  Dependence of a/r_c (inverse Sommer parameter in units of lattice spacing a)
on am_q (quark mass in lattice unit) has been observed in all lattice QCD
simulations with sea quarks including the ones with improved actions. How much
of this dependence is a scaling violation has remained an intriguing question.
Our approach has been to investigate the issue with an action with known
lattice artifacts, i.e., the standard Wilson quark and gauge action with
beta=5.6 and 2 degenerate flavors of sea quarks on 16^3 times 32 lattices. In
order to study in detail the sea quark mass dependence, measurements are
carried out at eight values of the PCAC quark mass values am_q from about 0.07
to below 0.015. Though scaling violations may indeed be present for relatively
large am_q, a consistent scenario at sufficiently small am_q seems to emerge in
the mass-independent scheme where for a fixed beta, 1/r_0 and sqrt{sigma} have
linear dependence on m_q as physical effects similar to the quark mass
dependence of the rho mass. We present evidence for this scenario and
accordingly extract the lattice scale (a = 0.0805(7) fm, a^{-1} = 2.45(2) GeV)
by chiral extrapolation to the physical point.
