Recent years have seen attempts to deploy robots for planetary exploration, seashore reconnaissance after a tsunami, volcano investigations, etc. These robots may have to move on soft terrain such as sand and soil. The movement of sand or soil particles under the wheels or tracks of the robots strongly affects traffic-ability. In other words, the development of a numeric simulator for terramechanics (e.g., DEM) will be useful if the spatial behavior of the particles can be measured. We are confronted by a simple difficulty: the particles inside sand and soil are not visible. Thus, only two-dimensional techniques that use transparent boards are available, excluding X-ray visualization using small iron beads. There are two issues with the X-ray technique: the profile of iron beads (e.g., density, diameter, and shape) is not similar to those of sand and soil, and only part of the fixed wheel is pushed into the sands in the experiment. A nuclear three-dimensional imaging technique called positron emission particle tracking (PEPT) was developed at the University of Birmingham. PEPT detects pairs of gamma rays emitted by a positron-emitting radionuclide of a tracer particle, which produces an image of the tracer. Thus, the overarching goal of this study is to explore the three-dimensional terramechanics between terrain particles and a wheel or track using PEPT. As an initial step, this paper introduces an imaging technique for standard sand (Toyoura sand) under a rotating wheel using PEPT and presents some images of sand particles under several conditions.