Geology Reference
In-Depth Information
6.1.5 Depth penetration
Arrays are usually chosen at least partly for their depth penetration, which
is almost impossible to define because the depth to which a given fraction
of current penetrates depends on the layering as well as on the separation
between the current electrodes. Voltage electrode positions determine which
part of the current field is sampled, and the penetrations of the Wenner and
Schlumberger arrays are thus likely to be very similar for similar total
array lengths. For either array, the expansion at which the effect of a deep
interface beneath a homogeneous upper layer becomes evident depends on
the resistivity contrast, but is of the order of a quarter of the spacing between
the outer electrodes.
For any array, there is a depth at which the effect of a thin layer of different
resistivity in otherwise homogeneous ground is a maximum. This would be
expected to be less than the depth at which the interface in a two-layer Earth
first becomes apparent, and plots of the effects of such a layer, shown in
Figure 6.6 for the Wenner, Schlumberger and dipole-dipole arrays, confirm
this. By this criterion, the Wenner is the least, and the dipole-dipole the
most, penetrative array. However, the Wenner curve is the most sharply
peaked, suggesting better resolving power. This is confirmed by the signal-
contribution contours (Figure 6.5), which are slightly flatter at depth for the
Wenner than for the Schlumberger array, indicating that the Wenner locates
flat-lying interfaces more accurately. The signal-contribution contours for
the dipole-dipole array are almost vertical at considerable depths in some
places, indicating that the array is best suited to mapping lateral rather than
vertical changes.
6.2 DC Practicalities
The currents used in surveys described as 'direct current', or
DC
,arenever
actually unidirectional. Reversing the direction of flow allows the effects
of unidirectional natural currents to be eliminated by simply summing and
averaging the results obtained in the two directions.
DC surveys require current generators, voltmeters, ammeters and electri-
cal contact with the ground. Cables and electrodes are cheap but vital parts
of the systems, and it is with these that much of the noise is associated.
6.2.1 Metal electrodes
The electrodes used to inject current into the ground are nearly always metal
stakes, which in dry ground may have to be hammered in to depths of
more than 50 cm (but not to more than 10% of the inter-electrode spacing)
and be watered to improve contact. Where contact is very poor, salt water
and multiple stakes may be used. If this fails to lower contact resistances
