In all cases, surface heat flow was determined as the product of measured down-hole temperature gradients and corresponding thermal conductivity values recorded from local core samples. The acquisition of all thermal data was undertaken by Hot Dry Rocks Pty Ltd. Temperature data were recorded using a precision logging instrument (thermistor) in 1m depth increments. Each hole was logged twice over the course of several months after drilling to ensure thermal equilibration.
Thermal conductivity data were recorded at room temperature using a divided bar apparatus to measure samples cut from core at several levels within each hole. Surface heat flow was estimated for each site by Hot Dry Rocks Pty Ltd using one-dimensional (1D) heat flow modeling software. Heat flow for each bore was assumed to be conductive and was estimated by comparison of modeled (predicted) and observed temperature values.
In nine out of ten cases, the relative confidence in the modeled result was found to be high with a good fit between observed and predicted temperature profiles. In one case (Bangor 1) this confidence was reduced due to the probable influence of advection, most likely related to shallow groundwater flow. In this case a basal heat flow value of 64 ± 4.2 metersWm-2 was determined for depths below 205 meters and is considered to best reflect the assumed regional conductive condition.
Comparisons of new and existing surface heat flow results indicate that all values are spatially consistent, enabling interpolation of a smoothly varying thermal field. Variations between heat flow estimates clearly indicate the existence of a significant thermal anomaly in central Eastern Tasmania. The size, distribution and location of this anomaly are consistent with the current geological model of buried high-heat-producing granite batholiths at depth in this location. In total, an area of 4170 square kilometers is estimated to have anomalously high heat flow (90 meters Wm-2). This area, which remains open to the west, is the prime target area for future geothermal development in Tasmania. Within this zone smaller areas of higher heat flow can be identified, the most significant of which defines an area of 620 square kilometers in the south central region where heat flow is predicted to be above 100mWm-2.
Integration of the surface heat flow field with available geological data enables the generation of targets for future potential hot rock development. Current activity is focusing on a 50 square kilometers area located along the eastern boundary of the 100mWm-2 contour in the south central region. In this area anomalously high heat flow values are observed to coincide with buried granite bodies, predicted from gravity data interpretation to be around 3-4 kilometers depth. The potential of this initial target will be further investigated by application of heat flow modeling to predict temperature conditions at depth as part of the company’s ongoing evaluation program.
