A new ‘schlieren’ technique application for fluid flow visualization at cold seep sites

A new optical instrument for the investigation of submarine fluid flows based on a ‘schlieren’ technique was developed and successfully deployed at cold seep sites. With this application it is possible to visualize the discharge and distribution of fluids in the ambient bottom water and to resolve microstructures and mixing processes at a scale of centimeters. The system is sensitive to small refractive index anomalies caused by temperature and salinity variations. Density anomalies of Δσt=0.049 are detectable evaluated by in-situ temperature variations of ΔT=0.1°C and salinity variations of ΔS=0.045 psu. In flume experiments the smallest detectable density variation was even lower with Δσt=0.023. The technique has been successfully applied in two different environments. First field experiments were performed to observe submarine groundwater discharge in Eckernförde Bay (western Baltic Sea) at shallow water depths. Subsequently, the ‘schlieren’ technique was successfully brought to a cold seep location at the Cascadia convergent margin (800 m water depth). The discharge of fluids was recorded in both field experiments which enabled a qualitative seep site identification.