Fluid escape structures in the Gulf of Cadiz. Evidence of structural control from combined seismic reflection and sidescan sonar interpretation

The Gulf of Cadiz is situated in a tectonically complex and active region, close to a major plate boundary. The tectonic regime in the area is characterized by a combination of important strike-slip movement and compressional tectonics related to the Africa-Eurasia NW-directed convergence, responsible for the formation of the Gibraltar Arc. Extensive mud volcanism, pockmarks, mud diapirism and carbonate chimneys related to hydrocarbon rich fluid venting are observed throughout the area. There is an extensive coverage of seismic reflection profiles in the area that includes industry data, a few deep-multichannel lines (IAM, ARRIFANO and BIGSETS) and many single-channel lines (both Sparker and Airgun data). During the TTR-12 (July/2002) and the GAP (Nov-Dec/2003) cruises, several single and multi channel seismic lines were acquired in this area that complement the existing database collected during previous TTR Cruises. These lines have re-processed to enhance the deeper structure.A combined interpretation of the available side-scan sonar imaging obtained by the Naval Research Laboratory in 1992 and the available seismic lines (both single channel and multichannel) shows clear evidence of the structural control of the mud volcanism in the study area. In particular, it appears that some of the mud volcanoes are located at the intersection between NW-SE strike-slip faults and thrusts of variable orientation, reflecting the curvature of the Gibraltar Arc

What do you think this is? "Conceptual uncertainty" in geoscience interpretation

Interpretations of seismic images are used to analyze sub-surface geology and form the basis for many exploration and extraction decisions, but the uncertainty that arises from human bias in seismic data interpretation has not previously been quantified. All geological data sets are spatially limited and have limited resolution. Geoscientists who interpret such data sets must, therefore, rely upon their previous experience and apply a limited set of geological concepts. We have documented the range of interpretations to a single data set, and in doing so have quantified the �conceptual uncertainty� inherent in seismic interpretation. In this experiment, 412 interpretations of a synthetic seismic image were analyzed. Only 21% of the participants interpreted the �correct� tectonic setting of the original model, and only 23% highlighted the three main fault strands in the image. These results illustrate that conceptual uncertainty exists, which in turn explains the large range of interpretations that can result from a single data set. We consider the role of prior knowledge in biasing individuals in their interpretation of the synthetic seismic section, and our results demonstrate that conceptual uncertainty has a critical influence on resource exploration and other areas of geoscience. Practices should be developed to minimize the effects of conceptual uncertainty, and it should be accounted for in risk analysis

Investigation of the role of gas hydrates in continental slope stability west of Fiordland, New Zealand

Sediment weakening due to increased local pore fluid pressure is interpreted to be the cause of a submarine landslide that has been seismically imaged off the southwest coast of New Zealand. Data show a distinct and continuous bottom‐simulating reflection (BSR)—a seismic phenomena indicative of the presence of marine gas hydrate—below the continental shelf from water depths of c. 2400 m to c. 750 m, where it intersects the seafloor. Excess pore fluid pressure (EPP) generated in a free gas zone below the base of gas hydrate stability is interpreted as being a major factor in the slope's destabilisation. Representative sediment strength characteristics have been applied to limit‐equilibrium methods of slope stability analysis with respect to the Mohr‐Coulomb failure criterion to develop an understanding of the feature's sensitivity to EPP. EPP has been modelled with representative material properties (internal angle of friction, bulk soil unit weight and cohesion) to show the considerable effect it has on stability. The best estimate of average EPP being solely responsible for failure is 1700 kPa, assuming a perfectly elastic body above a pre‐defined failure surface in a static environment

Three-dimensional seismic investigations of the Sevastopol mud volcano in correlation to gas/fluid migration pathways and indications for gas hydrate occurrences in the Sorokin Trough (Black Sea)

New 3-D seismic investigations carried out across the Sevastopol mud volcano in the Sorokin Trough present 3-D seismic data of a mud volcano in the Black Sea for the first time. The studies allow us to image the complex three-dimensional morphology of a collapse structured mud volcano and to propose an evolution model. The Sevastopol mud volcano is located above a buried diapiric structure with two ridges and controlled by fluid migration along a deep fault system, which developed during the growth of the diapirs in a compressional tectonic system. Overpressured fluids initiated an explosive eruption generating the collapse depression of the Sevastopol mud volcano. Several cones were formed within the depression by subsequent quiet mud extrusions. Although gas hydrates have been recovered at various mud volcanoes in the Sorokin Trough, no gas hydrates were sampled at the Sevastopol mud volcano. A BSR (bottom-simulating reflector) is missing in the seismic data; however, high-amplitude reflections (bright spots) observed above the diapiric ridge near the mud volcano at a relatively constant depth correspond to the approximate depth of the base of the gas hydrate stability zone (BGHSZ). Thus we suggest that gas hydrates are present locally where gas/fluid flow occurs related to mud volcanism, i.e., above the diapir and close to the feeder channel of the mud volcano. Depth variations of the bright spots of up to 200 ms TWT might be caused by temperature variations produced by variable fluid flow

Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Vøring Plateau, offshore Norway

This paper presents a new data set, including single-channel airgun seismic lines, OKEAN long range side-scan sonar data and gravity cores, acquired on the edge of the Vøring Plateau and in the region of the Storrega Slide. The data acquisition was part of the 8th Training Through Research (TTR-8) expedition with R.V. Professor Logachev. The acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom-simulating reflectors (BSRs) at about 350 ms TWT subbottom depth. The lower BSR zone is located on the slope of the Vøring Plateau in the immediate vicinity of the northern headwall of the Storegga Slide and, in some places, below the slide deposits. This zone runs parallel to the general trend of the continental slope. The spatial distribution of the upper BSR zone, located upslope in an area where diapir-like structures are found, does not demonstrate any topographic control. Interpretation of high-backscatter patches on the OKEAN sonographs associates the observed structures with fluid escape features on the seabed. Most of them are pockmarks, but in a few places, diapirs are cropping out and form dome-like elevations. After analysing the behaviour of the BSR (sometimes crosscutting) and its acoustic characteristics (reversed polarity), and after applying seismic inversion processing to estimate the acoustic velocity change across the BSR, this reflector is interpreted to represent the base of the local gas hydrate stability field (GHSF). This information was used to derive the regional geothermal gradient. Spatial variations of the inferred geothermal field appeared to be negligible. The observation of two separated BSR zones suggests different environmental controls for the growth of the hydrates. Enhanced reflectors observed in the intermediate zone can be explained by the presence of strata-bound free gas accumulations and migration combined with overlying permeability barriers. Therefore, a model for gas hydrate formation in relation to directional fluid migration processes, fluid escape features and the presence of diapiric structures is proposed in this paper. Our model is supported by sedimentological analyses of seafloor samples, which demonstrate the presence of stiff clays with evidence of gas migration, and by paleontological studies of the cores retrieved from the pockmarks. The presence of a BSR in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event. This observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm/year. Finally, the effect of hydrate dissociation on slope instability is considered. Assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re-distributed through the whole sedimentary section above the paleo-BSR (with consuming heat and increasing pore pressure, dissociation tends to shift the PT conditions back to equilibrium values), it appears that the dissociation of hydrates due to sliding would cause only 0.2% increase of the pore pressure, which would hardly contribute to further slope instability