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Geoelectrical methods are used extensively in groundwater mapping for investigation of the vulnerability of aquifers and shallow aquifers themselves. A geoelectrical measurement is carried out by recording the electrical potential arising from current input into the ground with the purpose of achieving information on the resistivity structure in the ground. In a homogeneous ground (halfspace) the current flow radially out from the current source and the arising equipotential surfaces (red surfaces) run perpendicular to the current flow lines (blue lines) and form half spheres.
Also known as time-domain electromagnetics / TDEM, is a geophysical exploration technique in which electric and magnetic fields are induced by transient pulses of electric current and the subsequent decay response measured. TEM / TDEM methods are generally able to determine subsurface electrical properties, but are also sensitive to subsurface magnetic properties. TEM/TDEM surveys are a very common surface EM technique for mineral exploration, groundwater exploration, and for environmental mapping, used throughout the world in both onshore and offshore applications.
Magnetotellurics (MT) is an electromagnetic geophysical method for inferring the earth's subsurface electrical conductivity from measurements of natural geomagnetic and geoelectric field variation at the Earth's surface. Investigation depth ranges from 300 m below ground by recording higher frequencies down to 10,000 m or deeper with long-period soundings.
Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum, and detects the reflected signals from subsurface structures. GPR uses high-frequency (usually polarized) radio waves, usually in the range 10 MHz to 2.6 GHz. A GPR transmitter (Tx) emits electromagnetic energy into the ground. When the energy encounters a buried object or a boundary between materials having different permittivities, it may be reflected or refracted or scattered back to the surface. A receiving antenna (Rx) can then record the variations in the return signal.
ZTEM is an Airborne-EM method in which the time variations of the vertical magnetic field component (Hz) are measured using an induction coil attached to an Airplane. At a reference site, the horizontal magnetic field components (Hy, Hx) are measured and processed together with Hz component to estimate the vertical magnetic transfer functions Tz. ZTEM methods is appropriate to detect horizontal contrasts in the electrical conductivities.