Abstract

The second vertical derivative is often used in gravity interpretation to enhance localized near-surface features. Since high-frequency noise is amplified considerably in a true second derivative map, some smoothing is always necessary. One of the methods available to the interpreter is the optimum second derivative technique which utilizes Wiener theory to design filters from an analysis of the power spectrum to suit the degree of noise in the data.The power spectrum of the Bouguer gravity field obtained from over 5100 gravity stations covering an area of approximately 21,000 km 2 in Archean greenstone belts of northern Ontario was computed. The radially averaged spectrum decreased monotonically with frequency and flattened at the high-frequency end. The spectrum, separated into its signal and noise components, was used to design an optimum second vertical derivative filter which has a peak at 0.35 cycles/grid interval with low- and high-frequency cut-off at 0.12 and 0.44 cycles/grid intervals, respectively.The resultant second derivative map corresponds remarkably well with the known surface geology. In most of the mapped regions the zero contours coincide with the lithological boundaries; positive and negative anomalies match surface exposures of the mafic and felsic rock units, respectively. In the central part of the area which covers the Birch-Uchi greenstone belt, where the geology has been well mapped, the second derivative map is quite successful in delineating the successive groups of mafic to felsic metavolcanic rocks that represent the product of cyclic volcanism.The distribution of the mineral deposits in the study area can be related to the positive or negative second derivative anomalies. For example, most of the known gold and silver mineralization occurring in mafic metavolcanics is associated with positive second derivative anomalies. Similarly, the polymetallic Zn-Cu-Ag deposits are mainly located in the negative second derivative anomalies caused by felsic metavolcanics.A properly designed second derivative map can thus be an important supplement to geologic mapping in the identification of lithological units, in the study of structure, and as an indirect tool in regional mineral exploration.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.