An Experimental Study of the Transient Electromagnetic Response of Connected Conductor Networks

The oft-held view of electromagnetic induction as a geophysical technique simply to register bulk electrical conductivity of the ground warrants further attention. The role played by ground inductance L, for example, is seldom fully appreciated. The time-domain electromagnetic response of an ordinary loop of wire demonstrates the important role of inductance. A complete buried loop can generate a sizable electromagnetic response, as any successful beachcomber knows. However, a through-going cut made in the loop does not remove any conducting material from the ground but severely affects the inductance of the loop and renders negligible its electromagnetic response. These ideas can be extended to interconnected conductor networks which are idealizations of fractured rock formations, heterogeneous aquifers, or partial melt in the upper mantle. A field study was performed using several metal wire and ring networks. Network percolation has little impact on the bulk transient electromagnetic response. Instead, significant factors are the total area of closed loops coupled to the primary flux and mutual inductance between loops. A so-called "critical" cut made in a wire mesh network to render it non-percolating actually has a negligible effect on the bulk response if it does not cut through any major loops contained within the network. There may be huge implications of these findings for magnetotelluric data interpretation.