Apparatus for and methods of electromagnetic surveying of elongated underground conductors

Apparatus for surveying an underground pipeline or other elongated element which carries, or can be made to carry, AC, the pipeline or element then forming a conductor around which there is a substantially cylindrical electromagnetic field, the apparatus comprising a coil array including a first pair of axially parallel coils arrangeable, in use, one above the other with their axes horizontal and transverse to the conductor;

• a phase comparator and indicator circuit for distinguishing from the outputs of a selection of coils of said array (such outputs being related to their position and attitude and the electromagnetic field) on which side of the conductor the array lies, a depth correction circuit including means responsive to the combined output of the first pair of coils to provide a correction factor dependent on the depth of the conductor, and a variable gain amplifier whose inputs are the correction factor and the output of one of said first pair of coils, and whose output is that coil output modified by the correction factor in such manner that variations in depth alone of the conductor leaves said output substantially unaffected although it will change according to variations of the electromagnetic field, a discontinuity in that field as the apparatus is progressed along the element corresponding to a fault;
• and indicator or recording means responsive to said variable gain amplifier output by which any such discontinuities can be determined.