FAST Methods and Equipment
Ground-penetrating radar uses electromagnetic waves in the frequency range of 10–1,000 MHz to detect changes in the electrical properties of the subsurface. Changes in the electrical properties of the ground create interfaces that reflect part of the transmitted signal, which is detected by the receiving antenna and stored for data processing and analysis. For archaeological and forensic exploration, higher resolution frequencies are common (200–1,000 MHz), which can provide increased resolution at shallower depths. The KGS operates a MALA ProEx system with three available frequency ranges: 200 MHz, 400 MHz, and 800 MHz shielded antennae. Depth of investigation ranges from 0.1 m to 3 m with these higher frequency antennae.
Magnetometers measure Earth's magnetic field. Earth's magnetic field ranges across the globe from approximately 30,000 nT at the equator to 60,000 nT at the magnetic poles. Proximal or buried objects with high magnetic susceptibility or remnant magnetization — such as buried metallic pipes, rebar, steel drums, or materials that have been burned — enhance the localized magnetic field. The KGS currently has two magnetometers. The first is a Bartington Single Fluxgate gradiometer with a 1 m sensor separation. The second is a Geometrics G858 cesium fluxgate gradiometer, which can be adapted to measure the magnetic field at different depths (0.5 m to 2 m).
Electrical resistivity applies direct current or low-frequency alternating current at the ground surface through metal probes to measure the potential difference between them. The electrical resistivity is calculated from that information using Ohm's law. When evaluating the shallow subsurface, three dominant factors control how resistive a material is: the amount of water present, the salinity of the water, and the soil grain size. The KGS operates an ABEM Terrameter LS2 with up to 48 cable takeouts as well as a Geoscan RM85 resistance meter. Both systems can be adapted to measure the electrical properties for a wide range of depths (0.5 m to 50 m).
Electrical Conductivity (FDEM)
Electromagnetic induction measures the change in mutual impedance between a pair of coils on or above Earth's surface. The transmitter coil is used to generate an electromagnetic field at a specific frequency, also referred to as the primary field. The primary field causes electrical currents to flow in conductive materials in the subsurface. The flow of currents in the subsurface, called eddy currents, generates a secondary magnetic field. The magnitude of the secondary field depends upon the type and distribution of conductive material in the subsurface. Both the induced secondary field and the primary field are detected at the receiver coil. The KGS operates a GEM2 system, which can measure up to 10 frequencies at once at depths ranging from approximately 0.5 m to 10 m.