Using the homogeneous half-space as a universal interpretation model, all of the secondary field data obtained with a single-frequency airborne electromagnetic (EM) system that satisfies the superposed dipole condition can be converted to the model parameters da (apparent resistivity) and da (apparent depth). These parameters have been investigated for their behavior above various conductivity models and at various flight altitudes, first for theoretical examples and then for several applications in the field.
The values of ρa and da are good approximations of the true resistivity and true depth of an extended, buried conductor only where the shielding effect of the cover is small. Moreover, a depth value has a meaning only within the lateral limits of a target conductor. A method is described to locate these lateral limits and to select acceptable depth and resistivity values by means of the “area of da-confidence,” which is derived from the horizontal gradient of log ρa and the maxima of da.
The results of the resistivity/depth mapping method are presented in the form of two contour maps. Examples of the practical application of the method, over known sulfide ore bodies and over a salt water intrusion, show that reliable data can be obtained on the depth, dip, and extent of these kinds of conductors, as well as on the approximate resistivity of the conductors and the host rock.