| 1. |
- Fiandaca, Gianluca, et al.
(author)
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Doubling the spectrum of time-domain induced polarization by harmonic de-noising, drift/spike removal and tapered gating
- 2016
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In: 22nd European Meeting of Environmental and Engineering Geophysics, Near Surface Geoscience 2016. - Netherlands : EAGE Publications BV.
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Conference paper (peer-reviewed)abstract
- The extraction of spectral information in the inversion process of time-domain (TD) induced polarization (IP) data is changing the use of the TDIP method. Data interpretation is evolving from a qualitative description of the subsurface, able only to discriminate the presence of contrasts in chargeability parameters, towards a quantitative analysis of the investigated media, which allows for detailed soil-And rock-Type characterization. Two major limitations restrict the extraction of the spectral information of TDIP data in the field: i) the difficulty of acquiring reliable early-Time measurements, and ii) the selfpotential drift in the measured potentials distorting the shape of the late time. For measuring at early times, we developed a new method for removing harmonic noise from the data. Furthermore, a new scheme for spike removal was developed and tapered windows are used in the data gating. For measuring at late times, we developed a drift-removal scheme that model the polarization effect and allows for preserving the shape of the IP responses at late times. Overall, the removal of harmonic noise, spikes, self-potential drift, tapered windowing, and the uncertainty estimation allows for doubling the usable range of TDIP data to almost four decades in time.
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| 2. |
- Fiandaca, Gianluca, et al.
(author)
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Heterodox transients in time-domain induced polarization
- 2022
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In: Geophysics. - : Society of Exploration Geophysicists. - 0016-8033 .- 1942-2156. ; 87:1, s. 35-47
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Journal article (peer-reviewed)abstract
- Negative induced polarization (IP) time-domain transients, sign-changing or non-monotonically decaying transients are currently often considered as measurement errors and removed in the data processing. These transients, here named heterodox in the sense of other than generally accepted signals, might originate from measurement errors, inductive effects, or poor signal processing, but synthetic modelling and field measurements show that these transients are physically possible. A simple theoretical explanation of the basic mechanism for their origin can be found through the superposition of contributions from regions with different sensitivities and such heterodox transients can be identified through the processing of full-waveform IP data. A mathematical classification of orthodox and heterodox IP transients into six different types is introduced based on the temporal development of the sign of their amplitude and derivative. The basic mechanism for IP transients with heterodox shapes is further investigated by considering the subsurface Cole-Cole parameter sensitivities and time-varying IP potential for 2D synthetic models. The time-domain forward response and sensitivities are computed through a time transformation that accounts for the current waveform. This approach allows for quantitative unbiased estimates of the time-domain transients and sensitivities, different from the estimates obtained when using multiple direct-current forward computations, as often done in the inversion of time-domain IP data. Time-domain IP transients may differ from the traditionally expected decaying-like transients when the electrode geometry has IP potential sensitivities with different signs for areas with different IP parameters. Hence, previously disregarded IP transients containing valuable information of the subsurface can be kept for inversion and contribute to the final parameter distribution. Increased understanding of theoretically possible IP transients makes way for more accurate processing of data in the future, reducing the time and resources needed for spectral inversion of time-domain data.
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| 3. |
- Fiandaca, Gianluca, et al.
(author)
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Non-Standard Responses in Time-Domain Induced Polarization Measurements
- 2019
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In: Proceedings of the 25th European Meeting of Environmental and Engineering Geophysics. - : European Association of Geoscientists & Engineers.
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Conference paper (peer-reviewed)abstract
- Today, negative induced polarization (IP) time-domain responses and responses with non-standard shapes are generally considered as measurement errors and removed in data processing before inversion. However, synthetic and field measurements show that these responses are in fact physically possible and that a simple theoretical explanation of the basic mechanism for their origin can be found by means of superposition of contributions from regions with different sensitivities. The basic mechanism for IP responses with non-standard shapes is investigated by considering the subsurface Cole-Cole parameter sensitivities and time varying IP potential for 2D synthetic models. The time-domain forward response and sensitivities are computed from the finite-element solutions of the frequency-domain differential equation, through a time transformation that takes the current waveform into account. This approach allows for quantitative unbiased estimates of the time-domain responses and sensitivities, which are different from the estimates that can be obtained when using multiple DC forward computations, as often done in the inversion of time-domain IP data. With the increased understanding, previously disregarded IP responses, which can contain valuable information of the subsurface, can be kept for the inversion process and thus contribute to the final parameter distribution.
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| 4. |
- Johansson, Sara, et al.
(author)
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Field Application of Resistivity and Spectral Time Domain IP to Investigate Geoelectrical Signatures of Free-phase PCE
- 2014
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In: 20th European Meeting of Environmental and Engineering Geophysics. - Netherlands : EAGE Publications BV. ; , s. 02-02
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Conference paper (peer-reviewed)abstract
- Soils beneath former dry cleaning sites are often contaminated with the chlorinated hydrocarbons PCE and its degradation products, which are highly toxic and cancerogenic. In this study, resistivity and time domain induced polarization (DCIP) were measured at a former dry cleaning site in order to investigate if the source contamination of PCE could be detected. 80 meter long 2D-profiles were measured across the site with the ABEM Terrameter LS instrument. The DC and full IP decay data were inverted using the 2-D DC/IP inversion algorithm developed by Fiandaca et al. (2013), giving access to the spectral information contained in the IP decays. The inversion parameters DC resistivity (ρ), chargeability (m0), the relaxation time (τ) and the frequency factor (C) were compared to reference data from boreholes. The reference data consisted of geological classification and concentration of free-phase chlorinated hydrocarbons, obtained from soil samples. A high resistive anomaly was observed at a location along the profile where soil sampling have confirmed high concentrations of free-phase chlorinated hydrocarbons. High IP-effects, in terms of the spectral parameters m0 and C, was seen next to this area. It is suggested that the IP-effects arise in a zone next to the free-phase source plume.
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| 5. |
- Johansson, Sara, et al.
(author)
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Influence of non-aqueous phase liquid configuration on induced polarization parameters: Conceptual models applied to a time-domain field case study
- 2015
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In: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851. ; 123, s. 295-309
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Journal article (peer-reviewed)abstract
- Resistivity and induced polarization (IP) measurements on soil contaminated with non-aqueous phase liquids (NAPLs) show a great variety in results in previous research. Several laboratory studies have suggested that the presence of NAPLs in soil samples generally decrease the magnitude of the IP-effect, while others have indicated the opposite. A number of conceptual models have been proposed suggesting that NAPLs can alter the pore space in different ways, e.g. by coating the grain surfaces and thus inhibiting grain polarization, or by changing the pore throat size and thus affecting the membrane polarization mechanism. The main aim of this paper is to review previously published conceptual models and to introduce some new concepts of possible residual NAPL configurations in the pore space. Time domain induced polarization measurements were performed at a NAPL contaminated field site, and the data were inverted using the Constant Phase Angle (CPA) model and the Cole–Cole model respectively. No significant phase anomalies were observed in the source area of the contamination when the CPA inverted profiles were compared with soil sampling results of free-phase contaminant concentrations. However, relatively strong phase and normalized phase anomalies appeared next to the source area, where residual free-phase presence could be expected according to the chemical data. We conclude that depending on the NAPL configuration, different spectral IP responses can be expected. In previous research, the NAPL configurations in different samples or field sites are often unknown, and this may to some extent explain why different results have been achieved by different authors. In our field case, we believe that the NAPL forms a more or less continuous phase in the pore space of the source zone leading to an absence of IP anomalies. The increase in phase and normalized phase angle observed next to the source zone is interpreted as a degradation zone. The ongoing biodegradation may have led to a fractionation of the continuous NAPL in the outer part of the original source zone, leading to residual presence of isolated NAPL droplets in the soil pores. With such NAPL configurations, an increased polarization can be expected according to the electrochemical- and membrane polarization mechanisms. More research is needed to confirm the effects of different NAPL configuration on spectral IP parameters.
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| 6. |
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| 7. |
- Johansson, Sara, et al.
(author)
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Spectral induced polarization of limestones: time domain field data, frequency domain laboratory data and physicochemical rock properties
- 2020
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 220:2, s. 928-950
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Journal article (peer-reviewed)abstract
- With advances in data acquisition and processing methods, spectral inversion of time domain (TD) induced polarization (IP) data is becoming more common. Geological interpretation of inverted spectral parameters, for instance Cole–Cole parameters, often relies on results from systematic laboratory measurements. These are most often carried out with frequency domain (FD) systems on sandstone samples. However, the two different methods of measuring the spectral IP response differ in both measurement technique and scale. One of the main objectives of this study is, thus, to perform a direct comparison of inverted spectral parameters from TD IP field data with FD IP spectra from laboratory measurements. To achieve this, field measurements were carried out before a ∼50-m-long rock core was drilled down along one of the measurement lines. Solid parts of the core were vacuum-sealed in plastic bags to preserve the natural groundwater in the samples, after which the core samples were measured with FD spectral IP in laboratory. The results showed that the inverted Cole–Cole parameters closest to the borehole were comparable to the IP spectra measured at the core samples, despite differences in measurement techniques and scale. The field site chosen for the investigation was a limestone succession spanning over a well-known lithological boundary (the Cretaceous–Palaeogene boundary). Little is known in previous research about varying spectral IP responses in limestones, and an additional objective of this study was, therefore, to investigate possible sources of these variations in the laboratory. The IP spectra were interpreted in light of measured lithological and physicochemical properties. The carbonate texture differed strongly across the Cretaceous–Palaeogene boundary from fine-grained calcareous mudstone (Cretaceous) to more well-lithified and coarse-grained wackestone and packstone (Palaeogene). Both laboratory and field spectral IP results showed that these differences cause a large shift in measured bulk conductivity across the boundary. Furthermore, carbonate mound structures with limestone grains consisting mainly of cylindrical bryozoan fragments could be identified in the inverted Cole–Cole parameters as anomalies with high relaxation times. A general conclusion of this work is that limestones can give rise to a wide range of spectral responses. The carbonate texture and the dominant shape of the fossil grains seem to have important control over the electrical properties of the material. A main conclusion is that the inverted Cole–Cole parameters from the field scale TD IP tomography were comparable to the magnitude and shape of FD IP spectra at low frequencies. This opens up large interpretational possibilities, as the comprehensive knowledge about relationships between lithological properties and IP spectra from laboratory research can be used for field data interpretation.
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| 8. |
- Lévy, Léa, et al.
(author)
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Managing the remediation strategy of contaminated megasites using field-scale calibration of geo-electrical imaging with chemical monitoring
- 2024
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In: Science of the Total Environment. - 0048-9697. ; 920
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Journal article (peer-reviewed)abstract
- Groundwater contamination is a threat to drinking water resources and ecosystems. Remediation by injection of chemical reagents into the aquifer may be preferred to excavation to reduce cost and environmental footprint. Yet, successful remediation requires complete contact between contamination and reagents. Subsurface heterogeneities are often responsible for diffusion into low-permeable zones, which may inhibit this contact. Monitoring the spatial distribution of injected reagents over time is crucial to achieve complete interaction. Source zone contamination at megasites is particularly challenging to remediate and monitor due to the massive scale and mixture of contaminants. Source zone remediation at Kærgård Plantation megasite (Denmark) is monitored here, with a new methodology, using high-resolution cross-borehole electrical resistivity tomography (XB-ERT) imaging calibrated by chemical analyses on groundwater samples. At this site, high levels of toxic non-aqueous phase liquids (NAPL) are targeted by in-situ chemical oxidation using activated persulfate. It may take numerous injection points with extensive injection campaigns to distribute reagents, which requires an understanding of how reagent may transport within the aquifer. A geophysical (XB-ERT) monitoring network of unprecedented size was installed to identify untreated zones and help manage the remediation strategy. The combination of spatially continuous geophysical information with discrete but precise chemical information, allowed detailed monitoring of sulfate distribution, produced during persulfate activation. Untreated zones identified in the first remediation campaign were resolved in the second campaign. The monitoring allowed adjusting the number of injection screens and the injection strategy from one campaign to the next, which resulted in better persulfate distribution and contaminant degradation in the second campaign. Furthermore, geophysical transects repeated over the timespan of a remediation campaign allowed high-resolution time-lapse imaging of reagent transport, which could in the future improve the predictability of transport models, compared to only using on a-priori assumptions of the hydraulic conductivity field.
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| 9. |
- Olsson, Per-Ivar, et al.
(author)
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Doubling the spectrum of time-domain induced polarization by harmonic de-noising, drift correction, spike removal, tapered gating, and data uncertainty estimation
- 2016
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In: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 207:2, s. 774-784
-
Journal article (peer-reviewed)abstract
- The extraction of spectral information in the inversion process of time-domain (TD) induced polarization (IP) data is changing the use of the TDIP method. Data interpretation is evolving from a qualitative description of the subsurface, able only to discriminate the presence of contrasts in chargeability parameters, towards a quantitative analysis of the investigated media, which allows for detailed soil- and rock-type characterization. Two major limitations restrict the extraction of the spectral information of TDIP data in the field: i) the difficulty of acquiring reliable early-time measurements, in the millisecond range and ii) the self-potential drift in the measured potentials distorting the shape of the late time IP responses, in the second range. Recent developments in TDIP acquisition equipment have given access to full waveform recordings of measured potentials and transmitted current, opening a breakthrough for data processing. For measuring at early times, we developed a new method for removing the significant noise from powerlines contained in the data through a model-based approach, localizing the fundamental frequency of the powerline signal in the full-waveform IP recordings. By this, we cancel both the fundamental signal and its harmonics. Furthermore, a novel and efficient processing scheme for identifying and removing spikes TDIP data is developed. The noise cancellation and the de-spiking allow the use of earlier and narrower gates, down to a few milliseconds after the current turn-off. Furthermore, tapered windows are used in the final gating of IP data, allowing the use of wider and overlapping gates for higher noise suppression without signal distortion. For measuring at late times, we have developed an algorithm for removal of the self-potential drift. Usually constant or linear drift-removal algorithms are used, but these algorithms fail in removing the background potentials due to the polarization of the electrodes previously used for current injection. We developed a drift-removal scheme that model the polarization effect and efficiently allows for preserving the shape of the IP responses at late times. Uncertainty estimates are essential in the inversion of IP data. Therefore, in the final step of the data processing, we estimate the data standard deviation based on the data variability within the IP gates and the misfit of the background drift removal Overall, the removal of harmonic noise, spikes, self-potential drift, tapered windowing and the uncertainty estimation allows for doubling the usable range of TDIP data to almost four decades in time (corresponding to four responses in frequency), and will significantly advance the science and the applicability of the IP method.
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| 10. |
- Olsson, Per-Ivar, et al.
(author)
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Doubling the spectrum of time-domain induced polarization: removal of non-linear self-potential drift, harmonic noise and spikes, tapered gating, and uncertainty estimation
- 2016
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In: Proceedings of the 4th international workshop on induced polarization. ; , s. 1-4
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Conference paper (peer-reviewed)abstract
- This paper presents an advanced signal processing schemefor time-domain induced polarization full waveform data.The scheme includes several steps with an improvedinduced polarization (IP) response gating design usingconvolution with tapered windows to suppress highfrequency noise, a logarithmic gate width distribution foroptimizing IP data quality and an estimate of gatinguncertainty. Additional steps include modelling andcancelling of non-linear background drift and harmonicnoise and a technique for efficiently identifying andremoving spikes. The cancelling of non-linear backgrounddrift is based on a Cole-Cole model which effectivelyhandles current induced electrode polarization drift. Themodel-based cancelling of harmonic noise reconstructs theharmonic noise as a sum of harmonic signals with acommon fundamental frequency. After segmentation ofthe signal and determining of noise model parameters foreach segment, a full harmonic noise model is subtracted.Furthermore, the uncertainty of the background driftremoval is estimated which together with the gatinguncertainty estimate and a uniform uncertainty gives atotal, data-driven, error estimate for each IP gate. Theprocessing steps is successfully applied on full field profiledata sets. With the model-based cancelling of harmonicnoise, the first usable IP gate is moved one decade closerto time zero. Furthermore, with a Cole-Cole backgrounddrift model the shape of the response at late times isaccurately retrieved. In total, this processing schemeachieves almost four decades in time and thus doubles theavailable spectral information content of the IP responsescompared to the traditional processing.
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