The Gas Technology Institute estimates over $1bn US spent on excavations per annum! That is before any repairs or maintenance can be done.
"Recent developments using Broadaband Electo-Magnetics (BEM) sensors to inspect pipeline systems have proven effective..." Source: GTI FOCUS Fact Sheets. http://www.gastechnology.org/webroot/app/xn/xd.aspx?it=enweb&xd=gtihome.xml