Introduction to the resistivity surveying method. The resistivity of ...

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46 Please refer to the instruction manual for the RES3DINV program for the data format. An online version of the manual is available by clicking the “Help” option on the Main Menu bar of the RES3DINV program. The set of files that comes with the RES3DINV program package has a number of field and synthetic data files. You can carry out an inversion of some of these files to get a feel of how the program works. 3.6 Examples of 3-D field surveys In this section, we will take a look at the results from a few 3-D field surveys over areas with complex geology. 3.6.1 Birmingham field test survey - U.K. This field test was carried out using a multi-electrode system with 50 electrodes commonly used for 2-D resistivity surveys. The electrodes are arranged in a 7 by 7 square grid with a unit spacing of 0.5 metre between adjacent electrodes (Figure 34). The two remote electrodes were placed at more than 25 metres from the grid to reduce their effects on the measured apparent resistivity values. To reduce the survey time, the cross-diagonal survey technique was used. The subsurface is known to be highly inhomogenous consisting of sands and gravels. Figure 35a shows the horizontal sections of the model obtained at the 6th iteration. The two high resistivity zones in the upper left quadrant and the lower right corner of Layer 2 are probably gravel beds. The two low resistivity linear features at the lower edge of Layer 1 are due to roots from a large sycamore tree just outside the survey area. The vertical extent of the gravel bed is more clearly shown in the vertical cross-sections across the model (Figure 35b). The inverse model shows that the subsurface resistivity distribution in this area is highly inhomogenous and can change rapidly within a short distance. In such a situation a simpler 2-D resistivity model (and certainly a 1-D model from conventional sounding surveys) would probably not be sufficiently accurate. Figure 34. Arrangement of electrodes in the Birmingham 3-D field survey. Copyright (1999-2001) M.H.Loke
47 Figure 35. Horizontal and vertical cross-sections of the model obtained from the inversion of the Birmingham field survey data set. The location of observed tree roots on the ground surface are also shown. Copyright (1999-2001) M.H.Loke
Page 1 and 2: Electrical imaging surveys for envi
Page 3 and 4: iii Table of Contents 1. Introducti
Page 5 and 6: v List of Figures Figure Page Numbe
Page 7 and 8: 1 1 Introduction to resistivity sur
Page 9 and 10: 3 carry out the more data intensive
Page 11 and 12: 5 2 2-D electrical imaging surveys
Page 13 and 14: 7 Note that as the electrode spacin
Page 15 and 16: 9 Figure 7. The apparent resistivit
Page 17 and 18: 11 function basically tells us the
Page 19 and 20: 13 Table 2. The median depth of inv
Page 21 and 22: 15 investigation) used in drawing t
Page 23 and 24: 17 electrode spacing along the surv
Page 25 and 26: 19 2.5.7 Summary If your survey is
Page 27 and 28: 21 significantly better results. Ho
Page 29 and 30: 23 Figure 15. Subdivision of the su
Page 31 and 32: 25 2.7.2 Odarslov Dyke - Sweden A d
Page 33 and 34: 27 2.7.4 Landslide - Cangkat Jering
Page 35 and 36: 29 same depth with more data points
Page 37 and 38: 31 2.7.8 Marine bottom resistivity
Page 39 and 40: 33 collected at 5 hours after the p
Page 41 and 42: 35 2.7.11 Wenner Gamma array survey
Page 43 and 44: 37 sediment samples. The lower resi
Page 45 and 46: 39 Figure 29. The arrangement of th
Page 47 and 48: 41 columns of electrodes. 3.3 3-D r
Page 49 and 50: 43 The free 3-D resistivity forward
Page 51: 45 Figure 33. The models used in 3-
Page 55 and 56: 49 Figure 36. The model obtained fr
Page 57 and 58: 51 Figure 38. The 3-D model obtaine
Page 59 and 60: 53 Break 3 (No. 7), 16-20. Griffith
Page 61 and 62: 55 electrode is less than the x-loc
Page 63 and 64: 57 Figure 40. Inversion models for
Page 65 and 66: 59 of regions that are internally a
Page 67: 61 calculated apparent resistivity

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