Transient Electromagnetic Measurements to Late Delay Times Over the Woodlawn Ore Body

1977 ◽  
Vol 8 (1) ◽  
pp. 1-5 ◽  
Author(s):  
G. Buselli
Keyword(s):  
Transient Electromagnetic ◽  
Ore Body ◽  
Electromagnetic Measurements ◽  
Delay Times

The effect of near‐surface superparamagnetic material on electromagnetic measurements

Geophysics ◽  
1982 ◽  
Vol 47 (9) ◽  
pp. 1315-1324 ◽  
Author(s):  
G. Buselli
Keyword(s):  
Transient Response ◽  
Soil Cover ◽  
Lateritic Soil ◽  
Near Surface ◽  
Transient Electromagnetic ◽  
Electromagnetic Methods ◽  
Electromagnetic Measurements ◽  
Delay Times ◽  
Residual Response ◽  
Made In

The development of an instrument that enables transient electromagnetic (TEM) measurements to be made to voltage levels of 1 μV/A and less has enabled the detection of an anomalous transient response in some areas with lateritic soil cover. This anomalous transient causes apparent resistivity values derived from the measured transient decay to decrease at late delay times in areas where the known geology indicates the values should increase with delay time toward the resistivity value of the basement. The main cause of the anomalous transient has been identified as the response of superparamagnetic material in the lateritic soil cover. Both field and laboratory measurements of the voltage M induced by this transient, show a [Formula: see text] time dependence. This is the same behavior reported previously for magnetic viscosity over a longer time scale. Measurements of magnetic susceptibility of material separated magnetically from soil samples taken at areas where a residual response is measured, show that over a wide temperature range (from −196°C to 590°C) the susceptibility increases with temperature, confirming the presence of superparamagnetic particles. The anomalous transient response is localized to within 3 m of the transmitter loop; it is consequently detected only by loop configurations where the receiver loop is in proximity to the transmitter loop. The effects caused by the presence of a superparamagnetic response within 3 m of the transmitter loop apply to all electromagnetic methods, whether the measurements are made in the time or frequency domain.

The effect of local distortions on time‐domain electromagnetic measurements

Geophysics ◽  
2004 ◽  
Vol 69 (1) ◽  
pp. 87-96 ◽  
Author(s):  
Andreas Hördt ◽  
Carsten Scholl
Keyword(s):  
Magnetic Field ◽  
Electric Field ◽  
Time Domain ◽  
Step Response ◽  
Model Parameters ◽  
Difficult Case ◽  
Storage Site ◽  
Transient Electromagnetic ◽  
Electromagnetic Measurements ◽  
Distortion Tensor

Based on the time‐domain integral equation, we derive expressions for the effect of an anomalous body close to the receiver or close to the transmitter on transient electromagnetic measurements. Similar to magnetotellurics, the distortion of electric fields at late times can be described by a constant distortion tensor relating the secondary electric field to the primary field components that would be obtained in the absence of the body. The distortion of a single electric field transient is a static shift only for particular configurations over a layered half‐space. In the general case, the perturbation is time dependent because the direction of the total electric field vector varies with time. The theory nicely explains spatial variations in electric field transients measured during a high‐redundancy long‐offset transient electromagnetics (LOTEM) survey over an underground gas storage site. An inversion example with synthetic data illustrates how distortion can be corrected. The elements of the distortion tensor are determined simultaneously with the model parameters. Ambiguity is reduced by a regularization of the distortion parameters. In the example, the background model is recovered well, even for the difficult case where only one transmitter is used. The distortion of the magnetic field time derivatives caused by bodies close to the receiver is proportional to the time derivative of the primary electric step response. The distortion is generally not limited to early times and cannot be neglected in general. Transmitter overprint effects resulting in static shifts of vertical magnetic field time derivatives may also be understood from the theory.

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