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Linking time domain spectral IP to hydraulic properties for environmental and engineering applications

Periodic Reporting for period 1 - LINKIP (Linking time domain spectral IP to hydraulic properties for environmental and engineering applications)

Reporting period: 2018-01-02 to 2020-01-01

Access to clean drinking water is a necessary requirement for life. With increasing difficulty in securing safe drinking water from surface waters, it is anticipated that the use of groundwater will increase. There are, however, several threats to the quality of this groundwater, such as the migration of pesticides and nitrates from agricultural applications into the water, waste leachates, and industrial contaminants (chemical industry, gasworks, impregnation plants, dry cleaning, etc.). To plan for protection, mapping of both groundwater resources and potential contaminant sources is needed.
The objective of the LINKIP (link induced polarisation) project was the improvement of the geophysical induced polarisation (IP) method to characterise the underground and to provide better understanding of the subsurface and mapping properties which are essential for a long term sustainable management of groundwater resources.
The aim was to evaluate how much spectral content in the direct current IP (DCIP) data can be enhanced using the newest optimised data acquisition hardware in combination with the latest developments in data processing algorithms. A further aim was also to assess the possibility to link information related to hydraulic properties (as e.g. permeability) and pollution status properties to such geoelectrical models of the subsurface. For that, also the link between time domain and frequency domain IP measurements was studied to get knowledge about to what extent these two domains can be made equivalent in practical application.
The outcomes of the LINKIP project show that in theory and in a laboratory environment both IP measurement domains can reveal the same information but are limited by the gathered data quality. One link between both domains can be the transformation to a relaxation time distribution which reveal almost identical results for both domains providing very good data quality. In general, the results from our field measurements using both domain methods, yielded similar findings, but due to different measurement techniques and instrument generations, discrepancies in the phase shift results have shown up and leave room for further investigations. By using the DCIP method at a test site with known hydrogeological properties and by applying the latest processing and inversion algorithm a comparability between the spectral content of the IP method and the permeability could be found. These results suggest that the number of drillings can be reduced by using the IP method to get similar information about (e.g.) the permeability of water-bearing layers in a faster and more cost-efficient manner, supplemented by drilling followed by hydraulic tests in selected spots for verification and calibration. Using IP to monitor biological remediation processes from the surface seems also promising after serval laboratory experiments with E.coli bacteria, even though the signals from the bacteria appear to be small and, so far challenging to measure in the field.
The work performed in the LINKIP project was very diverse. Several long-term laboratory experiments were conducted. So, the influence of E. coli bacteria to a sandy environment was studied with the colleagues from the Applied Microbiology lab at Lund University (Figure 1). These experiments were confirmed by further tests. The results reveal that bacteria have a direct impact on IP measurements, which must be considered when monitoring in-situ remediation processes at contaminated sites.
The comparison of IP laboratory measurements in both domains (time and frequency) at variety of samples were done at the laboratory at St. Petersburg State University, Russia together with the local researchers. The results are in very good agreement between both domains after transforming them into a relaxation time distribution (Figure 2 for a sandstone sample). Differences between both domains are mainly caused by the data measurement quality so, great attention must be paid to it.
Extensive field measurements at eight test sites were carried out to investigate the comparability between time domain and frequency domain in the field and to link the IP data to hydraulic properties. That has been done together with colleagues from France, Austria, and Germany. The results show that similar results can be achieved for both domains. Whereas the resistivity is often almost identical (Figure 3, left side), the phase shift variations are significantly larger and more problematic when the underground provides only small polarisation effects (Figure 3, right side). Furthermore, we could also observe discrepancies in the TD data for different settings which needs to be investigated further and affect an objective comparison between TD and FD in the field.
Much time was spent for the data processing and analysing. For that, the newest processing tools were used, respectively new tools were developed or adapted. By working together with our project partner from the Hydrogeophysics Group at Aarhus University Denmark and by doing a (“short-term” inter alia due to Covid-19) secondment there, the calculation of the hydraulic permeability could be done from the time domain IP results. That could be aligned with field data where the permeability was measured directly in the boreholes. We could observe that the values lie in the same range (Figure 4) and show a very good agreement between the permeability, calculated from our IP results, and the permeability, quantified by slug tests in boreholes.
All the results were disseminated in various ways. They were widely discussed with the colleagues at Lund University, all cooperation partners and scientific board committee members. They were also presented at scientific conferences and workshops and at an own organised Biogeophysics-Seminar with international guests. Companies and authorities could be reached by two workshops in Denmark (Biofilm-Symposium) and Sweden (Grundvattendagarna – groundwater days). Two peer-research papers are submitted and two more are in preparation. We also published two articles at a Project Repository Journal to reach the public, decision makers and stakeholders. By posting a small movie about the E. coli bacteria experiment at twitter we also reached more than 43,500 readers.
The here used approach of comparing both IP domains and to transfer them to a relaxation time distribution show very impressively that in both theory and practice, the same spectral information is available. Also, an extended study about the comparability in the field is new and valuable for the research community as well for the instrument developer and companies as it provides them the opportunity to adapt and modify their research and devices.
Furthermore, our results are of interest for engineering companies and authorities as they enable them to answer hydraulic investigation questions with the help of IP in a faster and cost-efficient manner. To monitor remediation processes at contaminated sites is valuable for authorities and problem owners who are in charge for the cleaning of their sites. The results can be also helpful for water treatment plants owner/ authorities as it might be possible to measure and monitor sand filter facilities.
Comparison between lab IP data, measured in time and frequency domain at a sandstone sample
E. coli bacteria (orange) attached to a grain of sand (grey). The magnification is 1: 4 300.
Comparison between field IP data, measured in time (bottom line) and frequency domain (top line).
Calculation of the comparable permeabilities from time domain IP data and borehole tests.