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 is moved through the ground in a region which includes at least one electrically conductive in-ground line and which is subject to static magnetic fields including the magnetic field of the earth, a method of tracking a positional relationship between the boring tool and said line as well as a directional heading of the boring tool within said region, an arrangement for tracking a positional relationship between the boring tool and said line as well as a directional heading of the boring tool within said region, said arrangement comprising:

• a) means for generating a time varying magnetic field from said line;
• b) a detecting arrangement at said boring tool for detecting a composite magnetic signal which includes one component affected by said static magnetic fields and another component affected by said time varying magnetic field such that the static magnetic field component varies as a function of said directional heading and the time varying component of the detected composite magnetic signal varies as a function of said positional relationship;
• c) processing means for processing said composite magnetic signal in a way which separates the static magnetic field component and the time varying magnetic field component from the composite magnetic signal for use in determining said directional heading and said positional relationship.