Type of field curves:A- Two layers caseAscendingDescendingB- Three layers caseHKQA

C- Four layers caseKHKQHKHAQQQHAAAK

Depth of Current Penetration Versus Current Electrode SpacingAs shown previously, when two current electrodes are moved in close proximity to one another current flows along arc-shaped paths connecting the two electrodes. If the earth has a constant resistivity, about 50% of the current flows through rock at depths shallower than the current electrode spacing.

What this implies is that by increasing the electrode spacing more of the injected current flows to greater depths, as indicated in the figure above. Because the measured potential difference will fall as the electrode spacing is increased, to get measurable voltages over these longer separation requires a larger electrical current. Thus, the maximum distance that current electrodes can be separated by is in part dictated by the size of the generator used to produce the current. Assuming for a moment that we have a large enough generator to produce a measurable current in the ground at large current electrode spacing, this increase in the depth of current penetration as current electrode spacing increases suggests a way in which we could hope to decipher the resistivity structure of an area. Because current flows mostly near the Earth's surface for close electrode spacing, measurements of apparent resistivity at these electrode spacing will be dominated by the resistivity structure near the surface. If the current and potential electrodes are spread apart and the apparent resistivity remeasured, these measurements will incorporate information on deeper Earth structure.

Suitable Current Penetration:In homogenous medium the current penetration depend on the distance between the current electrodes (AB). AB Current PenetrationSo, For example for two layer case if the distance between AB is too small than the depth of penetration (Z), the apparent resistivity represent the resistivity of the first layer only.AB

AB>>ZZABAB/2AB=ZZABAB/2if the distance between AB is approximately equal to the depth of penetration (Z), the apparent resistivity represent the resistivity of the first and second layers.

Depth of Investigation:The depth of investigation is depend on several factors:1- Electrode spread.2- Subsurface geological situation.3- Type of lithology.4- Existence of very high (or very low) resistive layers.5- Type of arrays.So there are different simple equations established for calculating depth of investigation.1- Z = 1/3 * AB/22- Z = 2/3 * AB/23- Z = 1/5 * ABSo for example when the total spread (AB) = 120 m. we obtained the following depths:1- Z = 1/3 * 120/2= 20 m.2- Z = 2/3 * 120/2= 40 m.3- Z = 1/5 * 120 = 24 m.So we can say by 120 m spread we obtained depths range between 20 - 40 m.Depend on the above factors