Since cost is no obstacle, I would explore GPR Ground Penetrating Radar. In my past experience GPR shows disturbed soil in lines or waves this could indicate previous intrusion i.e. the Fiber Optic Casing with the copper indicator Tracer. In a past Life I did many last mile Fiber to the Site (End User) installations. Of course we knew the start and end points of the fiber casing. However we often did not know the exact location of the travel of the fiber which causes issues when installing power poles and other semipermanent structures. We found a solution using inductive coupled RF this is not a perfect solution to your problem but it is work around. You can buy a low powered transmitter on ISM Frequencies we often used the 13.56MHz, we used a Magnetic Loop Antenna to loosely couple that power to the tracer wire then we used a very small receiver that was very inexpensive. (Not that cost is a factor, but because open trench construction sites are a hazardous environment. Things get damaged, lost or treated roughly, so an inexpensive item can easily become a consumable item. If any of this is of interest to you contact me and I can walk you through it. All this is not a perfect solution however it does work and might just be an answer for you to consider.