It goes without saying, that the highest priority of every method of electrical prospecting works, determinated, used, receiving-generator installation, is the efficiency (resolution, preciseness, sensitivity, deepness, etc.). But technological priorities are also of high importance.
It is quite fast, convenient and cheap but not efficient. The matter is that a current-line is a mixed source (disturbing TE and TM-polarization of electro-magnetic field), but such one, the field of which loses TM-polarization quite quickly. Powerful TE-field, connected with a summed-up longitudinal conductivity of a geological section, is the strongest disturbance at studying of weak lateral heterogeneous medium (like oil-deposit is), and also at determining of such typical characteristics as thin layer with high resistivity.
It is highly willingly to get rid of TE-field, that is connected with inductive disturbance from a horizontal electric line, and leave on only TM-field, that is disturbed by means of groundings. In such case electro-magnetic field becomes more sensitive to anomaly conductivity, and also to more delicate influence of other geoelectrical parameters (for example, those that occurs in halos above the deposit). It is possible to compensate TE-field completely only by means of radial set of lines (a well-known circle electrical dipole ). But this type of source is not-mobile.
Now we suggest a compromise variant, in which a compound of TE-field is compensated to a large extent, and configurations stays as mobile as a usual horizontal electric line. You can easily understand our suggestion from a pic. 1, where it is depicted a usual installation with a horizontal electric line (two groundings on both ends) and a suggested installation with 3 groundings. In this case the current goes from the edge groundings to the central one. Such source can be called as a counter electric line (CEL). Thereby we eliminate, not completely but to a certain extent, the capability to an inductive disturbance, keeping galvanic disturbance at the same time.
Pic.1 Usual and counter electric line.
Let us demonstrate the advantages of a suggested source by means of mathematical modeling and taken a well-known problem of defining of thin layer with high resistivity as an example. This problem is often connected to the problems of search and prospecting of oil deposit. You can see such model with the parameters in the pic.2. It is usual practice that for a traditional installation they use more solid layer parameters of higher resistance which have weak ground but which are absolutely “must” components for getting even weak-measured signal. Let’s disturb this medium by a traditional installation (2000 m, current of 50A) and by a counter electric line (1000+1000m, current of 50A in each segment, directed into opposite to each other). We can observe an electric signal of 100-meter receivingline at a distance of 2000 m from the source center.
Pic.2 a medium including high resistivity thin layer
We have used a checked universal program “Vybor-TS” (pic.3) and have done proper calculations of processes establishment. In pic.3 you can see the curves of EMF for a traditional and counter electric line, for a model described in pic.2 (code-3/1), and also for a heterogeneous medium.
Pic.3. Curves of EMF formation from a receiving line.
It can be seen in pic.3 the total weakening of a signal from a counter electric line and more rapid fall. It is clear as a part of magnetic type field (TE) has dropped drastically, and TM-field goes down much faster. It’s interesting to watch the absolute value of anomaly (influence of high resistivity layer) signal. We can see (pic.4) that an absolute anomaly signal has even gone up on the first stage of formation. Finally, in pic.5 there performed some graphs of a relative anomaly effect (in %). In case with differential installation an anomaly effect has gone up by 4-5 times, and it’s easy to explain as only TM-field gets into contact with thin high resistivity layer. We have increased drastically a part of TM-field within a common field by means of differential configuration of feed current.
Pic.4 The curves of anomaly signals
Pic.5 The curves of relative anomaly signals
Thereby, counter electric line is really more effective than a traditional current line. At the same time the mobility of suggested feed installation is equal to a traditional one.