  We study (1+1)-dimensional turbulence in the framework of the
Martin-Siggia-Rose field theory formalism. The analysis is focused on the
asymptotic behaviour at the right tail of the probability distribution function
(pdf) of velocity differences, where shock waves do not contribute. A
BRS-preserving scheme of phase space reduction, based on the smoothness of the
relevant velocity fields, leads to an effective theory for a few degrees of
freedom. The sum over fluctuations around the instanton solution is written as
the expectation value of a functional of the time-dependent physical fields,
which evolve according to a set of Langevin equations. A natural regularization
of the fluctuation determinant is provided from the fact that the instanton
dominates the action for a finite time interval. The transition from the
turbulent to the instanton dominated regime is related to logarithmic
corrections to the saddle-point action, manifested on their turn as
multiplicative power law corrections to the velocity differences pdf.
