A Passive Natural-Source Twin-Purpose Borehole Technique

Vertical Gradient Magnetometry (VGM)

Abstract

Two major factors hinder mineral exploration at this present time: (1) a conventional resistivity borehole log is often more indicative of the resistivity of the pore-filled fractured rock in the close locale (<1m) of the borehole than the actual resistivity of the layer in which the probe is located; and (2) ground-based em techniques, both natural and controlled source, are often unable to locate a mineralized zone beneath another mineralized zone.
In this paper, the theory is presented for the basis of a conceptually new type of borehole technique based on the ratio of the measurement of the natural horizontal magnetic field variation to its gradient with depth, down the hole, viz. V_{x, d}(ω)=H_{x, d}(ω)/(∂H_{x, d}(ω)/∂z). Defining the “downhole apparent resistivity”, ρ_a(ω, d) by
ρ_a(ω, d)=ωμ₀|V_{x, d}(ω)|²
it is shown that, for a 1D earth structure, as ω tends to infinity, then ρ_a(ω, d) tends to the actual resistivity of the layer in which the probe is located. Also, ρ_a(ω, d) is independent, in the 1D case, of any structure above it, and weakly dependent in the 2D case. The technique has the benefit of most borehole methods of being far superior at resolving structure at depth below it, e.g., a second good-conducting zone, than equivalent ground-based methods (e.g., MT and/or GDS).
Application of the technique to some theoretical 1D and 2D structures is presented, as well as a discussion of the feasibility of constructing the necessary sensor for the proposed technique.