  Studies of absorption spectra of high-z QSOs have revealed that the
intergalactic medium at z ~ 2-3 is enriched to ~ 0.1%-1% solar for gas
densities more than a few times the mean cosmic density, but have not yet
produced an accurate metallicity estimate, nor constrained variations in the
metallicity with density, redshift, or spatial location. This paper discusses
the ``pixel optical depth'' (POD) method of QSO spectrum analysis, using
realistic simulated spectra from cosmological simulations. In this method,
absorption in Ly-alpha is compared to corresponding metal absorption on a
pixel-by-pixel basis, yielding for each analyzed spectrum a single statistical
correlation encoding metal enrichment information. Our simulations allow
testing and optimization of each step of the technique's implementation. Tests
show that previous studies have probably been limited by C IV
self-contamination and O VI contamination by HI lines; we have developed and
tested an effective method of correcting for both contaminants. We summarize
these and other findings, and provide a useful recipe for the POD technique's
application to observed spectra. Our tests reveal that the POD technique
applied to spectra of presently available quality is effective in recovering
useful metallicity information even in underdense gas. We present an extension
of the POD technique to directly recover the intergalactic metallicity as a
function of gas density. For a given ionizing background, both the oxygen and
carbon abundance can be measured with errors of at most a factor of a few over
at least an order of magnitude in density, using a single high-quality
spectrum.
