Over the last 15 years significant advancements in induced polarization (IP) research have taken place, particularly with respect to spectral IP (SIP), concerning the understanding of the mechanisms of the IP phenomenon, the conduction of accurate and broadband laboratory measurements, the modelling and inversion of IP data for imaging purposes, and the increasing application of the method in near-surface investigations. We here summarized the current state of the science of the SIP method for near-surface applications and describe which aspects still represent open issues and should be the focus of future research efforts.Significant progress has been made over the last decade in the understanding of the microscopic mechanisms of IP; however, integrated mechanistic models involving the different possible polarization processes at the grain/pore scale are still lacking. A prerequisite for the advances in the mechanistic understanding of IP was the development of improved laboratory instrumentation, which has led to a continuously growing database of SIP measurements on various soil and rock samples. We summarize the experience of numerous experimental studies by formulating key recommendations for reliable SIP laboratory measurements. To make use of the established theoretical and empirical relationships between SIP characteristics and target petrophysical properties at the field scale, sophisticated forward modelling and inversion algorithms are needed. Considerable progress has been made also in this field, in particular with the development of complex resistivity algorithms allowing the modelling and inversion of IP data in the frequency domain. The ultimate goal for the future are algorithms and codes for the integral inversion of 3-D, time-3 lapse and multi-frequency IP data, which defines a 5-D inversion problem involving the dimensions space (for imaging), time (for monitoring) and frequency (for spectroscopy). We also offer guidelines for reliable and accurate measurements of IP spectra, which are essential for improved understanding of IP mechanisms and their links to physical, chemical and biological properties of interest. We believe that the SIP method offers potential for subsurface structure and process characterization, in particular in hydrogeophysical and biogeophysical studies.

Multi-electrode geoelectrical imaging has become very popular and is used for many different purposes. For some of these, the inclusion of IP data would be desirable as it would allow the interpreter to distinguish between, e.g. sand formations with saltwater infiltration and clay formations or help delineate landfills. However, present-day IP measuring techniques require the use of nonpolarisable potential electrodes and special wire layout and are thus cumbersome and expensive. In this paper, we suggest making IP measurements with multi-electrode cables and just one set of steel electrodes. The polarisation potentials on the potential electrodes are corrected for by subtracting the polarisation potential measured when no primary current and no IP signal are present. Test measurements indicate that the polarisation potentials vary slowly and that the correction procedure is feasible. At two sites in southern Sweden, we have compared measurements with only stainless steel electrodes and measurements with both stainless steel and Pb -PbCl nonpolarisable electrodes using one or two sets of multicore cables, respectively. Almost no difference between the two data sets was observed. At one site, the charge-up effect on the potential electrodes was not important, while at the other site, the correction procedure was crucial. Though only two sites have been studied so far, it seems that time-domain IP imaging measurements can be taken with only steel electrodes and ordinary multicore cables. Coupling in the multicore cables has not presented any problems at the investigated sites where grounding resistances were moderate, making the coupling effect small. High grounding resistance sites have not yet been investigated. D 2002 Published by Elsevier Science B.V.

Measuring induced polarization in the time domain with relatively compact multi‐channel multi‐electrode systems is attractive because of the simplicity of the procedure and thus its efficiency in the field. However the use of this technique is sometimes discouraged by the bad quality of the measurements in cases of high electrode contact resistances that can render data interpretation infeasible or at least unreliable. It is proposed that capacitive coupling in the multi‐core electrode cables has a significant role in creating this problem. In such cases separation of current and potential circuits by using separate multi‐conductor cable spreads can yield significant improvement in data quality. The procedure is relatively simple and can be implemented with common resistivity and time‐domain IP equipment. We show here three field examples from Southern Sweden, all measured as 2D electrical imaging sections. The first one is an example where the use of a single cable spread is sufficient thanks to moderate electrode contact resistance and high signal levels. The following two examples are from sites where induced polarization measurements could not yield consistent results using only a single multi‐conductor cable spread. Useful results were subsequently obtained by using separate cable spreads. The first example is a 280 m long line measured over an old covered municipal waste deposit where the waste body stands out as a zone of high chargeability. The second example is a 120 m line measured on a sandy glaciofluvial structure that is host to an aquifer of regional importance. The improvement led to discrimination between materials of different grain sizes, with potential bearing for understanding the aquifer. The third example is a 300–400 m line measured across an esker lying on clay till. The improvement led to a clear visualization of the esker and to the identification of a possible fault in the underlying gneissic bedrock. In all cases pseudosections and examples of chargeability decay curves are shown and discussed as tools for assessing data quality. Inversion results are shown together with background geological information and it is concluded that they are in good agreement.

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A resistivity and time-domain induced polarization (IP) survey was conducted at a landfill site under restoration at Här-löv in Southern Sweden. The covering of the landfill had begun some years ago, without keeping precise records of the work done, as is usual in such procedures. The survey was conducted in two steps, on two adjacent areas. First, a number of geoelectrical sections were made on a partly covered area that had been investigated earlier by auger drilling, in order to assist restoration. Then, a second area that should have received its final cover was imaged, and some defects in the cover could be detected and repaired. The resistivity and time-domain IP results were consistent with the results of the geotechnical drillings, and they enabled quasi-continuous mapping along the profiles. Three-dimensional visualization showed the overall consistency of the two-dimensional lines, and helped to generate a global view of the site. In spite of some ambiguities, cover and waste could be distinguished in most cases. In particular, fine-grained cover materials could be clearly distinguished from other cover materials.