Towards a Shallow Heat Flow Probe for Mapping Thermal Anomalies

Abstract

Conductive heat flow is arguably the only measurable surface expression of the thermal state of the crust at any given location. However, the geothermal component of heat flow (average ~0.06 W/m2) is effectively masked by solar irradiation (average daily peak ~300 W/m2) at shallow levels. Heat flow measurements must therefore be made in boreholes at least 100 m deep, below the level of influence of the seasonal surface temperature cycle. Such boreholes are drilled at considerable cost, or pre-existing boreholes are accessed opportunistically. Hot Dry Rocks (HDR) is developing and trialing a tool to detect variations in geothermal heat flow from measurements made within the top 1.5 m of the earth. The strategy is to record time-series data and use frequency-domain filtering to reveal regional variations in the geothermal (DC) signal underlying the time-varying solar signal. The goal is to detect variations on the order of 0.01 W/m2. This represents several orders of magnitude greater sensitivity than existing shallow temperature probes. The technical challenges revolve around achieving the necessary precision, accuracy, durability, reliability, thermal bulk (or lack of!), cost, usability and power efficiency for the probe; as well as designing appropriate field procedures, data processing algorithms and interpretation strategies. To date, HDR has designed, manufactured and calibrated a set of 12 prototype tools, and deployed six of these in a remote part of South Australia for extended field trials. Initial results have been encouraging. HDR hopes that the shallow heat flow probe will eventually become a useful geophysical tool for mapping the extent and magnitude of a thermal anomaly prior to expensive drilling. HDR has applied for patent protection for the probe.