Thermostatic controlled heat tape is superior in many ways to any light bulb arrangement. Wrap the heat tape around the pipe and secure the thermostat (typically a recognizable lump of plastic at the power cord end of the tape) to the pipe where the pipe goes into the cement. Insulate around the entire thing. The tape will heat the pipe which will heat the water. Don't worry about convection currents in the water not being your friend as the metal pipe will conduct heat downward below the surface for a distance until the heat is lost to the cold cement or earth. This is not a guarantee but a pretty effective method.

A more surefire heating method with metallic pipe is to pass an electric current through the pipe from one end to the other. As the resistance of the pipe is pretty low you need a darned good step down transformer to provide the current. Typically old fashioned transformer type welders are used for this turned down to a low heat for longer term use or higher amps for a quicker temporary thaw job.

Insulation alone will not prevent freezing, long term, for water in a pipe with no flow. Eventually enough heat will be lost through the insulation to let the temp of the water drop below freezing and potentially burst your pipe in a situation where the average temp stays below freezing for a protracted period.

Pat