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We present initial results from a deep neutral hydrogen (HI) survey of theHALOGAS galaxy sample, which includes the spiral galaxies NGC891, NGC925,NGC4414, and NGC4565, performed with the Robert C. Byrd Green Bank Telescope(GBT). The resulting observations cover at least four deg$^2$ around thesegalaxies with an average 5$\sigma$ detection limit of 1.2$\times$10$^{18}$cm$^{-2}$ over a velocity range of 20 km s$^{-1}$ and angular scale of 9.1$'$.In addition to detecting the same total flux as the GBT data, the spatialdistribution of the GBT and original Westerbork Synthesis Radio Telescope(WSRT) data match well at equal spatial resolutions. The HI mass fraction belowHI column densities of 10$^{19}$ cm$^{-2}$ is, on average, 2\%. We discuss thepossible origins of low column density HI of nearby spiral galaxies. Theabsence of a considerable amount of newly detected HI by the GBT indicatesthese galaxies do not have significant extended diffuse HI structures, andsuggests future surveys planned with the SKA and its precursors must go\textit{at least} as deep as 10$^{17}$ cm$^{-2}$ in column density tosignificantly increase the probability of detecting HI associated with thecosmic web and/or cold mode accretion.

A GBT Survey of the HALOGAS Galaxies and Their Environments I: Revealing the full extent of HI around NGC891, NGC925, NGC4414 & NGC4565