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

More documents

Recommendations

Info

17 ρa results from using a not permitted formula (only valid for a homogeneous earth) and because a homogeneous underground has no boundaries and therefore nothing depending on a depth-scale, the apparent resistivity thus defined is merely a function of L/2 and of course the potential electrode spacing a (Schlumberger a → 0, Wenner a=L/3). In other words: There is no apparent resistivity in the underground at any depth. Thus the question often asked during a measurement to the operator at the instrument : “How deep are you now?” is senseless.
1.5. The fundamental principle for geoelectric sounding on a layered earth 18 At first we shall explain by a simple model the current density within a layered underground. Case 1 (Fig.12) For explanation we only look at the electrodes A and B on the earth’s surface and their distance L. The layer below the surface has a resistivity ρ1. Its thickness be h. It is underlain by a second layer infinitely extended. We assume that this second layer is an insulator (ρ = ∞). When we observe the distance L in relation to h, we can see that the current can ex- tend normally within the first layer, that means be hardly influenced by the insulator. Case 2 (Fig.13) We have again the same electrode distance L, but the thickness h of the first layer has been reduced. By this of course the geology is changed, the measuring configuration however is still the same. When we look again at the distance L in relation to h, we can see that the current lines seem somehow pressed to the surface. As result we can derive: If h becomes smaller at the same electrode configuration, the current lines will be com- pressed more and more. Therefore the current density increases and con- sequently does the voltage at the potential electrodes. To zoom the insulator means increasing the current density "below our feet" at the centre point.
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: Case 2 (Fig. 10) 16 Now we observe
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 and 36: L/2 a/2 1,5 6,28 2 11,8 2,5 18,9 34
Page 37 and 38: 36 Fig.27 Fig.28 Before we start th
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

Create successful ePaper yourself

Delete template?

Save as template?