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The signal level and shape of induced polarization responses are significantly affected by the current pulse duration and waveform. If not accounted for, this data dependency on the current will propagate trough the inversion to results rendering unquantifiable subsurface models. While this problem has been addressed in full-response induced polarization modelling, questions remain as to how to accurately retrieve quantitative induced polarization inversion models from the types of apparent integral chargeability data often used in data interpretation. Although several methodologies have been proposed for handling and inverting apparent resistivity and integral chargeability, these cannot compensate for the data dependency on the current waveform and pulse duration. This paper presents a novel inversion method for such data. The method considers current waveform and receiver transfer functions for retrieving quantitative IP models unbiased by transmitter waveform. The method uses the constant phase angle model, expressed in terms of the medium resistivity and phase. Specifically, four field data sets for the same profile but with different 100 per cent duty cycle pulse durations (4, 2, 1 and 0.5 s) serve as examples of data sets giving models dependant on current waveform when inverted with standard approaches. The novel inversion method presented here gives quantifiable models independent on the current waveform and pulse duration. These results resemble models retrieved with existing, full-response induced polarization inversions. The results still contain some degree of uncertainty in relation to underlying assumptions and parametrizations. Managing this source of uncertainty is considered in terms of full-response induced polarization inversions with constant phase angle and maximum phase angle inversions.

There is an increasing demand for trustworthy engineering geological conceptual models in urban areas due to an increasing trend in the underground infrastructure construction. Good-quality site investigations can reduce the risk of encountering unexpected geological conditions during construction. Geoelectrical measurements can be used as a tool for providing an overview of the site conditions and serve as a base for planning a geotechnical drilling program and for integration of the results. Geophysical surveys in urban environments may encounter problems due to strict logistical constraints and may be severely affected by electric and electromagnetic noise. Careful processing of the data is necessary to obtain a reliable estimation of the electrical properties of the ground, both electrical resistivity and chargeability. A large three-dimensional dataset was acquired in the suburban area of Stockholm (Sweden), with the aim of investigating a weak zone in the crystalline bedrock, which had been pointed out by prior geological and geotechnical surveys. Full waveforms of potential dipoles were recorded and processed for removing harmonic noise and background drift. Moreover, a statistical algorithm for handling the quality of the full-waveform shapes has been proposed. The goodness-of-fit test identifies full waveforms with noise that derives from direct current injections, caused by grounding spots of the adjacent metro line. The processed dataset is inverted for electrical resistivity and integral chargeability. The results image a large three-dimensional volume of the underground. The inverted distribution of geophysical quantities marks out the presence of a wide zone of weak rock, which was not identified by geotechnical probing in the site investigation but documented during the construction phase. Such zones can potentially cause severe problems during the construction of underground infrastructure.

Electrical resistivity tomography has potential as a complementary long-term monitoring method in embankment dams; however, the 3D character of the geometry including the shape of the embankment, its internal zoned construction and the reservoir water make interpretation challenging. To tackle this problem, a qualified inversion model considering the 3D environment is necessary. In this paper, prior information about the resistivity of different parts of a test embankment dam was used as constraints in order to increase the capability of defect detection in a complex 3D context. Five small defects were incorporated into the core of the model. Laboratory measurements were made on samples of the materials intended to be used for the construction of a test embankment dam, and resistivity values provided from the laboratory measurements were used in the forward modelling. A measurement sequence of around 8000 synthetic data points using extended gradient, crossline bipole-bipole and corner arrays between horizontal-horizontal, vertical-vertical, and vertical-horizontal lines were modelled and inverted all at once. The structural constraints were applied to increase the accuracy of inversion, using the L1 and L2 methods. Different mesh qualities with different boundaries for each region and 3D geometric factor calculation were applied for the inversion to evaluate the effects of region control incorporated in the inversion process. The results showed that L1 and L2 norm inversions combined with region control can determine the location of very small defects and finding the defects located near each other, respectively. Removing the region control from the inversion caused unrealistic resistivity prediction for some regions and the inability to discover the dam defects. Therefore, the proposed methodology can decrease non-uniqueness in the inversion and make time-lapse ERT a valuable monitoring tool that complements other instrumentation techniques and based on these results it was concluded promising to continue with the construction of the test dam using the same type of defects and electrode set-up, for verification under field-conditions.

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. (Less)

Teknisk geologi vid Lunds tekniska högskola har i ett nyligen slutfört projekt, finansierat av Stiftelsen Bergteknisk Forskning (BeFo), undersökt om geofysiska mätningar med DCIP (kombinerade resistivitets- och IP-mätningar) kan ge bättre information om bergmassan i samband med planeringen av tunnlar och andra bergarbeten samt berguttag i bergtäkter. Resultaten bekräftar att DCIP i den undersökta miljön kan användas för att indikera lervittrade zoner, svaghetsstrukturer och uppkrossade zoner. Detta visar på att det går att underlätta och förbättra prognosarbetet, genom att ännu en möjlighet ges att bedöma bergmassans kvalitet innan byggfasen inleds.

Ground improvement with lime-cement pillars is becoming increasingly common in the Nordic countries for exploitation of areas with poor stability. However, there is no non-destructive method for quality control of the ground improvement. Significant changes in the electrical properties after mixing of the binders make electrical resistivity tomography (ERT) a potential method. In connection with lime-cement pillar trials for the Vastlanken project in Gothenburg, Sweden, a series of different single borehole ERT measurements were performed. Three cases are compared in this paper: untreated ground, treated uncured ground and treated cured ground. The raw data pseudosections show a significant general drop in resistivity between the untreated and treated uncured data sets, while the curing process increase the resistivity significantly close to the borehole. Full 3D inversions have been carried out for all three cases. In model space the cured pillar is still causing a clear increase in resistivity around the borehole, while the decrease between the untreated and uncured case is less obvious than in data space. With the large contrast between the untreated and the treated uncured in data space it was expected to be visible in model space, improved inversion methods and settings could help resolve this. (Less)

Combined resistivity and time-domain direct current induced polarization (DCIP) measurements are traditionally carried out with a 50\% duty cycle current waveform, taking the resistivity measurements during the on-time and the IP measurements during the off-time. One drawback with this method is that only half of the acquisition time is available for resistivity and IP measurements, respectively. In this paper, this limitation is solved by using a current injection with 100\% duty cycle and also taking the IP measurements in the on-time. With numerical modelling of current waveforms with 50\% and 100\% duty cycles we show that the waveforms have comparable sensitivity for the spectral Cole–Cole parameters and that signal level is increased up to a factor of 2 if the 100\% duty cycle waveform is used. The inversion of field data acquired with both waveforms confirms the modelling results and shows that it is possible to retrieve similar inversion models with either of the waveforms when inverting for the spectral Cole–Cole parameters with the waveform of the injected current included in the forward computations. Consequently, our results show that on-time measurements of IP can reduce the acquisition time by up to 50\% and increase the signal-to-noise ratio by up to 100\% almost without information loss. Our findings can contribute and have a large impact for DCIP surveys in general and especially for surveys where time and reliable data quality are important factors. Specifically, the findings are of value for DCIP surveys conducted in urban areas where anthropogenic noise is an issue and the heterogeneous subsurface demands time-consuming 3D acquisitions. (Less)

The present work is an example of a Direct-Current resistivity and time-domain Induced Polarization (DCIP) survey, carried out in an urban environment. The main goal of the survey is to map a weak zone in the bedrock, through which a tunnel will be constructed. Geophysical measurements in urban environments can be very complicated due to the restrictions of available space and the high levels of noise. A way to deal with the latter is careful processing of the recorded waveforms, if they are sampled at a reasonably high frequency (sampling >= 1 KHz). A deeper understanding of raw data leads to a more accurate use of the measured quantities, improving the reliability of electrical resistivity and IP estimations.

Characterization of varying bedrock properties is a common need in various contexts, ranging from large infrastructure pre-investigations to environmental protection. A direct current resistivity and time domain induced polarization (IP) survey aiming to characterize properties of a Cretaceous limestone was carried out in the Kristianstad basin, Sweden. The time domain IP data was processed with a recently developed method in order to suppress noise from the challenging urban setting in the survey area. The processing also enabled extraction of early decay times resulting in broader spectra of the time decays and inversion for Cole–Cole parameters. The aims of this study is to investigate if large-scale geoelectrical variations as well as small-scale structural and compositional variations exist within the Kristianstad limestone, and to evaluate the usefulness of Cole–Cole inverted IP data in early time ranges for bedrock characterization. The inverted sections showed variations within the limestone that could be caused by variations in texture and composition. Samples from a deep drilling in the Kristianstad basin were investigated with scanning electron microscopy and energy dispersive X-ray spectroscopy, and the results showed that varying amounts of pyrite, glauconite and clay matrix were present at different levels in the limestone. The local high IP anomalies in the limestone could be caused by these minerals otherwise the IP responses were generally weak. There were also differences in the texture of the limestone at different levels, governed by fossil shapes and composition, proportions of calcareous cement and matrix as well as amount of silicate grains. Textural variations may have implications on the variation in Cole–Cole relaxation time and frequency factor. However, more research is needed in order to directly connect microgeometrical properties in limestone to spectral IP responses. The results from this study show that it is possible to recover useable spectral information from early decay times. We also show that under certain conditions (e.g. relatively short relaxation times in the subsurface), it is possible to extract spectral information from time domain IP data measured with on-off times as short as 1 s.

The application of geophysics may assist in solving environmental, geotechnical and exploration problems. One commonly used method in such applications is electrical resistivity tomography (ERT). In order to interpret the results, inversion is needed to create models estimating distribution of resistivity in the ground. Several inversion programs are available for geophysicists to use. It is therefore important to investigate if different software's give similar results for a given data set. In this paper, three different inversion software's have been compared through inversion of the forward response of the same synthetic model. The programs used are Res2Dinv, Aarhusinv and BERT. Two synthetic forward models have been used. As expected, Res2Dinv generally tends to exaggerate structures vertically and Aarhusinv to exaggerate them vertically. BERT seems to be somewhere in between, but does not seem to resolve sharp features in the synthetic forward model, resulting in rounded anomalies. A square-like feature close to the ground surface in the forward model was well resolved by all inversion software. However, it was not possible to resolve inclined blocks in any of the inversions. Possible explanations might be poor data coverage at deeper depths in the model and model discretization that cannot resolve inclined geometries. (Less)

A DCIP survey, i.e. a geoelectrical survey with combinedmeasurement of DC resistivity and induced polarization(IP), was conducted in southern Sweden. The purposewas to identify soil depth and bedrock structures andvariations in rock quality.Data were acquired data with 100% waveform fromwhich IP of good quality data were extracted. Theinverted model sections show a wide variation in theelectrical properties of the bedrock that are expected torelate to variation in rock quality. The results, togetherwith results of other geophysical methods, form the basisfor a drilling program in order to identify variations inquality rock of importance to the construction of anunderground facility.

This paper presents an advanced signal processing scheme for time-domain induced polarization full waveform data. The scheme includes several steps with an improved induced polarization (IP) response gating design using convolution with tapered windows to suppress high frequency noise, a logarithmic gate width distribution for optimizing IP data quality and an estimate of gating uncertainty. Additional steps include modelling and cancelling of non-linear background drift and harmonic noise and a technique for efficiently identifying and removing spikes. The cancelling of non-linear background drift is based on a Cole-Cole model which effectively handles current induced electrode polarization drift. Themodel-based cancelling of harmonic noise reconstructs the harmonic noise as a sum of harmonic signals with a common fundamental frequency. After segmentation of the signal and determining of noise model parameters foreach segment, a full harmonic noise model is subtracted. Furthermore, the uncertainty of the background drift removal is estimated which together with the gating uncertainty estimate and a uniform uncertainty gives a total, data-driven, error estimate for each IP gate. The processing steps is successfully applied on full field profile data sets. With the model-based cancelling of harmonic noise, the first usable IP gate is moved one decade closer to time zero. Furthermore, with a Cole-Cole background drift model the shape of the response at late times is accurately retrieved. In total, this processing scheme achieves almost four decades in time and thus doubles the available spectral information content of the IP responses compared to the traditional processing.

A DCIP survey with different timing settings, electrode arrays, cable spreads and waveforms (50% and 100% duty cycle waveform) were carried out over the Liikavaara deposit in northern Sweden. All the measured data were inverted using AarhusInv. Measured data with 100% duty cycle waveform, which is much faster than the conventional 50% duty cycle waveform, present significant improvements in the quality of IP-data. It was also attempted to increase the data quality by separating current and potential cables. Tau and C parameters from time domain IP were compared in different time bases and spreads. These two parameters appear to be sensitive to the duration of current injections in which; decreasing the time base causes diminished anomalies in Tau and partly in C. Although measurement with separated spreads improved the data quality, it did not have any significant effect on these two parameters. Moreover correlation of sulphide content matched well with the inverted models, which confirms the applicability of the DCIP method in mineral exploration.

The duration of time domain (TD) induced polarization (IP) current injections has significant impact on the acquired IP data as well as on the inversion models, if the standard evaluation procedure is followed. However, it is still possible to retrieve similar inversion models if the waveform of the injected current and the IP response waveform are included in the inversion. The on-time also generally affects the signal-to-noise ratio (SNR) where an increased on-time gives higher SNR for the IP data.

An investigation of geological conditions is always a key point for planning infrastructure constructions. Bedrock surface and rock quality must be estimated carefully in the designing process of infrastructures. A large direct-current resistivity and time-domain induced-polarization survey has been performed in Dalby, Lund Municipality, southern Sweden, with the aim of mapping lithological variations in bedrock. The geology at the site is characterised by Precambrian granitic gneisses and amphibolites, which are intensely deformed, fractured, and partly weathered. In addition, there are northwest-trending Permian dolerite dykes that are less deformed. Four 2D direct-current resistivity and time-domain induced-polarization profiles of about 1-km length have been carefully pre-processed to retrieve time-domain induced polarization responses and inverted to obtain the direct-current resistivity distribution of the subsoil and the phase of the complex conductivity using a constant-phase angle model. The joint interpretation of electrical resistivity and induced-polarization models leads to a better understanding of complex three-dimensional subsoil geometries. The results have been validated by lithological descriptions from several drillings. In addition, direct-current resistivity and time-domain induced-polarization logging has been carried out in two different boreholes, showing a good match with the results of the surface direct-current resistivity and time-domain induced-polarization profiles. The direct-current resistivity and time-domain induced-polarization methodology proved to be a suitable technique for extensively mapping weathered zones with poor geotechnical characteristics and tectonic structures, which can lead to severe problems for infrastructure construction and/or constitute risk zones for aquifer contamination.

Resistivity and time domain IP measurements using a 100% duty cycle square waveform current injection signal, and measuring the IP responses during current transmission, was modelled numerically and tested in the field against a traditional IP current injection with a 50% duty cycle waveform and measuring during the current off-time. The results show that the approach is practically applicable and can reduce the measuring time substantially as well as increase the signal-to-noise ratio by measuring the IP-response during the current on-time.

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.

A geophysical site investigation using ERT and refraction seismic was done to detect the bedrock interface and possible weakness zones. For the purpose of a more realistic subsurface model a joint inversion was done.

Electrical resistivity tomography (ERT) is a commonly used geophysical method, which can conveniently be combined with measuring time-domain induced polarization. The combined measurement can be termed DCIP, and can be employed in a wide range of situations such as pre-investigations for construction, or contaminant monitoring to name a few. An integral part of a DCIP survey is the software used for processing and interpretation. There exists many such software packages. We compare the ubiquitous, commercial software, Res2DInv and two other common alternatives; the semi-academic AarhusInv and the academic, open source, BERT/GIMLi. The comparison is done by simulating measurements of resistivity and IP with a multiple gradient protocol over a model that represents a waste pond setting. The results show that there are quite a few similarities between the different softwares, but also some notable differences that one should be aware of during interpretation. This work is the first step towards an exhaustive comparison.

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.