Time series observations of vertical profiles of sediment temperature are presented for several locations at two distinct tidal flats. Surface sediment temperatures are shown to be strongly dependent on solar insolation during low-tide exposure, and that signal is communicated to the subsurface sediment temperatures. A vertical diffusion balance explains the observations well (up to 97% of the observed variance at some locations and 76% on average), and an estimate of thermal diffusivity is obtained for each location. A theoretical model relating sediment porosity to thermal diffusivity is presented and shown to agree with independent estimates of porosity. In addition, thermal diffusivity is shown to correlate with direct observations of sediment composition (percent sand) and surface strength. Results are suggested for application to remote classification of sediments using infrared time series images.