Mediterranean undercurrent and Meddies imaging by multichannel seismic data.

Multichannel seismic (MCS) imaging is becoming a new tool for oceanographic research. The evolution of the Mediterranean undercurrent (MU) along its outgoing way towards the North Atlantic Ocean has been imaging from MCS transects acquired in the Gulf of Cadiz within the IAM project (September 1993). Sequences of prominent, seaward dipping reflectors stuck to the Iberian continental slope depict the top, bottom and internal stratification of the MU can be observed in seismic images. A set of finely stratified (layer thickness of ~10 m), concentric reflectors forming a 60-100 km-wide and ~1 km thick elliptical lenses, that we associate to the presence of Meddies, may also be observed in three IAM transects. Additionally, a number of out-of-sequence, steeply dipping reflectors that might correspond to the propagation of internal waves is also imaged. Comparisons between seismic data and oceanographic CTD data from CANIGO project (Gulf of Cadiz, September 1997) show that seismic reflectivity is sensitive to abrupt temperature (and salinity) contrasts, confirming that there is a correlation between the reflection image and the oceanic thermahaline structure. These new oceanic seismic images, with horizontal resolution over two orders better than typical oceanographic data, confirms MCS technique as a promising new tool for ocean structure understanding.