Pradip Kumar Maurya

Permeability Estimation Directly From Logging‐While‐Drilling Induced Polarization Data

In this study, we present the prediction of permeability from time domain spectral induced polarization (IP) data, measured in boreholes on undisturbed formations using the El-log logging-while-drilling technique. We collected El-log data and hydraulic properties on unconsolidated Quaternary and Miocene deposits in boreholes at three locations at a field site in Denmark, characterized by different electrical water conductivity and chemistry. The high vertical resolution of the El-log technique matches the lithological variability at the site, minimizing ambiguity in the interpretation originating from resolution issues. The permeability values were computed from IP data using a laboratory-derived empirical relationship presented in a recent study for saturated unconsolidated sediments, without any further calibration. A very good correlation, within 1 order of magnitude, was found between the IP-derived permeability estimates and those derived using grain size analyses and slug tests, with similar depth trends and permeability contrasts. Furthermore, the effect of water conductivity on the IP-derived permeability estimations was found negligible in comparison to the permeability uncertainties estimated from the inversion and the laboratoryderived empirical relationship.

Geophysics‐Based Contaminant Mass Discharge Quantification Downgradient of a Landfill and a Former Pharmaceutical Factory

Contaminant mass discharge is a commonly applied tool to evaluate the environmental impact of contaminated sites on water resources. At large contaminated sites with heterogeneous sources, such as landfills, the number of wells available is often not sufficient, leading to a high uncertainty of mass discharge estimates. In this study, we tackle the uncertainty of the contaminant mass discharge due to low sampling densities by interpolating limited water-sample data with the support of surface direct current resistivity and induced polarization geophysical data. The method relies on finding a conceptual link between the bulk conductivity imaged from geophysics and the contaminant concentrations. We investigate the link between (1) imaged bulk and electrical water conductivity, (2) water conductivity and conservative ionic species, (3) water conductivity and redox-sensitive species, (4) water conductivity and semipersistent organic species, and (5) water conductivity and biodegradable organic compounds. The method successfully identify similarities between the distribution of the bulk conductivity and chloride and pharmaceutical compounds in a landfill leachate plume and between the bulk conductivity data and benzene and chlorinated ethenes for a contaminant plume from a former pharmaceutical factory. Contaminant concentrations were interpolated through regression kriging, using geophysical data as the dependent variable. The distribution of concentration determined with the novel method showed a lower mean relative estimation error than the traditional method of kriging only contaminant concentration data. At large sites, the method can improve contaminant mass discharge estimates, especially if surface geophysical measurements are integrated in the site investigation at an early stage.

Re-parameterization of the Cole-Cole Model for Improved Spectral Inversion of Induced Polarization Data

The induced polarization phenomenon, both in time-domain (TD) and frequency-domain (FD), is often parameterized using the empirical Cole-Cole model. We here suggest three re-parametrizations of the Cole-Cole model, namely the maximum phase angle (MPA) model, the maximum imaginary conductivity (MIC) model and the minimum imaginary resistivity (MIR) model. We test these new parameterizations against the classic Cole-Cole model using a Markov Chain Monte Carlo (MCMC) inversion algorithm. The MCMC method allows us to study the probability distributions of each model parameter and gives us direct information on the parameter uncertainty. We show that models that are poorly resolved from inversion with the Cole-Cole model, e.g. due to low C values or strongly correlated parameters, can be resolved well with the new parameterizations and that the parameterizations work equally well for TDIP and FDIP data.

Comparison of Frequency-domain and Time-domain Spectral Induced Polarization Methods at Field Scale

In this paper we present a comparison study of time-domain (TD) and frequency-domain (FD) spectral induced polarization (IP) methods in terms of field procedures, data quality, as well as spectral information retrieved from inversion. For this purpose we collected TDIP and FDIP surface measurements on three profiles with identical measurement setup at two field sites with different lithology. Both TD and FD data were processed and inverted using similar approaches. This comparison study shows that, given the right circumstances, both methods (TD and FD) can be readily applied on the field scale, thereby providing results that compare not only qualitatively, but also quantitatively. Furthermore, the surface inversions are in agreement with the boreholes’ inversions/lithology, proving the effectiveness of the surface imaging. However, due to the underlying technical constraints and with the instruments used in this comparison, TD measurements had a 10 fold speed advantage over the FD method, a wider spectral range and more flexibility in the array choice. Therefore we conclude that TD measurements are a suitable tool to recover spectral IP information in the field, provided that the measurement procedures are planned accordingly.

Mapping Possible Flowpaths of Contaminants through Surface and Cross-borehole Spectral Time-domain Induced Polarization

In the Capitol Region of Denmark, several sites are contaminated due to various human activities. A large fraction of these sites are in clayey moraines, where the flow of pollutants predominantly occurs in sand lenses or sandy layers. Boreholes are normally drilled in order to describe the geology, but boreholes alone do not always provide the necessary resolution to map out such sand lenses, which is why the Capitol Region initiated a project to evaluate different cross-borehole geophysical methods for mapping sand lenses/layers. A test site was established in an uncontaminated gravel pit near Hedehusene, Zeeland, Denmark (Kallerup grusgrav).

3D Resistivity and Induced Polarization for Leachate Plume Identification at a Challenging Field Site

In the present study, we have used 3D ERT for detailed mapping of a polluted site at a challenging river site in the southern part of the Denmark. The survey layout involves several crossings of the meander of a river where a leachate plume from an old chemical factory site is expected to reach the river. Both resistivity and time domain induced polarization data were acquired, and full 3D inversion was performed for the resistivity data and 2D inversion of the IP data. Two sets of 3D data were acquired; one at a larger scale covering total area 410x90m2 and one at small scale covering 126x42m2, which enabled both details in the shallow part while covering a large area. 3D DC data were inverted using the inversion package BERT and 2D DC-IP data were inverted using AarhusInv. 3D DC inversion models show the presence of low resistivity anomaly at the northern bank of the stream interpreted as a contaminant plume. The contaminant plume is also seen in a deeper aquifer, which is separated by a low permeable clay layer to the upper aquifer. We could not discriminate the clay-lithology from the contamination using only resistivity model however, including the 2D inversions of the IP data we clearly separate the clay layer from the contaminated aquifer.

Detailed Mapping of a Leachate Plume from a Landfill Using Full-decay Time-domain DC-IP

Leaching of organic and inorganic contamination from landfills are a serious environmental problem as surface waters and aquifers are being contaminated. In order to assess these risks and investigate the migration of leachate from the landfill, several time-domain DCIP profiles have been collected at a heavily polluted landfill in Grindsted, Denmark. The DCIP data were inverted using a new 2D DCIP inversion code inverting for a full cole-cole model while modelling the full system transfer response including waveforms, gates and low-pass filters. The inverted profiles describe both the variations along the groundwater flow as well as the plume extension across the flow directions. Chemical analysis of borehole data agrees well with the observations indicating a leachate plume which gradually sinks and increases in size while migrating from the landfill in the groundwater flow direction. High chargeabilities are seen on the landfill itself but there are only small IP effects in the leachate migrating from the landfill. The information obtained from the DCIP results will be combined with chemical analysis in order to compute contaminant mass discharge. Hence, the DCIP data provides invaluable spatial information on the leachate characteristics, which cannot be obtained from boreholes alone

Detailed Leachate Mapping Using a 441 Electrode Full 3D Setup Using an Extendable Fishbone Layout

3D surveying of resistivity and induced polarization has often utilized only a small number of electrodes determined by the number of addresses in the instrument. This is often in the range of 64-128 electrodes if mounted using a switching unit. Here we use a flexible and expandable 3D system that allows for arbitrary use of any set of parallel cables in a fish-bone structure. The measurements are performed at a large highly contaminated historic landfill in Denmark for a detailed mapping of the leachate plume migrating from the fill. A total of 441 stainless steel electrodes were installed along 7 lines (63 electrodes each). Each line was 410 meters long and the inter-line spacing is 25 meters making the total area 410 by 150 m. Six switch boxes enabled data acquisition with all 441 electrodes using a resistivity meter designed for only 64 electrodes. We collected data both in-line, and crossline to build a full 3D dataset consisting of several thousand 4-electrode measurements. The inversion identify a very conductive leachate plume migrating from the fill, with a fluid resistivity as low as 0.5 ohmm. Due to the dense 3D cover we identidfied a division of the main plume into two sub-plumes diverting as the distance to the landfill increses.