The Lockman-SpReSO survey I. Description, target selection, observations and preliminary results

Context. Extragalactic surveys are a key tool in better understanding the evolution of galaxies. Both deep and wide surveys are improving the emerging picture of physical processes that take place in and around galaxies and identifying which of these processes are the most important in shaping the properties of galaxies. Aims. The Lockman-SpReSO survey aims to provide one of the most complete optical spectroscopic follow-ups of far-infrared (FIR) sources detected by the $Herschel$ Space Observatory in the Lockman Hole field. Such a large optical spectroscopic sample of FIR-selected galaxies will supply valuable information about the relation between fundamental FIR and optical parameters (including extinction, star formation rate and gas metallicity). In this article, we introduce and provide an in-depth description of the Lockman-SpReSO survey and of its early results. Methods. We have selected FIR sources from the observations of the $Herschel$ telescope over the central 24 arcmin $\times$ 24 arcmin of the Lockman Hole field with an optical counterpart up to 24.5 $R_{\rm C}$(AB). The sample comprises 956 $Herschel$ FIR sources plus 188 interesting additional objects in the field. The faint component of the catalogue ($R_{\rm C}$(AB)$\geq$20) was observed using the OSIRIS instrument on the 10.4 m Gran Telescopio Canarias (GTC) in MOS mode. The bright component was observed using two multifibre spectrographs: the AF2-WYFFOS at the William Herschel Telescope (WHT) and the Hydra instrument at the WYIN telescope.