Study On The Electrical Resistivity Tomography Method With High-efficiency Acquisition And High-precision Imaging

Electrical resistivity tomography (ERT) method is one of the oldest and commonly used geophysical exploration methods. Unfortunately, the problem of traditional ERT method hard to have efficient acquisitions and accurate interpretations is highlighted when the surveys expand to large scale explorations,urban explorations and high resolution imaging surveys. The classical survey design of ERT method requires placing of electrodes in the form of a grid with measurements in different directions. However,it is always very time-consuming and costly when it comes to large scale surveys. The urban environment is an obvious challenge for traditional ERT method because the concrete pavement is very resistive and does not allow achieving continuous galvanic contact between the steel electrodes and the soil. For imaging subsurface structures, the low signal-to-noise ratio, non-uniqueness and probing depth problem bring great difficulties for the subsequent interpretation.By referring to the international and domestic acquisition experiences of ERT surveys, this paper presented an efficient 'radial type' electrode layout. Compared to the usual ones, this unconventional layout type increases efficiency significantly while it can obtain the appropriate data density for the detecting object. In this paper, the layout type is proved by a case study of investigating a chamber tomb which is carried out in Sifangling site, Hepu Tombs in Han Dynasty, Guangxi Province.In urban explorations, the concrete pavements severely limit the development of ERT method by blocking the direct current. This paper introduced a new type of ERT method based on the capacitive coupling theory transmitting the current through the pavements. In general, this resistivity measurement without galvanic coupling has a higher ratio of measurement speed to data density than the traditional DC resistivity measurement. Hence, it is economically preferred for urban explorations. The evaluation of the detection results using these two different resistivity measurements is given. The advantages and the shortages of this new measurement are concluded.For the very first time, the capacitive coupling system is tested by an archaeological study of detecting the ancient city walls beneath Hangzhou city.ERT method is an advantageous method because of rich geo-electric information.However, the inversion data from different array types could create various geo-electric image results and bring huge difficulty to delimit the anomaly and explanation. The probing depth, data coverage, signal intensity and sensitivity of resistivity variation are different. For the high-precision electrical exploration, no single array is enough. In this paper, through simulation and the inversion of integrating ERT data of an archaeological site is observed and adopted the technique to compensate the weakness of single array type.In the final chapter, it is necessary to study the critical factors for affecting the maximum probing depth of ERT method by theoretical calculation since the traditional ERT method is always invalid when facing the task of detecting small-sized anomaly occurring in deep subsurface. Taking the study of detecting the boulders in a Shenzhen subway tunnel construction as example,the simulations of models show that the traditional ERT method is not available for detection of small-sized boulders and the cross-hole resistivity tomography method can reveal these boulders. A case study of applying the cross-hole resistivity tomography method is also given.