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This is the first of a series of investigations into far-IR characteristicsof 11 planetary nebulae (PNs) under the Herschel Space Observatory Open Time 1program, Herschel Planetary Nebula Survey (HerPlaNS). Using the HerPlaNS dataset, we look into the PN energetics and variations of the physical conditionswithin the target nebulae. In the present work, we provide an overview of thesurvey, data acquisition and processing, and resulting data products. Weperform (1) PACS/SPIRE broadband imaging to determine the spatial distributionof the cold dust component in the target PNs and (2) PACS/SPIREspectral-energy-distribution (SED) and line spectroscopy to determine thespatial distribution of the gas component in the target PNs. For the case ofNGC 6781, the broadband maps confirm the nearly pole-on barrel structure of theamorphous carbon-richdust shell and the surrounding halo having temperatures of26-40 K. The PACS/SPIRE multi-position spectra show spatial variations offar-IR lines that reflect the physical stratification of the nebula. Wedemonstrate that spatially-resolved far-IR line diagnostics yield the (T_e,n_e) profiles, from which distributions of ionized, atomic, and molecular gasescan be determined. Direct comparison of the dust and gas column mass mapsconstrained by the HerPlaNS data allows to construct an empirical gas-to-dustmass ratio map, which shows a range of ratios with the median of 195+-110. Thepresent analysis yields estimates of the total mass of the shell to be 0.86M_sun, consisting of 0.54 M_sun of ionized gas, 0.12 M_sun of atomic gas, 0.2M_sun of molecular gas, and 4 x 10^-3 M_sun of dust grains. These estimatesalso suggest that the central star of about 1.5 M_sun initial mass isterminating its PN evolution onto the white dwarf cooling track.

The Herschel Planetary Nebula Survey (HerPlaNS) I. Data Overview and Analysis Demonstration with NGC 6781