TIME DOMAIN ELECTROMAGNETICS (TDEM)
Time-domain electromagnetic (TEM) surveys involve induction of electromagnetic (EM) fields in the sub-surface via a square or rectangular transmitter loop which has no electrical connection with the earth. In turn, the sub-surface EM fields induce secondary EM fields in a receiver coil and a receiver attached to this coil measures a transient decay which is diagnostic of ground conditions.
In ground EM surveys the receiver is at the surface and can measure both vertical and horizontal components of the B-field and/or dB/dt field.
Due to the physics of the technique it is inherently better at detecting conductors in resistive environments rather than resistors in conductive environments.
The depth of exploration for ground EM is down to 1000m.
• Mineral Exploration
• Detection and mapping of sub-surface electrical conductors such as a massive sulphide ore body.
• Detecting and mapping groundwater that is more conductive than its host.
• Location of buried hazards and groundwater contaminants.
AUDIO-FREQUENCY MAGNTOTELLURIC AND MAGNETOTELLURIC METHODS
In the audiomagnetotelluric (AMT) method, the electrical conductivity structure of the subsurface is imaged by using Earth’s natural electromagnetic (EM) fields as a source (e.g. Berdichevsky & Dmitriev 2008). The principles of the technique were first introduced by Tikhonov (1950) and Cagniard (1953). In AMT, the primary incident EM fields are considered as planar in geometry and propagating vertically downward. In the earth, EM waves travel diffusively so that high-frequency (short wavelength) waves penetrate a relatively short distance, while low-frequency (long wavelength) waves reach greater depths. The main advantage of AMT is the wide survey depth range from hundreds of metres down to several kilometres. The magnetotelluric method (MT) uses even lower frequencies and the survey depth can therefore even be hundreds of kilometres. AMT-MT measurement requires a simultaneous time series of orthogonal components of magnetic (Bx , By and Bz ) and electric (Ex and E y ) fields resulting from large-scale solar-driven ionospheric and magnetospheric electric currents. The energy sources for frequencies above 1 Hz are EM waves caused by distant lightning storms and they propagate within the Earth–ionosphere waveguide (Garcia & Jones 2002). A schematic diagram of the audio-magnetotelluric site is presented in following figure:
The magnetotelluric method allows to illustrate the conductivity distribution of rock complexes at depths in the range of several meters to hundreds of kilometers. The deeper the unit, the greater the thickness it must have to be properly located and recognized.
The measuring station is mobile and can be placed anywhere the operator can reach on foot. As this method is not logistically demanding, it allows for the examination of a large area in a relatively short time. The measurements are non-invasive and do not cause environmental damage.
The audio-magnetotelluric method (AMT) is an important high resolution, non-seismic geophysical technique that measures variations in the Earth’s natural electromagnetic fields to detect electrical resistivity variations in the subsurface at shallow to intermediate depths. Resistivity values are controlled mainly by porosity, fractures, water content, concentration of dissolved solids, and permeability. The AMT method provides detailed information about variations in subsurface electrical resistivity values that can be used for interpretation of lithological and/or structural differences along the profile line. The method is capable of imaging the subsurface with resolutions good enough to detect features a few meters across. The shallow subsurface is imaged from high frequency measurements, and the deeper subsurface is imaged from low frequency measurements. The depth of investigation can extend to 1 km, although the depth of investigation is reduced by the presence of near-surface conductive sediments.
Controlled Source Audio-Frequency Magneto Tellurics (CSAMT) :
Controlled source audio-frequency magnetotellurics (CSAMT) is a high-resolution electromagnetic sounding technique that uses a fixed grounded dipole or horizontal loop as an artificial signal source.
CSAMT is similar to the natural source magnetotellurics (MT) and audio-frequency magnetotellurics (AMT) techniques, with the main difference being the use of an artificial signal source for CSAMT.
The source provides a stable signal, resulting in higher-precision and faster measurements than are usually obtainable with natural-source measurements in the same spectral band. However, the controlled source can also complicate interpretation by adding source effects, and by placing certain logistical restrictions on the survey. In most practical field situations these drawbacks are not serious, and the method has proven particularly effective in mapping the earth’s crust in the range of 20 to 2000 meters.
A CSAMT source usually consists of a grounded electric dipole one to two km in length, located four to ten km from the area where the measurements are to be made. The frequency band for typical instruments is between 0.125 and 8,000 Hz, with measurements most commonly made in the 16 to 8,000 Hz range.
Magnitude and phase are normally measured for one electric (E) and one magnetic (H) field component (for example Ex and Hy, with Ex parallel to the transmitting dipole and Hy perpendicular), as shown below.
CSAMT has been successfully used in the following applications:
• Mineral exploration
• Petroleum exploration
• Geothermal resource mapping
• Groundwater exploration
• Mapping geological structure and lithology
• Mapping of old mine sites
• Mapping of industrial contaminants
• Engineering projects
TDEM (Time Domain Elecromagnetic)
Electromagnetic (EM) methods measure subsurface conductivity using low frequency electromagnetic induction. A transmitter and receiver coil are used in EM instruments. The depth of discovery is a function of antenna frequency, separation and direction. Soil conductivity is a function of the electrical properties of underground materials and the chemistry of pore liquids and is the inverse of electrical resistance. For this reason, EM methods are very useful in mapping changes in lithology. In addition, EM methods can be used to detect and map embedded ferrous and non-ferrous metals. Enerson uses both Frequency and Time Domain Electromagnetic devices for various applications.
What is the difference between IP/R and CSAMT :
In summary, there are following differences between CSAMT:
- CSAMT gives only resistivity, while IP gives chargeability and resistivity.
- CSAMT has much higher depth penetration and resolution than DC resistivity from IP survey because:
- The penetration and resolution in IP are related to distance between receiver and source. There is a trade off between penetration and resolution. In order to get greater penetration with IP the array spacing must be expanded, the dipole size increased and this reduces resolution.
- CSAMT overcomes this problem since its penetration is not dependent on array spacing. The penetration depends on the ground resistivity, the frequency and the signal/noise ratio.
- In IP survey the measured signal is an average signal of a large volume of ground between the source and the receiving dipole. This results in low resolution at depth;
- In CSAMT survey the measurements are made in the far-field zone and the signal is going directly vertically into the ground. The resistivity information is related directly to the resistivity of the ground beneath the receiver dipole.
Scalar and Tensor Arrays used in AMT and MT
While tensor AMT electric- and magnetic-field tensor measurements provide more comprehensive information at each station, production rates using the scalar AMT array helps maximize production coverage (distances of one or more line-kilometers per day). With an eight channel GDP-32 24 up to eight analog signals can be acquired. A typical AMT scalar setup might employ up to seven 50 meter electric field dipoles (each measuring Ex, for a total setup length of 350 meters) and a single magnetic field sensor (Hy). Geothermal and water basin scalar AMT surveys in the past have used six 100 meter electric field dipoles with the ANT/6 magnetic field sensor, for a single setup length of 600 m. Select high-resolution mining related programs have used seven 25 meter electric field dipoles with the ANT/6 magnetic field sensor, for a single setup length of 175 meters. When operating in rough terrain where access is poor along survey lines, one useful approach is to use multiple GDP-3224 AMT setups that are GPS-synchronized. With GPS synchronization, the “Remote Reference” Robust Processing option can be used with multiple receivers collecting Time-Series files based the same GPS clocktime.
Scalar CSAMT and AMT surveys are good for reconnaissance type surveys to cover large areas in relatively short time. They are generally used for metallic mineral, groundwater and shallow geothermal exploration.
The advantage of full tensor AMT survey over scalar AMT surveys can be summarized as follows:
· Full Tensor AMT measurements are better for geological strike calculation and removing static shift effect.
· Deeper investigation depth with the use of magnetic coils with 10kHz – 0.001Hz frequency band.
· A true 3D inversion and modelling is possible is full tensor AMT surveys
· Using Remote Reference (RR) station to reduce uncorrelated noise at site measurements.
Application of the magnetotelluric method.
MT surveys are most often used to identify deep structures such as geothermal systems, drinking water and oil deposits.
They are also helpful in identifying problematic structures for seismic methods such as salt domes. It is also used in situations where seismic acquisition is not possible. This is based on the fact that magnetotelluric measurements are non-intrusive. They are logistically easier to perform than seismic surveys. This is caused by the fact that a measuring station can be placed almost anywhere where human access is possible. MT probes also allow for rapid coverage of large research areas.
The results of magnetotelluric surveys can also be used to determine the best location of seismic profiles. They will be useful also to supplement the results of this method and obtain more complete information on the structure and stratigraphy of the medium.
Enerson uses following Zonge equipment for TEM and scalar CSAMT/AMT surveys
– GDP™ 24-bit integrated, multi-method receiver
– ANT/6 Antenna (for CSAMT/AMT surveys)
– TEM/3 Antenna (for TDEM surveys)
– GGT-10 Transmitter (10kVA up to 1000 V, 20 Amp for CSAMT and TDEM surveys)
– ZeroTEM Transmitter ZT-30 (30 amp, battery powered – for TDEM surveys)
– Electrodes Non-polarizing Cu/CuSO4
Enerson uses following Metronix equipment for tensor AMT/MT surveys
– ADU‐07e 10 ch. AMT/MT receivers
– MFS‐06e Magnetic coils
– Electrodes Non-polarizing Cu/CuSO4