Running Waves and Standing Waves in Geomagnetic Depth Sounding

The process of electromagnetic induction in a layered half-space depends on the horizontal scale-length of the source field, whether it has the form of a running wave or a standing wave, and whether it consists of mixed modes. The theory as traditionally applied to geomagnetic depth sounding depends on approximating natural source-fields by single-mode running waves.Magnetometer array data can be interpreted to show whether source fields are in fact running waves or standing waves. Phase maps of array data give wave-length estimates for running waves; amplitude maps of array data give field gradients (but usually not wave-length estimates) for standing waves. Commonly superpositions exist of both running waves and standing waves, indicating possible errors in traditional depth-sounding estimates which are based on the theory of a single-mode running wave.Lack of source-field knowledge is a more serious disadvantage in traditional geomagnetic depth-sounding than it is in a newer method, which involves the ratio of vertical component to horizontal gradient measured from array data.