Australian Surface Temperature Corrections for Calculation of Thermal Gradient

Authors: Ed GERNER, Anthony BUDD
Keywords: Australia, exploration, surface temperature
Conference: World Geothermal Congress Session: Exploration
Year: 2015 Language: English
Abstract: For any temperature modelling it is important to have robust data upon which to build an interpretation. In many cases this temperature 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 temperatures 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. The ground surface temperature is also an important boundary condition when building a 3D thermal model. Importantly, ground surface temperature refers to the temperature in the top layer of the rock or soil, which is generally slightly higher than the average air temperature. Howard and Sass (1964) extrapolated surface temperature from 11 near surface gradient readings distributed across 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. Beardsmore and Cull, 2001). 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 by Howard and Sass (1964); however when examined in a spatial context it is apparent that there are a number of factors influencing the difference between the air temperature and the ground surface temperature that are unaccounted for if using an average value. A map of the variation in the difference between the air temperature and the ground surface temperature has been produced which can be used as a guide for corrections when working with temperature data. Beardsmore, G. R. and Cull, J. P. 2001. Crustal Heat Flow: a guide to measurement and modelling. Cambridge University Press, Melbourne, Australia. Holgate, F. and Gerner, E. 2010. OzTemp Well temperature data. Geoscience Australia. catalogue No. 70604 Howard, L.E. and Sass J.H. 1964. Terrestrial heat flow in Australia. Journal of Geophysical Research, 69, pp1617-1626.
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