Practical Airborne Em (Aem) Interpretation Using a Sphere Model

Abstract

The most popular model for quantitative interpretation of airborne EM data is the conductive half‐plane. This is a good shape for representing many long‐strike conductors, but a different model is needed for interpreting localized anomalies. A conductive sphere is an obvious choice. Although the mathematical solution has been available for decades and dozens of papers have been published on the subject, no sets of type curves or interpretation diagrams for this simple model and actual EM prospecting system are yet available in the open literature. For this paper, response profiles for an airborne EM system of “double dipole” type (coil axes in the direction of flight) have been computed from the mathematical solution using all necessary terms of the series. The anomaly profiles are of simple shape and it has been found that the profile half‐width has a constant relationship with the depth to the center of the sphere. The profile half‐widths along with the in‐phase and quadrature anomaly amplitudes allow a direct interpretation of depth, radius, and conductivity of the sphere. Where the sphere is also permeable, interpretation of the accompanying magnetic survey profile can allow this extra parameter to be taken into account. Finally, sample AEM profiles from the New Insco deposit are interpreted. True depth as obtained from drilling lies between the depth interpreted by two extreme models, viz., a sphere and a half‐plane.

The Impulse-Response Moments of a Conductive Sphere in a Uniform Field, a Versatile and Efficient Electromagnetic Model

“…The full EM response of a sphere in free space excited by a dipolar field has been extensively studied by workers such as Debye (1909), March (1953), Grant andWest (1965), Nabighian (1970;1971), Singh (1973), Lee (1975), Lodha and West (1976) and Wait (1982). Although the moments of the sphere response in a dipolar field are faster to calculate than the full EM solution (Smith and Lee, 2000) an even simpler model like the sphere in free space excited by a uniform field would provide a further improvement in speed.…”

Quantitative Interpretation of Horizontal‐loop Em Measurements Using a Permeable Sphere Model*

“…For computing the response function we have ignored the magnetic relaxation phenomenon. For K = 1 the values of the real and the imaginary parts check well with those given by Lodha and West (1976) and Best and Shammas (1979). We have also presented curves for the uniform field case which is valid when the first multipole (n = 1) is dominating.…”

“…The first serious and commendable attempt in this direction was made by Lodha and West (1976) when they gave a practical interpretation scheme for an airborne EM system of "double dipole" type (coil axes in the direction of flight). Following the expressions given by Grant and West (1965, pp.…”

“…Though the solution to the problem of computing the EM response of a conducting sphere is classical (Debye 1909), its application to geophysical exploration started about three decades ago (Wait 1951, 1953, 1960, March 1953, Ward 1959. More complicated models involving nonuniform conductivity distribution, nonzero permeability contrast, conducting surroundings, multi-frequency and time-domain excitations were later considered by Wait (1969), Negi (1962Negi ( , 1967, Verma (1972), Verma and Singh (1970), Nabighian (1970Nabighian ( , 1971), Fuller (1971), Singh (1973), Lodha and West (1976), Best and Shammas (1979) and others.…”

Inversion of helicopter electromagnetic data to a magnetic conductive layered earth

“…Lodha and West (1976) and Huang and Zhu (1984) presented methods for determining the magnetic susceptibility, conductivity, size, and depth of a magnetic conductive sphere in a dipole source field. Fraser (1973) described a technique for the estimation of the amount of magnetite contained in a nonconductive vertical dike.…”