Description: This paper applies nonlinear Bayesian inversion to marine controlled source electromagnetic (CSEM) data collected near two sites of the Integrated Ocean Drilling Program (IODP) Expedition 311 on the northern Cascadia Margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents. The Cascadia margin, off the west coast of Vancouver Island, Canada, has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics. Gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling. Here, we invert time-domain CSEM data collected at Sites U1328 and U1329 of IODP Expedition 311 using Bayesian methods to derive subsurface resistivity model parameters and uncertainties. The Bayesian information criterion is applied to determine the amount of structure (number of layers in a depth-dependent model) that can be resolved by the data. The parameter space is sampled with the Metropolis–Hastings algorithm in principal-component space, utilizing parallel tempering to ensure wider and efficient sampling and convergence. Nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude. Marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties. One-dimensional inversion of time-domain CSEM data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure. The data sets can be generally explained by models with 1 to 3 layers. Inversion results at U1329, at the landward edge of the gas hydrate stability zone, indicate a sediment unconformity as well as potential cold vents which were previously unknown. The resistivities generally increase upslope due to sediment erosion along the slope. Inversion results at U1328 on the middle slope suggest several vent systems close to Bullseye vent in agreement with ongoing interdisciplinary observations.

Description: 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.

Description: Kongsberg (EM120, EM1002) and ELAC (SB3050) multibeam systems of low to medium frequencies and various subbottom profilers were used to analyze the seafloor of the Baltic Sea between twenty and one hundred meter water depth. The working areas are characterized by soft mud allowing for significant penetration by both subbottom and multibeam signals, especially if lower frequencies were used. Locally shallow gas was found transforming the low-reflectivity mud acoustically into a strong volume scatterer. Single beam subbottom profiles across these shallow gas areas show distinct blanking effects below one and four meters below the seafloor. We demonstrate that low frequency multibeam systems are ideally suited to map those shallow gas areas over the entire swath of 140°. First the depth of the working areas was successfully determined with the shallow to mid-water 95kHz multibeam system. No backscatter anomaly was found while crossing the transition zone between mud and gas-bearing mud. In contrast a 12kHz survey over the same location reveals several meters deeper soundings. The resulting bathymetric data mimics the subbottom morphology of a till structure rather than the seafloor. The reason is strong penetration into the mud up to ten meters, even though the system was manually optimized for correct bottom detection. This makes the 12kHz system prone to subsurface mapping of strong reflectors within very soft sediments. High scattering gas bubbles embedded in the mud could be mapped by backscatter anomalies and misinterpretation of the shallow gas front as bottom echoes occurred. Angular range backscattering strength analysis suggests distinct differences between gassy and non-gassy areas and demonstrates the sensitivity of the low frequency multibeam sounder on free gas even on the very outer beams of the swath. The data is groundtruthed by subbottom profiling and geochemical sampling both indicating free gas. Even small gas pockets of only a few meters extension can be resolved demonstrating the advantages of high resolution and large coverage multibeam mapping compared to single beam surveys. Similar results were gathered using a mobile 50kHz system. (a) Backscatter amplitude chart of EM120. The red rectangle focuses on a transition zone between blue color/no-shallow-gas and red color/shallow-gas area; the inlet shows amplitude data from the 95kHz system not showing any transition. (b) PARASOUND subbottom data. The transition zone (red arrow) between shallow gas and no shallow gas plots exactly at the same location as seen in the multibeam data (a).

Description: This study highlights the potential of using a low frequency multibeam echosounder for detection and visualization of shallow gas occurring several meters beneath the seafloor. The presence of shallow gas was verified in the Bornholm Basin, Baltic Sea, at 80 m water depth with standard geochemical core analysis and hydroacoustic subbottom profiling. Successively, this area was surveyed with a 95 kHz and a 12 kHz multibeam echosounder (MBES). The bathymetric measurements with 12 kHz provided depth values systematically deeper by several meters compared to 95 kHz data. This observation was attributed to enhanced penetration of the low frequency signal energy into soft sediments. Consequently, the subbottom geoacoustic properties contributed highly to the measured backscattered signals. Those appeared up to 17 dB higher inside the shallow gas area compared to reference measurements outside and could be clearly linked to the shallow gas front depth down to 5 meter below seafloor. No elevated backscatter was visible in 95 kHz MBES data, which in turn highlights the superior potential of low frequency MBES to image shallow sub-seafloor features. Small gas pockets could be resolved even on the outer swath (up to 65°). Strongly elevated backscattering from gassy areas occurred at large incidence angles and a high gas sensitivity of the MBES is further supported by an angular response analysis presented in this study. We conclude that the MBES together with subbottom profiling can be used as an efficient tool for spatial subbottom mapping in soft sediment environments.

Description: Using high-resolution seismic data, this study aims at investigating the evolution and morphological diversity of subsea permafrost features on the eastern Laptev Sea shelf, Arctic Siberia. Several seismic facies were recognized. These relate to the major environmental changes, which affected the Laptev Sea area before, during, and after the last global transgression. Because this shallow shelf was part of the Beringian landmass, we consider a prominent subsurface seismic basal reflector as the top of the former terrestrial permafrost table. Five zones differing in geometry, reflection patterns, depths, and continuity of the permafrost top are identified. Where visible, the upper 70 m of the sediments consists of epigenetically and syngenetically frozen ice-poor sandy deposits at the base, possibly of early last glacial age, marine isotope stages (MIS) 5 and 4. These are followed by late glacial, ice-rich facies interpreted to be MIS 3 to 2. The early Holocene (MIS 1) features well-stratified lagoonal and taberal deposits. As verified by radiocarbon-dated sediment cores, these deposits are overlain by middle to late Holocene sediments with an increasingly marine signature.

Description: A combined high resolution seismic, sub-bottom profiling, and multi-beam echo-sounding survey in the Skagerrak (Danish sector of the North Sea) together with gas analyses at a station along the profile exhibit the expulsion of gas (mainly methane) and the presence of gas-charged sediments at shallow depth. The echo-soundings yield detailed insight into the distribution and shape of typical sea-floor features associated with gas seepage, such as pockmarks. The pockmarks reach dimensions of 800 m in length, 300 m in width, and 15 m in depth, with the long axis running parallel to the slope of the Norwegian Trench. Processing of the multi-channel high resolution seismic data and the digitally recorded sub-bottom profiler signals indicate an internal compressional velocity of about 1050 m s-1 within the gas-charged sediments reaching from the sea-floor to a sub-bottom depth of about 23 m. Using the lateral distribution and thickness of the gas-charged sediments in conjunction with a mean concentration of gas of 3000 ppb, the present amount of trapped gas is estimated to be 6·45 × 1011 g CH4. The flux of methane through the sea-bed into the water column appears to be 7·2 × 1010 g CH4 per year. To explain the small difference in size between the methane pool in near-surface sediments and the annual flux through the sea-bed, a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active.