Wireless underground sensor networks (WUSNs) are an emerging and promising research area. The aim of WUSNs is to establish an efficient wireless communication in the underground medium. A magnetic induction (MI)-based waveguide technique has been proposed to overcome the very harsh propagation conditions in WUSNs. In this approach, several resonant relay circuits are deployed between the two nodes to be connected. This technique allows for an extension of the transmission range. In this work, we investigate digital transmission schemes for MI-WUSNs. We analyze the influence of transmission parameters like symbol duration and modulation scheme and propose methods for their optimization.