  We use the SPARC code for MHD simulations with monolithic flux tubes of
varying subsurface topology. Our studies involve the interactions of waves
caused by a single source with subsurface magnetic fields. Mode conversion
causing acoustic power to trickle downwards along the flux tube has been
described before and can be visualized in our simulations. We show that this
downward propagation causes the flux tube to act as an isolated source,
creating a characteristic surface wavefield. Measuring this wavefield at the
surface reveals subsurface properties of the magnetic field topology. Using
time distance helioseismology, we demonstrate how to detect such a flux tube
signal based on a group travel-time delay of $\varDelta t = 282.6$ sec due to
the wave packet spending time subsurface as a slow mode wave. Although the
amplitude is small and generally superimposed by the full wave field, it can be
detected if assumptions about $\varDelta t$ are made. We demonstrate this for a
simulation with solar like sources. This kind of study has the potential to
reveal subsurface information of sunspots based on the analysis of a surface
signal.
