Description: We present high-resolution resistivity imaging of gas hydrate pipe-like structures, as derived from marine controlled-source electromagnetic (CSEM) inversions that combine towed and ocean-bottom electric field receiver data, acquired from the Nyegga region, offshore Norway. Two-dimensional CSEM inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures, located in proximity to a major gas hydrate pipe-like structure, known as the CNE03 pockmark. The resistivity model resulting from the CSEM data inversion resolved the CNE03 hydrate structure in high resolution, as inferred by comparison to seismically constrained inversions. Our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean-bottom and towed receiver CSEM data simultaneously. The approach applied here can be utilised to map and monitor seafloor mineralisation, freshwater reservoirs, CO2 sequestration sites and near-surface geothermal systems.

Description: According to classic plume theory, the Tristan da Cunha mantle plume played a major role in the rifting of the South Atlantic margins by impinging at the base of the continental lithosphere shortly before or during the breakup of the South Atlantic. Onshore the Tristan mantle plume is associated with the emplacement of the Parana-Etendeka flood basalt province ~132 Ma, and offshore with the age-progressive Walvis Ridge and associated young seamount province. The global data is lacking coverage and therefore resolution around the Tristan da Cunha for shallower mantle depths. The search for deep-reaching geophysical/thermal anomalies beneath Tristan da Cunha was the starting point for a multi-disciplinary geophysical experiment to acquire passive marine electromagnetic, seismic and bathymetric data within the framework of the SPP-1375 “South Atlantic Margin Processes and Links with onshore Evolution” (SAMPLE) funded by the German Science foundation. The experiment included two expeditions with the German research vessel MARIA S. MERIAN in 2012 and 2013. Between February 2012 and January 2013, a network of 24 ocean bottom seismometers (OBS) from the German DEPAS pool (Deutscher Geräte-Pool für Amphibische Seismologie) and 26 ocean-bottom magneto-telluric stations from GEOMAR Kiel and the University of Tokyo were deployed around the archipelago of Tristan da Cunha. The seismological stations are equipped with a Güralp CMG-40T wideband seismometer (60 s) incorporated in a titanium pressure housing, a hydrophone, and a GEOLON MCS (Marine Compact Seismocorder) data logger from SEND GmbH Hamburg, Germany. Each sensor channel is sampled at 50 Hz (2 OBS at 100 Hz), preamplifier gain of the hydrophone channel is 4 and 1 for the three seismometer components. The seismometers are connected to a cardanic levelling mechanism, which was activated every 21 days to level the sensors. Based on previous experiments and triangulation of some stations, the accuracy of the positions is estimated to 500 m. To complement the ocean-bottom network and the existing observatory on Tristan da Cunha (TRIS), two land seismometers and one magnetotelluric station were installed on Nightingale Island located southwest of the main island each of which was equipped with a Güralp-3ESP seismometer (60 s) and an EarthData data logger. As recording parameters, we chose 100 Hz sampling rate and low preamplifier gain (0.4). One of the stations (NIG01) recorded earthquake data for the entire year, while the second station failed after few days due to water damage. Unfortunately, the permanent station TRIS failed also during the experiment.

Description: Multibeam echosounder (MBES) data recorded during RV MARIA S. MERIAN cruise MSM20-2 between 17.01.2012 and 15.02.2012 around Tristan da Cunha. The Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge by impinging at the base of the continental lithosphere shortly before or during the breakup of the South Atlantic margins. MSM20-2 staged therefore a multi-disciplinary geophysical study of the Tristan da Cunha hotspot by acquiring passive marine electromagnetic and seismic data, bathymetric data as well as gravity data from which we will derive an electrical resistivity, velocity and density model down to a depth of several hundred kilometers. CI Citation: Paul Wintersteller (seafloor-imaging@marum.de) as responsible party for bathymetry raw data ingest and approval. Description of the data source: During the RV MARIA S. MERIAN cruise, MSM20-2 the Kongsberg EM120 multi-beam echo sounder was used. The echosounder uses a nominal sounding frequency of 12 kHz. 191 beams with a 2°/2° footprint are formed for each ping while the seafloor is detected using amplitude and phase information for each beam sounding. For further information, consult https://epic.awi.de/26725/1/Kon2007a.pdf. The EM120 was operated continuously during the cruise including transit routes in a 24-hour schedule. The angular coverage sector and beam pointing angles were set to vary automatically with depth according to achievable coverage. The beam spacing was set to equidistant. The ship's speed varied from 13 kn during the transits and during the deployments to 8 kn during the surveys around Tristan da Cunha, the Tristan da Cunha Fracture Zone and the Mid-Atlantic Ridge. The quality of the data varied considerably during the cruise, being extremely noisy during the fast speed lines (see section Data Quality Evaluation). The maximum opening angle for the swath also varied during the cruise, being set to 56-56 for the fast speed lines and up to 64-64 (port-starboard) for the slow speed survey lines, since most of the external beams were lost at fast speeds. This resulted in an extremely variable ratio width/depth for the swath. Responsible person during this cruise / PI: Marcia Mia (marcia.maia univ-brest.fr). Chief scientist: Marion Jegen-Kulcsar (mjegen@geomar.de) CR: https://www.tib.eu/de/suchen/id/awi%3Adoi~10.2312%252Fcr_msm20_2/ CSR: https://www2.bsh.de/aktdat/dod/fahrtergebnis/2012/20120063.htm ADCP: https://doi.pangaea.de/10.1594/PANGAEA.823159

Description: Raw data acquired by position sensors on board RV METEOR during expedition M196 were processed to receive a validated master track which can be used as reference of further expedition data. During M196 the motion reference unit Kongsberg SeaTex AS MRU-5 combined with Kongsberg SeaTex AS Seapath 320 and two C and C Technologies GPS receivers C-NAV3050 were used as navigation sensors. Data were downloaded from DAVIS SHIP data base (https://dship.bsh.de) with a resolution of 1 sec. Processing and evaluation of the data is outlined in the data processing report. Processed data are provided as a master track with 1 sec resolution derived from the position sensors' data selected by priority and a generalized track with a reduced set of the most significant positions of the master track.

Description: Raw data acquired by position sensors on board RV METEOR during expedition M196 were processed to receive a validated master track which can be used as reference of further expedition data. During M196 the motion reference unit Kongsberg SeaTex AS MRU-5 combined with Kongsberg SeaTex AS Seapath 320 and two C and C Technologies GPS receivers C-NAV3050 were used as navigation sensors. Data were downloaded from DAVIS SHIP data base (https://dship.bsh.de) with a resolution of 1 sec. Processing and evaluation of the data is outlined in the data processing report. Processed data are provided as a master track with 1 sec resolution derived from the position sensors' data selected by priority and a generalized track with a reduced set of the most significant positions of the master track.

Description: We present high-resolution resistivity imaging of gas hydrate pipe-like structures, as derived from marine controlled-source electromagnetic (CSEM) inversions that combine towed and ocean-bottom electric field receiver data, acquired from the Nyegga region, offshore Norway. Two-dimensional CSEM inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures, located in proximity to a major gas hydrate pipe-like structure, known as the CNE03 pockmark. The resistivity model resulting from the CSEM data inversion resolved the CNE03 hydrate structure in high resolution, as inferred by comparison to seismically constrained inversions. Our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean-bottom and towed receiver CSEM data simultaneously. The approach applied here can be utilised to map and monitor seafloor mineralisation, freshwater reservoirs, CO2 sequestration sites and near-surface geothermal systems.

Description: Deep crustal constraint is often carried out using deterministic inverse methods, sometimes using seismic refraction, gravity and electromagnetic datasets in a complementary or "joint" scheme. With increasingly powerful parallel computer systems it is now possible to apply joint inversion schemes to derive an optimum model from diverse input data. These methods are highly effective where the uncertainty in the system is small. However, given the complex nature of these schemes it is often difficult to discern the uniqueness of the output model given the noise in the data, and the application of necessary regularization and weighting in the inversion process means that the extent of user prejudice pertaining to the final result may be unclear. We can rigorously address the subject of uncertainty using standard statistical tools but these methods also become less feasible if the prior model space is large or the forward simulations are computationally expensive. We present a simple Monte Carlo scheme to screen model space in a fully joint fashion, in which we replace the forward simulation with a fast and uncertainty-calibrated mathematical function, or emulator. This emulator is used as a proxy to run the very large number of models necessary to fully explore the plausible model space. We develop the method using a simple synthetic dataset then demonstrate its use on a joint data set comprising first-arrival seismic refraction. MT and scalar gravity data over a diapiric salt body. This study demonstrates both the value of a forward Monte Carlo approach (as distinct from a search-based or conventional inverse approach) in incorporating all kinds of uncertainty in the modelling process, exploring the entire model space, and shows the potential value of applying emulator technology throughout geophysics. Though the target here is relatively shallow, the methodology can be readily extended to address the whole crust.

Description: The analysis of marine magnetotelluric data is often complicated by disturbing signals that are caused by small-scale periodic movements of the instrument. The motion-induced noise leads to a bias and/or severe scattering in the derived magnetotelluric transfer functions. Both the motion itself and its effects on the magnetic and telluric time-series are investigated in this study using an 80 d magnetotelluric data set that includes dynamic tilt records measured in the Pacific Ocean off Costa Rica. We apply a standard motion removal technique as well as a newly developed method to correct for motion-induced noise. The resulting magnetotelluric transfer functions are of significantly better quality than the uncorrected ones. Furthermore, the study of the properties of motion noise leads to conclusions about the optimal processing approach even in case of data sets where an explicit correction for that noise is not possible.