  Recent observations have revealed detailed structures of radio relics in a
wide range of frequencies. In this work, we perform three-dimensional
magnetohydrodynamical simulations of merger shocks propagating through a
turbulent magnetized intracluster medium, and employ on-the-fly Lagrangian
particles to explore the physical processes originating radio substructures and
their appearances in high and low-frequency observations. We employ two
cosmic-ray (CR) electron acceleration models: the fresh injection of electrons
from the thermal pool and the re-acceleration of mildly relativistic electrons.
We use the relative surface brightness fluctuations, $\delta S_{\nu}$, to
define a "degree of patchiness''. We find that: 1) Patchiness is produced if
the shock's surface has a distribution of Mach numbers, rather than a single
Mach number; 2) Radio relics appear patchier if the Mach number distribution
consists of a large percentage of low Mach numbers ($\mathcal{M}\lesssim2.5$);
3) As the frequency increases, the patchiness also becomes larger.
Nevertheless, if radio relics are patchy at high frequencies (e.g., 18.6 GHz),
they are necessarily also at low frequencies (e.g., 150 MHz); 4) To produce
noticeable differences in the patchiness at low and high frequencies, the shock
front should have a Mach number spread of $\sigma_{\mathcal{M}}\gtrsim0.3$-0.4;
5) The amount of the patchiness depends on the Mach number distribution as well
as the CR acceleration model. We propose $\delta S_{\nu}$ as a potential tool
for extracting merger shock properties and information about particle
acceleration processes at shocks in radio observations.
