Investigation of lithium pegmatite by VECS method in the West Finland.
The challenge to investigate morphology of a pegmatitic vein with the average LiO2 content about 1.0% was set to geophysical methods. The survey target of length several hundred meters and of width from 20 to 70 m has the sub-vertical dip. The thickness of the overlying rocks is no more than 10 m. The ZaVet-Geo Company has selected the relatively novel method such as Vertical Electric Current Soundings (VECS) for electrical prospecting.
The main advantages of VECS method as applied to these works are as follows:
- possibility for fast detecting all local heterogeneities of a medium resistivity. The location of a sought target becomes evident as early as the field the data are looked through before analysis of them in detail and prolonged interpretation in an office;
- economic efficiency. It is required to maintain only one source of the electromagnetic field for conducting field works.;
- resistance to electromagnetic noises at some observation points;
- relative indiscriminateness of the VECS method to the accuracy of measured signals. This circumstance turns out to be useful inasmuch as a road ran through the exploration area center with a power transmission line along the road. The level of electromagnetic noises from the transmission line in survey lines near the road reached 200 uV.
A circular electric dipole (CED) consisting of eight grounded horizontal current lines was used as a source of electromagnetic current. The CED radius was 200 m, the currents in all the eight lines were the same and the GTE-10S generator controlled in automatic regime these currents to be equal. The total current in the SED was 4.48 A. Measurements were conducted by the CEI-7 using the PDI-100 receiver (equivalent of a receiving loop 100×100 m). Electrical exploration works were performed on the area with dimensions 0,5×1 km within which 81 measurements of the electromagnetic field dBz/dt component were taken.
The Figure shows the areal EMF distribution in the working area. The positive signal is shown with red color and the negative signal is shown with blue color. The center of the target being studied is wherein the signal is zero (at this point, when moving along a survey line, sign change of measured signal takes place).
Areal distribution of the dBz/dt component at time 21 us.
For determination of morphology of the pegmatitic vein boundaries, 3D simulation was executed that allows the boundaries, volume, and location of the target being studied to be ascertained (Figure). According to the data obtained, the target resistivity was about 15 ohm•m. The host medium was characterized by the high resistivity ( >1000 ohm•m).
Signal acquired in the field is shown in the left Figure. The right Figure displays the combined field signal, signal obtained via 3D simulation, and the model outlines. The yellow isosurface is the signal from the model based on the calculation results. The model outlines are depicted with red color. The CED center is at the point with coordinates 0. 0.
The three-dimensional display of isosurfaces of the signal value. Left – measured dBz/dt field. Right – fitted dBz/dt field
Booklets
Electrical prospecting by the method of vertical electric current sounding (VECS) as applied to ore targets. 3.0 Мb, pdf-file