[S-CG45] Ocean Floor Geoscience

[SCG45-P18] Sampling process of seafloor sediments as a source of information on their rheology

*Nikita Dubinya^1,2, Sergey Tikhotskiy^1,2, Victor Nachev^1,3 (1.Moscow Institute of Physics and Technology, 2.Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences, 3.Sadovsky Institute of Geosphere Dynamics of the Russian Academy of Sciences)

Keywords:Seafloor sampling, Indentation, Rheology, Geomechanics

The study is related to the problem of offshore infrastructure construction with an especial focus on development of offshore hydrocarbon fields from the perspective of geomechanics. It is essential to have a model of mechanical properties of seafloor sediments for effective construction of safe infrastructure. The problem of creating such model is rather complicated as the rheological properties of seafloor sediments are nonlinear – mechanical behavior of sediments appears to be described by nonlinear visco-elasto-plastic models. At the same time there are limited opportunities to estimate these properties as engineering drilling – a valid source of information on mechanical properties – may be very expensive and complicated at offshore conditions.

Sediment sampling with various corers provides one of the most valuable sources of information regarding mechanical properties of seafloor sediments. The collected samples may be analyzed in laboratory conditions using various techniques. One of the problems with this procedure is related to difficulties in recreating mechanical conditions of the sample before coring procedure: any sample could have been significantly altered before arriving to the laboratory. There are various ways to deal with this problem – in the current study we focus on one of them.

In the current study we analyze the sampling process itself as a source of information regarding rheological properties of seafloor sediments. From the perspective of continuum mechanics the interaction between corer and seafloor may be considered as a contact problem – a rigid body (corer) penetrates into a visco-elasto-plastic medium (seafloor). It is clear that the dynamics of penetration is governed by a set of conditions, including initial conditions, setting distance, geometry and mechanical properties of the seafloor at the vicinity of the contact point. As rheological properties of the seafloor are included into this set of factors, one may postulate and try to solve the inverse problem: can mechanical properties of the seafloor sediments be estimated if all other factors mentioned above are known alongside with the changes of measurable corer acceleration with time?

In the current paper we analyze this question using numerical modeling. A numerical finite-element model has been constructed for the mentioned contact problem in the case of gravity tubular corer penetrating into plastic medium without hardening. A set of numerical experiments has been carried out to obtain the dependencies between changes in acceleration of the corer and mechanical properties of seafloor sediments: density, elastic moduli, and strength properties. The obtained dependencies revealed that it is possible to deal with the inverse problem mentioned above. There are nonlinear relationships between the corer penetration dynamics and mechanical properties of the seafloor. It appears that the postulated inverse problem may have more than one solution even without viscosity taken into account. Nevertheless, the obtained results prove that sampling process itself may provide information regarding the type of rheological model properly describing the mechanical behavior of seafloor sediments.