Exploration Geophysics - Volume 17, Issue 4, 1986

A general purpose time domain algorithm is provided for the calculation of down-hole zero offset synthetic seismograms. Unlike previous vertical synthetic seismograms, the algorithm allows both source and receiver to be arbitrarily located in a borehole, and can therefore, be used to model the results obtained by various recently developed acoustic well log or vertical seismic data gathering techniques, including the Yo-Yo arrangement. As it is developed from a solution of Robinson’s model with appropriate boundary and initial conditions, the algorithm provides information about variations in the source impulse due to transmission losses and also the properties of multiple reflections.

A program based on the algorithm employs ARMA and LATTICE filter structures which improve the efficiency of the calculation. The program allows the use of a flexible source wavelet which can easily be designed by the user. In addition, because of its simplicity, the program can be used on personal computers.

The theoretical background, a computational algorithm and an HP41CX calculator program for forward modelling of magnetic and gravity anomalies due to ellipsoidal bodies are presented, including gravity effects of prolate and oblate ellipsoids of revolution and two-dimensional elliptic cylinders. The ellipsoid is particularly useful for modelling strongly magnetic, compact orebodies, because of the flexibility and appropriateness of the geometric form and, in particular, because ellipsoids are the only bodies for which self-demagnetization can be treated exactly and analytically. Remanence, anisotropic susceptibility and self-demagnetization are all included in the analysis.

A petrophysical laboratory study was carried out on samples of skarn, lightly mineralized skarn ores, associated calc silicates, carbonate host rocks, intrusive granitoids and country rocks from mineralized areas at King Island, Duckmaloi and Mt Moss. The skarn categories were found to be relatively dense and porous, and to have some radiometric character in the form of low potassium content. Rocks from the skarn environments had variable magnetic susceptibility and remanence properties that were not generally diagnostic. Electrical resistivities of skarns, although somewhat lower than the other rock types, tended to be high but this property was not distinctive. Induced polarization was, as expected, sensitive to the presence of minor sulphides. Thermal conductivities could not differentiate the rock types. Acoustic ultrasonic velocities were high for most rocks and could not be used to distinguish types.

The Bureau of Mineral Resources has recently recorded expanding spread reflection profiles coincident with its regional near-vertical incidence seismic reflection lines primarily to obtain velocity estimates throughout the crust. Accurate shot corrections are required in the first stage of the processing of these expanding spread profiles. A simple least-squares procedure that uses the time differences between reversed shot-receiver paths has been developed for the calculation of shot static corrections. Numerical examples from an expanding spread recorded across the Nebine Ridge in south-east Queensland have been given to show that the standard errors in these corrections for high quality records should generally not exceed 1.0 ms.