  We have analyzed the properties of metals in the high redshift intergalactic
medium using a novel objective pixel optical depth technique on a sample of
extremely high signal-to-noise Keck HIRES and ESI spectra of 26 quasars between
redshifts 2.1 and 6.4. The technique relies on using the doublet nature of the
common ions CIV and SiIV that are the principal metal tracers in the
intergalactic medium outside of the Ly alpha forest. Optical depths are
statistically corrected for contamination by other lines, telluric absorption,
bad pixels, continuum fitting, etc. and for incompleteness, and we achieve in
this way an increased sensitivity of approximately 0.5 dex over previous
analyses. Unlike existing POD techniques, we do not compare the ion optical
depths with HI optical depths; we therefore avoid problems arising from
different velocity widths in the ion and HI. We have shown how the conventional
analysis can be reproduced using a percolation method to generate pseudo-clouds
from ion optical depths. We find that for the higher resolution HIRES data
there is a tight relation, tau ~ N^{0.7}, between the peak optical depth and
the column density. From the optical depth vectors themselves we show that
there is little evolution in the total amount of CIV from z = 2 to z = 5,
though there is a turndown of at least a factor of two in Omega(CIV) above z =
5. We do, however, see substantial evolution in the ratio, SiIV/CIV. Two
subsequent papers will investigate what fraction of the absorbers lie in
galatic wind outflows (Paper II) and what metallicity is associated with
regions of tau(Ly alpha) < 1 (Paper III).
