Geoscience Reference
In-Depth Information
separation traversing can be graphed to produce
a profile of the lateral changes in ρ
a
found along
the transect (Figure 5.11). In an agricultural
setting, assuming the resistivity survey had a
shallow depth of investigation (~2 m), the fairly
sharp step increase in ρ
a
shown in Figure 5.11
most likely represents some spatially abrupt soil
property change (e.g., a decrease in soil clay
content over a short distance).
The measurements of ρ
a
acquired during
a vertical electric sounding are usually plotted
versus some fraction, 1/
K
, of the electrode array
length (Figure 5.12). For the Schlumberger
array, the
K
value is typically 2; therefore, ρ
a
in the case of a Schlumberger array is normally
graphed with respect to half the array length.
The plotted vertical sounding curve depicted
in Figure 5.12, if obtained for agricultural pur-
poses, might indicate a soil profile trend from the
surface downward, in which soil resistivity first
decreases with depth and then increases with
depth. The presence of a clay-pan within the
soil profile is one scenario that would account
for this type of vertical resistivity trend. During
the past, changes in resistivity with depth were
determined using manual graphical procedures,
where type curves usually representing simple
two- or three-layer resistivity depth models were
overlaid and fit to the measured vertical electric
sounding plot. These older graphical procedures
are rarely used today.
Forward computer modeling techniques superseded these graphical procedures and provided
substantially improved capabilities for quantifying the vertical distribution of resistivity. The one-
dimensional forward modeling approach involves the following four steps:
ρ
a
Distance
fIGURe 5.11
A horizontal profile of apparent
resistivity along a transect, which can be produced
from data collected by a constant separation travers-
ing survey.
ρ
a
1
K
× Electrode Array Length
fIGURe 5.12
A plotted vertical electric sounding
curve.
1. The person responsible for data interpretation constructs a one-dimensional resistivity
depth model based on the available information regarding subsurface conditions.
2. Computer software is used to generate a synthetic vertical electric sounding curve corre-
sponding to the initial one-dimensional model of soil resistivity variation with depth.
3. The computer-generated synthetic vertical sounding curve is then compared to the vertical
sounding curve measured in the field.
4. The interpreter then adjusts the initial one-dimensional resistivity model, followed by the
generation of a new synthetic vertical sounding curve.
The last forward modeling step is repeated until there is a good fit between the measured vertical
electric sounding curve and the synthetic vertical electric sounding curve. Once a good enough fit
is achieved, the final one-dimensional resistivity depth model is considered to be a reasonable esti-
mate for the true vertical resistivity distribution.
In recent years, inverse modeling techniques have gained widespread acceptance for use in
determining resistivity variations with depth. The inverse computer modeling approach is com-
pletely automated, using iterative procedures coupled with optimization protocols to produce a
