RIASSUNTO
Abstract
Electrical resistivity tomography (ERT) is a new technique for providing depth, volumetric, and time dependent imaging of subsurface salt contamination. The technique has broad applications for planning remediation programs associated with co-produced salt water in flare pits, drilling sumps, blowouts, injection wells, etc. New, easily applied cost-effective techniques of low altitude, high resolution photography from kites, balloons, and masts can match visual airborne imaging of vegetation stress with the geoelectrical data sets. A final product of a 3-D rendered electrical image of subsurface salt contamination can easily be overlaid by a high resolution, orthorectified photographic base map-essentially, an easily understood blueprint for follow-up intrusive investigations or excavations. Three case studies are discussed.
Background
Chloride and sulfate contamination, while often considered tbe less ""glamorous"" problems to address than hydrocarbon contamination, frequently carry more severe consequences in Western Canada. This is due to the conservative nature of chlorides and sulfates (they do not sorb or biodegrade), the significant negative impact salts may have on agricultural land in a part of the world that already suffers from dryland salinity, the corrosive effects of salinized soil, and the high cost of remediating salt contaminated soil. Costs of intrusive characterization (drilling, soil sampling, analytical chemistry, etc.) and remediation of salinized soils are directly related to the volume of contaminated soil, and the surface area of contamination.
Fortunately, a number of geophysical techniques are available for mapping, cross-sectionally slicing, and volumetrically imaging subsurface salt contamination. Most recently, a renaissance of the nearly century old technique of one dimensional (1-D) resistivity soundings, as modified into modern electrical imaging techniques, has facilitated volumetric imaging in two (2-D), three (3-D), and even four (4-D) dimensions. 2-D, 3-D, and 4-D electrical resistivity tomography (ERT) surveys are now routinely performed as cost-effective, rapid and non-intrusive means of volumetric imaging subsurface salt contamination associated with flare pits, blowouts, casing failures, drilling sumps, injection wells, pipeline breaks, landfills, tank leaks, tanker spills, and so forth.
In its simplest definition, tomography is any technique that images a specified plane through an object, whether that object is the earth or the human body. Often in geophysics, though, the term ""tomography"" is reserved for imaging of an object located between two boreholes by the reconstruction of that object using multiple projections of energy, whether seismic, radar, electrical, or electromagnetic energy. The term ""imaging"" is often reserved for techniques which reconstruct an object strictly utilizing projections from surface. In this paper, the two terms tomography and imaging will be used interchangeably as the techniques addressed are here applied from surface, but the resulting image is clearly a tomographic reconstruction.
Photodocumentation of salt contaminated sites can be very beneficial during the characterization and remediation phases of investigation. Salt contaminated sites are particularly amenable to low level, high resolution aerial photographic techniques due to changes in vegetation color in both the visible light and infrared spectra. Small format aerial photography from kites, balloons, and masts is particularly attractive due to the low cost of such surveys, the extremely high quality of the photographic products, and the convenience of getting the proper visual images when, where and to the proper scale one desires for a particular project.
Digital photogrammetric techniques of merging high resolution aerial photography with subsurface geophysical data have, in particular, enhanced the utility of the technique. A final product of a 3-D rendered electrical image of subsurface salt contamination can easily be overlaid by a high resolution, orthorectified photographic base map-essentially, an easily understood blueprint for follow-up intrusive investigations or excavations.