  We study the solar-cycle variations of solar p-mode travel time for different
wave packets to probe the magnetic fields at the base of the solar convection
zone. We select the wave packets which return to the same spatial point after
traveling around the Sun with integral number of bounces. The change in
one-bounce travel time at solar maximum relative to minimum is approximately
the same for all wave packets studied except a wave packet whose lower turning
point is located at the base of the convection zone. This particular wave
packet has an additional decrease in travel time at solar maximum relative to
other wave packets. The magnitude of the additional decrease in travel time for
this particular wave packet increases with solar activity. This additional
decrease in travel time might be caused by the magnetic field perturbation and
sound speed perturbation at the base of the convection zone. With the
assumption that this additional decrease is caused only by the magnetic field
perturbation at the base of the convection zone, the field strength is
estimated to be about a 4-7x10^5 gauss at solar maximum if the filling factor
is unity. We also discuss the problem of this interpretation.
