Virginie Leroux

Measuring techniques in induced polarisation imaging

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

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

Measuring induced polarization in the time domain with relatively compact multi-channel multielectrode 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.

Dense resistivity and induced polarization profiling for a landfill restoration project at Härlöv, Southern Sweden

A resistivity and time-domain induced polarization (IP) survey was conducted at a landfill site under restoration at Härlöv 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.

Potential of Geoelectrical Imaging Techniques for Detecting Subsurface Gas Migration in Landfills - An Experiment

We measured eleven regularly spaced resistivity profiles over a ten by ten meter experimental plot where high landfill gas (LFG) emissions had been recorded on the surface. The measurements were repeated seven times during four days which made it possible to trace the development of resistive areas. The LFG emissions at the surface of the landfill were estimated with laser absorption spectroscopy and static chamber measurements at the same time as resistivity surveying. By studying the changes of resistivity with time it was indicated that the gas migration in the waste mass was a relatively fast process, changing within only a few hours. Our presentation will focus on details regarding temporal and spatial changes of measurement.

Constrained Time-lapse Inversion of 3-D Resistivity Surveys Data

Three-dimensional surveys and inversion models are required to accurately resolve structures in areas with very complex geology where 2-D models might suffer from artefacts. 3-D data sets collected at different times are inverted simultaneously using a least-squares methodology that uses roughness filters in both the space and time domains. The spatial roughness filter constrains the model resistivity to vary smoothly in the x, y and z directions. A temporal roughness filter is also applied that minimizes changes in the resistivity between successive temporal inversion models. This method can accurately resolve temporal changes in the resistivity even in the presence of noise. The use of the L1 and L2 norm constraints for the temporal roughness filter are examined using a synthetic model. The L1 norm temporal constraint produces significantly more accurate results when the resistivity changes abruptly with time. A test with field data from a landfill site with methane gas accumulation shows near surface resistivity changes that are probably due to surface temperature variations. The temperature variations cause changes in the gas volume and moisture content in the near surface landfill materials.

Full Wave-form Time-domain IP Data Acquisition

SUMMARY We present and discuss some examples of full wave form resistivity-IP data recorded at one site in Sweden and one in Malaysia. The examples show that highly detailed recordings can be made, and that various types of noise can be clearly distinguished. With such detailed information about the character of the signal and noise it is relatively easy to develop algorithms that can remove the noise without biasing the extracted data. Tests using a pure square wave as measuring signal show that information relating to the chargeability of the ground are as clearly visible as in traditional time-domain IP-data, and it should thus lend itself well to extraction of the spectral IP information. An advantage of using square wave for timedomain SIP over frequency domain SIP is that data acquisition can be much faster, furthermore it would be more time efficient than a traditional time-domain IP signal. The results also show that the transmitted current wave form is almost ideal, whereas recorded output voltage varies to a significant extent. This may be due to, for example electro-chemical processes, and a constant voltage transmitter would not have produced current wave forms with such regular shape.

An Evaluation of the Potential of the Geoelectrical Resistivity Method for Mapping Gas Migration in Landfills

Methane is a powerful greenhouse gas and growing concern regarding global climate changes over the last years has pointed out the need to quantify and control the leaking of methane into the atmosphere. Landfill gas is regarded as one of the major sources for methane migration to the atmosphere. In this study we present research work with the objective to evaluate the use of geoelectrical resistivity to detect gas migration in landfills. Extensive field experiments were conducted at the Filborna landfill site in Helsingborg, Sweden, in August 2008. In general, the resistivity measurements showed results corresponding to results reported from previous investigations in waste. However, also large variations in resistivity were indicated. Relatively high variability and high mean resistivity in the surface-near layers clearly indicate influence on the resistivity in the upper zone of the landfill. The variability and high resistivity may partly be explained by appearance and migration of landfill gas.

Improvement in Time-domain IP Data Quality with Multi-electrode Systems by Separating Current and Potential Cables

Good quality time-domain IP data can be obtained using multi-electrode resistivity-IP equipment and standard electrode cable layouts at sites with favourable electrode contact. High contact resistances often result in low signal levels and capacitive coupling problems making the IP signals drown in noise. We tested an approach for measuring with separated cable spreads for current transmission and potential measurement with an instrument with transmitter, receiver and relay switch housed in the same box. Stainless electrodes and standard non-shielded multi-electrode cables with were used throughout. Data quality assessment was done via pseudosections, IP decay curves and full waveform plots. The results show that it is possible to improve the IP data quality at a site with unfavourable electrode grounding conditions. The results suggest that most of the coupling problems arise in the multi-core electrode cables, and that the problems can be reduced dramatically by separating the cable spreads for transmitting current and measuring potentials. The procedure used is relatively simple and applicable for current field measurements. Inversion of the recorded data results in low residuals and produces models that fit well with the geology at the site, although more detailed information would be needed to fully explain the observed phenomena.