Philip Heath

Some comments on potential field tensor data

The measurement of potential field tensor (vector gradient) data is rapidly becoming a new tool for geophysical exploration. Data potentially provide significant improvements in resolution, noise suppression, data interpolation, and discrimination of subtle geological basement and regolith features, particularly in areas of high remanence, low latitudes, and in steep terrains. We have constructed algorithms in MATLAB for the three-dimensional inversion of potential field tensor data using Monte Carlo and Downhill Simplex approaches. We use these algorithms to invert simulated magnetic and gravity tensor data generated from simple geological structures, such as linear dykes and faults. The algorithms have a set target misfit (e.g., RMS misfit equal to one), and the final geological models are illustrated in three dimensions.

Geophysical modelling of the Gawler Province, SA – interpreting geophysics with geology

Multi-scale edge detection was applied to potential field data over the Gawler Province in the central part of South Australia. Also known as worming, the multi-scale edge analysis technique can aid identification of structural controls and depth extents of anomalies. A geological interpretation of the multi-scale edge detection results was then undertaken; integrating drill-hole information, ground mapping and tectonic understanding with geophysical modelling to gain a better comprehension of the dominant structures present. The multi-scale edge detection process provides potential solutions for the lack of outcrop, particularly that which is representative of three-dimensional architecture. The latter is particularly important in understanding how terrains are juxtaposed or dissected tectonically which, in turn, influences the style of any mineral system that may be present. Moreover, correct identification of structural geometry and cross-cutting relationships allows a more confident assessment of fault kinematics and potential dilatancy. In particular, the degree of uranium mineralisation in iron-oxide-copper-gold systems in the Gawler Province may be dependent on the interconnectivity of fault plumbing in three dimensions to nearby uraniferous Mesoproterozoic granitoids.

Quantifying the differences between gravity reduction techniques

Gravity data processing (reduction) generally utilises the best-available formulae. New and improved formulae have been introduced over time and the resulting newly processed gravity will not match the old. Additionally, mistakes made in the gravity reduction process, as well as the incompatibilities between various equations, will inevitably lead to errors in the final product. This can mean that overlapping gravity surveys are often incompatible, leading to incorrect geological interpretations. In this paper I demonstrate the magnitude of change that results when different information is introduced at various stages of the gravity reduction process. I have focussed on differences relating to calibration factors, time zones and time changes, height, geodetic datums, gravity datums and the equations involved therein. The differences range from below the level of detection (0.01 mGal) to over 16.0 mGal. The results not only highlight the need to be diligent and thorough in processing gravity data, but also how it is necessary to document the steps taken when processing data. Without proper documentation, gravity surveys cannot be reprocessed should an error be identified. This paper demonstrates the magnitude of change that results when different information (relating to calibration factors, time zones and time changes, height, geodetic datums and gravity datums) is introduced at various stages of the gravity reduction process. The differences range from below 0.01 mGal to over 16.0 mGal.

Geoscientific investigation of a remanent anomaly ? Teetulpa, South Australia.

We investigate a small (100 metre width) 150 nT amplitude magnetic anomaly delineated in a high-resolution aeromagnetic survey in the Teetulpa Goldfield of the southern Flinders Ranges. We believe that the anomaly is due to a kimberlite pipe, part of a field already known in the general region. As is quite common with kimberlites, the magnetization is clearly dominated by remanence. Modelling the anomaly reveals that the source is very shallow, and would have outcropped at some stage. Follow-up ground geophysical, geochemical and biogeochemical investigations are planned to establish a methodology for integrated studies as follow-up to high resolution aeromagnetic surveys.

New Gravity in the Musgrave Ranges, South Australia

A gravity survey has been undertaken on the Alcurra, Agnes Creek and Tieyon 1:100K mapsheets, in the eastern portion of the Musgrave Ranges, South Australia. A total of 821 readings were collected, incorporating 689 new stations, 88 repeats and 44 base measurements. The data highlights new features giving insight into the underlying geological structure, including a gravity high near Doug’s Well.