Landfills generate and contain gas, methane gas, which is both a heating resource and an environmental risk, when leaking into the atmosphere. Resistivity investigations can be used to map and monitor this gas and thus provide, for example, information on where best to locate gas extraction wells or carry out remediation work.
CHALLENGE
A municipal solid waste landfill can comprise about 40-60% methane gas. Whilst this gas is a resource that can be used for heating, it is also a greenhouse gas. With climate change high on the agenda for many countries and institutions, it is increasingly important to quantify and manage the leaking of landfill gas into the atmosphere, and/or to carefully collect the gas for use in heating systems thus reducing the level of emissions.
The distribution and movement of methane gas within a landfill site can be quite heterogenous, which can make the placement of gas wells somewhat difficult. A landfill area is often large and discrete, point-by-point, investigation techniques are time consuming and gain relatively limited information.
SOLUTION
Resistivity methods have been used for several years for different landfill applications. When measuring the moisture movement (with low resistivity zones) in previous MaLaGa-projects, high resistivity anomalies were also observed and connected to gas movement. To further investigate this, a thorough investigation was set up at the Filborna landfill site, in Helsingborg, Sweden.
Several parallel investigation lines were monitored using a remote-controlled ABEM LS system, with a 3D investigation design for both measurements and interpretation. Measurements were made several times a day, to provide near- continuous monitoring. To check the gas emissions at the surface, a laser measurement system was used and results compared to the resistivity investigations.
More to read
More to read on the internal structure of landfills can be found here:
We would like to thank MaLaGa (malagageophysics.com) and Dr. Mats Svensson, Tyréns AB and Torleif Dahlin, Engineering Geology, Lund Universty for sharing this case.
PROJECT
Method: Resistivity
Solution: ABEM Terrameter LS
Measurement: Parallel profiles with measurement, and processing undertaken in 3D, and monitored remotely. The electrode configuration was pole-dipole with 100 m to the remote electrode. Measurements have also been made with gradient configurations.
Inversion & Visualization SW: Res2dInv and Res3dInv when several parallel lines were investigated.
Client
The MaLaGa-project (Mapping Landfill Gas) has been a collaborative research project between universities, public and private sector mainly between 2012 and 2015, but which is still ongoing. The main participating organizations in the project are the Department of Engineering Geology, Lund University, and Tyréns AB. The MaLaGa-project’s main aim was to develop techniques for monitoring and characterization of solid waste landfills, based on geophysical measurements.
RESULTS
Measurements from the MaLaGa-project provided several clear examples of being able to map the physical extent of landfill areas, both in plan and in depth.
The results also demonstrate that measurements undertaken for mapping landfills may also be used to:
Locate old landfills
Provide a basis for further investigation, for example providing a picture of where soil sampling should be performed.
Determining former landfills’ extents
Estimate areas with different types of material in a landfill
The 3D volume shows the percentage change in resistivity between five different measurement occasions. The arrows indicate a possible directions of gas movement.
