At Etowah, archaeologists use remote sensing to detect buried features without excavation. Ground-penetrating radar uses high-frequency radio waves transmitted into the ground and records variations in the return signal that may indicate buried archaeological features such as structures, fire pits, and burials. Its depth range is limited by the electrical conductivity of the ground, the transmitted center frequency, and the radiated power, and at Etowah clay-laden soils hamper penetration. Electrical resistivity survey maps subsurface archaeological features and patterns through electrical resistance by staking the area into a grid, inserting metal probes into the ground, and logging readings at regular intervals as a technician moves the instrument along closely spaced parallel lines. Magnetometers and gradiometers measure the level of magnetization of magnetic material such as iron, allowing archaeologists to detect magnetic anomalies of various types at a much larger depth than conventional metal detectors. A magnetometer measures a magnetic field and provides data on its strength and direction, while a gradiometer evaluates the difference between two measurements, strength and direction, and a fluxgate gradiometer provides an electrical signal proportional to the intensity of the external magnetic field acting along its axis.