Publication 19
A passive, natural-source, twin-purpose borehole technique: Vertical
Gradient Magnetometry (VGM)
Alan G. Jones
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 (< 1 m) 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. Vx,d(w) = Hx,d(w)/{dHx,d(w)/dz). Defining the "downhole apparent resistivity",
rho_a{w,d) by
rho_a(w,d) = w mu0 |Vx,d(w)|^2
it is shown that, for a 1D earth structure, as w tends to infinity, then rho_a(w,d)
tends to the actual resistivity of the layer in which the probe is located. Also,
rho_a{w,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.
Source
Journal of Geomagnetism and Geoelectricity, 35, 473-490.
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Alan G Jones / 28 February 2010 /
alan-at-cp.dias.ie