Estimation of Soil Superficial Conductivity in a Zone of Mature Oil Contamination Using DC Resistivity

Abstract

At several SAGEEP conferences we presented resistivity technology for oil contamination study. Soil resistivity and groundwater salinity measurements were recalculated into petrophysical parameters: clay content, porosity and cation exchange capacity. For uncontaminated sandy-clay soil these parameters are close to real ones. For contaminated soil these parameters are apparent ones, but can be used as good contamination indicators. The real cause of petrophysical anomalies is an increase of superficial conductivity greater than electrolytic conductivity change in pore space of contaminated soil, as it was found by Abdel Aal et al. (2004) with spectral IP method. We suppose that DC resistivity also can give information about superficial conductivity, which is the main cause of resistivity and petrophysical anomalies at contaminated sites. Superficial conductivity for soil is estimated on clay content (calculated on soil resistivity), groundwater salinity and clay conductivity. Clay conductivity depends on pore water salinity and cation exchange capacity of clay. Anomalous clay content reflects an increase of internal surface area whereas anomalous cation exchange capacity reflects an increase of surface charges at the mineral grainelectrolyte interface. Practical examples of superficial conductivity estimation at oil contaminated sites are demonstrated and compared with resistivity and petrophysical anomalies.

Imageamento Elétrico 3D em área contaminada por hidrocarbonetono polo industrial de Cubatão - SP

“…Os resultados obtidos corroboram o modelo geoelétrico condutivo de pluma de LNAPLs (light non-aqueous phase liquids ) desenvolvido por Sauck (2000), sendo apoiado por diversos estudos em laboratório e em campo (Atekwana et al, 2000(Atekwana et al, , 2001(Atekwana et al, , 2004a(Atekwana et al, , 2004bCassidy et al, 2001;Legall, 2002;Werkema Jr, 2002;Abdel Aal et al, 2003, 2006Shevnin et al, 2003Shevnin et al, , 2006Werkema Jr et al, 2003, 2006Davis et al, 2006), tendo sido possível verificar correlação entre as anomalias geofísicas condutivas e asáreas impactadas por hidrocarboneto.…”

Petrophysical Analysis of Resistivity Data

“…In 2004 geoelectrical study was made in dry period and groundwater level (GWL) was at the depth 2.3 m. Humidity in vadose zone was 10% (sand with no more than 6% of clay). Contamination was determined on resistivity values and on petrophysical maps including superficial conductivity (or its inverse value -RSC) as the most evident parameter of contamination having highest resolution in this case (Shevnin et al, 2006b). Superficial conductivity is estimated on clay content, groundwater salinity and clay conductivity.…”