Vertical Electrical Sounding Lab

Overview

Vertical electrical sounding is the most used geophysical method in Iraq for groundwater, for the depth of gypsum and salt, and for foundation and dam sites. The field work is simple, but the interpretation is usually done with old curve-matching charts or desktop programs that most students and field geologists do not have. This lab does the full interpretation on a phone, without a connection, from the moment the last reading is taken.

Readings can be entered as apparent resistivity, as resistance ΔV/I, or as voltage and current, typed or pasted from a spreadsheet. The number of layers can be chosen or found automatically, any thickness or resistivity known from a borehole can be fixed, and the model can be edited by hand to see how the curve responds.

What you enter · What you get

How it works in the field

Method and equations

Accuracy and error limits

• The forward solution agrees with the exact image series of a two-layer earth to better than 0.0001 %. The uncertainty of the result is that of the data and of the method: with a fit of 2–5 % the resistivity of thick layers is usually within ±10–20 % and their depth within ±10–25 %, as shown for each layer.
• What is not covered: equivalence (a thin conductive layer is resolved only by its conductance h/ρ and a thin resistive layer only by its transverse resistance hρ, so different models fit equally well), suppression of a thin layer whose resistivity lies between its neighbours, dipping or laterally changing layers, and anything deeper than about a third of the largest AB/2. The lithology is a suggestion from resistivity ranges; clay and saline water give the same low values.
• To reduce the error: use six to ten readings per decade of AB/2, overlap two readings when MN is changed, keep AB/2 at least five times MN/2, extend AB/2 to three times the target depth, fix layers known from nearby boreholes, compare the result with the logs, and repeat soundings across lateral changes rather than trusting a single one.

Who uses it

Standards and references

1. Koefoed, O. (1979). Geosounding Principles 1: Resistivity Sounding Measurements. Elsevier, Amsterdam.
2. Guptasarma, D. & Singh, B. (1997). New digital linear filters for Hankel J0 and J1 transforms. Geophysical Prospecting, 45(5), 745–762. Filter coefficients from the libdlf library (CC BY 4.0).
3. Pekeris, C.L. (1940). Direct method of interpretation in resistivity prospecting. Geophysics, 5(1), 31–42.
4. Marquardt, D.W. (1963). An algorithm for least-squares estimation of nonlinear parameters. Journal of the Society for Industrial and Applied Mathematics, 11(2), 431–441.
5. Maillet, R. (1947). The fundamental equations of electrical prospecting. Geophysics, 12(4), 529–556.
6. Archie, G.E. (1942). The electrical resistivity log as an aid in determining some reservoir characteristics. Transactions of the AIME, 146, 54–62.
7. Telford, W.M., Geldart, L.P. & Sheriff, R.E. (1990). Applied Geophysics, 2nd ed. Cambridge University Press.
8. Zohdy, A.A.R., Eaton, G.P. & Mabey, D.R. (1974). Application of surface geophysics to ground-water investigations. USGS Techniques of Water-Resources Investigations, Book 2, Chapter D1.