THE SMOOTH SOUNDING GRAPH. A Manual for Field Work ... - BGR

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35 The necessary holes for potential electrodes are drilled (f.i. by a cylindri- cal tube) on those points, which have special signs on the measuring tape. One has to pay attention to the holes, which are drilled slightly greater in comparison to the diameter of the potential electrodes. After the potential electrodes have been placed into the bore-holes and additionally pressed closely, in order to maintain the transition resistance between potential electrodes and ground very low, the cables are connected to the measuring equipment. Finally the electrodes are brought into the initial position and also connected to the cable drums and measuring equipment respec- tively. In order to avoid any influence on the potential electrode voltage the sur- rounding of the centre point should be kept free ("holy" district). This district - to say it again - should be large enough for, even if the cables are perfectly insulated, "weak" points cannot be avoided where the metal core is connected to the drums (Fig.27). Another version of building up the lay-out not using the 100m-measuring tapes is often applied by field parties where enough labourers are avail- able, i.e. at least two (better three) of them on the A-direction and the B- direction as well. By help of a normal tape measure (25m long) small sticks are put into the ground at the L/2-distances from the centre point according to the table of K-factors. These sticks carry the L/2 values in [m] (Fig.28). No leakage currents then can creep along the measuring tape because this does not exist. Errors are avoided because each single stick shows the exact distance.
36 Fig.27 Fig.28 Before we start the measurement we will repeat the preparations looking at Fig.26 and adding some details: 1 The chief surveyor checks the well at the farm. The depth of the water table may be of interest for the interpretation of the sounding graph. 2 The "holy" district has to be chosen very carefully. It is recom- mended to plant 4 potential electrodes at the very beginning: MN = 1m and M ' N' = 10m and connect them to the equipment by a double wire (Fig.27). This brings great advantages : a) during measuring with the smaller distance MN the potential electrodes M’ and N’ get already "accustomed" to the ground; b) the overlapping of the Schlumberger branches (see chapter 1.5) can be done by the operator at the instrument, because calling the labourers at the current electrodes back and forward again often leads to errors. 3+4 Assistants marking the lay-out by a tape measure.
Page 1 and 2: THE SMOOTH SOUNDING GRAPH A Manual
Page 3 and 4: Preface 2 This manual shall be a pr
Page 5 and 6: 1. Basic rules 4 The first chapter
Page 7 and 8: This current density is marked as j
Page 9 and 10: 8 Fig.2 Fig.3 Fig.4 Fig.5
Page 11 and 12: 10 1.3. The four-electrode arrangem
Page 13 and 14: 12 Fig.6 Fig.7 Fig.8
Page 15 and 16: 14 If we compare equations (10) and
Page 17 and 18: Case 2 (Fig. 10) 16 Now we observe
Page 19 and 20: 1.5. The fundamental principle for
Page 21 and 22: Case 3 (Fig.14) 20 The electrode di
Page 23 and 24: with just the same factor K. 22 Aft
Page 25 and 26: 24 Fig17 layers with different resi
Page 27 and 28: 26 Simulating this zooming by enlar
Page 29 and 30: 28 1.6. Shifting of potential elect
Page 31 and 32: 30 Fig.21 Fig.22 Fig23
Page 33 and 34: 32 2.1. How to carry out a field me
Page 35: L/2 a/2 1,5 6,28 2 11,8 2,5 18,9 34
Page 39 and 40: 38 fence, ditch) especially, if the
Page 41 and 42: 40 by experience. The measurement i
Page 43 and 44: 42 2.2. Possible errors influencing
Page 45 and 46: 2.2.4. Crossing a ditch (Fig.26/29)
Page 47 and 48: 46 accuracy is not so important the
Page 49 and 50: 2.2.8. Insulation and leakage curre
Page 51 and 52: 50 Fig.31. They are ascending with
Page 53 and 54: 52 If j' is negative, i.e. the dist

THE SMOOTH SOUNDING GRAPH. A Manual for Field Work ... - BGR

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