Synchronised seismic and oceanographic data were acquired during the GO (Geophysical Oceanography) project cruise in the Gulf of Cadiz in April - May 2007. The small volume (117 cu-inch) mini GI-gun seismic source used during the GO experiment provided high resolution seismic data which revealed features of the internal structure of the ocean. The seismic acquisition design gave a usable bandwidth of 50-250 Hz with a vertical resolution of 1.25 m which is similar to that achieved by co-located CTD casts. We focus on the reflections observed on seismic data covering an area to the east of the Portimao canyon in water depths of 600-800 m. To test the hypothesis that measurable reflections can be generated by suspended sediment we perform forward modelling of seismic response based on the temperature, salinity and sediment concentration derived from light attenuation measurements, available from CTD casts. Modelling based solely on temperature and salinity data show that thermohaline structure does not always explain reflections in water column, but these discrepancies are resolved when the contribution from suspended sediment is included.