Offshore 3D-seismic acquisition has been a standard for high-precision structural imaging in the oil and gas industry for many years. Recently this technique has been adapted by only a few teams to the resolution required for archaeological marine investigation. In contrast to sonar techniques, the 3D-seismic method produces images below the sea-floor. We investigate the harbour of the Viking age proto-town of Hedeby in Northern Germany with the SEAMAP-3D system. SEAMAP-3D allows for rapid acquisition and employs an automated data processing sequence. We observe a wealth of archaeologically relevant detail and compare our results with previous work.