  We consider an inverse boundary value problem for a model time-harmonic
equation of acoustic tomography of moving fluid with variable current velocity,
sound speed, density and absorption. In the present article it is assumed that
at fixed frequency the coefficients of this equation are already recovered
modulo an appropriate gauge transformation using some reconstruction method
from boundary measurements presented in the literature. Our main result
consists in formulas and equations that allow to get rid of this gauge
non-uniqueness and recover the fluid parameters using boundary measurements at
several frequencies.
