  We have used solar oscillation frequencies and frequency splittings obtained
over solar cycles 23 and 24 to investigate whether the base of the solar
convection zone shows any departure from spherical symmetry. We used the
even-order splitting coefficients, $a_2$-$a_8$, and estimated the contributions
from each one separately. The average asphericity over the two solar cycles was
determined using frequencies and splittings obtained with a 9216-day
time-series. We find that evidence of asphericity is, {\em at best}, marginal:
the $a_2$ component is consistent with no asphericity, the $a_4$ and $a_6$
components yield results at a level a little greater than $1\,\sigma$, while
the $a_8$ component shows a signature below $1\,\sigma$. The combined results
indicate that the time average of the departure from the spherically symmetric
position of the base of the convection zone is $\lesssim 0.0001R_\odot$. We
have also used helioseismic data obtained from time-series of lengths 360 days,
576 days, 1152 days, and 2304 days in order to examine the consistency of the
results and evaluate whether there is any time variation. We find that the
evidence for time variation is statistically marginal in all cases, except for
the $a_6$ component, for which tests consistently yield $p$ values of less than
$0.05$.
