A combined seismic reflection and refraction study of a landfill and its host sediments
ABSTRACT In an attempt to delineate the base of a landfill and map the geometries of the host sediments, we have recorded a high-resolution seismic profile. To obtain sufficient resolution in the heterogeneous landfill environment, common midpoint (CMP) spacing was set to 0.125 m and subsurface coverage (i.e. fold) was maintained at ≥120 in the central region of the survey. Despite the high density and high redundancy of the data, severe source-generated noise (i.e. direct, refracted, guided and surface waves) and strong lateral velocity variations made it difficult to identify reflections on processed shot and CMP gathers. However, a quasi-continuous sequence of reflections R1–R3 was eventually traced along the length of the profile. After time-to-depth converting the stacked seismic reflection section using poorly resolved initial stacking velocities, no consistent correlations with boundaries identified in nearby boreholes and on three-dimensional georadar data were apparent. In a first attempt to obtain more reliable velocities, ∼183,000 first-arrival times were tomographically inverted. Unfortunately, the resultant velocity model was found to be incompatible with knowledge supplied by the borehole and georadar data and the seismic reflection section. By including the known depths to a key geological horizon and the R1–R3 traveltimes as constraints, a second suite of tomographic inversions produced a satisfactory model. This model included a thin capping layer of humus and sandy clay (velocities of 400–1000 m/s) overlying a distinctly lower velocity landfill (200–600 m/s) along the northern half of the profile and a southward thickening sequence of fluvial deposits (600–900 m/s) along the southern half. A southward thinning layer of compact lacustrine sediments and basal till (2000–3800 m/s) and a nearly horizontal bedrock interface (4000–5400 m/s) was mapped beneath the entire profile. Although independent applications of the seismic reflection and refraction techniques were not successful in meeting the survey objectives, a combination of the two approaches suitably constrained by borehole information finally provided the required details on the landfill and surrounding sediments. Nevertheless, our study has highlighted the limitations of employing 2-D seismic refraction and reflection methods for resolving problems in highly heterogeneous 3-D media.
- SourceAvailable from: Cedric Schmelzbach
[Show abstract] [Hide abstract]
- "Because the resolution of information provided by electromagnetic methods may substantially decrease with increasing depth, seismic surveys can complement electromagnetic studies in that they generally yield higher resolution details throughout the depth ranges of interest (Sumanovac, 2006). By combining seismic-refraction and seismic-reflection surveying, complementary images allow improved interpretations to be made (De Iaco et al., 2003; Metwaly et al., 2005; Improta and Bruno, 2007; Schmelzbach et al., Figure 1. (a) Satellite image (Google Earth) of the Okavango Delta in northwestern Botswana showing the locations of the regional HTEM survey and the Jao (Jedibe Island) and HR2 seismic surveying sites (yellow squares). "
ABSTRACT: Electrical resistivity models derived from exceptionally high-quality helicopter transient electromagnetic data recorded across the Okavango Delta in Botswana, one of the world's great inland deltas or megafans, include three principal layers: (1) an upper heterogeneous layer of dry and water-saturated sand, (2) an intermediate electrically conductive layer that likely comprises saline-water-saturated sand and clay, and (3) a lower fan-shaped electrically resistive layer of freshwater-saturated sand/gravel and/or crystalline basement. If part of the lower layer comprises a freshwater aquifer, it would be evidence for a recently proposed Paleo Okavango Megafan and a major new source of freshwater. In an attempt to constrain the interpretation of the lower layer, we acquired two high-resolution seismic refraction and reflection data sets at each of two investigation sites: one near the center of the delta and one along its western edge. The interface between unconsolidated sediments and basement near the center of the delta is well defined by an similar to 1800 to similar to 4500 m/s increase in P-wave velocities, a change in seismic reflection facies, and a strong continuous reflection. This interface is about 45 m deeper than the top of the lower resistive layer, thus providing support for the Paleo Okavango Megafan hypothesis. Subhorizontal seismic reflectors are additional evidence for a sedimentary origin of the upper part of the lower resistive layer. In contrast to the observations at the delta's center, the interface between unconsolidated sediments and basement along its western edge, which is also defined by a similar to 1800 to similar to 4500 m/s increase in P-wave velocities and a continuous reflection, coincides with the top of the resistive layer.Geophysics 02/2014; 79(3). DOI:10.1190/geo2013-0278.1 · 1.76 Impact Factor
[Show abstract] [Hide abstract]
- "Reflection and refraction seismics have been tested on landfills as well (Lanz et al., 1998; Green et al., 1999; Balia and Littarru, 2010), but until now they have shown high uncertainties. De Iaco et al. (2003) have illustrated that the result of a conventional reflection seismic survey (CRSS) at a landfill is extremely difficult to interpret because strong scattering events and strong lateral velocity variations can influence the interpretation of the reflections and the source-generated noise. Our aim is to image the subsurface of landfills to obtain an indication of the possible flow pathways. "
ABSTRACT: A significant problem with landfills is their aftercare period. A landfill is considered to be safe for the environment only after a relatively long period of time. Until it reaches such a condition, it has to be periodically treated. Not only are treatments very expensive, but they could be dangerous as well; for example, when barriers limiting the waste break. So far, there is no established technique that can predict the leachate and gas-emission potential of a landfill, especially in time-lapse monitoring. This potential depends on the channeling of fluids due to the presence of high-density waste areas and the redistribution of the channels with time. We propose to use seismic interferometry (SI) applied to active reflection seismics to help improve the image of the waste areas (scatterers) and to monitor the subsurface changes in time. Normally, application of SI to reflection recordings from active sources at the surface would result in an erroneous retrieved result, but secondary illumination of the receivers from strongly scattering subsurface, like a landfill, would remedy this problem. We conduct modeling studies to examine the possible benefits of this approach compared to using the conventional seismic reflection method. We show that the reflections retrieved from SI can be used to obtain a clearer image of the shallower scatterers. In addition, we illustrate that time-lapse monitoring using reflections retrieved by SI shows a more repeatable result than the conventional approach in case of source nonrepeatability.Geophysics 10/2013; 78:EN107-EN116. DOI:10.1190/geo2013-0099.1 · 1.76 Impact Factor
[Show abstract] [Hide abstract]
- "• Self-potential (Bavusi et al., 2006; Buselli and Lu, 2001; Faraco Gallas et al., 2011; Mota et al., 2004; Naudet et al., 2003, 2004). • Refraction seismics (Cardarelli and Bernabini, 1997; Carpenter et al., 1991; De Iaco et al., 2003; Lanz et al., 1998). "
ABSTRACT: Electrical resistivitymethods arewidely used for environmental applications, and they are particularly useful for the characterization and monitoring of sites where the presence of contamination requires a thorough understanding of the location and movement of water, that can act as a carrier of solutes. One such application is landfill studies, where the strong electrical contrasts between waste, leachate and surrounding formations make electrical methods a nearly ideal tool for investigation. In spite of the advantages, however, electrical investigation of landfills poses also challenges, both logistical and interpretational. This paper presents the results of a study conducted on a dismissed landfill, close to the city of Corigliano d'Otranto, in the Apulia region (Southern Italy). The landfill is located in an abandoned quarry, that was subsequently re-utilized about thirty years ago as a site for urban waste disposal. The waste was thought to be more than 20 m thick, and the landfill bottom was expected to be confined with an HDPE (high-density poli-ethylene) liner. During the digging operations performed to build a nearby new landfill, leachate was found, triggering an in-depth investigation including also non-invasivemethods. The principal goal was to verify whether the leachate is indeed confined, and to what extent, by the HDPE liner.We performed both surface electrical resistivity tomography (ERT) and mise-à-la-masse (MALM) surveys, facing the severe challenges posed by the rugged terrain of the abandoned quarry complex. A conductive body, probably associated with leachate,was found as deep as 40 mbelowthe current landfill surface i.e. at a depth much larger than the expected 20 mthickness of waste. Given the logistical difficulties that limit the geometry of acquisition, we utilized synthetic forward modeling in order to confirm/dismiss interpretational hypotheses emerging from the ERT and MALM results. This integration between measurements and modeling helped narrow the alternative interpretations and strengthened the confidence in results, confirming the effectiveness of non-invasive methods in landfill investigation and the importance of modeling in the interpretation of geophysical results.Journal of Applied Geophysics 08/2013; 98:1-10. DOI:10.1016/j.jappgeo.2013.07.010 · 1.30 Impact Factor