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Hot Dry Rocks Pty Ltd has developed the Heat Needle, a precise instrument for monitoring ground temperature, measuring soil thermal properties, and revealing conductive heat flow at the Earth's surface.

What is the Heat Needle for?

The Heat Needle is at once a precise and accurate ground thermometer, a device for measuring soil thermal diffusivity and thermal conductivity, and an instrument to detect subsurface heat sources such as geothermal systems, accumulations of radioactive material, sewerage pipes, underground coal fires, and others.  With the Heat Needle, HDR aims to provide cost-effective heat flow mapping for the first time through an instrument easily embedded into the top metre of the ground.

Features and Specifications

  • Proprietary temperature sensors at 0, 10, 30, 50, 70, 90, 110 cm depth

  • Onboard power and memory for 12 months stand-alone recording

  • Thermal conductivity measured in situ with an active heating coil

  • Thermal diffusivity measured in situ by Fourier signal analysis

  • Weather-proof

  • Calibrated over temperature range 0-50°C

  • Calibrated to ±0.0003°C precision, ±0.003°C absolute accuracy

  • Deployed using a handheld electric drill

  • Removed using a mechanical jack

  • Reusable and easily transportable

  • Typical deployments 4-12 weeks, depending on the application


The Heat Needle records ground temperature with high accuracy and precision at regular intervals over several weeks. We process the results in the frequency domain to reveal the in situ thermal diffusivity and long-period trends in thermal gradient. The temperature response of the ground to an active heat pulse reveals in situ thermal conductivity. Gradient and thermal conductivity together give the conductive heat flow signal, from which seasonal trends can be filtered to reveal the 'geothermal' signal. Comparison of results from individual Heat Needles across a survey area reveals lateral heat flow variations due to subsurface causes.

If you have any questions about the Heat Needle, or to discuss a potential deployment, please contact us directly

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