Plattform Platform	Reise-Nr. Cruise-No.	Zeitraum Period	Projekt Project	Arbeitsgebiet Working area
Poseidon	POS524	07.06.2018 - 26.06.2018		Iceland Sea

Fahrtleiter Chief Scientist	Institut Institute	Auslaufhafen Port of departure	Einlaufhafen Port of return	Stationskarte Station map	Schiffsroute Trackchart
Holz Sebastian	Helmholtz Centre for Ocean Research Kiel (GEOMAR)	Reykjavik	Bergen

Ziel der Reise / Objectives of Cruise:

The main objective of cruise POS524 was to study the suitability of the MARTEMIS system (as well as other EM systems / configurations) for the characterization of an active hydrothermal system. The hydrothermal system had previously been studied during several cruises (POS229, POS253, POS291), during which high temperature venting (200 – 250?C) was observed but no massive sulfide mineralization, which would be expected to some degree in such a geotectonic setting, were found. Thus, as second objective it was projected that EM investigations would not only give insight into the hydrothermal structure, but could potentially also reveal covered mineralizations at depth. To facilitate interpretations, EM investigations were accompanied by geophysical (heat probe) and geological (gravity core) measurements for ground truthing and to gain further structural insight. Successful measurements with the MARTEMIS coil system were carried out covering the hydrothermally active area with about 15km of profiles. Additional measurements were carried out along 2.5km of profile around a second site of interest to the north of the hydrothermal field and, additionally, along a 2.5km long profile connecting the two working areas, which most likely cover background sediments. Signals transmitted with the MARTEMIS coil were also recorded by 12 stationary, remote OBEM receivers, which had been installed prior to the first experiment. This "Coil2Dipole" experiment with a moving coil as transmitter and stationary OBEMs as receivers, has an increased depth of penetration (~100m) as compared to the pure MARTEMIS coil measurements (~30m) and will hopefully yield complementary information for the EM interpretation. As part of these experiment, measurements of the self-potential were also carried out. Additionally, we performed a novel EM experiment, in which the coil frame of the MARTEMIS system was used to carry two perpendicular pairs of transmitter electrodes, thus, allowing for a CSEM experiment with two independent transmitter dipole polarizations, which will yield an increased depth of investigation (~250m). This "Dual Polarization" experiment was carried out along a 3.7km long W ↔ E profile crossing the active hydrothermal vent field. Transmitted signals were recorded by nine stationary OBEM receivers along the profile. Active EM experiments were concluded by running a second MARTEMIS experiment along 20km of profiles. A first review of the coil data revealed some problems caused by a damaged cable of the receiver coil and it is not clear if it will be possible to use this MARTEMIS data set. However, the data measured with the remote OBEM receivers during this last deployment, will yield an additional Coil2Dipole data set. EM investigations were accompanied by measurements with a 2.2m long heat probe at 14 stations. They may yield valuable data for the interpretation of EM data, potentially aiding in distinguishing between anomalies in the EM data caused by hydrothermal activity or anomalies caused by mineralizations. Finally, geological samples were collected with a 3m long gravity corer for ground truthing and to gain further structural insight. In total about 20m of core material was collected at 8 stations. Temperature measurements were directly performed on deck and sampling of the pore fluids will help in the interpretation of EM data by yielding in situ salinities of fluids taken from the active field as well as fluids taken in background areas. The combination of experiments proved to be convenient in terms of handling on the ship, as EM experiments on the one hand and measurements with heat probe and gravity coring on the other hand were usually performed in an alternating manner each other day. This alternating style of experiments gave each method time for adjustments and repairs as well as time to take a first look at measurements.

Messungen / Measurements