Magnetometers measure the strength and direction of a magnetic field. Archaeologists uses magnetometers because these instruments are able to measure and record deviations in the earth's magnetic field that ferromagnetic material causes. Although magnetometers work in lab settings, archaeologists usually use magnetometers for conducting searches in nature. These instruments measure the strength of a magnetic field in terms of tesla units. Archaeologists implement magnetometers when conducting ground and marine searches. The first practical magnetometer was invented in the mid-19th century.
Archaeologists use electrical resistance meters to conduct electrical resistance surveys. These meters help detect anomalies -- such as underground objects and sediments -- that differ from their surroundings by resisting the flow of electrical currents. Electrical resistance meters work by injecting electrical currents into the ground, which creates an electrical field. Electrical resistance surveys differ from other remote sensing surveys since they usually have a deeper and wider probing space. The probing space is the underground area that receives electrical currents from the meter.
Also referred to as GPRs, ground-penetrating radars transmit electromagnetic pulses from the instrument's antenna -- which is aboveground -- to below the surface. Afterward, the radar measures the time that elapses between sending the transmission to when the pulses return to the antenna; GPRs measure the pulses' travel times in nanoseconds. Archaeologists are able to determine what is underneath the surface by the velocity of the electromagnetic pulses. The various physical and chemical properties of objects and sediments underneath the ground determine the electromagnetic pulse's velocity. The frequency of a GPR's antenna and soil affect how far the radar's pulses can penetrate.
Electromagnetic conductivity meters conduct electromagnetic conductivity, or EM conductivity searches. Archaeologists use these meters to obtain data from the conductive properties of underground rocks and soil. Moisture, clay content, porosity and concentration of dissolved electrolytes are factors that affect the amount of soil and rock conductivity. Two commonly used EM conductivity meters are the EM 31 and EM 38. Archaeologists use EM conductivity meters when the area's soil conductivity is too high to use ground-penetrating radars. These meters measure conductivity in units of millisiemens per meter.