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Highlights • The developed joint inversion quantifies both free gas and hydrate concentration. • The robust method uses sonic and conductivity logs as main input parameters. • For the test site it reveals two hydrate accumulations with very different characteristics. • The whole range of concentrations is shown that can explain the observed data. • The method is applicable to most continental margins when there is borehole control. Abstract Quantification of gas hydrates in marine sediments is crucial for understanding gas hydrate systems. By empirical relationships or effective medium modelling, gas hydrate concentrations can be derived from velocity and/or conductivity logs. However, these approaches do not take the co-occurrence of free gas and gas hydrate into account leading to large uncertainties in the calculated free gas and gas hydrate concentrations. To overcome this issue we adopt a joint elastic and electric self-consistent/differential effective medium model as the basis for a new joint inversion scheme that distinguishes between both phases. We apply this scheme to p-wave velocity and electric induction data measured by downhole-logging of boreholes at Formosa Ridge off Taiwan - a known hydrate province with an active gas conduit. Gaussian Mixture Modeling separates the background signal of the host medium from anomalies and allows to determine a background porosity as a probability density function of depth. We use this derived porosity to jointly invert electrical conductivity and velocity data for hydrate and free gas concentrations. At Formosa Ridge, we find two resistive anomalies, one in the shallow and another in the deep part of the borehole. Only the deep anomaly in conductivity coincides with a high-velocity anomaly. This is consistent with ∼30% hydrate with ∼1% free gas concentration. For the shallow anomaly, increased velocities due to hydrate concentrations of ∼15% are compensated by a decrease in velocity due to ∼1% of free gas. The metho

During research cruise AL595 (31.5. - 20.6.2023) onboard research vessel ALKOR, investigations were carried out at the Grimsey Hydrothermal Field offshore Northern Iceland as part of the Helmholtz InnoPool project “High CO2 – metabolic responses and bioeconomic opportunities”. For the first time, the Hover-AUVs Anton and Luise were successfully operated at water depths of up to 400m, which is close to the maximum operational depth of 500m specified for these Girona 500 AUVs. AUV Anton was used to measure high resolution multibeam data with a horizontal resolution of approximately 40cm cov- ering a total area of ca. 1.4km2. AUV Luise acquired five photo-mosaics with sub-centimeter resolution covering a total area of ca. 5.000m2. In addition, both AUVs carried CTD probes, which will allow to investigate the local distribution of hydrothermal activity. Both high-resolution bathymetry and pho- togrammetry data yield new insights into the morphology and overall structure of the vent site and its surrounding, which will be valuable for the interpretation of geophysical data previously acquired in the working area. Sampling with a multicorer (three successful deployments), a 300cm long gravity corer (five successful deployments), a BIGO lander (two successful deployments) and casts with the CTD- rosette (seven deployments) generated fluid, pore-fluid and sediment samples to be analyzed by the working groups Geomicrobiology and Biogeochemistry, Marine Natural Products and Marine Geochem- istry at GEOMAR and at Matís (Iceland, Natural Products only). Lab work to be carried out in the home labs will yield insights into the physiological adaptation of microbial communities and individual microbes to very high CO2 concentrations and will explore microbial utilization of CO2 for establishing CO2-based bioeconomic value chains.

During cruise MSM103 (12.9. 15.11.2021), carried out as part of the SOURCE project, multidisci-plinary investigations were carried out in the Gulf of St. Lawrence Gulf north of Prince EdwardIsland (PEI) to identify and quantify offshore aquifers in the region. For this purpose, seismic, elec-tromagnetic and hydroacoustic measurements were carried out and sediment samples were obtainedwith the gravity corer. The structure of the seabed and the seafloor were explored using acousticmethods. A large number of channel structures could be detected in sections, which are interpretedas parts of glacial or postglacial drainage systems. It is not yet possible to say whether these are con-nected to any existing deeper groundwater systems. In the hydroacoustic data, no escapes of freshwater or gas could be observed in the water column during the cruise. In several places, however, ablanking was evident in the seafloor, which indicates the occurrence of gases and could hint atpotential pathways for gas and / or freshwater in the seafloor. Here, an extended evaluation of thehydroacoustic data after the cruise will provide further insight. In sediment samples taken at a loca-tion north of PEI, a significant drop in salinity and resistivity could be detected in the upper 3m ofthe sediment, which provided a first direct indication of the presence of fresher groundwater. Fromthe electromagnetic measurements carried out in seven working areas, we hope to be able to con-tinue this punctual information over larger areas in order to obtain an extended insight into the pres-ence and distribution of freshwater deposits by the detection of anomalies of the electricalconductivity.

A GEOMAR (Kiel, Germany) research team has developed a passive electric field acquisition system for Autonomous Underwater Vehicles (AUVs) to optimize seafloor massive sulfides exploration. This sensor was made of two perpendicular and horizontal pairs of electrodes, and was successfully tested over active basalt-hosted hydrothermal site TAG (26°N, Mid-Atlantic Ridge) and several inactive sites in its vicinity. The resulting data underline the efficiency of combining deep-sea electric and magnetic measurements for searching for active and inactive hydrothermal vent fields. With these datasets, it becomes possible to determine the geological nature of the targets and to constrain the characteristics of fluid circulation at depth without involving costly and invasive underwater tools such as Remotely Operated Vehicles or even manned submersibles to collect samples. Data analysis also revealed that AUV attitude variations induce distortions of the electric signal. These distortions start prevailing for dives at altitudes higher than 90 m above the seafloor, as the distance between the AUV becomes too important to guarantee that the signal produced by the geological target still dominates. To improve the acquisition system and reduce the overall noise, we discuss solutions that limit the impact of such attitude variations. These solutions consist of minor adjustments, such as masts at AUVs stern to tow damping electrodes arrays. In such configurations, we believe that deep-sea passive electric measurements combined with high-resolution magnetic measurements can become a highly efficient seafloor exploration tool, including for sulfide deposits associated with inactive hydrothermal systems.

Highlights • Marine CSEM data was collected in the Danube Paleo-delta, Black Sea for gas hydrate targets investigation. • The receivers and the novel transmitter “Sputnik” developed at GEOMAR (Helmholtz Centre for Ocean Research Kiel) were used. • Inversions in terms of rotational invariants are conducted. • Two potential gas hydrate layers are derived. • Gas hydrate saturation is estimated using Archie's Law. Abstract The Black Sea is known to have extensive direct and indirect indicators of methane hydrates. Since the resistivity of the seafloor increases significantly in the presence of gas hydrates or free gas, marine controlled-source electromagnetics (CSEM) is a suitable method for the investigation of hydrates. We have collected CSEM data in a channel-levee system of the Danube paleo-delta (Bulgarian sector) at water depths of about 1500 m. The working area is within the gas hydrates stability zone and seismics suggests the presence of gas hydrates. The CSEM data were acquired with stationary receivers and a novel mobile, dual polarization transmitter system, and interpreted in terms of rotational invariants by means of 1-D inversions at common midpoints (CMP) to generate pseudo 2D resistivity sections. The inversion results reveal two resistive layers at shallow depths of 60–120 mbsf and greater depths of 270–400 mbsf. A comparison with seismics shows a good correlation of the shallow layer with high amplitude reflections and a velocity anomaly. The deeper layer can be tied to the current bottom simulating reflector (BSR). The comparison to a second CSEM experiment, which was conducted by the BGR (Federal Institute for Geosciences and Natural Resources, Germany) shows a good agreement of the derived sections down to a depth of about 350 m. Based on salinity and porosity models derived from boreholes in the Black Sea, we apply Archie's law to estimate potential gas hydrate saturations of up to 23% for the shallow resistor and up to 7% for the deeper layer. Di

The formation of marine massive sulfides is connected to offshore hydrothermal circulation, where minerals from the underlying crust are mobilized and transported towards the seafloor. These sulfides have a high electrical conductivity, which can be detected by electromagnetic systems. To study potential occurrences of massive sulfides at the Grimsey Vent Field, which is located to the North of Iceland, two research cruises were in 2018 and 2019. Even though the Grimsey Vent Field is hydrothermally active, previous geoscientific studies were not able to detect these massive sulfides in the seafloor.In order to detect potential occurrences of massive sulfides, several electromagnetic (EM) experiments were done. One of those experiments is the ”Coil2Dipole” experiment, in which the mobile MARTEMIS transmitter system (developed at GEOMAR), which uses a coil antenna, is used to generate a time varying magnetic field. After turning off the electrical current in the transmitter, the induced EM fields are measured with stationary Ocean Bottom EM-receiver (OBEM-receiver) on the seafloor. Through the receivers placed on the seafloor the signal is detectable up to distances of 250m, which yield depth of investigation of about 120m. A ranging system allows for the precise distance measurements between MARTEMIS and OBEM-receivers. This is important for the correct interpretation of acquired data, since the offset has great influence on detected signal.In previous studies (Safipour et al., 2017, Geophysics; Safipour et al., 2018, Geophysical Prospecting) it was demonstrated that the “Coil2Dipole” is suitable to detect conductive targets, like massive sulfides in the seafloor. In this study we carry out a qualitative analysis in terms of 1D modelling of the data. Generally, we observed increasing resistivities with larger depthof investigation, which we attribute to the compaction of the seafloor. In the vicinity of one OBEM-receiver, which is located to the West of the active ve

Seafloor massive sulfides (SMS) are regarded as a potential future resource to satisfy the growing global demand of strategic metals. Aside from mining and retrieving profitable amounts of massive sulfides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centers.In the past years we have used the marine transient electromagnetic induction system MARTEMIS, a coincident-loop TEM system developed at GEOMAR (Kiel, Germany), in various marine geological settings for the detection and characterization of SMS in the shallow seafloor down to a depth of ~30m. The system was also used in combination with remote EM receivers (Coil2Dipole experiment) to allow for investigations of conductive structures,which are covered by up to ~100m of sediments.During two research cruises (2015: POS483, 2017: POS509), several types of EM experiments were carried out at the Palinuro Seamount located about 141km to the SSE of Naples (Italy) in the Tyrrhenian Sea. At this hydrothermally practically inactive site, previous investigations had confirmed the occurrence of SMS by drilling down a depth of 5m, the maximum depth reached by the drill (BGS Rockdrill I). For all EM experiments we used the marine transient induction system (MARTEMIS) -a mobile marine coil system -as source. For TEM experiments a coincident loop receiver integrated into the MARTEMIS system was used to investigate the shallow conductivity structure of the seafloor down to a depth of about 30m. In the TEM experiments we were able to show that the lateral extent of the SMS body is larger than previously known from drilling. In a second set of experiments, which we have named "Coil2Dipole", we used remote OBEM (ocean bottom EM) receivers to measure theelectrical fields excited by the MARTEMIS coil source. In this experiment, OBEM receivers were able to detect the transmitted

Highlights • Joint seismic, CSEM and geochemical estimate of gas and hydrate saturation. • Converted shear wave analyses for gas hydrate distribution. • Wide angle full waveform inversion. • Paleo-BSRs and gas hydrate system in disequilibrium. • Numerical framework for joint inversion of multiple geophysical methods. Abstract Gas hydrates are naturally-occurring solid compounds of gas and water within almost all sediment-rich continental margins. Due to the large amounts of methane stored in submarine gas hydrates, they might serve as future reservoirs for offshore marine gas production. Assessing the reservoir characteristics requires reliable estimates of both the gas and gas hydrate concentration, which can be best addressed using geophysical and geological investigations. Here, we demonstrate the power of joint interpretation of interdisciplinary geophysical techniques and geological laboratory experiments. Regional 2D multichannel seismic data provide the broad overview of a hydrate-bearing area. High-resolution 2D and 3D seismic reflection data provide detailed images of two working areas, the buried S1 channel-levee system at 1500 m water depth (well within the gas hydrate stability zone) and a slope failure location, located at 665 m water depth (top limit of the hydrate formation) next to the S2 channel. Detailed compressional and shear wave (Vs) velocity-depth models were derived from four component ocean-bottom seismic data, the latter from P- to S-conversion upon reflection. Due to their steep reflection angles, shear wave events result in less resolved Vs models. Nevertheless, in case of a change in elasticity of the sediment matrix due to gas hydrate cementation, shear wave events can be used as an indicator. As such, Vs can give insight into the nature of hydrate formation throughout the GHSZ. We present new developments in the application of common reflection surface, normal-incidence-point tomography and full waveform inversion techniques to enhance

Large national programs in the United States and several Asian countries have defined and characterised their marine methane hydrate occurrences in some detail, but European hydrate occurrence has received less attention. The European Union-funded project “Marine gas hydrate – an indigenous resource of natural gas for Europe” (MIGRATE) aimed to determine the European potential inventory of exploitable gas hydrate, to assess current technologies for their production, and to evaluate the associated risks. We present a synthesis of results from a MIGRATE working group that focused on the definition and assessment of hydrate in Europe. Our review includes the western and eastern margins of Greenland, the Barents Sea and onshore and offshore Svalbard, the Atlantic margin of Europe, extending south to the northwestern margin of Morocco, the Mediterranean Sea, the Sea of Marmara, and the western and southern margins of the Black Sea. We have not attempted to cover the high Arctic, the Russian, Ukrainian and Georgian sectors of the Black Sea, or overseas territories of European nations. Following a formalised process, we defined a range of indicators of hydrate presence based on geophysical, geochemical and geological data. Our study was framed by the constraint of the hydrate stability field in European seas. Direct hydrate indicators included sampling of hydrate; the presence of bottom simulating reflectors in seismic reflection profiles; gas seepage into the ocean; and chlorinity anomalies in sediment cores. Indirect indicators included geophysical survey evidence for seismic velocity and/or resistivity anomalies, seismic reflectivity anomalies or subsurface gas escape structures; various seabed features associated with gas escape, and the presence of an underlying conventional petroleum system. We used these indicators to develop a database of hydrate occurrence across Europe. We identified a series of regions where there is substantial evidence for hydrate occurrenc

Large national programs in the United States and several Asian countries have defined and characterised their marine methane hydrate occurrences in some detail, but European hydrate occurrence has received less attention. The European Union-funded project “Marine gas hydrate – an indigenous resource of natural gas for Europe” (MIGRATE) aimed to determine the European potential inventory of exploitable gas hydrate, to assess current technologies for their production, and to evaluate the associated risks. We present a synthesis of results from a MIGRATE working group that focused on the definition and assessment of hydrate in Europe. Our review includes the western and eastern margins of Greenland, the Barents Sea and onshore and offshore Svalbard, the Atlantic margin of Europe, extending south to the northwestern margin of Morocco, the Mediterranean Sea, the Sea of Marmara, and the western and southern margins of the Black Sea. We have not attempted to cover the high Arctic, the Russian, Ukrainian and Georgian sectors of the Black Sea, or overseas territories of European nations. Following a formalised process, we defined a range of indicators of hydrate presence based on geophysical, geochemical and geological data. Our study was framed by the constraint of the hydrate stability field in European seas. Direct hydrate indicators included sampling of hydrate; the presence of bottom simulating reflectors in seismic reflection profiles; gas seepage into the ocean; and chlorinity anomalies in sediment cores. Indirect indicators included geophysical survey evidence for seismic velocity and/or resistivity anomalies, seismic reflectivity anomalies or subsurface gas escape structures; various seabed features associated with gas escape, and the presence of an underlying conventional petroleum system. We used these indicators to develop a database of hydrate occurrence across Europe. We identified a series of regions where there is substantial evidence for hydrate occurrence (