Imaging and characterization of heterogeneous landfills using geophysical methods

Abstract

Nowadays many countries use landfilling for the management of their waste or for treating old landfills. Emissions from landfills can be harmful to the environment and to human health, making the stabilization of landfills a priority for the landfill communities. Estimation of the emission potential for determination of the aftercare period and improvement of the treatment technologies for the minimization of the aftercare period are examples of problems landfill research groups are now facing. For handling these problems, the degradation processes inside the landfill, which highly depend on the heterogeneity of the landfill, must be well understood. Geophysical methods can be used to image and characterize the heterogeneity of the landfill body non-intrusively. Researchers have earlier used geophysical methods for landfills. However, till now, artifacts, uncertainties and low resolution remained problematic in imaging and characterizing the landfill body in detail. In addition, the results so far have been rather qualitative. In this thesis, we aim to improve the imaging and characterization of landfill bodies using seismic and electrical methods, by proposing new ways to deal with some major problems that were previously encountered. A landfill body is very heterogeneous with many high-density areas that act as obstructions to the fluid flow. Characterization of these high-density areas is important for the understanding of the preferential flow paths inside the landfill body and hence of the degradation processes. These high-density areas manifest themselves as scatterers in the recorded seismic wavefield. Till now, research works using the seismic method have had difficulties in imaging a landfill body mainly because of the presence of very strongly scattered seismic energy. We investigate the use of seismic interferometry (SI), by performing a number of modeling studies, to improve the imaging. The additional amount of traces and sources computed by SI provides more information, as an increased number of rays penetrate into the earth and are recorded. Furthermore, scatterers act as secondary sources illuminating the landfill from below, thus reducing possible artifacts due to the location of the seismic receivers and sources only at the earth's surface. We discuss in detail the concept of using SI for landfill application and its advantages. The SI approach is compared with the conventional reflection seismic survey (CRSS), considering different acquisition geometries and processing and acquisition errors. Our results have established the merits of SI in landfill applications. The next step was to test our modeling results on field data. For this, we acquire seismic datasets at two different landfill sites. Before applying SI to the CRSS data, we attempt to image the landfill body solely by CRSS. We have found that this is possible when special processing steps are adopted. Special care has been taken in velocity analysis. The developed methodology is proposed to be used for very heterogeneous subsurfaces in order to image the higher-density areas (scatterers) in detail. We apply this procedure in two fieeld datasets. We have succeeded to image higher-density areas, the top and bottom of the landfill, and the geological subsurface below the landfill body. The geometry used in acquiring the second field dataset was better for the purpose than the first one, resulting in an improved imaging of the landfill. SI is applied to the first acquired field dataset for validation of the modeling studies. Not only have we been able to improve the imaging of the landfill body, but we have also proposed a new method for the removal of surface waves that dominate the field seismic data. In our first field dataset, surface-wave signal from another source was recorded; that was not possible to be removed using conventional processing/filtering techniques. We use the method of adaptive subtraction (AS) involving SI to remove the surface waves and have, thereby, shown the improved imaging of reflections inside the landfill body. Finally, we explore the improvement of the characterization of the landfill body when using the velocity analysis results from all three approaches (CRSS, SI and AS). Besides imaging, the characterization of the higher-density areas and of the leachate- and gas/air-bearing zones inside the landfill is very important for understanding the degradation processes. Using a landfill-specific empirical relationship between the unit weight and shear (S)-wave velocity of the landfill materials, we have calculated the density distribution inside the landfill. These values can be used in models that predict the emission potential. For further characterization, we use electrical methods in conjunction with the seismic methods. In the first study, we are able to determine the heterogeneities (wet and dry pockets) through combined use of S-wave velocity and electrical resistivity (ER). S-wave velocity can resolve the high-density areas (scatterers) that act as an obstruction to the fluid flow. ER helps to identify the pockets that represent accumulation of water (low-resistivity values) above the high-density areas. In the second field study, we acquire S- and compressional (P)-wave reection seismic datasets. By jointly interpreting the results, leachate and gas/air bearing zones are distinguished. The conditions that must be met for interpreting leachate or gas/air (relatively dry) zones are thoroughly discussed, taking into account the P- and S-wave velocities. Interpreting results of ER and induced polarization (IP) measurements acquired at the same location as the seismic surveys, the structure of the landfill is better defined, wet and gas pockets are further characterized, and an indication for the type of the waste is obtained. The velocity analysis of extremely heterogeneous seismic data is challenging. We check the velocity distribution obtained from analysis of seismic reflection data by comparing it with the results of multichannel analysis of surface waves (MASW) and early-arrival waveform tomography. In the second field study, additional measurements (gas concentration and mechanical resistance to waste deformation) are used for validation of results from seismic and electrical methods. The methodology developed in this research presents a way for improved imaging and characterization of a heterogeneous landfill body through use of specially adapted seismic and electrical methods. New approaches are presented to overcome some problems that were encountered in the past and to provide more reliable, quantitative results.