AN ELEGANT, UNIVERSAL NOMENCLATURE FOR ELECTROMAGNETIC MOVING SOURCE‐RECEIVER DIPOLE CONFIGURATIONS *

The transmitter and receiver coils and the line joining them in an electromagnetic moving source‐receiver system can, in principle, have an infinity of mutual orientations. Let the direction of a survey line (traverse direction) be x and let the vertical plane through it be the x‐z plane and let the z‐axis be a line in this plane perpendicular to the z‐axis, then the y‐axis of an orthogonal coordinate system is automatically determined. A definite coordinate system, dependent on the mutual coil orientation alone, and independent of the spatial direction of the survey line or alterations in it, is thus associated with each point. The geometry of a moving source‐receiver configuration is uniquely defined by specifying (i) the direction cosines a, b, c of the transmitter coil axis T, (2) the direction cosines d, e, f of the line L joining the transmitter and the receiver coils and (3) the direction cosines g, h, i of the receiver coil axis R. Hence a set of indices conveniently written as T a, b, c L d, e, f R g, h, i describes the geometry and also enables “reconstruction” of the system uniquely and without the risk of confusion inherent in such incomplete descriptions as “vertical coil system”, “horizontal coil system” etc. Degeneracy arises if the traverse direction is vertical. How this is overcome is described in the paper. To the above indices may be appended the indices T, the distance between T and R and v, the frequency.

For example, the most common “horizontal coplanar coil system” has the indices T 001 L 100 R 001. An airborne wing‐tip system with coil axes in the flight direction is specified by T 100 L 010 R 100. Other examples are given in the paper. Maps and data sheets of moving source‐receiver systems should be marked with the appropriate set of indices.