Calypso sediment cores – Acquisition, logging and sub-sampling for geomechanical and petrophysical studies

In regard of the SEAMSTRESS project, a scientific cruise campaign (CAGE19-3) began Mid-October 2019 until beginning of November (3 weeks) in the Fram Strait, West of Svalbard. The objective of that cruise was to characterize the physical and geotechnical properties of sediments in the vicinity of pockmarks in the Vestnesa region. To do so, we sampled in-situ sediments by using a giant piston corer (core length = 10-20 m), a Gravity corer (core length = 3-6 m) and a Multi-Corer (core length = 1 m). Before and after the coring, a list of activities were performed by the science team on board, and later at the university facilities.

Orientation of Giant piston corer (GPC): For each deployment of the GPC, we installed a magnetic sensor upon the coring case in order to measure the orientation of the core. A first step of calibration was performed prior to the deployment of the magnetic sensor to estimate the location of the true North and magnetic North. This job was performed in collaboration with the ship crew and captain who provided us with the ship location and heading during the deployment of the GPC.

Geotechnical measurement on-board: After receiving, labeling and cutting the cores into 1-m long segments on deck, we performed a battery of geotechnical tests on the open sections such as: 1) shear strength, 2) density and 3) Water content measurements on the sediments.

Protocol for preparing the samples for geotechnical tests at NGI: In regard of the Seamstress project, we specifically stored vertically 15 sections (3 sections for each of the 5 targeted locations) that were chosen according to their depth and shear strength. Those sections were stored vertically to preserve the original conditions as much as possible and to avoid disturbing the sediment layers. Eight of those vertical sections have been sent to NGI in Oslo where they are being subjected to oedometer and triaxial tests (i.e., to study strain and derived-stress variations along the continental margin).

Lab work (MSCL and X-Ray): Once the cruise was over, further analysis were performed on the core sections in the NT-IG Laboratory. Every core collected during the expedition were thus scan by X-ray and multi-sensor core logger. Those profiles allow us to have a clear picture of the core and detailed data about the density, magnetic susceptibility and gamma ray of the sediments.

Subsampling for petrophysical analyses at the French Research Institute for Exploration of the Sea (IFREMER): In addition to sediment coring, in-situ pressure and temperature were measured using the piezometer instrument from IFREMER at 3 stations.  For further analysis of these data, a selection of sub-samples (depth and core section) were sent to IFREMER. Subsampling was made based on the depth of main anomalies along temperature and pressure logs from the piezometer. 10-cm long cylinders centered on the specific depths were prepared by looking at the CT scan images and processed MSCL logs.

Text and picture Rémi Vachon