  Recent work investigating resonant nonlinear magneto-optical rotation (NMOR)
related to long-lived ($\tau\ts{rel} \sim 1 {\rm s}$) ground-state atomic
coherences has demonstrated potential magnetometric sensitivities exceeding
$10^{-11} {\rm G/\sqrt{Hz}}$ for small ($\lesssim 1 {\rm \mu G}$) magnetic
fields. In the present work, NMOR using frequency-modulated light (FM NMOR) is
studied in the regime where the longitudinal magnetic field is in the
geophysical range ($\sim 500 {\rm mG}$), of particular interest for many
applications. In this regime a splitting of the FM NMOR resonance due to the
nonlinear Zeeman effect is observed. At sufficiently high light intensities,
there is also a splitting of the FM NMOR resonances due to ac Stark shifts
induced by the optical field, as well as evidence of alignment-to-orientation
conversion type processes. The consequences of these effects for FM-NMOR-based
atomic magnetometry in the geophysical field range are considered.
