Application of Multi-Electrode Resistivity Imaging System On Engineering Prospecting

Abstract:

Based on the two examples of bridge foundation inspecting and tunnel engineering prospecting, this paper gives brief introduction to multi-electrode resistivity imaging, its working principle, related instrument, electrode array configuration as well as 20-year data processing experience. Also, we compare features and applications between the conventional and modern multi-electrode resistivity imaging systems. The conclusion is, apply different electrode array configuration according to specific geophysical prospecting project. Many years of practice proves that: multi-electrode resistivity imaging plays an advantageous role in engineering prospecting, which mostly due to its features of lightweight, portability, huge sampling data, sufficient geological information and reliability. In spite of its effectiveness in engineering prospecting, this paper also points out the unsolved problems of multi-electrode resistivity imaging.

Keywords:Multi-electrode resistivity imaging, α2 electrode array, A-MN-N electrode array, bridge foundation, tunnel prospecting, application efficiency

Preface:

Multi-electrode resistivity imaging is a practical, efficient geophysical prospecting approach. After years of research and practice, it is now widely applied in mineral exploration and engineering construction in China. As the country is growing and infrastructure is being paid more and more attention, bridge and tunnel engineering gradually becomes key section to railway, highway and large water conservancy & hydroelectric projects. Most of these prospecting are carried out in deep mountains and complicated landforms, to which ordinary geological prospecting approach is difficult to apply. We have to select some suitable geophysical prospecting approaches to get knowledge of the thickness of the covering layer and of the weathered crust, depth of the bedrock surface, existing features of the stratum and of the geologic structure, potential hazard of hydrogeology and others. In this article, the writer involves the working principle, method and application of multi-electrode resistivity imaging according years of experience.

 

1. Application of Multi-Electrode Resistivity Imaging in China

1.1 Ministry of Geology and Mineral Resources Initiates Research

In the late 80s, 21st century, Ministry of Geology and Mineral Resources began research on multi-electrode resistivity imaging and development of its technical application. Multi-electrode resistivity imaging system is developed from general resistivity meter, same on working principle but apply different array configurations. It is a new prospecting method, with tens of or hundreds of electrodes arranged in field at one time. Each electrode is a survey point, and is connected to one cable which itself connect to a programmable multiplex electrode switcher and a mainframe, and thus to realize swift and automatic data acquisition. The data can be further processed in the computer, and depictures a geologic map with various properties. Multi-electrode resistivity imaging can apply in various aspects, featuring automatic multi-electrode data acquisition, dense survey point, low cost, high efficiency, rich geologic information, high resolution and precision, great and reliable prospecting capacity, etc.

1.2 Working Method & Configuration

In recent years, electronics and computer technologies roared. Benefit from this, multi-electrode resistivity imaging instrument also stepped into a new level, from conventional to intelligent multi-electrode resistivity imaging. Electrode array also developed from the original three (Wenner α, Wenner β, Wenner γ) to more than 10. Taking the BTSK WGMD-6 for example, the system supports maximum 18 electrode array, namely, Wenner α, Wenner β, Wenner γ, Delta A, Delta B, Alpha2, SP M, SP MN, Charging MN, Charging M, etc., which are applicable for fixed cross-section scan measurement; and A-M, A-MN, AB-M, AB-MN, MN-B Rectangle A-MN, A-MN-B, cross-hole dipole are applicable to extendable cross-section continuous roll-along measurement. We should consider different geologic objects and depth, and choose a most effective and suitable electrode array as the configuration.

Generally, we utilize fixed cross-section roll-along scan measurement Alpha2 (α2) and extendable cross-section continuous roll-along measurement A-MN-B (Fig.1). Both of them are 4-electrode configuration, potential electrode MN always keep fixed and thus help to eliminate the impact of various effective areas which is caused by increase of MN interval, but guaranteed the measuring precision and best measured results. The difference between these two arrays is, that the pseudo-section of data by α2 array is an inverted trapezoid, while by A-MN-B is a rectangle and enable the cross-section be continuously and infinitely extended.

1.3 Data Processing and Results

Measured data of multi-electrode imaging system needs to be transferred to the computer through data communication software, and then processed by other data processing software. The data pictured in the following examples is inversed by Res2D INV software—a senior multi-electrode data inversion software. During the process, we applied the 'Least Squares' to inverse the apparent resistivity data at real time. Suppression coefficient increases as depth. In order to stabilize the inversion process, we have the suppression increase 1.2times as the depth every time, stacking 4~6times, tolerance less than 5% and at the same time rectifying the landform.

Finally, we attain a profile of apparent resistivity contour. The contour profile directly and vividly pictures electric property of the cross-section and also the structure, facilitating interpretation and improves precision.

2 Field Survey Example of Multi-Electrode Res Survey

2.1 Bridge Foundation Prospecting

(1) Project: Coal field Geological and Geophysical Exploration of Jilin Province, Changchun, preformed this survey on the No.4 newly-built bridge for a railway project, with the purpose of getting a clear picture of the bridge foundation, such as electric property of the uneven geologic bodies, their distribution and geological structure, etc, and therefore leave it a reference for future prospecting tasks and bridge design and building.

(2) Geologic characteristics: the river bed is about 150m wide), Ⅰterrace. Length of the bridge is about 400meters, relative height is 5 meters. We have already known the general geological structures: the covering layer is Quaternary strata mainly consisting of scree, sand and clay, and the bedrock is Cambrian stratum mainly of sandstone and marlite, etc.

(3) Instrument applied: WGMD-2 Multi-Electrode Resistivity Imaging System by Chongqing Bengteng Digital Control Technology Institute (BTSK). We designed a survey line along the main axis of the bridge foundation, 6 profiles on the survey line, array configuration is A-MN-B, 60 involved electrodes, electrode spacing 5m, layer numbers 22, reaching deep into 40m.

(4) Survey results: Fig.2 shows the side view of the No.4 bridge foundation. In terms of electric property: ① At the upper section, apparent resistivity is higher in the middle (>500Ω•m) and lower in the two sides. Therefore, we think it is Quaternary of scree and sands. At the ride side, since it is close to the mountain root, apparent resistivity contour is between 100~300Ω•m, it should be Quaternary deposits mainly consisting of sand and clay.
② In the middle section, there are apparent resistivity less than 500Ω•m which should be eroded bedrock formed by limestone and siltstone. Eroding intensity is in direct proportion to the value of the apparent resistivity. The more it is eroded, the lower the apparent resistivity, the vice versa. In the Fig.2, the apparent resistivity is less than 200Ω•m, forming a regional close contour which is expected to be a karst cave. The later borehole surveying did prove they are karst caves.
③ At the bottom section, apparent resistivity is higher than 500Ω•m. They are expected to be solid unweathered bedrock, its surface is about 20~50m deep.
As the above interpretation, multi-electrode resistivity imaging data can tell us the potential hazard of geologic structure of the bridge foundation.

2.2 Tunnel Prospecting

(1) Project: this prospecting practice it to inspect thickness of the covering layer and of the weathered bedrock, depth of solid unweathered bedrock surface, structure and wall rock types around the tunnel, etc.
(2) Geologic characteristics: No.1 tunnel is 600m long, located in the mountain area, its entry at a relative height of about 120m. The mountain is covered with trees; top of it is the thin Quaternary. Bedrock is Jurassic Andesite. Because of complicated landform, the tunnel has to walks deep 100m. WGMD-2, as a conventional multi-electrode resistivity imaging system, is impossible to accomplish this project.

(3) Instrument applied:BTSK WGMD-6 Multi-Electrode 3D Resistivity Imaging System. This system is completed in functions, portable and convenient. Maximum high-voltage reaches 800V, current up to 3A, dramatically improves measuring precision and imaging depth. We take the tunnel as a reference axis, parallel to which lays a survey line, 110 involved electrodes, electrode spacing 10m, layer number 48 and the maximum depth is expected to be about 120m. Electrode array is α2 array.

(4) Survey results: ① Fig.3 is a geologic map of the first section of No. 1 tunnel. From its electric property, we can get known that apparent resistivity contour of the top layer are mostly less than 500Ω•m and dense but parallel to the surface, which should be surface soil and weathered bedrock crust, same in thickness with the weathered crust.
② At the left top of Fig.3, there is a closed circle of low-resistivity region; it is fault fragment zone filled with water.
③ Other resistivity contours of more than 500Ω•m are electric property of the solid unweathered bedrock.
④ Then let’s see where the tunnel is, except at its entry, the fragment zone and wall rock which are lower in resistivity, the other sections are imbedded in the solid unweathered bedrock, reliable rocks.
Till then, multi-electrode resistivity imaging system has given us a clear geologic map of the tunnel and helps us achieving this project successfully.

Unsolved problems:
Also, during the years, we found some problems waiting for solution: Based on the final survey results, how to better reflect the features of the earth body through landform rectification? Electrical interference still exists between different strata, affecting actual survey depth and result, and so on. All these problems need to solve in the future, through research or in practice. Wherever it develops, it is predictable, that multi-electrode resistivity imaging approach will reach its application to more fields, as new technology and working method are being put out. Conclusion:Thanks to the unstoppable progresses made in science, multi-electrode resistivity imaging system is also being developed, making great progress in measuring precision and function, and extending its applicable fields, in particularly in engineering prospecting, on aspects of portability, mass sampling data, rich geologic information, high resolution, simple data interpretation, excellent survey ability, gaining more and more attention and priority.