high-conductivity materials such as clay soils and soils that are salt contaminated. Performance is also limited by signal scattering in heterogeneous conditions (e.g. rocky soils). Other disadvantages of currently available GPR systems include: Interpretation of radar-grams is generally non-intuitive to the novice. Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys. Relatively high energy consumption can be problematic for extensive field surveys. Radar is sensitive to changes in material composition, detecting changes requires movement. When looking through stationary items using surface-penetrating or ground-penetrating radar, the equipment needs to be moved in order for the radar to examine the specified area by looking for differences in material composition. While it can identify items such as pipes, voids, and soil, it cannot identify the specific materials, such as gold and precious gems. It can however, be useful in providing subsurface mapping of potential gem-bearing pockets, or "vugs." The readings can be confused by moisture in the ground, and they can't separate gem-bearing pockets from the non-gem-bearing ones. When determining depth capabilities, the frequency range of the antenna dictates the size of the antenna and the depth capability. The grid spacing which is scanned is based on the size of the targets that need to be identified and the results required. Typical grid spacings can be 1 meter, 3 ft, 5 ft, 10 ft, 20 ft for ground surveys, and for walls and floors 1 inch–1 ft. The speed at which a radar signal travels is dependent upon the composition of the material being penetrated. The depth to a target is determined based on the amount of time it takes for the radar signal to reflect back to the unit’s antenna. Radar signals travel at different velocities through different types of materials. It is possible to use the depth to a known object to determine a specific velocity and then calibrate the depth calculations. Power regulation In 2005, the European Telecommunications Standards Institute introduced legislation to regulate GPR equipment and GPR operators to control excess emissions of electromagnetic radiation. The European GPR association (EuroGPR) was formed as a trade association to represent and protect the legitimate use of GPR in Europe. Similar technologies Ground-penetrating radar uses a variety of technologies to generate the radar signal: these are impulse, stepped frequency, frequency-modulated continuous-wave (FMCW), and noise. Systems on the market in 2009 also use Digital signal processing (DSP) to process the