A parametric study of the vertical electric source

Abstract

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.