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Enhancing VLF data for qualitative interpretation

An example of massive chalcopyrite exploration

Ankara University, Turkey

Corresponding author: basokur@eng.ankara.edu.tr

The first 20% of the full text of this article appears below.

Economic ores usually consist of some combination of semiconductor sulfide minerals that respond to applied electric and electromagnetic fields. Consequently, electric and electromagnetic methods are the main exploration tools to locate the sulfide minerals. Models obtained from two- or three-dimensional inversion of induced polarization (IP) or controlled source audio-magnetotellurics (CSAMT) data can provide a good description of orebodies (Basokur et al., 1997). However, these methods are cumbersome in field procedures, survey costs are relatively high, and acquisition systems are expensive. For these reasons, they are not suitable for reconnaissance surveys and for exploration of small targets. The very low frequency (VLF) method, on the other hand, is appropriate under these conditions. It is fast, can often be implemented by 1–2 people, and the receiver unit is less expensive than those used with other ground EM methods. Moreover, the method uses existing VLF radio transmitters as electromagnetic sources.

Interpretation of VLF data is generally qualitative because the use of one or a few frequencies in a very narrow frequency band does not permit accurate modeling of the subsurface, especially in the depth direction. Some idealized models may help the quantitative interpretation. However, the success of such interpretation depends on the similarity of the model to the exact shape of the target body which, moreover, is generally embedded in a homogeneous subsurface structure. For these reasons, qualitative interpretation is still valuable for mining exploration.

	Defining apparent resistivity

 
Qualitative interpretation is based on apparent resistivity, which is not a directly measured field quantity. Instead it is a variety of quantities (e.g., potential differences, electric and magnetic components) that are measured and converted to "apparent resistivity" to give the data physical significance. In the VLF method, the radio transmitters generate a primary field in the very low frequency range with a horizontal magnetic component H_y and a . . . [Full Text of this Article]