Method and apparatus for locating underground cast iron pipe joints

A method for locating underground cast iron pipe joints comprising the steps of:

• determining the location and orientation of an underground cast iron pipe;
• passing an oscillating electrical current through an electrically driven resonant coil disposed substantially vertically above said underground cast iron pipe, forming an oscillating magnetic field;
• inducing an electrical current in each of two resonant sensor coils disposed within said oscillating magnetic field;
• measuring a magnetic reluctance between said electrically driven resonant coil and each of said resonant sensor coils, producing an initial magnetic reluctance;
• and
• measuring said magnetic reluctance while moving said coils above and parallel to said cast iron pipe, whereby measured magnetic reluctances substantially different from said initial magnetic reluctance constitutes proximity to a cast iron pipe joint.