Exploration Geophysics - Volume 7, Issue 2, 1976

A 45 m overburden layer having a conductivity 10 times that of the underlying host rock covers a sulphide body under study. It is estimated that this overburden will attenuate surface IP signal potentials to below 1/25 of that produced if the overburden is replaced by host material. By placing the pair of current electrodes down existing boreholes (75 m apart) at a depth of 1.6 times the thickness of the overburden this attenuation is completely overcome. Type curves showing diagnostic features are presented for traverses with potential electrodes remaining on the surface.

Airborne gamma ray spectrometer test flights were made over the Mary Kathleen Uranium Ltd orebody with a large crystal assembly and modern digital recording system. The decrease in response with height is shown theoretically to be a combination of an exponential air absorption factor and a geometrical factor. The results of these tests indicate that for this specific detecting system and gamma ray source, the overall attenuation effect with height becomes exponential for the total count and thorium channels with apparent attenuation coefficients of 7.4 × 10^-3 and 7.6 × 10^-3 m^-1, respectively.

The uranium channel count rate shows a departure from an exponential relationship; this departure can be allowed for by a height dependent component in the stripping (Compton scatter) correction factor.

Ground surveys with a calibrated portable spectrometer enable abundance conversion factors to be derived for the airborne system.

A summary of the four papers given at a seminar at Macquarie University on "The North-West Shelf Oil and Gas in the Australian Energy Scene" is presented. Barnett compared the historical costs for platforms in the Gulf of Mexico and the North Sea, and compared these to published estimates for the North-West Shelf. He then argued that the Australian government should not give preferential tax treatment to oil producers and that establishing world parity pricing was sufficient incentive for the industry. Lodge discussed the engineering challenges and the cost of developing a gas field in such an isolated location as the North-West Shelf. Charlton outlined the marketing prospects for the North-West Shelf gas. He concluded that a pipeline across Australia was not an economic proposition and that a liquefied natural gas export scheme was the only economically sound alternative. Oldf ield asserted that a liquefied natural gas project to ship the gas to Sydney was not viable, but that a trans-Australia pipeline was.

The design is presented of an M:M multichannel seismic filter which passes and rejects information on the basis of the moveout of that information and which produces as many output traces as are input. The design is discussed in two stages, the first dealing with a filter which is able to handle specific moveouts only and the second pertaining to filters which work according to specified moveout ranges or windows. Two different window types are considered, viz. linearly bounded and hyperbolically bounded windows. Two examples of the filters are discussed and the results of their application to synthetic data are presented. Potential uses of the filter are: (1) prefiltering common depth point gathers prior to velocity analysis, and (2) filtering stacked or 100% coverage data, i.e. data for which there is no redundancy.