Exploration Geophysics - Volume 12, Issue 3, 1981

A facility for the calibration of down-hole radiometric logging instruments has been constructed at the Australian Mineral Development Laboratories in South Australia. Three calibration models of a borehole passing through a zone of homogeneous uranium ore have been completed and are available to users. The radiometric grades determined for the models are 0.209%, 0.920% and 0.054% equivalent U3 O8.

Automated density profiling can be used to analyse real data errors, and to anticipate the effect of errors. The best possible density-profiling precision can also be predicted for any given topographic cross-section. Although the terrain correction is by-passed in the automatic process, it is useful to display this quantity, as its size is a function of steepness and height, and therefore an indicator of the likely effectiveness of density profiling.

Density profiling over ridges less than 50 m high is feasible provided there are no perturbations in the gravity field, and the cross-sectional shape is known accurately. A comprehensive density-profiling analysis is demonstrated for a 35 metre ridge.

Kimberlites occur as intrusive igneous rocks, generally as dykes, and pipes and some small diatremes or volcanic vents. The pipes are generally irregular structures and have not been found to exceed 2 km in diameter.

The physical properties of kimberlites are variable. Physical properties from a large number of sources are tabulated.

Western Australian pipes have shown some similarity in features to the Orapa (Botswana) pipes and the choice of geophysical prospecting methods for these pipes should be according to these similarities.

Geological interpretation of aerial photography and satellite imagery are useful in the screening and classification of magnetic anomalies in kimberlite environments. Aerial photography may serve as a basic tool for studying the structural environment of kimberlites and the identification of intrusions, particularly dykes. Fracture analysis may aid in locating kimberlites.

There may be considerable variation in the amplitude of magnetic response over kimberlite pipes. Within pipes that exhibit targe magnetic response, there are large local in-homogeneities, probably caused by a combination of the explosive emplacement mechanism, different volcanic pulses, and different weathering.

The gravity method is useful for detecting extensions of existing kimberlite pipes, and defining the position of the intrusion at depth. Over large pipes where kimberlite is weathered, a negative anomaly of a few tenths of a milligal may be expected. The unweathered kimberlite at depth gives a positive anomaly, and must be more dense than the average density of basalt. Since the anomalies over quite large pipes in basalt are small (1 or 2 milligal), accurate terrain corrections and allowances for any substantial variations in the thickness of the overburden and for any regional gravity gradient are important.

Kimberlite material is electrically conductive compared with host country rocks, and resistivity methods are useful in delineating large bodies. Electromagnetic techniques have proven effective in some areas in defining the extent of kimberlite pipes and shape of dykes. Despite success in some Scandinavian countries and Canada the electromagnetic method for kimberlite prospecting in tropical and subtropical countries has not shown convincing results because of the conductive overburden in these areas.

Induced polarisation has sometimes served to define sharply the vertical edge of weathered pipe rock.

Owing to the enrichment of radioelements including potassium, thorium and uranium in kimberlite the radio-metric technique might be expected to be of use in kimberlite prospecting. However, weak total-count anomalies have been obtained over pipes, with some potassium radioactivity measured either over the pipe or along its contact.