pyGIMLi: An open-source library for modelling and inversion in geophysics

Induced polarization is a geophysical method looking to image and interpret low-frequency polarization mechanisms occurring in porous media. Below 10 kHz, the quadrature conductivity of metal-free sandy and clayey materials exhibits a distribution of relaxation times, which can be related to the pore size distribution of these porous materials. When the polarization spectra are fitted with a Cole-Cole model, we first observe that the main relaxation time is controlled by the main pore size of the material and that the Cole-Cole exponent c is never much above 0.5, a value corresponding to a Warburg function. The complex conductivity is then obtained through a convolution product between the pore size distribution and such Warburg function. We also provide a way to recover the pore size distribution by performing a deconvolution of measured spectra using the Warburg function. A new dataset of mercury porosimetry and induced polarization data of six siliciclastic materials supports the hypothesis that the Cole-Cole relaxation time is strongly controlled by the pore size, and especially the characteristic pore size corresponding to the peak of the pore size distribution from mercury porosimetry. The distribution of the pore throat sizes of these materials seems fairly well recovered using the Warburg decomposition of the spectral induced polarization spectra but additional data will be needed to confirm this finding.

https://hal.archives-ouvertes.fr/hal-01196405/document

Spectral induced polarization porosimetry

We have compared various induced polarization (IP) models for permeability prediction of the same general form that were all based on two parameters, the first being an electric substitute of effective porosity (the formation factor) and the second being an electric proxy of pore-normalized surface area (the imaginary part of electric conductivity). These models (empirically derived and based on mechanistic formulations) were applied to an extensive database acquired on sandstones and unconsolidated sandy materials. Whereas previous studies on permeability prediction mainly concentrated on either sandstone or unconsolidated sediments, we investigated a database composed of 94 samples including sandstones and unconsolidated material. Most of the samples in the database were saturated with a NaCl solution with an electric conductivity close to 100  mS/m. Samples with a saturating fluid that deviated from this composition were corrected using recently published relationships describing the IP depende...

Permeability prediction based on induced polarization: Insights from measurements on sandstone and unconsolidated samples spanning a wide permeability range

Laboratory measurements of soil samples are necessary to assess the effect of mineralogy, grain size distribution, moisture content, and electrolyte composition on the resistivity spectrum of soil material. Laboratory results are also required for the interpretation of field data. Induced polarization phenomena in glacial soils are poorly understood and so far no convenient laboratory techniques are available for its measurement. Coarse grain size and the need to measure unsaturated samples and to monitor the homogeneity of the sample require a sample holder-electrode construction that differs from those presented in clay mineral studies. This study presents a spectral induced polarization laboratory system that is suitable for measuring fine- and coarse-grained and both saturated and unsaturated soil samples. The noise caused by the electrode-electrolyte interface is studied in detail. It is shown that easy-to-use platinum or acid-free steel potential electrodes are convenient over a broad frequency band ranging from 0.016 Hz up to more than 1000 Hz. The laboratory experiments and comparisons between laboratory and field results also indicate that sampling and sample packing procedures have only a minor influence on the phase spectrum of glacial soils.

Laboratory technique for measurement of spectral induced polarization response of soil samples

A comparative analysis of statistical and machine learning techniques for mapping the spatial distribution of groundwater salinity in a coastal aquifer

. Field and laboratory spectral induced polarization (SIP) measurements are integrated to characterize the hydrogeological conditions at the Schillerslage test site in Germany. The phase images are capable of monitoring thin peat layers within the sandy aquifers. However, the field results show limitations of decreasing resolution with depth. In comparison with the field inversion results, the SIP laboratory measurements show a certain shift in SIP response due to different compaction and sorting of the samples. The SIP data are analyzed to derive an empirical relationship for predicting the hydraulic conductivity (K). In particular, two significant but weak correlations between individual real resistivities (ρ') and relaxation times (τ), based on a Debye decomposition (DD) model, with measured K are found for the upper groundwater aquifer. The maximum relaxation time (τmax) and logarithmically weighted average relaxation time (τlw) show a better relation with K values than the median value τ50. A combined power law relation between individual ρ' and τ with K is developed with an expression of A · (ρ')B · (τlw)C, where A, B and C are determined using a least-squares fit between the measured and predicted K. The suggested approach with the calculated coefficients of the first aquifer is applied for the second. Results show good correlation with the measured K indicating that the derived relationship is superior to single phase angle models as Borner or Slater models.

/pdf/spectral-induced-polarization-measurements-for-predicting-se3un42ubn.pdf

Spectral induced polarization measurements for predicting the hydraulic conductivity in sandy aquifers

Automated vertical electrical sounding (VES) inversion schemes were applied for hydrogeological characterisation at a semi-arid area in the southeastern part of the East Nile Delta, Egypt. The apparent resistivity measurements were integrated with borehole, hydrogeological and structural information for a better description of the subsurface conditions. The final results were presented in the form of cross-sections, maps and 3D diagrams to visualise the geological and hydrogeological situations. The resistivity values derived from the VES data were used to predict the hydraulic conductivity (K) values by using two empirical relations, which depend on the direct relation between K and the aquifer transverse resistance (T) and the power relation between K and the aquifer resistivity (ρ). The estimated values were compared with the observed K values obtained from the pumping tests. The power law relation produces more reliable predicted K values that show a good correlation with the data compiled for the hydraulic gradient of the main Pleistocene aquifer in the East Nile Delta. The results also indicate that the joint use of an automated 1D interpretation algorithm and the power law empirical relationship constitute a valuable tool to characterise the hydrogeophysical properties of aquifers in the study area.

Hydraulic conductivity estimation using direct current (DC) sounding data: a case study in East Nile Delta, Egypt

Flash flood hazard assessments, mitigation measures, and water harvesting efforts in desert environments are often challenged by data scarcity on the basin scale. The present study, using the Wadi Atfeh catchment as a test site, integrates remote sensing datasets with field and geoelectrical measurements to assess flash flood hazards, suggest mitigation measures, and to examine the recharge to the alluvium aquifer. The estimated peak discharge of the 13 March 2020 flood event was 97 m3/h, which exceeded the capacity of the culverts beneath the Eastern Military Highway (64 m3/h), and a new dam was suggested, where 75% of the catchment could be controlled. The monitoring of water infiltration into the alluvium aquifer using time-lapse electrical resistivity measurements along a fixed profile showed a limited connection between the wetted surficial sediments and the water table. Throughflow is probably the main source of recharge to the aquifer rather than vertical infiltration at the basin outlet. The findings suggest further measures to avoid the negative impacts of flash floods at the Wadi Atfeh catchment and similar basins in the Eastern Desert of Egypt. Furthermore, future hydrological studies in desert environments should take into consideration the major role of the throughflow in alluvium aquifer recharge.

Modeling Flash Floods and Induced Recharge into Alluvial Aquifers Using Multi-Temporal Remote Sensing and Electrical Resistivity Imaging

To understand the impact of sea level rise (SLR) and aquifer intensive use would cause to the groundwater level and saltwater intrusion, an integrated relationship between salinity and electrical resistivity of a Quaternary aquifer is established in the northern part of East Nile Delta (El Sharkia area), Egypt. Historical data of groundwater salinity are mapped and compared with the current water samples to better understand the salinity spatial variability. The salinity maps show that salinization has increased sustainably. Additionally, the transition/dynamic zone related to SLR and/or excessive pumping can be addressed. In particular, the surface DC resistivity soundings were carried out to demonstrate the vertical and horizontal salinity distributions in the area. In the course of this study, the 1D model generation using a hybrid genetic algorithm (GA) was applied and tested using borehole information. The constructed geoeletrical cross-sections emphasize and delineate the extension of saltwater intrusion. Cleary, it is found that due to excessive pumping from shallow wells over the last decades, the subsurface resistivity and TDS vertical distributions can change rapidly within a short distance. Additionally, the results show that despite the dominance of brackish and saltwater at the northern part of the area, perched low conductive lenses are observed reflecting a low level of groundwater salinization. To obtain a link between water salinity and aquifer resistivity, an empirical relationship was derived to predict the salinity variations at different depths. In comparison with the measured total dissolved solids (TDS), the predicted salinity map appears realistic. These results demonstrate the important role of the integration between resistivity and salinity measurements for mapping the groundwater salinization with depth, and call for further research to plan and manage the area’s water resources.

Use of salinity and resistivity measurements to study the coastal aquifer salinization in a semi-arid region: a case study in northeast Nile Delta, Egypt

Mohamed Attwa

Papers

Spectral induced polarization measurements for predicting the hydraulic conductivity in sandy aquifers

Hydraulic conductivity estimation using direct current (DC) sounding data: a case study in East Nile Delta, Egypt

Modeling Flash Floods and Induced Recharge into Alluvial Aquifers Using Multi-Temporal Remote Sensing and Electrical Resistivity Imaging

Use of salinity and resistivity measurements to study the coastal aquifer salinization in a semi-arid region: a case study in northeast Nile Delta, Egypt

Evaluation of DC, FDEM and IP resistivity methods for imaging perched saltwater and a shallow channel within coastal tidal flat sediments