The sensing coverage area of surveillance wireless sensor networks, which is determined by applying the Neyman-Pearson detection rule, is considered. Using a graph model for the perimeter, the weakest breach path is found through Dijkstra's shortest path algorithm. The breach probability is a critical parameter for assessing the required number of sensor nodes and the surveillance performance of the network. Analysis indicates that for the Neyman-Pearson detection model, the most significant parameter is the false alarm rate, which is inversely proportional to the breach probability. Furthermore, the width of the field has a significant impact on the breach probability.