A Borehole Electromagnetic Scanning Device With Ferrite Core Coil Antenna: Numerical Simulations and Field Experiments

Subsurface electromagnetic sensing is used in the mining, tunneling, environmental, groundwater, energy, and geoscience research sectors of the global economy. For underground applications, these methods require focused, compact antenna. We introduce the concept of a ferrite core transmitting coil to time-domain electromagnetic sensing, with an emphasis of forward scanning detection of subsurface hazards from a borehole. First, we develop an analytical expression for mutual coupling between a ferrite core coil transmitting antenna coupled with an air core receiver coil to demonstrate the focusing potential gained by adding a ferrite core. Next, we complete 3-D numerical simulations for a scanning borehole transient electromagnetic (BTEM) device. These simulations include radial scanning and demonstrate the improved resolution achievable with a ferrite core coil transmitting antenna. Finally, a ferrite core coil radial scanning in-hole device was constructed, and field experiments were completed in a shallow borehole. A small high-conductivity object was hidden in the subsurface and then detected with the device within radial plots of subsurface resistivity. Our combined numerical and field experiments present the ferrite core coil antenna as a method for focusing electromagnetic fields with one applicant being hazard prediction ahead of a borehole extended from a tunneling face. We hope these electromagnetic scanning methods find many Earth science and engineering applications.