Detecting metal objects in magnetic environments using a broadband electromagnetic method

We analyze the use of the broadband electromagnetic (EM) method in detecting metallic objects, such as unexploded ordnance (UXO), buried in magnetic environments. Magnetic rocks close to the sensor often contribute a larger in-phase response than does the target at depth, making target detection and identification difficult. On the other hand, magnetic rocks contribute little quadrature response, which gives rise to the concept of using quadrature response and apparent conductivity to detect metallic objects in highly magnetic environments. To test this concept, we employed numeric models, physical experiments, and field studies. A layered half-space simulated conductive overburden and magnetic basement. Sphere models are used for isolated magnetic rocks and metal targets. The responses of the layered earth, magnetic rocks, and metal objects were added to obtain the approximate total response. We then inverted the EM data into apparent magnetic permeability and conductivity. The EM response at the lowest frequency was used initially to estimate apparent magnetic permeability, which let us calculate the apparent conductivity using the EM data at all frequencies. The simulations and field examples show that broadband EM sensors can detect small metal targets in magnetic environments, mainly by the quadrature component of the responses and the apparent conductivity.