Laboratory and field investigations on freeze and gravity core sampling and assessment of coring disturbances with implications on gas bubble characterization†

The quantification of greenhouse gas emissions from aquatic ecosystems requires knowledge about the spatial and temporal dynamics of free gas in sediments. Freezing the sediment in situ offers a promising method for obtaining gas-bearing sediment samples, unaffected by changes in hydrostatic pressure and sample temperature during core withdrawal and subsequent analysis. This article presents a novel freeze coring technique to preserve the in situ stratigraphy and gas bubble characteristics. Nondestructive X-ray computed tomography (CT) scans were used to identify and characterize coring disturbances of gravity and freeze cores associated with gassy sediment, as well as the effect of the freezing process on the gas bubble characteristics. Real-time X-ray CT scans were conducted to visualize the progression of the freezing process. Additional experiments were conducted to determine the freezing rate to assess the probability of sediment particle/bubble migration, and gas bubble nucleation at the phase transition of pore water to ice. The performance of the freeze coring technique was evaluated under field conditions in Olsberg and Urft Reservoir (Germany). The results demonstrate the capability of the freeze coring technique for the preservation of gas-bearing sediments and the analysis of gas bubble distribution pattern in both reservoirs. Nevertheless, the obtained cores showed that nearly all gravity and freeze cores show some degree of coring disturbances