“…Some authors have classified them into two categories: hardware and software-based methods [34,35]. In an attempt to group these methods based on technical nature further research efforts have been made [36,37,38,39,40,41,42] which has led to the classification of available leakage detection systems into three major groups, namely internal, non-technical or non-continuous and external methods [2,4]. In this study, we will classify different methods into the following categories: exterior, visual or biological, and interior or computational methods.…”