One approach to treating atrial fibrillation relies on freezing tissue of the heart wall. This surgical technology requires sub-millimeter spatial resolution when dynamically tracking the freezing of pulmonary vein; conventional techniques such as ultrasound lack the necessary precision. Here we use an electrothermal "3ω" method to track propagating freezing fronts in nearly real time. The heater line is excited with multiple frequencies simultaneously, and the freezing front detected as it passes through the various penetration depths due to the contrast between thermal conductivities on either side of the front. Comparison of water freezing experiments with video images further suggests the accuracy of the method. Analysis and experiments show how the uncertainty, time response, and measurement range depend on the frequencies and thermal conductivity contrast. Finally, the method is demonstrated on biological tissue as further proof of principle for medical applications.