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.

Airborne resistivity and susceptibility mapping in magnetically polarizable areas

The apparent resistivity technique using half‐space models has been employed in helicopter‐borne resistivity mapping for twenty years. These resistivity algorithms yield the apparent resistivity from the measured in‐phase and quadrature response arising from the flow of electrical conduction currents for a given frequency. However, these algorithms, which assume free‐space magnetic permeability, do not yield a reliable value for the apparent resistivity in highly magnetic areas. This is because magnetic polarization also occurs, which modifies the electromagnetic (EM) response, causing the computed resistivity to be erroneously high. Conversely, the susceptibility of a magnetic half‐space can be computed from the measured EM response, assuming an absence of conduction currents. However, the presence of conduction currents will cause the computed susceptibility to be erroneously low. New methods for computing the apparent resistivity and apparent magnetic permeability have been developed for the magnetic conductive half‐space. The in‐phase and quadrature responses at the lowest frequency are first used to estimate the apparent magnetic permeability. The lowest frequency should be used to calculate the permeability because this minimizes the contribution to the measured signal from conduction currents. Knowing the apparent magnetic permeability then allows the apparent resistivity to be computed for all frequencies. The resistivity can be computed using different methods. Because the EM response of magnetic permeability is much greater for the in‐phase component than for the quadrature component, it may be better in highly magnetic environments to derive the resistivity using the quadrature component at two frequencies (the quad‐quad algorithm) rather than using the in‐phase and quadrature response at a single frequency (the in‐phase‐quad algorithm). However, the in‐phase‐quad algorithm has the advantage of dynamic range, and it gives credible resistivity results when the apparent permeability has been obtained correctly.

A parametric study of the vertical electric source

Measurement of the vertical magnetic field caused by a vertical electric source (VES) is an attractive exploration option because the measured response is caused by only 2-D and 3-D structures. The absence of a host response markedly increases the detectability of confined structures. In addition, the VES configuration offers advantages such as alleviating masking resulting from conductive overburden and the option of having a source functioning in a collapsed borehole. Applications of the VES, as in mineral exploration, seafloor exploration, and process monitoring such as enhanced oil recovery, are varied, but we limit this study to a classic mining problem—the location of a confined, conductive target at depth in the vicinity of a borehole. By analyzing the electromagnetic responses of a thin, vertical prism, a horizontal slab and an equidimensional body, we investigate the resolving capabilities, identify survey design problems, and provide interpretational insight for vertical magnetic field responses arising from a VES. Data acquisition problems, such as electrode contact within a borehole, are not addressed. Current channeling is the dominant mechanism by which a 2-D or 3-D target is excited. The response caused by currents induced in the target is relatively unimportant compared to that of channeled currents. At low frequencies, the in‐phase response results from galvanic currents from the source electrodes channeled through the target. The quadrature response, at all frequencies, results from currents induced in the host and channeled through the target. At high frequencies, in‐phase currents are also induced in the host and channeled through the target. Hence, the quadrature response and the high‐frequency in‐phase response are quite sensitive to the host resistivity. Time‐domain magnetic field responses show the same behavior as the quadrature component. Interpretation of low‐frequency vertical magnetic field measurements is straightforward for a source placed along strike of the target and a profile line traversing the target. The target is located under a sign reversal or null in the field for a flat‐lying or vertical target. A dipping target has an asymmetrical response, with reduced amplitude on the downdip lobe. The target is located between the maximum lobe and the null. Although the vertical magnetic field caused by a VES for a 2-D or 3-D structure is purely anomalous, the host layering can affect signal strength by more than an order of magnitude. A general knowledge of the location of the target and host layering is helpful in maximizing signal strength. In practice boreholes are not vertical. An angled source can introduce a response because of the horizontal component that can overwhelm the VES response. For low‐frequency, in‐phase, or magnetometric resistivity (MMR) measurements made with a source angled at less than 30 degrees from the vertical, the host response caused by a horizontal electric source (HES) is negligible, and the free space response is easily computed and removed from the total response leaving a response that can be interpreted as that being caused by a VES. The high‐frequency, in‐phase response and the quadrature response at any frequency caused by a HES are strongly dependent on the host resistivity and dominate the scattered response. The measured response, therefore, must be interpreted using sophisticated techniques that take source geometry and host resistivity into account.