Time-domain electromagnetic technology (TDEM) is effective in determining the electrical conductivity in soils from a few meters to 1000 meters and more. From the moment that the conductivity is strongly correlated to soil properties, the TDEM represents an important tool for soil mapping as well as for determining lateral and vertical changes of the properties in them. This method, which can be used in the design of drilling test or profiling, is an important tool in hydrological research in sedimentary/alluvial environments and in the characterization of sequences with the presence of lenses of brackish water into the coastal environment.
The method can also be used in aggregate studies, characterization of contaminated area, mining and other applications involving geological mapping.
The operating principle-SEDT TDEM method consists in making circulate cyclically, for short periods of time, an alternating electric field around a transmitting coil or a long wire, grounded (Lotem). During the connection time, it is originated a stable primary magnetic field in the subsurface. When instantly the current is cut in the transmitting coil (and therefore stops the primary magnetic field), the Electro Magnetic field (EM), induced in the subsurface, causes eddy currents (EMF), which spread both through field and coming drivers. As a result of loss of heat resistance, these currents decrease with time, causing a secondary magnetic decreasing field in the surface.
Since the secondary magnetic field is generated when the primary field is switched off, it can be measured with relative ease. When in the subsurface there are bodies of high electrical conductivity, the attenuation of eddy currents is significantly lower than in bad drivers. Therefore, measuring the decreasing ratio of secondary field provides a way to detect the presence of conductive bodies in the ground and estimate its conductivity.
The common measurement device consists of placing the receiver coil in the center of the transmitter coil (usually square), and even use the same coil for both functions. In this configuration, the extention of the decreasing field in the center of the coil is equivalent to the measurement of resistivity according to its depth (similar to the method of direct current SEV).
The depth of investigation is based on the delay time of the decreasing field and it is independent of the distance between the transmitter and receiver coils. With the increment of time, the current is propagated to greater depths. It is a fast method (within minutes of measurement for each drill test) and it can reach, depending on the size / geometry of the coils (loops) used and the transmitter power, from a few tens of meters (NanoTEM for high resolution studies) to a few kilometers deep (LoTEM which uses a large dipole transmitter and a high power generator).
The measurement process is repeated several times, quickly storing the measures with a summation process (stacking), which highlights the created voltages and tends to cancel out the background noise in the area. Increasing the period of the signal transmitter and / or time of measurement, the induced currents can reach deeper fields, and thus provide information on greater depths. The TDEM method can be used with different configurations. The most common configurations are with loops of transmitter and receiver in central mode (loops whose center is the same), matching mode (same loop) and in "offset" mode (loops whose center is not the same). In the same way, a long grounded cable can also be used as a transmitter. When the receiving aerial is installed in the center of the loop, near to a well or perforation, is called parametric SEDT.
The values of a TDEM drilling test are presented as curves of apparent resistivity variation according to time, and its interpretation is carried out by conductivity sections and resistivity 2D sections, which can be compiled for the characterization and geometric definition of the electro stratigraphic sequence.
The main advantages of TDEM are:
- A great sensitivity to conductor terrains.
- A great power of vertical resolution (an important element in geological. environment with vertical resistivity variations as the strata in sedimentary/flood environments).
- A good detection of low resistivity anomalies in a conductor terrain.
- A practical implementation due to the lack of contact with the ground and, therefore, a fast implementation on a cleared terrain.
The applications of TDEM method is based on the characteristics of the instrument and can vary from surface applications (NanoTem) to deeper ones (Long Offset Transient Domain).
In the field of hydrogeology, geological research, geothermal, etc... The most common method is the TDEM (common applications up to 1000 m depth).
The depth of investigation depends on the size / number of turns in the coil or length of the dipole transmission (and geometry under transmitter) and transmitter power. This can reach and exceed 500 - 1000m, with instrumentation designed for surface and intermediate studies, and some type instrumentation Lotem km (Long Offset Transient Domain). Penetration is a function of surface resistivity and electro-stratigraphic sequence investigated. With TRX team can investigate up to 300-400 m depending on the electro-stratigraphic sequence.
Logistics is practical and fast if small loops (5-10 m) are used with larger number of turns in the coil (8). In this case, each set-up and measurement (drill test) could take up to 20-30 minutes. For larger loops (100m) the operation would be slower and would take a 30% of additional time. An average of 10 daily TDEM drilling tests is common in areas without further limitations than the associated to vegetation and topography.
It is a robust and widely accepted method. Like all electromagnetic methods is less sensitive to noise from geological type than other methods. It is also an active method, the generally high signal / noise ratio allows to acquire reliable data in any environment. Excellent characterization of vertical electrical variations in sub horizontal stratified mediums (sedimentary and alluvial sequences) and characterization of brackish cribs in coastal environments.