As an inhomogeneous mixture of pure ice, brine, air and solid salts, the physical properties of sea ice depend on its highly temperature-dependent microstructure. Understanding the microstructure and the way it responds to variations not only in temperature but also salinity is crucial in developing an improved understanding of the role sea ice plays in the climate system. In principle, the internal structure of sea ice can be studied through measurements of any physical property that exhibits strong contrasts between solid ice and brine. However, it has proved extremely difficult to obtain such measurements reliably without disturbing the natural state of the ice. We have recently developed an application of cross-borehole d.c. resistivity tomography to make in situ measurements which resolve the anisotropic resistivity structure of first-year landfast sea ice. We will present results from measurements made in 2008 at Barrow, Alaska, and in 2009 at Scott Base, Antarctica. The sea ice in these two regions forms in different environments: at Barrow, relatively quiescent conditions typically lead to a predominance of columnar ice, while more turbulent conditions and underwater ice formation in McMurdo Sound tend to produce a larger component of frazil or platelet ice. Interpretation of the resistivity measurements in association with simultaneously collected temperature data allows development of simple models that track the evolution of different ice microstructures during the period of spring warming. Over this time period ice transport properties undergo major transitions that are difficult to observe through more invasive techniques.