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: The architectural complexity of a paleovalley 350 m deep has been revealed by acquisition and conventional processing of a high-resolution seismic-reflection survey in northern Alberta, Canada. However, processing degraded much of the high quality of the original raw data, particu-larly with respect to near-surface features such as commer-cial methane deposits, and that motivated use of additional processing algorithms to improve the quality of the final images. The additional processing includes development of a velocity model, via tomographic inversion, as the input for prestack depth migration (PSDM); application of a variety of noise-suppression techniques; and time-variant band-pass filtering. The resulting PSDM image is of poorer quality than the newly processed time-reflection profile, thus emphasizing the importance of a good velocity function for migration. However, the tomographic velocity model highlights the ability to distinguish the materials that constitute the paleovalley from the other surrounding rock bodies. Likewise, the reprocessed seismic-reflection data offer enhanced spatial and vertical resolution of the reflection data, and they image shallow features that are newly apparent and that suggest the presence of gas. This gas is not apparent in the conventionally processed section. Consequently, this underscores the importance of (1) ensur-ing that primarily high-frequency signals are kept during the processing of near-surface reflection data and (2) experimenting with different noise-suppression and elimi-nation procedures throughout the processing flow.
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ABSTRACT: A geophysical investigation using electromagnetics (EM), electrical resistivity and ground penetrating radar was carried out on the abandoned Gaborone landfill that was decommissioned in 1992 after being active for ten years. The aim of the study was to map the physical boundaries of the decommissioned landfill, map the distribution of waste and identify zones of leachate within and below the landfill. The results of EM conductivity measurements show a wide distribution of conductive materials, which represents a zone of active leaching which is mostly concentrated in the centre of the landfill. In-phase EM measurements also identified zones occupied by metallic waste that are less distributed over the landfill. Results of the resistivity survey indicated a three layer resistivity structure within and surrounding the landfill. The top layer is a more resistive cover material (68 - 127 ohm-m) and varies in thickness from over the landfill. The second layer is a low resistivity zone (3-40 ohm-m) and indicates a zone of high leachate activities. At the bottom is a more resistive layer (greater than 500 ohm-m) which is likely bedrock that underlies the abandoned landfill. The ground penetrating radar images also indicated a three layer structure over the landfill which is similar to the resistivity results. All the methods implied that the leachate has not penetrated the bedrock but the large amount of leachate suggests that it may leak into the unlined landfill in the future despite being in an arid environment. Of the three methods, the resistivity survey provided the most complete information on the subsurface conditions of and beneath the landfill.
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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