Mapping the Distribution of the Neutral Zone in Assist of Shallow Geothermal Applications in the United States

Abstract

At depths of 5 to 20 meters below the ground surface, the geological stratum with stable temperature, known as the neutral zone, is crucial for climate battery and shallow geothermal applications. Currently, in the United States, very limited and fragmented data concerning the neutral zone, including its depth and temperature, primarily due to the prohibitive upfront costs associated with drilling tests. However, with the comprehensiveness of national meteorological and geophysical databases, the need for effective large-scale modeling of the shallow ground temperature has become feasible and urgent. This study introduces an analytical model established on the subsurface energy balance equations. By considering the conjugated heat flow at ground surface, solar radiation, and latent heat transfer, this model enables the anticipation of the temperature and depth of neutral zone based on the provided geological parameters. The simulation results were validated through a comparison against the soil temperature datasets at different depths in five documented case studies in Lemont and Argonne, IL; Houston, TX; Harrisburg, PA; and Teaneck, NJ. Two-dimensional thermal isopleths at various depths were generated across the United States, accompanied by the annual temperature fluctuations at neutral zone. This model enables early engineering assessments to identify potential climate battery candidate regions and facilitates the development of shallow geothermal applications through advancements in remote sensing technology and agricultural datasets.