Can any one tell what method ( other than Nitrogen freezing ) can be used to detect oil leakages in underground oil filled transmission lines of 132 KV?​

Dear engineers, I need your help in analyzing this query: Our plant consists of 11x 635 KW, 415V Waukesha gensets, 2 x 3.5MW,11KV wartsila gensets, 11 KV main bus for...