We analyse low-column density extraplanar gaseous structures detected in optical \ion{Ca}{ii} and \ion{Na}{i} absorption towards quasars. In total 177 lines of sight were observed, providing a large sample for the analyses of low-column density IVC and HVC gas which is most likely located in the inner and outer halo of the Milky Way. The study allows us to directly compare the observed absorption column-density distribution of gas in the Milky Way halo with the overall column-density distribution of intervening absorbers towards QSOs. Moreover, the analysis enables us to identify the neutral and mildly ionised absorption structures having low gas column densities and small angular extent which remain unseen in the large 21\,cm IVC and HVC all-sky surveys, but possibly have a considerable absorption cross section.

We supplement our absorption-line data with new \ion{H}{i} 21\,cm observations to investigate the relation between intermediate- and high-velocity \ion{Ca}{ii} absorption features and halo 21\,cm emission. We discuss the first results of our analysis of the physical and statistical properties of the detected absorption and emission features. Additionally, follow-up \ion{H}{i} high-resolution synthesis observations of four of the absorbers are presented, unveiling low-column density small-scale structures embedded in the absorbers environment.