The Analytic Signal and Cross-Correlation Applied in Rapid, Detailed Analysis of Aeromagnetic and Radiometric Data

Abstract

The analytic signal, which is derived from two-dimensional magnetic data sets, can be used to map magnetic boundaries or changes in susceptibility. As air-rock boundaries in steep terrains represent a change in susceptibility, there is an ambiguity between terrain and geology. This was evident in the trace of the analytic signal for the Goonaloom Creek area and the ambiguity could not be resolved satisfactorily by comparison of the analytic signal trace and the known geology. When applied to a small scale, high definition aeromagnetic data set, the analytic signal trace was fragmented by grid corrugation. The Laplacian convolution filter suggested by McLeod et al., (1993), accentuated the noise artifacts. The application of the analytic signal to the mid-latitude data of this project gave no advantages over reduce-to-pole methods, although this may not be the case at low magnetic latitudes. Cross-correlation in the Fourier plane is defined as the multiplication of one transformed signal by the complex conjugate of the second transformed signal. In a commercial image processing package such as ER Mapper, cross correlation is easily implemented. Cross-correlation of the total magnetic intensity and potassium-40 counts is useful in targeting areas of where hydrothermal activity has occurred. In the case of the Goonaloom Creek survey, the correlation anomaly can be interpreted as porphyry mineralization zoning under the Lowell and Guilbert (1970) model. This may also be the case for an area two kilometres south of Mt. Hastings mine west of Biggenden, Queensland. The cross-correlation of the first vertical derivative of the total magnetic field intensity and potassium-40 defines areas of possible structurally-controlled, near-surface mineralization.