Application of Forward Modelling in the Design of Laboratory Waste Columns and Optimization of Electrical Resistivity Tomography (ERT) Measurements

Old landfills have been under continuous investigation since the 1960s due to their potential for soil and groundwater contamination, gas explosion risks, and methane emissions contributing to climate change. Economic factors such as land reclamation, waste-to-energy, and resource recovery also drive studies before installing gas and leachate wells or excavating closed landfills. Additionally, insights from old landfills help improve the design and technology of new ones.

Numerous geophysical methods have been employed for characterizing and monitoring old landfills, with Electrical Resistivity Tomography (ERT) standing out as a particularly valuable tool. ERT has been consistently used to identify different waste zones in landfills based on moisture content and waste chemistry. However, field studies on ERT’s sensitivity to landfill gas remain challenging due to the complex nature of waste and gas dynamics. This issue can be addressed through controlled small-scale laboratory experiments. Fortunately, one of ERT’s key strengths is its versatility in adapting to investigations at various scales. To achieve optimal results in small-scale laboratory experiments, careful column design and an appropriate current injection and measurement strategy are crucial, often necessitating the use of forward modelling to simulate the desired experiment.

In our efforts to quantify the contribution of landfill gases to the measured electrical resistivity of waste in gas hotspot regions of closed old landfills, we designed a small-scale resistivity column experiment. This experiment aimed to investigate the perturbation in the electrical resistivity of waste caused by controlled gas circulation under varying degrees of moisture saturation, gas pressure, and waste composition. In the initial stage, forward modelling was performed using ResIPy software to determine the optimal current and measurement strategy for a cylindrical column with a diameter of 120 mm and a height of 400 mm. Various combinations of electrode numbers, electrode spacing, and measurement configurations were tested to identify the setup that provides the best spatial and visibility resolution for targets of different sizes and resistivities across the column within a reasonable measurement duration. Although each strategy has its advantages, the results from the models provided a foundation for selecting an optimal design strategy and understanding the limitations of the column.