Previous theoretical and experimental studies on seismoelectric logging suggest that the electromagnetic head wave (EH wave) is much weaker than the electric field accompanying the Stoneley wave (ESt wave). Nevertheless, recent in situ measurements show that the EH wave amplitude can be greater than that of the ESt wave. We have addressed this issue according to the simulation of borehole seismoelectric wavefields and find that the amplitude ratio of EH to ESt waves is sensitive to the salinity contrast at the interfaces. Specifically, the EH wave amplitude can be greater than that of the ESt wave if the salinity of the borehole fluid is much higher than that of the pore fluid in a homogeneous porous formation. When an impermeable mud cake layer is taken into account between the borehole fluid and the formation, the amplitude ratio of EH to ESt waves can be even larger, although the amplitudes of the EH and ESt waves become smaller. For a radially stratified porous formation, the large amplitude ratio of EH to ESt waves also occurs if the salinity of the borehole fluid is much higher than that of the pore fluid in the inner layer, or if the salinity of the pore fluid in the inner layer is much higher than that in the outer layer. The large amplitude ratio of EH to ESt waves has potential for detecting interfaces with high salinity contrast, or it can be used as an indicator of mud cake.