Spectral Induced Polarization Imaging Research Articles

Spectral induced polarization imaging to investigate an ice-rich mountain permafrost site in Switzerland

Abstract. Spectral induced polarization (SIP) measurements were collected at the Lapires talus slope, a long-term permafrost monitoring site located in the western Swiss Alps, to assess the potential of the frequency dependence (within the frequency range of 0.1–225 Hz) of the electrical polarization response of frozen rocks for an improved permafrost characterization. The aim of our investigation was to (a) find a field protocol that provides SIP imaging data sets less affected by electromagnetic coupling and easy to deploy in rough terrains, (b) cover the spatial extent of the local permafrost distribution, and (c) evaluate the potential of the spectral data to discriminate between different substrates and spatial variations in the volumetric ice content within the talus slope. To qualitatively assess data uncertainty, we analyse the misfit between normal and reciprocal (N&R) measurements collected for all profiles and frequencies. A comparison between different cable setups reveals the lowest N&R misfits for coaxial cables and the possibility of collecting high-quality SIP data in the range between 0.1–75 Hz. We observe an overall smaller spatial extent of the ice-rich permafrost body compared to its assumed distribution from previous studies. Our results further suggest that SIP data help to improve the discrimination between ice-rich permafrost and unfrozen bedrock in ambiguous cases based on their characteristic spectral behaviour, with ice-rich areas showing a stronger polarization towards higher frequencies in agreement with the well-known spectral response of ice.

Investigation of cable effects in spectral induced polarization imaging at the field scale using multicore and coaxial cables

The spectral induced polarization (SIP) method has emerged as a well-suited laboratory technique to characterize hydrogeologic and biogeochemical parameters in soil samples. However, field applications of the SIP imaging method are still rare, which can be attributed to the particular care required to minimize the contamination of the data by electromagnetic coupling. To date, field procedures rely on the use of two different cables separating the current and potential dipoles to improve the quality of the SIP readings, although this increases the efforts in the field and might reduce the depth of investigation or the spatial resolution of the data. To overcome these limitations, we have investigated the use of a single coaxial cable, as an alternative to improve data quality and simplify field procedures. We evaluate SIP imaging data collected with the same measuring device using a coaxial cable and a combination of multicore cables of different lengths and manufacturers. Data sets collected with a single coaxial cable reveal a significantly lower number of outliers and a high spatial consistency between the phase-lag readings, even for measurements collected with a coaxial cable five times longer than the length of the profile. Furthermore, the data collected with coaxial cables reveal an improved quality for deeper measurements (with a lower signal-to-noise ratio) in comparison to data sets collected with separate cables. Our results demonstrate that the use of coaxial cables might permit the collection of SIP readings with high quality and similar field procedures to those used in resistivity surveys.

Retracted: Induction-free acquisition range in spectral time- and frequency-domain induced polarization at field scale

Spectral induced polarization imaging is a valuable method in near-surface hydrogeological and environmental investigations. Many field applications have been presented during the last 20 years, both in frequency and time domain. Despite the electromagnetic inductive effects always occurring in induced polarization measurements, they are usually disregarded in the spectral inversion of the data. Consequently, in order to invert only data with negligible inductive effects, the frequency and time ranges of both frequency-domain and time-domain data have often to be restricted, limiting the higher invertible frequency and the smaller invertible time. Consequently, this study investigates the induction-free acquisition range of both frequency-domain and time-domain acquisitions, computing the electromagnetic inductive effects of homogeneous media, with theoretical and field examples. It is found that it is much easier to get induction-free early times than induction-free high frequencies. For nested arrays it is difficult to measure a significant spectral range in the frequency domain, but also with the dipole-dipole array the inductive effects are not negligible with the typical settings used in field applications.

Delineation of subsurface variability in clay-rich landslides through spectral induced polarization imaging and electromagnetic methods

Although commonly triggered by heavy or long-lasting precipitation events, landslides in clay-rich formations are hardly predictable as their triggering mechanisms are still not fully understood. Hence, detailed information about the internal structure of landslides with high spatial resolution is fundamental for an improved understanding of triggering mechanisms and management. In this context, electrical resistivity tomography (ERT) is a well-established method used for the delineation of lithological interfaces in landslides and changes in water content. However, the quantitative interpretation of the ERT images can be challenging, particularly for clay-rich landslides, considering that the electrical signatures are dominated by the variations in the clay fraction rather than saturation. Moreover, large scale investigations also demand the development of faster surveying techniques. Hence, in this study we discuss the combined investigation of a landslide with low induction number electromagnetic (EMI) mapping and induced polarization (IP) imaging. We investigate the application of such methods to assess subsurface variability across different scales: near surface EMI mapping and their correlation with geomorphological data, whereas IP images are used to investigate the extension at depth from the shallow structures. Measurements are performed at a shallow clay-rich landslide in Lower Austria (Austria), characterized by lithological structures of the Flysch and Gresten Klippen Zone, with both formations known to be highly susceptible to landsliding. The interpretation of the imaging results for data collected along 15 profiles is performed using separately acquired hydrogeological and geotechnical data. We observe a relationship between the electrical properties and geotechnical parameters which permits to delineate areas associated to different weathering stages controlling the groundwater flow. Moreover, spectral induced polarization (SIP) data collected along one transect shows sensitivity to changes in textural composition, such as the fraction of clay minerals. The study shows the suitability of an integrated investigation with EMI mapping and IP imaging in combination with extensive geotechnical data for an improved characterization of subsurface variability and thus, understanding of clay-rich landslides.

Time‐lapse spectral induced polarization imaging of stimulated uranium bioremediation

ABSTRACTTomographic measurements of the spectral induced polarization (SIP) response of aquifer sediments were conducted at the U.S. Department of Energy’s (DOE) Integrated Field Research Challenge site (IFRC) in Rifle, Colorado (USA), where biostimulation research is ongoing with the purpose to immobilize uranium in tailings‐contaminated groundwater. The aims of the SIP surveys were to (a) collect data over a sufficiently broad bandwidth so as to determine the characteristic frequency of the spectral response (at which the strongest polarization response takes places), (b) investigate the distribution of spectral parameters (e.g., characteristic time constant) in an imaging framework and (c) evaluate the potential of these images to delineate changes in the hydraulic properties of the aquifer. Careful field procedures provided high‐quality SIP data from 0.060–256 Hz for three different periods during the remediation experiment. Data quality was evaluated by means of analysis of the discrepancy between normal and reciprocal measurements. A Cole‐Cole model was fitted to pixel values extracted from the inverted images in order to assess changes in the SIP response – particularly in time constant () and chargeability (m) – due to processes accompanying the stimulation of subsurface microbial activity. A significant increase in both and m was observed after halting acetate injection, consistent with the accumulation of semi‐conductive minerals (e.g., FeS) during biostimulation and the post‐injection rebound in aqueous Fe(II). The reliability of the imaged spectral parameters was assessed by means of a numerical study.

Delineation of subsurface hydrocarbon contamination at a former hydrogenation plant using spectral induced polarization imaging

Broadband spectral induced polarization (SIP) measurements were conducted at a former hydrogenation plant in Zeitz (NE Germany) to investigate the potential of SIP imaging to delineate areas with different BTEX (benzene, toluene, ethylbenzene, and xylene) concentrations. Conductivity images reveal a poor correlation with the distribution of contaminants; whereas phase images exhibit two main anomalies: low phase shift values (<5 mrad) for locations with high BTEX concentrations, including the occurrence of free-phase product (BTEX concentrations >1.7g/l), and higher phase values for lower BTEX concentrations. Moreover, the spectral response of the areas with high BTEX concentration and free-phase products reveals a flattened spectrum in the low frequencies (<40Hz), while areas with lower BTEX concentrations exhibit a response characterized by a frequency peak. The SIP response was modelled using a Debye decomposition to compute images of the median relaxation-time. Consistent with laboratory studies, we observed an increase in the relaxation-time associated with an increase in BTEX concentrations. Measurements were also collected in the time domain (TDIP), revealing imaging results consistent with those obtained for frequency domain (SIP) measurements. Results presented here demonstrate the potential of the SIP imaging method to discriminate source and plume of dissolved contaminants at BTEX contaminated sites.

Data error quantification in spectral induced polarization imaging

Induced polarization (IP) imaging is being increasingly used in near-surface geophysical studies, particularly for hydrogeologic and environmental applications. However, the analysis of IP data error has received little attention, even though the importance of an adequate error parameterization has been demonstrated for electrical resistivity imaging. Based on the analysis of data sets measured in the frequency range from 1 Hz to 1 kHz, we proposed a model for the quantification of phase data errors in IP measurements. The analyzed data sets were collected on an experimental tank containing targets of different polarizability. Our study is based on the common practice that the discrepancy of measurements taken in normal and reciprocal configuration can be considered as a measure of data error. Statistical analysis of the discrepancies between normal and reciprocal measurements revealed that the phase error decreases with increasing resistance (i.e., signal strength). We proposed an inverse power-law model to quantify the phase error as a function of the measured resistances. We found that the adequate implementation of the proposed error model in an inversion scheme leads to improved IP imaging results in laboratory experiments. Application to a data set collected at the field-scale also demonstrated the superiority of the new model over previous assumptions.

Inversion of 2D spectral induced polarization imaging data

ABSTRACTLaboratory measurements of various materials suggest that more information can be obtained by measuring the in‐phase and out‐of‐phase potentials at a number of frequencies. One common model used to describe the variation of the electrical properties with frequency is the Cole‐Cole model. Apart from the DC resistivity (ρ) and chargeability (m) parameters used in conventional induced‐polarization (IP) surveys, the Cole‐Cole model has two additional parameters, i.e. the time (τ) and relaxation (c) constants. Much research has been conducted on the use of the additional Cole‐Cole parameters to distinguish between different IP sources. Here, we propose a modified inversion method to recover the Cole‐Cole parameters from a 2D spectral IP (SIP) survey. In this method, an approximate inversion method is initially used to construct a non‐homogeneous starting model for the resistivity and chargeability values. The 2D model consists of a number of rectangular cells with constant resistivity (ρ), chargeability (m), time (τ) and relaxation (c) constant values in each cell. A regularized least‐squares optimization method is then used to recover the time and relaxation constant parameters as well as to refine the chargeability values in the 2D model. We present results from tests carried out with the proposed method for a synthetic data set as well as from a laboratory tank experiment.