Finding hidden water leaks

Thanks to advances in technology, engineers can examine old underground pipes in great detail. However, technology is of much less use when it comes to finding leaks. This is largely (9) ________ many damaged pipes are in inaccessible locations, a fact which (10) ________ it impossible for engineers to transport heavy scanning equipment there. Even in areas where equipment could eventually be installed, the costs in time and money usually rule this (11) ________ as an option. Supposing sensors did detect (12) ________ might be a hidden problem, researchers may end up deciding not to proceed with any repairs anyway. They would have to consider (13) ________ the leak was likely to be serious (14) ________ to be worth the effort. After all, there are often many warning signs visible at the surface. Therefore, it is careful observation engineers rely on, (15) ________ opposed to high-tech equipment. The best maintenance teams can even spot small damp patches hidden in old plaster, in (16) ________ of the fact that plaster and stains are often similar in colour and texture.