  We employ an analytical model that incorporates both wavelength-dependent and
wavelength-independent depolarization to describe radio polarimetric
observations of polarization at $\lambda \lambda \lambda \, 3.5, 6.2, 20.5$ cm
in M51 (NGC 5194). The aim is to constrain both the regular and turbulent
magnetic field strengths in the disk and halo, modeled as a two- or three-layer
magneto-ionic medium, via differential Faraday rotation and internal Faraday
dispersion, along with wavelength-independent depolarization arising from
turbulent magnetic fields. A reduced chi-squared analysis is used for the
statistical comparison of predicted to observed polarization maps to determine
the best-fit magnetic field configuration at each of four radial rings spanning
$2.4 - 7.2$ kpc in $1.2$ kpc increments. We find that a two-layer modeling
approach provides a better fit to the observations than a three-layer model,
where the near and far sides of the halo are taken to be identical, although
the resulting best-fit magnetic field strengths are comparable. This implies
that all of the signal from the far halo is depolarized at these wavelengths.
We find a total magnetic field in the disk of approximately $18~\mu$G and a
total magnetic field strength in the halo of $\sim 4-6~\mu$G. Both turbulent
and regular magnetic field strengths in the disk exceed those in the halo by a
factor of a few. About half of the turbulent magnetic field in the disk is
anisotropic, but in the halo all turbulence is only isotropic.
