Improvement in time‐domain induced polarization data quality with multi‐electrode systems by separating current and potential cables

Abstract

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.

Data Quality Quantification for Time Domain IP Data Acquired along a Planned Tunnel near Oslo, Norway

“…This agrees with results reported by Dahlin and Leroux (2012) for sites with favourable conditions. If the data is to be used for spectral time domain IP inversion in order to recover the ColeCole parameters (Gianluca et al 2012) the additional effort of measuring with separated cable spreads is likely to be well motivated for improving the quality of the models.…”

“…This is caused by larger standard deviation and more outliers in the single cable spread data. Considering the large electrode spread and the varying and partly rather high contact resistances the data quality for the single cable spread is surprisingly good compared to previous experience (Dahlin and Leroux 2012). The data quality of the IP data could probably be further improved by additional stacking of the data, and possibly advanced noise removal techniques.…”

“…Electromagnetic coupling in the multi-core electrode cables can have a significant role in creating this problem (Nielsen 2006). In such cases separation of current and potential circuits by using separate multi-conductor cable spreads can yield significant improvement in data quality (Dahlin and Leroux 2012). The procedure is relatively simple and can be implemented with common resistivity and time-domain IP equipment.…”

Investigations of a Cretaceous limestone with spectral induced polarization and scanning electron microscopy

“…The signal processing scheme successfully reduced periodic noise and allowed retrieval of data from early decay times. Use of early decay times increases the risk of coupling effects in the data superimposed on the information arising from the geological target (Dahlin & Leroux 2012). However, many of the decays appeared to be unaffected by such effects and all early decay times that were suspected to contain coupling effects were removed from the data set during the processing.…”

“…A separated cable layout for injecting current and measuring potentials was used in order to reduce capacitive coupling effects. In this layout, one multiconductor cable is used for the current circuit and another for the potential circuit to avoid current leakage between the two (Dahlin & Leroux 2012). Acid grade stainless steel electrodes were installed with an in-line spacing of 2.5 m and low electrode contact resistances were assured via Focus One electrode tests (see Ingeman-Nielsen et al 2016).…”