Tracking the positional relationship between a boring tool and one or more buried lines using a composite magnetic signal

In a system in which a boring tool including a drill string having a length which is fed out in a way which moves the boring tool through the ground in a region which includes at least one electrically conductive in-ground line, an apparatus for tracking a specific positional relationship between the boring tool and said line, said apparatus comprising:

• a transmitter arrangement for generating a magnetic locating field from one of either said boring tool or at least an underground length of said line in a predetermined way;
• a receiver arrangement carried by said boring tool if said field is generated from said line, or connected to said line if said field is generated from said boring tool, for detecting an intensity of said magnetic locating field which, in cooperation with the predetermined way in which the magnetic locating signal is generated, provides for an unambiguous measurement of the positional relationship between the boring tool and said line throughout said region using said magnetic locating signal;
• a drill string monitoring arrangement for monitoring a value of at least one parameter relating to the way in which said drill string is fed out;
• and
• a processor for correlating the magnetic locating field intensity with the value of said one parameter in a way which indicates certain changes in the positional relationship between said boring tool and said line without the need to establish an actual distance between the boring tool and said line as the boring tool moves through the ground within said region.