Underground electric feeder cable without conduit must be buried a minimum of 24 inches or 2 feet deep.
If the underground electrical feeder cable or underground electric wire is run through conduit then it can be buried only 18 inches deep.
I would recommend just burying the electric feeder cable or electric service wire 24 inches to prevent someone digging into it so easy.
24 inches deep is not that much more depth than the 18 inches and burying the underground electrical wire 24 inches deep with meet almost all codes and also protect the electric wire from any possible damage.