Magnetotelluric Survey

The magnetotelluric method or magnetotellurics (MT) is an electromagnetic geophysical exploration technique that images the electrical properties (distribution) of the earth at subsurface depths. The energy for the magnetotelluric technique is from natural source of external origin.

When this external energy, known as the primary electromagnetic field, reaches the earth’s surface, part of it is reflected back and remaining part penetrates into the earth. Earth acts as a good conductor, thus electric currents (known as telluric currents) are induced in turn produce a secondary magnetic field.

Magnetotellurics is based on the simultaneous measurement of total electromagnetic field, i.e. time variation of both magnetic field B(t) and induced electric field E(t). The electrical properties (e.g. electrical conductivity) of the underlying material can be determined from the relationship between the components of the measured electric (E) and magnetic field (B) variations, or transfer functions: The horizontal electric (Ex and Ey) and horizontal (Bx and By) and vertical (Bz) magnetic field components.

According to the property of electromagnetic waves in the conductors, the penetration of electromagnetic wave depends on the oscillation frequency. The frequency of the electromagnetic fields development of the theory determines the depth of penetration.

The basis for MT method is found by Tikhonov and Cagniard [1, 2]. In half a century since its inception, important developments in formulation, instrumentation and interpretation techniques have yielded MT as a competitive geophysical method, suitable to image broad range of geological targets.

Magnetic Geophysical Survey

Magnetic geophysical surveys measure small, localised variations in the Earth’s magnetic field. The magnetic properties of naturally occurring materials such as magnetic ore bodies and basic igneous rocks allows them to be identified and mapped by magnetic surveys. Strong local magnetic fields or anomalies are also produced by buried steel objects. Magnetometer surveys find underground storage tanks, drums, piles and reinforced concrete foundations by detecting the magnetic anomalies they produce.

Very-low-frequency (VLF) Methods

VLF survey methods use very-low-frequency, radio communication signals to determine electrical properties of shallow bedrock and near-surface soils, primarily as a reconnaissance tool. VLF profiles can be run quickly and inexpensively to identify anomalous areas warranting further investigation by other surveys, drilling or sampling. The technique is especially useful for mapping steeply dipping structures such as faults, fractures and shallow areas of potential mineralization. Depth of investigation varies from 4-5 meters in conductive soils to 40-60 meters in highly-resistive soils.


VLF interpretation is generally subjective in nature (in contrast to quantitative modeling which requires high data density and a well-constrained model). Anomalous areas are identified and a gross characterization attached to the anomaly (e.g., steeply dipping conductor or thickening conductive overburden).

Ground Penetrating Radar (GRP) Survey

Ground Penetrating Radar (GPR) is the general term applied to survey methods employing high-frequency electromagnetic waves to map below-ground lithology or buried objects. GPR methods are both high-resolution and highly site specific, which means they can produce excellent results but only in specific conditions.

EXACT has completed radar surveys of objects at depths from 2 inches to 15 meters under the right conditions. A variety of modern computer-driven instruments are used depending on the project.

Ground Penetrating Radar is among the high-resolution geophysical techniques available for shallow engineering and environmental projects. As such, GPR profiles are spectacularly revealing when successful (i.e. detection of gas pipes, cable, concrete, etc.).

A GPR survey involves using a specialized antenna to focus electromagnetic pulses (radar signals) of short duration into the ground. These signals propagate and are then reflected by discontinuities or interfaces in soil materials and return to be detected by a receiver antenna. The reflected signals are processed and displayed on a graphic recorder. The time axis is converted to depth by an estimated velocity function. As the antenna is moved along the surface, this display results in a cross-section record of distance traveled vs. depth (time) of the reflected radar signatures.


Colored cross-sections with interpreted locations of discovered subsurface objects are delivered. When the targets are three-dimensional, colored time-slice maps may be constructed. More sophisticated beam-forming approaches are available. A plan map of the project with cultural features and a narrative discussion of the work are sometimes included in the report.

Geoelectrical (IP & Resistivity)

Resistivity and Induced Polarization (IP) are commonly-used geophysical survey methods for measuring the electrical properties of subsurface rock. Both measurements are made by introducing a controlled electrical current into the ground using two current electrodes, thus energizing the ground, and then measuring the induced potential-field gradient voltage at (between) two non-polarizable receiver electrodes.

The distance between the pair of current electrodes and the pair of potential-field electrodes determines the depth of investigation (the measured data). The resulting voltages as a function of time (time-domain IP/TDIP), are recorded digitally and analyzed for the “induced polarization” effect. The measured IP phase indicates the ability of rocks to briefly hold an electrical charge after the transmitted voltage is turned off.

Electrical properties of the ground can be calculated from comparing the transmitted signal to the received signal. Ground resistivity (ability to conduct an electrical current) affects the strength of the received signal. A change in induced polarization (ability of ground material to polarize at interfaces) affects the shape or timing of the received waveform. When plotted, these properties reveal valuable information about faults, fractures, geologic structures, mineralization, and groundwater porosity for subsurface modeling.