The submarine groundwater discharge (SGD) site in Hanko on the southern coast of Finland has been previously studied for its geological structure and Rn-based groundwater discharge estimates (Virtasalo et al., 2019), for the impacts of focused groundwater flow from seafloor pockmarks on the heterogeneity of seabed biogeochemical processes and microbial community structure (Purkamo et al., 2022), and for the behavior of Li, S and Sr isotopes in the subterranean estuary (Ikonen et al., 2022). Here, we report results of a FloaTEM survey to characterize the vertical structure of the seawater-groundwater mixing. FloaTEM is a floating time-domain electromagnetic profiling instrument that measures continuously while towed by a small boat. Our survey was run on October 13, 2023, along shore-parallel survey lines over known pockmark locations. The FloaTEM system used had a 4x4 m transmitter coil and a receiver coil at an 8 m offset. The survey speed was 5–6 km/h. Resistivity of the brackish sea water was ca. 1.2 Ωm. Water depth was recorded using an echosounder and applied as a constraint for the top layer thickness in data inversion. The data was processed in Aarhus Workbench and a preliminary inversion was made by using a 1D laterally constrained inversion (LCI) scheme. The data was inverted with a smooth layer model type consisting of 15 layers with depth layer boundaries from 1–80 m. The survey results showed downward increasing resistivity values (1–100 Ωm) in organic-rich muddy sediments below the seafloor. The resistivity values were higher (>10 Ωm) beneath the pockmarks, indicating funneled upward groundwater flow. References: Ikonen J. et al. 2022, Applied Geochemistry 147, 105471. Purkamo L. et al. 2022, Geochimica et Cosmochimica Acta 334, 14–44. Virtasalo JJ. et al. 2019, Solid Earth 10, 405–423.