Introduction to the resistivity surveying method. The resistivity of ...

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2.7.11 Wenner Gamma array survey - Nigeria
Technically, the “normal” Wenner array is the Wenner Alpha array (Figure 2a). As
with any four electrodes array, there are two other possible variations that are usually referred
to as the Beta and Gamma configurations (Carpenter and Habberjam 1956). The Wenner Beta
array (Figure 2b) is in fact a special case of the dipole-dipole array. The Wenner Gamma
array (Figure 2c) has a relatively unusual arrangement where the current and potential
electrodes are interleaved. Compared to the Wenner Alpha and Beta (dipole-dipole) arrays,
the Wenner Gamma array is much less frequently used in field surveys. However, in some
situations, there might be some advantage in using this array. The depth of investigation is
significantly deeper than the Wenner Alpha array (0.59a compared to 0.52a, see Table 2), but
the voltage measured between the potential electrodes is only about 33% less than the Alpha
array. In comparison, the voltage that would be measured by the Wenner Beta array is onethird
that of the Alpha array which could be a serious disadvantage in noisy environments.
Figure 27a shows the Wenner Gamma array pseudosection from a groundwater survey
in the Bauchi area of Nigeria (Acworth 1981). In this region, groundwater is frequently found
in the weathered layer above the crystalline bedrock. The weathered layer is thicker in areas
with fractures in the bedrock, and thus such fractures are good targets for groundwater. In this
area, the surveys were carried out with the Wenner Alpha, Beta and Gamma arrays, together
with electromagnetic profiling measurements using a Geonics EM34-3 system (Acworth
1999). Here, only the result from the Wenner Gamma array data set is shown as an example.
To emphasize the boundary between the soil layer and the bedrock, the robust
inversion option was used (section 2.6.2). The inversion model in shown in Figure 27b. The
thickness of the lower resistivity weathered layer is generally about 10 to 20 metres. There is
a narrow vertical low resistivity zone with a width of less than 20 metres below the 190
metres mark which is probably a fracture zone in the bedrock. A borehole well that was
placed at the 175 metres mark which lies just at the edge of the fracture zone. It had yields
that were somewhat lower than expected (Acworth 1999). In such a situation, the 2D
resistivity model would be useful to pinpoint the exact location of the centre of the fracture
zone to improve the yield from the borehole. The placement of the well was largely based on
resistivity and EM profiling data, and many years before 2D resistivity inversion software and
fast microcomputers were widely available.
Figure 27. Bauchi Wenner Gamma array survey. (a). Apparent resistivity pseudosection. (b)
The inversion model with topography. Note the location of the borehole at the 175 metres
mark.
Copyright (1999-2001) M.H.Loke