  We performed 101Ru nuclear quadrupole resonance (NQR) measurements on the
s-wave superconductor CeRu2 and found oscillatory behavior in the spin-echo
amplitude at the +-1/2 <-> +-3/2 transitions but not at the +-3/2 <-> +-5/2
transitions. The modulation disappears in the superconducting state or in a
magnetic shield, which implies a geomagnetic field effect. Our results indicate
that the NQR spin-echo decay curve at the +-1/2 <-> +-3/2 transitions is
sensitive to a weak magnetic field.
