  The enclosure method was originally introduced for inverse problems of
concerning non-destructive evaluation governed by elliptic equations. It was
developed as one of useful approaches in inverse problems and applied for
various equations. In this paper, an application of the enclosure method to an
inverse initial boundary value problem for a parabolic equation with a
discontinuous coefficient is given. A simple method to extract the depth of
unknown inclusions in a heat conductive body from a single set of the
temperature and heat flux on the boundary observed over a finite time interval
is introduced. Other related results with infinitely many data are also
reported. One of them gives the minimum radius of the open ball centered at a
given point that contains the inclusions. The formula for the minimum radius is
newly discovered.
