VES

Method

Direct current resistivity method gathers data via injecting electric current into ground with two electrodes (stainless steel bars) and measuring voltage differences occurs at two different points. The distance between the current electrodes and the electrical resistivities of the geological units determine the depth of investigation. The measured data are first converted to the apparent resistivity data,

ρa=k ∆V/I,

here, ρa is a value that  the combination of the resistivities of all subsurface geological units . A and B electrodes where current injected, M and N are the electrodes where voltage difference measured. The AB interval defines the depth of investigation.

DC resistivity survey
vertical electrical survey

Field work

In the field;

• The lower the potential difference occurs, the worse the signal/noise becomes.

If 1D inversion is the tool;

• Selecting  large interval for MN is a simple but dangerous approach. ,

• The large MN interval in Schlumberger array makes  easier to measure the voltage difference but the measured value deviates from what it should be!

suggested ratio MN < ~AB/10

in the field condition, one must comply with this ratio , at least for first few measurement then the ratio can be changed as much as required.

The measurements made with large MN  do not cause problems in 2D or 3D inversion since the MN is also utilised in codes.

The survey data with different MN  is shifted to the first MN curve prior to the modeling steps. In other words, all curve segments are shifted to overlap with the curve obtained for the first MN (blue).

Raw DC resistivity curve
Segmented curves due to MN variation

Survey

A minimum of 3 measurements are recommended for each survey. Overlapping measurements along the profile always provide control over each other.
VES data set for 2D inversion
Overlapping schlumberger arrays along the profile

Orientation

 The AB expansion is directly related to the approach to be used in the data processing. If one is going to do 1D modeling, survey lines should be taken parallel to possible discontinuity (fracture, contact etc.). “Jumps” in curves is prevented . The F-distance is important,  a fictitious  layer at depth F should be expected in the 1D layered model.

1D inversion should not be used for fault imaging
Survey line parallel to fault zone
If one is going to do 2D modeling, the lines should be taken perpendicular to the potential discontinuity (fracture, contact etc.). “Jumps” on the curve should be left as is.

survey line should cross the fault zone
VES data set for 2D inversion

Depth of investigation

For the  survey design;  max(AB / 2) should be selected  3 (three) times or more of the targeted depth of investigation.  The figures show the relation between max(AB/2) and basement depth.  The resistivity of the basement can only be determined accurately with measurements larger than max(AB / 2) ~ 300m
depth of investigation is the 1/3 of max(AB/2)
Depth of investigation and max(AB/2)

1D vs. 2D Inversion

The result is always different when  1D and 2D inversion applied to the same data set. In geophysical surveys, lateral geological changes, which are the targets in many survey, cannot be ımaged correctly  in 1D modeling studies.

2D inversion of VES data

result of 1D inversion