For a thermoelectric device driven by the temperature difference between soil and ambient air, previous analytical work has been performed wherein a sinusoidal surface boundary condition was imposed; the results suggested that optimal placement of the lower terminal of the device should be at a nondimensional depth of x* = 2.28, resulting in a 7% increase in power over placement at a much greater depth. Analysis of temperature data taken in conjunction with operation of a thermoelectric device has shown that the optimal depth for the lower thermal reservoir is much shallower than originally thought (x* similar to 1), with increases in performance approaching 70% over that experienced at greater depths. Representative data are presented and interpreted, along with recommendations for future work. Published by Elsevier Ltd.