“DOI/Vmn charts” tab

The DOI/Vmn charts tab allows to explore the values for maximum sensitivity and depth of investigation for the different combinations of receiving dipoles/transmitting n-poles and to check how the Vmn signal changes when the background resistivity or the input current is modified.

Charts with maximum sensitivity, depth of investigation and expected Vmn signal.

Fig. 28 Charts with maximum sensitivity, depth of investigation and expected Vmn signal.

The tab consists of a horizontal bar at the top, where the commands for analysis are hosted, and two graphs, one for maximum sensitivity/depth of investigation (left in Fig. 28) and one for potential Vmn (right).

Select transmitters

This section allows to select the transmitting n-pole used to perform the analysis. The tool calculates maximum sensitivity, depth of investigation and potential Vmn related to all receiving dipoles in combination with that specific TX n-pole.

Set field parameters

The parameters here are related to the fieldwork scenario:

  • Background Rho: this is the value (in Ohm m) of the site background resistivity.
  • Injected current: this is the value (in mA) the user supposes will be able to inject into the ground at the transmitting dipoles.

To make the changes effective, click on the arrow button Apply. The defaults (100 Ohms m and 1000 mA) can be changed in the Field parameters section of the Preferences panel.

Sensitivity/depth of investigation chart

This plot represents the values ​​of maximum sensitivity (orange marker) and of maximum depth of investigation (red marker) for each combination of TX/RX. Positioning the mouse pointer inside the graph allows to view (horizontal bar at the top-right) the value of the z depth in meters at the selected point.

The depth of investigation estimate is performed, first, integrating the analytic sensitivity function – for the given n-pole – and then by calculating the median z depth, so that the area under the sensitivity curve is equal to the 50% of the total area (Edwards, 1977 [1] ; Baker, 1991 [2] ).

Note

For peculiar arrangements of the transmitting and receiving electrodes, in particular when dipoles are strongly orthogonal, the sensitivity curve with depth can change of sign and show multiple local extremes, which can make sometimes not very reliable the estimate of the depth of investigation. For these particular measurements, or for models that take into account complex scenarios of 3D distribution of resistivity, or, again, in case of marked topographic variations, it may be useful to save the project to DATA file through the command Save project of the Open and save project panel and perform a specific modeling with advanced tools, such as the ERTLab64 TM software.

Vmn signal graph

This plot represents the expected Vmn potential (in milliVolt) for each combination of the TX/RX. The scale of representation of potentials is logarithmic. The bars are green if the (absolute) value of Vmn is between the minimum and maximum thresholds specified in the Field parameters section of the Preferences panel. For signals below the minimum threshold the bar turns purple, as displayed in Fig. 28 : in the example this happens for the P1-P2 and P2-P3 dipoles of RX8 box for the Box1-Box2 transmission n-pole, electrodes P1-P2. Similarly, for signals above the maximum threshold (overload risk at the receiver) the bar turns to red. Positioning the mouse pointer inside the graph allows to view (horizontal bar at top-right) the value of the potential Vmn in milliVolt at the selected point.

Footnotes

[1]L.S. Edwards, 1977. A modified pseudosection for resistivity and induced-polarization. Geophysics, 42, 1020-1036.
[2]Barker, R.D., 1991. Depth of investigation of collinear symmetrical four-electrode arrays. Geophysics, 54, 1031-1037.