonic sediments imaged by MCS matches with the tomographic 3.5 km/s iso-velocity contour. Block bounding faults are imaged as lateral velocity contrasts. Velocities within the basement blocks vary between 3.5 km/s and 6.5 km/s. Lower crustal velocities are only observed within the middle and westernmost basement blocks. 2) The PR rises up to 5.5 km depth on the southernmost profiles. Twenty kilometers north, the basement deepens 1 km, and the basement expression of PR dissipears. 3) The 6.5 km/s isovelocity contour matches the MCS retrieved S reflector. Beneath the S reflector, the velocities increase from 6.5 to 7.5 km/s within less than 1 km depth interval. Low velocity zones (~ 7 km/s) are observed at the intersections between the block bounding faults and S reflector suggesting that the faults act as conduits for the water flow in the upper-mantle

European Geosciences Union General Assembly 2014 (EGU2014), 27 april - 2 may 2014, Vienna, Austria.-- 1 page, In June thru September, 2013, a 3D reflection and a long offset seismic experiment were conducted at the Galicia rifted margin by investigators from the US, UK, Germany, and Spain. The 3D multichannel experiment covered 64 km by 20 km (1280 km2), using the RV Marcus Langseth. Four streamers 6 km long were deployed at 12.5 m hydrophone channel spacing. The streamers were 200 m apart. Two airgun arrays, each 3300 cu in, were fired alternately every 37.5 m, to collectively yield a 400 m wide sail line consisting of 8 CMP lines at 50 m spacing. The long offset seismic experiment included 72 short period OBS’s deployed below the 3D reflection survey box. Most of the instruments recorded all the shots from the airgun array shots. The 3D seismic box covered a variety of geologic features. The Peridotite Ridge (PR), is associated with the exhumation of upper mantle rocks to the seafloor during the final stage of the continental separation between the Galicia Bank and the Grand Banks of Newfoundland. The S reflector is present below most of the continental blocks under the deep Galicia basin. S is interpreted to be a low-angle detachment fault formed late in the rifting process, and a number of rotated fault block basins and ranges containing pre and syn-rift sediments. Initial observations from stacked, but not yet migrated, 3D seismic data show that the PR is elevated above the present seafloor in the South and not exposed through the seafloor in the North. The relative smoothness of the PR surface for the entire 20 km N-S contrasts with the more complex, shorter wavelength, faulting of the continental crustal blocks to the east. The PR does not seem to show offsets or any apparent internal structure. However, migration will be required to see internal structure of the PR. Between the PR and the western most rifted continental crustal blocks, is a sedimentary basin about as wide as the PR and very different from the sedimentary basins bounded by the continental crustal blocks. The unusual basin is the deepest and thickest of the basins, and is characterized by a very bright reflector with reverse polarity relative to that of the seafloor. This reverse polarity bright reflector might be caused by the presence of water and perhaps overpressure below the reflector. This material may have sloughed off the PR as it was emplaced at the seafloor during syn- and post-rift time