Petrophysical characterisation of parna using ground and downhole geophysics at Marinna, central New South Wales

Abstract

Parna, an aeolian sediment of dominantly clay size, forms a significant component of the regolith developed in western New South Wales. These materials blanket parts of the contemporary landscape and, in places, they form surficial deposits in excess of 8 m thick. Recent studies have suggested that parna constitutes a significant store and source of soluble salts, hence its significance to land management. This paper describes the mineralogical and petrophysical characteristics of parna, making specific reference to materials adjacent to a recognised parna section at Marinna, near Junee, which is located in the Murrumbidgee catchment. Rising water tables and dryland salinity have been identified as major concerns to the overall health of this catchment. Therefore information on the distribution and movement of salt within the upper reaches of this catchment is important, as is the role of parna in controlling these dynamics. Results from multiparameter borehole geophysical studies indicate that parna is characterised by sub-horizontal variations in conductivity and natural gamma activity. These properties are attributed to contrasts in the porosity and permeability of these materials and to variations in their composition. Significant vertical conductivity contrasts have been noted between parna and underlying saprolite, with the results from EC1:5 measurements and downhole induction logs suggesting that conductivity is primarily controlled by textural variations rather than the soluble salt and moisture content of these materials. Ground electromagnetic surveys (EM31) suggest that areas covered by parna may exhibit marked lateral variations in average conductivity attributable to differences in the thickness and proportion of parna in these materials.