We present 888 visual-wavelength spectra of 122 nearby type II supernovae ( SNe II ) obtained between 1986 and 2009 , and ranging between 3 and 363 days post explosion .
In this first paper , we outline our observations and data reduction techniques , together with a characterization based on the spectral diversity of SNe II .
A statistical analysis of the spectral matching technique is discussed as an alternative to non-detection constraints for estimating SN explosion epochs .
The time evolution of spectral lines is presented and analysed in terms of how this differs for SNe of different photometric , spectral , and environmental properties : velocities , pseudo-equivalent widths , decline rates , magnitudes , time durations , and environment metallicity .
Our sample displays a large range in ejecta expansion velocities , from \sim 9600 to \sim 1500 km s ^ { -1 } at 50 days post explosion with a median H _ { \alpha } value of 7300 km s ^ { -1 } .
This is most likely explained through differing explosion energies .
Significant diversity is also observed in the absolute strength of spectral lines , characterised through their pseudo-equivalent widths .
This implies significant diversity in both temperature evolution ( linked to progenitor radius ) and progenitor metallicity between different SNe II .
Around 60 % of our sample show an extra absorption component on the blue side of the H _ { \alpha } P-Cygni profile ( “ Cachito ” feature ) between 7 and 120 days since explosion .
Studying the nature of Cachito , we conclude that these features at early times ( before \sim 35 days ) are associated with Si ii \lambda 6355 , while past the middle of the plateau phase they are related to high velocity ( HV ) features of hydrogen lines .