Exploration Geophysics - Volume 2, Issue 4, 1971

Convolution filters are normally applied to data sets in one of two modes. The first is to terminate the operation, at a distance equal to half the filter length from the ends of the data. This results in loss of information. The second is to run the filter off the end of the data set, by adding sufficient zeros to the ends of the data.

The effects of this second method are shown to be serious, in cases where there is significant information in the ends of the data set. An alternative method is proposed in which the convolution operator is modified when operating on the end portion of the data.

The performance of Induced Polarization in Australia over the last decade has not fulfilled expectations. A general lack of theoretical understanding of the physical principles involved has resulted in poor assessment of the limitations of interpreted data.

Hydrogeological evaluation of subsurface aquifers involves measurement of electrolyte resistivity and subsequent determination of solution salinity. Resistivity of the water may be evaluated by quantitative interpretation of electrical well logs or by direct measurement on recovered samples. Determination of a reliable relationship between electrolyte resistivity and salinity presents a more difficult problem. Approximate solutions have been attempted frequently on theoretical and experimental bases. An empirical relationship derived from previously collected data provides the most satisfactory solution for any particular situation (be it region, valley, basin or aquifer).

Measured resistivities of sphalerite - galena - pyrite ores evince an inverse correlation with metallic mineral concentration whereas induced polarization parameters show a positive correlation. The resistivity of massive sulphide ore appears much greater than that of constituent minerals suggesting limited usefulness of geophysical methods which locate electrical conductors. The high polarizability of concentrated ores corroborates the applicability of induced polarization for locating massive sulphide deposits. The low resistivity and high polarizability of zinc rich samples suggest that sphalerite may contribute significantly to the electrical properties of lead-zinc-silver ores.

Laboratory measurements of phase angle spectra suggest that this parameter may be of diagnostic value in surface and downhole variable frequency I.P. surveying.