Developing the 'Heat Needle'—a Tool for Cost Effective Heat Flow Mapping

Conductive heat flow is arguably the only measurable surface expression of the thermal state of the crust at any given location. The Heat Needle is a tool designed to detect variations in geothermal heat flow from measurements made within the top meter of the earth. At these shallow levels, the geothermal component of heat flow (average ~0.06 W/m 2 ) is dominated by solar irradiation (average daily peak ~300 W/m 2 ). To overcome this, the Heat Needle is designed to record time-series temperature data at the surface and at 20 cm intervals from 10 cm to 110 cm subsurface, and to use frequency-domain filtering to increase the geothermalto-solar signal ratio. Vertical thermal conductivity is derived from a combination of vertical thermal diffusivity measurements and radial heat injection tests. The goal is to develop the Heat Needle as a cost effective geophysical tool for mapping the extent and magnitude of thermal anomalies prior to test drilling. The target is to detect surface heat flow variations on the order of 0.01 W/m 2 . This requires several orders of magnitude greater sensitivity than existing shallow temperature and heat flow mapping methods. The technical challenges of the Heat Needle revolve around achieving the necessary precision, accuracy, durability, reliability, thermal bulk, cost, usability and power efficiency for the probe; as well as designing appropriate field procedures, data processing algorithms and interpretation strategies. As of September 2014, a laboratory process had been refined to calibrate temperature sensors to an absolute accuracy of ±0.0024°C (1 ). Radial heat injection tests had been performed and analyzed. Eight Heat Needles had been deployed and successfully recovered during a field trial of approximately nine-months duration in a remote part of South Australia. That survey generated close to one million, high precision (±0.0003°C), high accuracy (±0.0024°C) temperature records as a robust data set for developing and testing algorithms for data reduction and interpretation. A subsequent twelve-month trial over four geothermal sites had commenced in Mexico.