Half-Schlumberger array for multi- electrode resistivity survey

Transcription

1 Alexey Bobachev, Moscow University The useu Half-Schlumberger array for multi- electrode resistivity survey! Introduction to Pole-Dipole array! Disadvantages! Advantages! Difference between AMN and MNB sounding curves as indication of 2D object! Distortion of VES curves by 2D inhomogeneities! Field example WEB:

2 Introduction to Pole-Dipole array Electrode configuration A Ι AB V UMN A M N Forward pole-dipole array A MNB AMN M N Reverse pole-dipole array M N B AM MN

3 Pole-Dipole and Schlumberger arrays Apparent resistivity for pole-dipole and Schlumberger arrays is the same in 1D case ρa AMN = ρa MNB = ρa AMNB(Schlumberger) ρa AMNB General case ρa AMN + = 2 ρa MNB

4 Reference point for Pole-Dipole array Pole-Dipole is asymmetrical array A M N O X Pseudo depth Res2dInv, Res2dMod Electre x2ipi

5 Length of infinity line for Pole-Dipole array B B AOmax

6 Disadvantages of Pole-Dipole array! Unconventional array for modelling, measurements and presentation of results! Infinity line! Small value of measured signal! Great number of possible measurements! All measurements are doing twice for forward and reverse array! High sensitivity to inhomogeneities! Segmented VES curve! Common used software does not support Pole- Dipole array! A lot of problems! High current! Special software to make optimal SEQ files! Long time of measurements! Complicated app. resistivity pseudo section! Special data preprocessing

7 Problems using infinity line Additional good wire ( m) Extra work and space to set up infinity Infinity electrode should be with minimal grounding resistance Danger of electrical current leakage from wire, especially in wet weather Probability of accident break of wire by somebody Danger of electrical shock from infinity wire

8 Small value of measured signal AMN U MN = Wenner U MN

9 Great number of possible measurements SEQ files are generated by Electre II for array with 64 electrodes Wenner - N quadripoles = 651 AMN N q = 1488 (for 32 lines)

10 High sensitivity to inhomogeneities AMN Wenner Data by Henri Robain, pseudo section from IPI2win

11 Merger of segmented curve On level of right segment Mean

12 Pseudo section after different segments merger Right segment Mean Data by Henri Robain, pseudo section from IPI2win

13 Advantages! Maximal depth of research! Optimal using of rolling array along profile! Two VES curves for each location! Regularity of geological distortion

14 Depth of research for Pole-Dipole and Wenner arrays MN 1a Wenner 3a 9a AMN 21a AO, AB/3 m a = 1 m X,m SEQ file for AMN array is generated by x2ipi (N q =441)

15 Depth of research for Pole-Dipole and Wenner arrays for field example (a=4m, N el =64) AMN Wenner Data by Henri Robain, pseudo section from IPI2win

16 Rolling array along profile 1 64 Wenner MNB AMN AO, AB/3 m a = 1 m X,m

17 Comparing pseudo sections for AMN and MNB arrays ρa AMNB ρa AMN + = 2 ρa MNB ρ A a MN ρa MNB

18 Comparing pseudo sections for AMN and MNB arrays (field example) D transf. MNB AMN Data by Henri Robain, pseudo section from IPI2win and IPI_2d

19 Regularity of geological distortion VES curves ln( ρ ) App. resistivity a ln( AO) AMN Wenner Data by Henri Robain, pseudo section from IPI2win

20 Difference between AMN and MNB sounding curves as indication of 2D object by Res2dMod

21 One vertical boundary D=Ro_a(AMN)-Ro_a(MNB) R=d D / d X Modelling by Ie2dl, Modin & Bobatchev, transformation by Ipi_2d

22 Two vertical boundaries

23 Wide 2D object

24 Fault zone

25 Two 2D objects

26 Distortion of VES curves by 2D shallow depth inhomogeneities! P&C effects for pole dipole array VES curves! Distortion of Wenner-Alpha and Wenner-Beta array! Median polish of data

27 Pole-dipole Distortion of VES data Wenner-alpha

28 Pole-dipole Distortion of VES data Wenner-alpha

29 P-effect distortion of VES curve by anomalous object near DIPOLE-element of array 1 3 U MN MN 0 ρ α MN = K E = MN ρ U I MN AB ρ ΜΝ ΜΝ j 0 j j MN MN App. res is tivity Depth, m X, m 10 Current lines distribution by DC_Flow

30 C-effect distortion of VES curve by anomalous object near POLE-element of array A A* H, м H, м X, м Anomalous potential Current lines and potential distribution by DC_Flow

31 Comparing P and C effects P - effect! Great amplitude C - effect! Small amplitude! Does not depend on spacing! Depends on MN length! Decreases slowly with spacing! Usually the same for both AMN and MNB array! Does not change VES curve form (reference point MN center)! Different sign and usually different amplitude for AMN and MNB array! Changes VES curve form (reference point MN center)

32 Distortion of Wenner-Alpha and Wenner-Beta array a) Model Wenner α Wenner α Wenner β Wenner β App. resistivity, Ohm.m b) Variants of arrays a, m c) VES curves for Wenner and α β Modeling by Res2Dmod

33 Field example for Wenner-Alpha (N( el =160) V-transformation (d Ln(Ro)/d Ln (a)) 2D inversion by Res2Dinv Data by Henri Robain, pseudo section from IPI2win

34 P and C effects for different arrays Array Current electrodes Potential electrodes Schlumberger, Pole-Dipole C-effect P-effect Pole-Pole C-effect C-effect Dipole-Dipole P-effect P-effect Wenner Mainly C-effect

35 Median polish to decrease P and C effect X X X X X

36 Median polish on field data (N( el =64, 2 x shift) MNB AMN Data by Henri Robain, pseudo section from IPI2win, data processing by Median

37 D-transformation before and after Median polish Field data After polish by Median Inversion field data for MNB array by Res2dInv Data by Henri Robain, pseudo section from IPI_2d, 2D inversion by Res2dInv

38 Field example! Date: Mai, 1999! Place: South of Madagascar, semi-arid area! Goals: Hydrogeological research. Problems of water resource: rarity and high mineralization! Geological section:! Basement vertical-layered, weathered, metamorphic precambrian rocks (saprolite), depth (0-12 m)! Upper part laterite and sand! Equipment: Syscal R2, 64-electrodes array! Array: Pole-Dipole, distance between electrodes 3 m, MN 3 and 9 m, AO max =94.5 m

39 Median polish on field data (N( el =64, 2 x shift) MNB AMN Data by Vero Rabemanana, pseudo section from IPI2win, data processing Median

40 2D inversion AMN+MNB array After polish Without polish Data by Vero Rabemanana, pseudo section from IPI2win, 2D inversion by Res2dInv

41 Conclusion The practical use of Half-Schlumberger array for multi-electrode resistivity measurement is more complicate in comparing with Wenner (α,β) array for field measurement, data processing and inversion. On the other hand, Half-Schlumberger array allows to receive maximum geophysical information by multielectrode resistivity survey and to improve quality of interpretation, especially for deepest part of geological section.