The cases of bombs often include ferromagnetic materials, so can be detected using magnetic methods. The Grad601 and Grad-01-1000L single axis gradiometers, and Grad-13 three-axis gradiometer, can be used to map and detect magnetic anomalies from the surface. Alternative methods of detection use the Mag-13 three-axis magnetometer, to which total field calibration is applied. This enables accurate total field measurements to be obtained from vector magnetometers. Mag-13 sensors and Grad-13 gradiometers can also be used in boreholes, enabling underground investigations to be carried out. At sea, similar threats affect the installation of infrastructure, such as pipelines, undersea cables, or offshore wind turbines. The same equipment is used on underwater arrays to perform mapping.