  Although monitoring and covering are fundamental goals of a wireless sensor
network (WSN), the accidental death of sensors or the running out of their
energy would result in holes in the WSN. Such holes have the potential to
disrupt the primary functions of WSNs. This paper investigates the hole
detection and healing problems in hybrid WSNs with non-identical sensor sensing
ranges. In particular, we aim to propose centralized algorithms for detecting
holes in a given region and maximizing the area covered by a WSN in the
presence of environmental obstacles. To precisely identify the boundary of the
holes, we use an additively weighted Voronoi diagram and a polynomial-time
algorithm.Furthermore, since this problem is known to be computationally
difficult, we propose a centralized greedy 1/2-approximation algorithm to
maximize the area covered by sensors. Finally, we implement the algorithms and
run simulations to show that our approximation algorithm efficiently covers the
holes by moving the mobile sensors.
