  We use high spectral resolution (R=45000) and high signal-to-noise ratio (S/N
\~ 35-70 per pixel) spectra of 19 high-redshift (2.1<z_em<3.2) quasars to
investigate the metal content of the low-density intergalactic medium using
pixel-by-pixel procedures. This high quality homogeneous survey gives the
possibility to statistically search for metals at HI optical depths smaller
than unity. We find that the gas is enriched in carbon and oxygen for neutral
hydrogen optical depths tau(HI)>1. Our observations strongly suggest that the
CIV/HI ratio decreases with decreasing tau(HI) with
log[tau(CIV)]=1.3xlog[tau(HI)]-3.2. We do not detect CIV absorption
statistically associated with gas of tau(HI)<1. However, we observe that a
small fraction of the low density gas is associated with strong metal lines as
a probable consequence of the IGM enrichment being highly inhomogeneous. We
detect the presence of OVI down to tau(HI)~0.2 with log[tau(OVI)/tau(HI)] -2.0.
We show that OVI absorption in the lowest density gas is located within ~300
km/s from strong HI lines. This suggests that this OVI phase may be part of
winds flowing away from overdense regions. This effect is more important at the
largest redshifts (z>2.4). Therefore, at the limit of present surveys, the
presence of metals in the underdense regions of the IGM is still to be
demonstrated.
