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

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36 As a final note, it is possible to invert data collected with the Wenner Alpha, Beta and Gamma arrays along the same line simultaneously with the RES2DINV program as a single data set. This can be done by using the "non-conventional array" option in the program where the positions of all the four electrodes in an array are explicitly specified. This might be an interesting method to combine the advantages of the different variations of the Wenner array. 2.7.12 Mobile underwater survey - Belgium This example is one of the most unusual data sets that I have come across, and a worthy challenge for any resistivity imaging inversion software. It is not only the longest in physical length and number of electrode positions, but also uses an unusual highly asymmetrical non-conventional electrode arrangement collected by an underwater mobile surveying system. Mobile surveying systems have an advantage of faster surveying speed, but on land they suffer from the problem of poor ground contact or low signal strength. Mobile land surveying systems consists of two main types, a pulled array type of system with a direct contact between the electrodes and the ground (Bernstone and Dahlin, 1999) and an electrostatic type of system with no direct ground contact (Panissod et. al, 1998). The type that requires direct contact can only be used on open ground where reasonably good contact can be obtained with the soil. The electrostatic type does not require direct ground contact and thus can be used in many areas where normal resistivity surveying systems cannot be used (for example in built-up areas) but has the problem of a more limited depth of penetration due to the limited amount of current induced. An underwater environment provides an almost ideal situation for a direct contact type of mobile system since there is no problem in obtaining good electrode contact! Figure 28a shows the data from the first two kilometres of an eight kilometres survey line along a river. This survey was carried out by Sage Engineering of Belgium. The purpose of the survey was to map the near surface lithology of the riverbed where there were plans to lay a cable. The particular data subset in Figure 28a is slightly more than 2 kilometres long has an electrode position at almost every metre. It has a total of 1994 electrode positions and 1760 data points, whereas the inversion model used has 5312 blocks. On a 550 Mhz Pentium III computer, it took slightly over 14 hours to process this data set! Figure 28. (a) The apparent resistivity pseudosection for the first two kilometres of an underwater survey along a riverbed by Sage Engineering, Belgium. (b) The inversion model after three iterations. In the inversion model (Figure 28b), most of the riverbed materials have a resistivity of less than 120 ohm.m. There are several areas where the near-surface materials have significantly higher resistivities of over 150 ohm.m. Unfortunately, geological information in this area is rather limited. In the high resistivity areas, the divers faced problems in obtaining Copyright (1999-2001) M.H.Loke
37 sediment samples. The lower resistivity materials are possibly more coherent sediments (possibly sand with silt/clay), whereas the higher resistivity areas might be coarser and less coherent materials. Shallow seismic reflection surveys are frequently used in rivers/lakes/marine environments for engineering site surveys. A mobile resistivity survey might be a useful addition in some situations, such as in seismically opaque areas. In theory, both surveys can be carried out simultaneously to reduce costs. Besides these examples, 2-D imaging surveys have been carried for many other purposes such as detecting leakage of pollutants from landfill sites, areas with undulating limestone bedrock, mapping of the overburden thickness over bedrock, leakage of water from dams, and the saline water intrusion in coastal aquifers. The resistivity imaging method has also been used in underwater surveys in lakes and dams. 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: 35 2.7.11 Wenner Gamma array survey
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 and 52: 45 Figure 33. The models used in 3-
Page 53 and 54: 47 Figure 35. Horizontal and vertic
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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