Drone borne magnetic gradiometry in archaeological applications: a Metaponto case-study

Applied geophysics offers non-invasive techniques to uncover and characterize buried structures or characterize the type and quality of materials in archaeology. Among the various geophysical methods, the magnetic method stands out due to its effectiveness, speed, cost-efficiency, and non-invasive nature. This method leverages the magnetic susceptibility contrasts between archaeological features and the surrounding soils, making it particularly useful for detecting and mapping subsurface remains. The magnetic method is highly sensitive to both natural sources, such as soils and rocks, and man-made objects, including ditches, storage pits, foundations, and walls. This sensitivity allows geophysicists to identify significant anomalies that indicate the presence of archaeological targets. The method’s ability to produce high-resolution data quickly and efficiently makes it a preferred choice for large-scale surveys. Magnetic anomalies generated by archaeological targets are typically weak, dispersed over small areas, and often interfere with each other. Therefore, high-resolution magnetic data, collected with closely spaced survey lines near the ground, are essential for their identification. The survey area must be large enough to provide informative anomalies, especially for regular and elongated shapes of buried structures like buildings or roads. One of the most advantageous applications of the magnetic method in archaeology is the use of gradiometric surveys. Gradiometric surveys involve measuring the magnetic gradient, which enhances the detection of shallow sources and improves the resolution of the data. This approach is particularly beneficial in archaeological contexts where the anomalies are often weak and spread over small areas. By using a pair of sensors to measure the magnetic field at different heights, gradiometric surveys can effectively filter out temporal variations and regional magnetic fields, focusing on the anomalies generated by archaeological features. Often, the two sensors are arranged in a vertical direction and spaced at a fixed distance, called the ‘baseline’. The choice of the length should be smaller than the distance between the sensor closer to the ground and the source depth. Therefore, depending on several conditions, the baseline for hand-held magnetometers ranges between a minimum of 0.25 m and a maximum of 1 m. Recently, applied geophysics have further revolutionized methods of data acquisitions through the adoption of Unmanned Aircraft Vehicles (UAVs) equipped with new miniaturized magnetometers. UAV-based magnetic surveys can cover extensive areas at low altitudes, providing high-resolution datasets that are crucial for identifying subtle archaeological features. This technology is especially useful in challenging environments where ground-based surveys are impractical. We conducted a UAV magnetic investigation in the archaeological site of Metaponto, arranging the magnetic sensors of the Geometrics Micro-Fabricated Atomic Magnetometer (MFAM) as a gradiometer. Then, aerial data was compared with a ground dataset used to validate the quality of the measurements collected with the drone.