We study the self-affine properties of intensity fluctuations of underground muons generated by primary cosmic rays of rigidity≥1600 GV. The muon time series were recorded at a vertical depth of 570 Hg/cm2 during the years 1981--1983. This time interval includes a period of intense solar activity in the summer of 1982 which was characterized by the creation of large solar wind shocks and associated production of major interplanetary disturbances. The purpose of our analysis is to obtain from the behavior of the muon intensity some insight into the large-scale behavior of interplanetary magnetic field fluctuations and their effects on the propagation of high-rigidity galactic cosmic rays. The results give good evidence for fractal behavior of large-scale muon fluctuations in the range from a few hours to more than 10 days, with scaling exponent H varying according to the time interval analyzed. We find that the large time scale muon fluctuations evolve from a preshock condition in late 1981 to early 1982 characterized by a scaling exponent H=0.28 and power spectrum f-1.6 to a condition with H=0.07 and an f1.1 power spectrum during the most intense solar activity in 1982, followed by a slow return in 1983 to the original preshock conditions (H=0.14, f-1.3).