Jesper Pedersen

Mapping of landfills using time-domain spectral induced polarization data: the Eskelund case study

This study uses time-domain induced polarization data for the delineation and characterization of the former landfill site at Eskelund, Denmark. With optimized acquisition parameters combined with a new inversion algorithm, we use the full content of the decay curve and retrieve spectral information from time-domain IP data. Thirteen IP/DC profiles were collected in the area, supplemented by el-log drilling for accurate correlation between the geophysics and the lithology. The data were inverted using a laterally constrained 1D inversion considering the full decay curves to retrieve the four Cole-Cole parameters. For all profiles, the results reveal a highly chargeable unit that shows a very good agreement to the findings from 15 boreholes covering the area, where the extent of the waste deposits was measured. The thickness and depth of surface measurements were furthermore validated by el-log measurements giving in situ values, for which the Cole-Cole parameters were computed. The 3D shape of the waste body was pinpointed and well-defined. The inversion of the IP data also shows a strong correlation with the initial stage of the waste dump and its composition combining an aerial map with acquired results.

Data repeatability and acquisition techniques for time-domain spectral induced polarization

The Time Domain Induced Polarization (TDIP) technique is widely used in applied geophysics, particularly for environmental issues, for instance for delineating landfills or detecting leachate percolation. Because the reliability of IP data remains an issue at the field scale, this paper deals with the factors controlling data quality and compares different arrays and acquisition parameters for optimal collection of data in the field. The first part focuses on repeatability experiments carried out in the former Horlokke landfill (Denmark), in order to infer the degree of which a signal can be reproduced over time. Results show a good repeatability, with on average less than 10% of difference in raw data. Also, from the results it is inferred that the paramount parameter controlling repeatability is the IP signal level; a value of 2 mV is a sufficient threshold to ensure repeatability within 10% of data difference, although system dependant. The second part focuses on survey design and underlines the importance of keeping the geometrical factor low. This points to the choice of a relevant measurement protocol, which depends on the threshold of the geometrical factor, again depending on expected chargeability and resistivity, threshold voltage and injected current. Furthermore, acquisition parameters such as the duration of the pulse injection and data sampling have a significant effect on both the signal-to-noise ratio and resolution. A comprehensive comparison between three protocols, the gradient array, the linear grid and the dipole-dipole array, is shown and the choice of an acquisition sequence is discussed.

Improved Geoarchaeological Mapping with Electromagnetic Induction Instruments from Dedicated Processing and Inversion

Increasingly, electromagnetic induction methods (EMI) are being used within the area of archaeological prospecting for mapping soil structures or for studying paleo-landscapes. Recent hardware developments have made fast data acquisition, combined with precise positioning, possible, thus providing interesting possibilities for archaeological prospecting. However, it is commonly assumed that the instrument operates in what is referred to as Low Induction Number, or LIN. Here, we detail the problems of the approximations while discussing a best practice for EMI measurements, data processing, and inversion for understanding a paleo-landscape at an Iron Age human bone depositional site (Alken Enge) in Denmark. On synthetic as well as field data we show that soil mapping based on EMI instruments can be improved by applying data processing methodologies from adjacent scientific fields. Data from a 10 hectare study site was collected with a line spacing of 1–4 m, resulting in roughly 13,000 processed soundings, which were inverted with a full non-linear algorithm. The models had higher dynamic range in the retrieved resistivity values, as well as sharper contrasts between structural elements than we could obtain by looking at data alone. We show that the pre-excavation EMI mapping facilitated an archaeological prospecting where traditional trenching could be replaced by a few test pits at selected sites, hereby increasing the chance of finding human bones. In a general context we show that (1) dedicated processing of EMI data is necessary to remove coupling from anthropogenic structures (fences, phone cables, paved roads, etc.), and (2) that carrying out a dedicated full non-linear inversion with spatial coherency constraints improves the accuracy of resistivities and structures over using the data as they are or using the Low Induction Number (LIN) approximation.

Mapping of Landfills Using Time-domain Spectral Induced Polarization Data - The Eskelund Case Study

This study uses time domain IP for the delineation and the characterization of the former Eskelund landfill, Denmark. With optimized acquisition parameters combined with a new inversion algorithm it is now possible to use the full content of the decay curve and retrieve spectral information from the time domain IP data. Thirteen IP/DC profiles were collected, supplemented by en el-log drilling for an accurate correlation between the geophysics and the lithology. The data were inverted using a laterally constrained 1D inversion considering the full decay curves to retrieve the four Cole-Cole parameters. For all profiles, the results reveal a highly chargeable unit with an impressive agreement to 15 boreholes. The thickness and the depths have been validated by el-log measurements giving in-situ values for the Cole-Cole parameters. The 3-D shape of the waste body has been delineated with a very high accuracy, and the south boundary of the landfill has been redefined based on these results. The inversion of the IP data also allows reconstruction of the waste history combining aerial maps with the results. Thus, this case study shows the relevance and the full benefits of using the TDIP data for the recognition and the characterization of landfill areas.

Mapping Soil Heterogeneity Using Spatially Constrained Inversion of Electromagnetic Induction Data

Precision agriculture applications rely on highly detailed information on variations in soil properties. This demand has led to recent developments in EMI instrumentation, resulting in stable and high data quality. The reliability of the data enables real non-linear inversion providing true formation resistivity with depth. With the aim of mapping soil heterogeneity, an EMI survey has been carried out in the central part of Jutland, Denmark. The collected data were carefully processed prior to inversion in order to remove any couplings from man-made conductors such as buried electrical cables, which can cause errors in the lithological interpretation. The data were subsequently modelled with a 1D layered spatially constrained inversion which takes the entire EMI data geometry into account. The inversion results gives a detailed quasi tree-dimensional image of the survey area, revealing several small scale resistivity variations which corresponds well with existing geological knowledge.

ANTHROPOGENIC WETLANDS DUE TO OVER-IRRIGATION OF DESERT AREAS; A CHALLENGING HYDROGEOLOGICAL INVESTIGATION WITH EXTENSIVE GEOPHYSICAL INPUT FROM TEM AND MRS MEASUREMENTS

During the last century, many large irrigation projects were carried out in arid lands worldwide. Despite a tremendous increase in food production, a common problem when characterizing these zones is land degradation in the form of waterlogging. A clear example of this phenomenon is in the Nubariya depression in the Western Desert of Egypt. Following the reclamation of desert lands for agricultural production, an artificial brackish and contaminated pond started to develop in the late 1990s, which at present extends for about 2.5 km2. The available data provide evidence of a simultaneous general deterioration of the groundwater system. An extensive hydrogeophysical investigation was carried out in this challenging environment using magnetic resonance sounding (MRS) and ground-based time-domain electromagnetic (TEM) techniques with the following main objectives: (1) understanding the hydrological evolution of the area; (2) characterizing the hydrogeological setting; and (3) developing scenarios for artificial aquifer remediation and recharge. The integrated interpretation of the geophysical surveys provided a hydrogeological picture of the upper 100 m sedimentary setting in terms of both lithological distribution and groundwater quality. The information is then used to set up (1) a regional groundwater flow and (2) a local density-dependent flow and transport numerical model to reproduce the evolution of the aquifer system and develop a few scenarios for artificial aquifer recharge using the treated water provided by a nearby wastewater treatment plant. The research outcomes point to the hydrological challenges that emerge for the effective management of water resources in reclaimed desert areas, and they highlight the effectiveness of using advanced geophysical and modeling methodologies.