The classical approach for evaluating the extent and nature of salt contamination of geologic media typically involves a rapid surface reconnaissance of vegetative stress (or absence), and other visual salt contamination markers to identify the outer boundaries of salt contamination followed by a sampling program to determine the intensity, nature and sometimes the vertical extent of salt contamination. This classical approach provides a simple and reasonably rapid investigative paradigm but suffers from at least two shortcomings: (1) an inability to discern salt contamination that is not currently causing surface impacts on soil or vegetation when the reconnaissance is performed, and (2) lack of a real-time quantitative measure of the degree of saline impact. The electrical properties of geological materials are a function of the physical and mineralogical properties of both geologic solids and the chemistry of their contained pore fluids. The presence of dissolved NaCl greatly increases the electrical conductivity of geologic media. Accordingly, electrical geophysical methods provide an alternative means for rapidly and cost effectively evaluating the lateral and vertical extent of salt contaminated soils and other geologic media. Despite their utility and long history of application, electrical geophysical methods are not necessarily well known or widely applied to characterize areas of salt contamination. This is unfortunate because these methods provide a means to capture a synoptic view of potential salt contamination that includes the presence of salt contamination that may not be directly impacting surface conditions at the time of geophysical survey.
The most commonly applied geophysical methods are frequency domain electromagnetic induction surveys (a/k/a terrain conductivity surveys) and
electrical resistivity profiling (a/k/a electrical resistivity tomography). Terrain conductivity surveys provide a very rapid means of not only finding the lateral boundaries of salt contamination, but also provide data regarding the degree of salt contamination and can help identify the origin of the salt release through imaging electrically conductive infrastructure, such as steel pipelines. Electrical resistivity tomography provides a means of imaging the vertical distribution of salt contaminated soil and groundwater; with suitable acquisition geometry and appropriate geological conditions electrical resistivity tomography can yield information on soil and groundwater electrical properties at depths in excess of 100-m, and if porosity values are known or can be reasonably estimate provides a means for estimating groundwater salinity. It should be remembered that electrical geophysical methods only provide a measure of the electrical properties of soil and groundwater. To determine the presence and magnitude of salt contamination, it is essential to collect samples of soil and groundwater for appropriate analyses. That being said, knowledge of the pattern of soil electrical properties permits much better sample targeting. This presentation reviews actual examples of the application of electrical geophysical methods to characterize the extent and nature of salt contamination surrounding petroleum production operations and illustrates the utility of such work in identifying otherwise cryptic salt contamination.
Primary Author / Conference Presenter:
J. Berton Fisher
Tulsa, Oklahoma, USA