ABEM Terrameter SAS 1000 / SAS 4000
- 80 -
10.2
MULTIPLE GRADIENT ARRAY SURVEYING
The following text is mainly based on Dahlin and Zhou (2002).
10.2.1
Surveying Principles
Multi-electrode gradient surveying is carried out by injecting current with a separation (
s+2
)
a
and simultaneously or sequentially picking up all the potential differences between the
potential electrodes with the spacing
a
(Figure 1). Here
s
is an integer, which means the
maximum number of potential readings for a current injection. Figure 2a gives a schematic
representation of this array, where the sensitivity function of the measurement with the first
potential electrode pair is also shown in the background. Generally, the bigger separation of
current electrodes the deeper penetration of the configuration, and the smaller spacing of
potential electrodes the more details of potential variation are obtained. In practical
applications the selection of spacing
a
and
separation
s
will be a trade-off between noise
sensitivity, horizontal detail and depth penetration. Traditional gradient surveying often
comprised measurements with the current electrodes at one fixed location only. In a multi-
electrode gradient survey a large number of current electrode combinations are used, scanning
across the electrode layout with several different spacings
a
and
separations
s
, similar to
multi-electrode surveying with other electrode arrays.
a
n-factor
s-factor
M
N
B
A
It can be observed that the gradient array is essentially a pole-dipole array (Figure 2b) when
the potential dipole is close to one of the current electrodes, at least for large
s
-factors. When
the potential dipole is centred between the current electrodes it is identical to the
Schlumberger array (Figure2c). Thus, the gradient array may be expected to combine the
characteristics of the pole-dipole and Schlumberger arrays, however without the need of a
remote electrode that can be cumbersome to arrange in some environments.
Figure 26. Sketch of gradient array layout showing the position of the
electrodes for a measurement with s-factor 8 and n-factor 2.
