Ambient ground-motion data were collected using phased seismic arrays in fall 2002 and spring 2007 within the Mississippi embayment and at a single station external to the embayment. These data allowed us to determine the wave-field composition of ambient noise for understanding wave-propagation mechanisms giving rise to spectral peaks using Nakamura’s H/V technique. Ambient ground motions in the frequency band of 0.1–0.33 Hz (10–3 sec period) were dominated by spatially localized Rayleigh- and Love-wave microseisms generated by high-ocean waves along the North American seaboard in the time periods of analysis. Seismic waves important in forming the H/V peak near 4 sec period are composed of relatively high-phase velocity Rayleigh and Love waves that convert to propagating homogeneous shear waves in the thick unconsolidated sediments of the embayment. The H/V resonant period is controlled by both constructive and destructive interference of these shear waves. A simple relationship for the H/V peak is given using a propagator matrix formulation that predicts the resonance frequency of a layered medium for surface wave motion at the base of the system. The amplitude of the observed H/V peak, however, does not give an accurate estimate of shear-wave amplification because it depends on the slowness of the incident wave. The inconsistency in estimated average shear-wave velocities using the H/V method and differential travel times of local earthquake Sp phases in the Mississippi embayment may be explained by misidentification of Sp-wave conversion points from deeper interfaces.