A PARAMETRIC STUDY OF THE VERTICAL ELECTRIC SOURCE

Measurement of the vertical magnetic field caused by a vertical electr ic source (VES) is an attractive exploration option because the measur ed response is caused by only 2-D and 3-D structures. The absence of a host response markedly increases the detectability of confined struct ures. In addition, the VES configuration offers advantages such as all eviating masking resulting from conductive overburden and the option o f 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 th is study to a classic mining problem-the location of a confined, condu ctive 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 capabilitie s, identify survey design problems, and provide interpretational insig ht for vertical magnetic field responses arising from a VES. Data acqu isition 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 cu rrents. At low frequencies, the in-phase response results from galvani c currents from the source electrodes channeled through the target. Th e quadrature response, at all frequencies, results from currents induc ed in the host and channeled through the target. At high frequencies, in-phase currents are also induced in the host and channeled through t he target. Hence, the quadrature response and the high-frequency in-ph ase response are quite sensitive to the host resistivity. Time-domain magnetic field responses show the same behavior as the quadrature comp onent. Interpretation of low-frequency vertical magnetic field measure ments is straightforward for a source placed along strike of the targe t and a profile line traversing the target. The target is located unde r a sign reversal or null in the field for a flat-lying or vertical ta rget. A dipping target has an asymmetrical response, with reduced ampl itude on the downdip lobe. The target is located between the maximum l obe 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 i n maximizing signal strength. In practice boreholes are not vertical. An angled source can introduce a response because of the horizontal co mponent that can overwhelm the VES response. For few-frequency, in-pha se, or magnetometric resistivity (MMR) measurements made with a source angled at less than 30 degrees from the vertical, the host response c aused by a horizontal electric source (HES) is negligible, and the fre e space response is easily computed and removed from the total respons e leaving a response that can be interpreted as that being caused by a VES. The high-frequency, in-phase response and the quadrature respons e 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 t ake source geometry and host resistivity into account.