Running Waves and Standing Waves in Geomagnetic Depth Sounding

Abstract

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.

Diurnal characteristics of transfer functions at pulsation periods

“…The geomagnetic response function Z'(0, k~) however, stiU remains a function of kT by virtue of equation (4). Such a dependence can be exploited using regional magnetometer array data to estimate the layered distribution of conductivity (Lilley 1975;Lilley & Sloane 1976). The factor C(w, kT) is related to the horizontal gradient of the inducing field as:…”