Australian Surface Temperature Corrections for Thermal Modelling,

ABSTRACT For any temperature modelling it is important to have robust thermal gradient data upon which to build an interpretation. In many cases this data can be taken from a continuous temperature log, particularly when using data specifically collected for geothermal exploration. However, in Australia there are a large number of discrete downhole temperature measurements from other sources (eg Holgate and Gerner, 2010). To make the best use of this data it is necessary to have established surface temperatures from which to calculate an overall temperature gradient. On an even broader scale Importantly, ground surface temperature refers to the temperature in the top layer of the rock or soil which in Australia is usually warmer than the average air temperature. Howard and Sass (1964) extrapolated surface temperature from 11 near surface gradient readings distributed across Australia (seven of which were in the Yilgarn province of Western Australia) to calculate a mean difference of +3° C relative to the air temperature across the whole of Australia. This value has been widely relied on since (eg. To provide the greatest spatial coverage of the continent, extrapolated surface temperature data from more than 100 wells logged by Geoscience Australia and soil temperature data at 1 m depth collected by the Bureau of Meteorology have been used to test the assumption that the +3° C correction is appropriate for the whole continent. Mean annual maximum temperature and the mean annual minimum temperature grids (Bureau of Meteorology) were used to calculate a mean annual average air temperature grid to which the soil temperatures and the extrapolated surface temperatures were compared. The average of the differences between the air temperature and the ground surface temperatures is broadly consistent with the value determined b