This dataset contains a variety of hydrographic measurements including temperature, salinity, sound velocity, current speed/ direction and seismic data. Hydrographic profilers provided measurements of temperature, salinity, sound velocity and density. Four mooring stations were also installed as part of this project, with three minilogger chains providing temperature data and four moored ADCPs measuring current veloicty. The project ran from February 2006 to September 2009, however all of the data were collected between 17 April 2007 and 14 May 2007 during two cruises which took place in the Gulf of Cadiz. The research was conducted using two research vessels, the RRS Discovery (cruise D318) and the RV Poseidon (cruise PO350). The RRS Discovery cruise D318 was split into two legs, D318a, which took place between 17 April 2007 and 23 April 2007 and D318b, which took place between 29 April 2007 and 14 May 2007. For the second leg of cruise D318, the RRS Discovery was joined by the RV Poseidon. Hydrographic measurements were taken using a variety of instruments, including expendable bathythermographs (XBT), expendable CTDs (XCTD), conductivity-temperature depth (CTD) profilers, acoustic doppler current profilers (ADCP) and VEMCO minilogger chains. Airguns and streamers were used in the recording of the seismic data. The main objectives of the Geophysical Oceanography (GO) project were A) To evaluate and improve new research methods in the developing field of seismic oceanography by exploiting the opportunity of two-ship operations between RSS Discovery and RV Poseidon and B) To study the internal wave field and mixing processes in the Gulf of Cadiz and demonstrate quantitative links between seismic and oceanographic measurements. The cruise was coordinated by Durham Univerity and funded under an EU grant as part of the Framework 6 NEST programme. Eight scientific institutions were involved in the project. These were: the University of Durham, the Proudman Oceanographic Laboratory (POL), the Helmholtz Centre for Ocean Research Kiel (IFM-GEOMAR), the French Research Institute for Exploitation of the Sea (IFREMER), the Italian National Agency for New Technologies, the Energy and Sustainable Economic Development (ENEA), the Spanish National Research Council (CSIC), the University of Western Brittany and the University of Lisbon. Data from the programme are held at the British Oceanographic Data Centre.