Time-lapse airborne EM surveys across a municipal landfill

In contrast to the majority of historical landfills, modern municipal landfills are highly engineered and follow a contain and seal strategy of leachate management. The purpose of the management system is to render the waste products inert and environmentally safe. A requirement for monitoring and assessment of the installation on the scale of decades is a consequence of the strategy. Data obtained from two repeated fixed-wing airborne electromagnetic surveys across an active, municipal solid waste landfill are considered here. The time interval between the surveys is 4years . In theory such data may be used to both test the isolation performance of the installation and to monitor mass (leachate) transport behaviour within the landfill structure. Single frequency (3.1kHz) data obtained at a similar density (100m flight line spacing) over the 4year span are presented and compared. These data have an expected mean depth of investigation of about 15m within the landfill. Half-space conductivity models are determined from the survey data by an inversion procedure. Conductivities within the landfill are observed to be three orders of magnitude above background. From the initial survey data, a specific distribution of high conductivity material can be identified in three of the landfill cells (peak values of 170mS∕m ). Four years later, a considerable redistribution of material is apparent in the results obtained across two of the cells (peak values of 317mS∕m ). A third cell shows no change. A subtraction of the two time-lapse conductivity models allows the dynamic components of the conductivity distribution (all increases with time) to be mapped within individual cells. All larger conductivity increases (>20mS∕m) are confined to the operational landfill.