Continuous Temperature Logging in Air across the Deep Vadose Zone

Abstract

Accurate temperature measurements with good depth resolution will be helpful in appreciating processes that influence temperatures in the vadose zone. In some areas, such as the Albuquerque Basin, groundwater flow perturbs the temperature regime in the saturated zone, making the deep vadose zone a potentially better region to investigate ground surface temperature changes. When it is not logistically or economically possible to span the vadose zone with a pipe filled with water, the temperature measurements must be made in air. Because of the very small heat capacity of air, these temperature measurements are difficult. This paper describes a new temperature sensor with a relatively fast time constant in air that provides accurate temperature measurements in a continuous logging mode. Temperature gradient characteristics below the 5‐m depth, based on a 1‐m depth measurement interval, are reproducible at a number of logging speeds. Absolute temperatures between the logs at any given depth below about 25 m typically agree to within 0.01°C. The temperature logging system should enable one to gather considerable information regarding the thermal regime in the vadose zone.