  In some underwater acoustic waveguides with specially selected sound speed
profiles striations or fringes of the interference pattern are determined by a
single parameter \beta\ called the waveguide (or Chuprov) invariant. In the
present paper it is shown that an analytical description of fringes may be
possible in a waveguide with an arbitrary sound speed profile. A simple
analytical expression is obtained for smooth lines formed by local maxima of
the interference pattern. This result is valid at long enough ranges. It is
derived proceeding from a known relation connecting the differences of ray
travel times and the action variables of ray paths.
