On the Use of Boreholes in Controlled Electromagnetic Source Soundings of the Ocean Crust,

In this report the theory of Chave and Cox (1982) is extended to the case where either the source or receiver is located in the oceanic crust, as in a borehole. Numerical calculations are presented which indicate that very little is gained by the use of boreholes unless low conductivity ( 0.005 S/m) material is penetrated for a significant distance, an unlikely occurrence unless the hole is deeper than 1500-2000 m. In particular, a VED source located in a borehole with receivers placed on the seafloor yields generally weaker signals than HED sources and receivers both located on the seafloor. A seafloor-based HED source with borehole receiver shows greater promise in terms of signal strength, however engineering limitations for this arrangement are more severe.