Microplasticity effects caused by seismic wave from two sources of opposite polarity on frequencies about 300. Hz and 1000. Hz are detected in the borehole-to-borehole measurements in loam. For both sources, microplasticity manifestations on seismic records are presented in the form of the ladder-like changes in amplitude course. The stress plateaus on seismic trace interrupt the amplitude course, transform wavefront, and shift the arrival time and amplitude maximum along the time axis. Transformation character depends on the initial deformation type (compression or tension) that generates the source with positive or negative polarity. For the opposite-polarity sources, the arrival time and wave form appreciably differ from each other. Increase in strain amplitude leads to wavefront microplasticity increase and the significant arrival time difference on records with the positive and negative polarity. The combined elastic-microplastic process forms the wavefront steepness and its duration. This contribution to wave propagation physics can be useful in solving the applied problems in material science, seismic prospecting, diagnostics etc