  The NO$\nu$A experiment is an electron neutrino appearance neutrino
oscillation experiment at Fermilab. Electron neutrino events are identified by
the electromagnetic (EM) showers induced by electrons in the final state of
neutrino interactions. EM showers induced by cosmic muons or rock muons, are
abundant in NO$\nu$A detectors. We use a Muon-Removal Technique to get pure EM
shower samples from cosmic and rock muon data. Those samples can be used to
characterize the EM signature and provide valuable checks of the MC simulation,
reconstruction, PID algorithms, and calibration across the NO$\nu$A detectors.
