Exploration Geophysics - Volume 21, Issue 3-4, 1990

Gravity investigations in mountainous areas have been traditionally performed along profiles located in the valleys due to the extremely rugged topography. Errors up to 10 mGal can be caused due to fault structures or low density sediments filling the valleys. Gravity stations have therefore to be distributed in a grid system with many stations high up mountain flanks and tops. This station pattern necessitates remarkable changes in measuring techniques and reduction procedures for a gravity survey. In this case the site coordinates are a source for considerable gravity errors besides the station heights, as the accuracy of topographic corrections also depends on the error of station coordinate determination. Topographic corrections have to be performed by high resolution techniques and the radius of the correction area must be extended to 167 km. The gravity effect of lakes and glaciers has also to be considered. The problems are discussed using case histories from the Eastern Alps. The improvements gained by expanded reduction procedures are demonstrated.

The equation for the gravity anomaly of an inclined fault of finite strike length with quadratic density function is derived. The coefficients of the density function and the strike length are assumed to be known. Synthetic anomaly profiles of the fault model for the half strike lengths Y=2, 5, 10, 25, 50 and 100 km and for various values of dip and depths to top and bottom of the fault surface are calculated. Distances are measured from an arbitrary reference point, and the origin of the fault model, dip, and depths to top and bottom, are treated as unknown parameters. These parameters are solved by the Marquardt’s algorithm. The convergence of the method is shown by plotting the values of the objective function, the damping parameter lambda and the various parameters, with respect to iteration number. The same anomaly profiles are interpreted for different initial models. The method converges for all cases. A field profile across the Weardale granite, in northeast England, is interpreted as two faults by the present method.

Ground probing radar (GPR) is a relatively new geophysical technique thai has evolved from radio-frequency soundings ot ice to determine depth to bed rock. The technique attracted Interest as a viable geophysical tool in the early 1970s when a number ol successful experiments were reported by researchers including Cook (1974, 75) and Morey (1974). The Geomechanics Division of CSIRO has bean carrying out field investigations with GPR since suitable equipment was obtained in 1986. The Division’s experience with GPR in a wide variety of sites and geological conditions has clearly shown that the radar method is capable ot generating valuable information concerning shallow subsurface structure although the technique has been found to be highly site-dependent with depths of penetration rarely exceeding 10 m. Consequently the role of GPR in exploration is limited. It is best suited to geotechnical or engineering geophysics applications such as the detection underground services and cavities, foundation studies and the detection of the water table at shallow depths in sandy soils. Despite these limitations, GPR is capable of producing high quality data. These data can be processed with the use of seismic-style filters and migration schemes to yield accurate images of subsurface structure.

Standard radioactive sources of known element abundances are essential for the calibration of airborne gamma-ray spectrometers. The conventional approach to calibration has been the determination of 3-channel stripping ratios and sensitivity constants from large airport calibration pads, and a well calibrated test strip, respectively. However, calibration pads are expensive to build and suitable test strips are difficult to find and expensive to calibrate. Also, both pads and test strips are subject to variations in radiation output due to variable moisture content and radon loss, and this can lead to considerable calibration errors. This paper describes an alternative approach to the calibration of airborne gamma-ray spectrometers. We have built a set of portable calibration sources, and have used these to simulate the response of an airborne spectrometer to infinite sources over a range of altitudes. The calibration has produced conventional 3-channel stripping ratios and sensitivity constants as a function of altitude, as well as good quality, pure multichannel spectra which are fundamental to any research into improved processing techniques. The sources are inexpensive, and since they are completely sealed, they do not suffer from variations in radiation output.

The first twenty years of ASEG publications have been reviewed. The review has taken the form of summaries of developments over the period in a number of topics; these include petroleum exploration, coal exploration, potential fields, electrical and electromagnetic methods, and regional and deep crustal geophysics. The survey gives a good indication of the development of ideas and methods in these topics, and shows that the ASEG journals and conferences have played a significant role in stimulating and promulgating the practice of geophysics in Australia.