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.
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ABSTRACT: A sequential hybrid approach was presented here to invert crosshole seismic first-arrival times. The proposed tomographic scheme combined a simple simulated annealing algorithm with a linearized smoothness-constrained least-squares inversion. The simulated annealing was implemented to obtain a background velocity distribution used by the linearized inversion for the initial guess. The linearized component was based on the functional description of traveltimes. This indicates a nonlinear function, the eikonal equation, providing traveltimes for a given slowness model. Thus an explicit ray tracing was not required by the linearized scheme. The velocity updates were obtained by a matrix inversion based on an iterative conjugate gradient-like LSQR algorithm. Second-difference regularization was used to stabilize the solutions. The Jacobian matrix giving the partial derivatives with respect to the model parameters was constructed by a finite-difference approximation based on the perturbation of the cell slowness. The traveltimes for both the hybrid and linearized schemes were calculated by a fast finite-difference eikonal solver. The hybrid scheme was tested by using both synthetic and field data sets based on the crosshole geometry. According to the tests studies, the tomograms resulted from the hybrid approach better imaged the subsurface velocity distribution. Also the hybrid optimization was characterized by quicker convergence rate than the conventional optimization based on only the linearized inversion. The tests with the synthetic data set also showed that the hybrid approach yielded a solution having lower rms residual, smaller Euclidean distance and lower relative errors in the cell velocities.Journal of Geophysics and Engineering 03/2011; 8(1):99. · 0.72 Impact Factor
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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. · 1.33 Impact Factor
- Geophysics 01/2013; 78:EN107-EN116. · 1.72 Impact Factor