Electrical resistivity method in hydrogeophysical surveys : theory, inversion and modelling

Abstract

The capacitively-coupled (CC) resistivity method is an emerging geophysical technique for near-surface investigations and therefore has the potential to be widely used in hydrogeophysical surveys. In this thesis, the focuses are put on resolving two major problems associated with the CC resistivity (line antenna) method, i.e., theories and inversion techniques. First, a new expression of the associated geometric factor is developed to improve the accuracy of the measured apparent resistivity. A quadru-line model is then derived to describe the characteristics of the CC resistivity (line antenna) system and the validity of this model is verified experimentally. Relevant measurement biases, including the one induced by the gap between the line antenna and the ground surface, are also examined using the quadru-line model and experiments. To improve the accuracy of exiting indirect inversion method, the adopted equivalent point electrode array is suggested to be optimized by minimizing the sensitivity (1D) difference between the line and point electrode arrays. The direct inversion of resistivity (line electrode) measurements is also realized by incorporating line electrodes into an open source resistivity inversion package. The usefulness and effectiveness of the proposed inversion methods are supported by numerical examples. While using the electrical resistivity method for hydrological related studies, a key step involved in the quantitative analyses is how to relate the measured electrical resistivity to soil hydrological properties. In this study, an attempt is first made to develop an easy-to-calibrate electrical conductivity (EC) model for unsaturated soils. Comparisons between measurements and theoretical predictions demonstrate that the proposed model can provide sufficiently accurate description of the soil ECs at different saturations. An electrical-hydraulic conductivity (EC-K(h)) relationship for unsaturated soils is also derived. Theoretical predictions of K(h) based on the measured soil EC are in good agreement with the measured results. Apart from that, the proposed EC-K(h) relationship is applied to quantify the anisotropy of K(h) of unsaturated soils using the measured directional ECs.

Date of Award	2014
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology