LeSchack and Lewis discuss the use of the time lag method for determining the thermal diffusivity of a homogeneous and isotropic layer of earth material. Their expression for diffusivity is
Kx = thermal diffusivity of layer (cm2/s),
x = thickness of layer (cm),
P = period of the annual temperature wave (s) (assumed to be sinusoidal),
tx − t0 = time lag between temperature wave at the ground surface and at the base of the layer.
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