  Marchenko focusing functions are in their essence wavefields that satisfy the
wave equation subject to a set of boundary, initial, and focusing conditions.
Here, we show how Marchenko focusing functions can be modeled by finding the
solution to a wavefield reconstruction inversion problem. Our solution yields
all elements of the focusing function including evanescent, refracted, and
trapped waves (tunneling). Our examples indicate that focusing function
solutions in higher dimensions are however challenging to compute by numerical
means due to the appearance of strong evanescent waves.
