Aestus GeoTrax Survey™: Proprietary Electrical Resistivity Imaging
Historically, environmental consultants, geophysicists, and academics that have tried using conventional electrical resistivity imaging (ERI) technology and/or other geophysical methods to image contaminant plumes have been largely dissatisfied with the results. Our early experiments with standard electrical resistivity imaging and other technologies also yielded unsatisfactory results. The desire to develop a method for solving these complex environmental site problems was the genesis of the founding of Aestus in 2001.
Aestus GeoTrax Survey™ Proprietary Electrical Resistivity Imaging Technology Overview
Aestus GeoTrax Survey™ uses technology based in part on conventional electrical resistivity imaging techniques that were developed decades ago. However, we have worked with researchers at Oklahoma State University (OSU) to vastly improve this technology and make it useful for the environmental industry. Aestus is currently the sole worldwide licensee of trade secret intellectual property from OSU that uses a proprietary form of electrical resistivity imaging to provide higher data quality and a more robust and accurate subsurface image relative to standard methods.
This intellectual property allows Aestus to successfully scan the subsurface for anomalies of interest (environmental contamination, preferential flowpaths, voids, etc.) at sites on which other geophysical survey technologies such as ground penetrating radar, conventional electrical resistivity imaging techniques, and electromagnetic surveys have failed to achieve the project objectives.
Aestus GeoTrax Survey™ has proven to be an effective tool for detecting potential subsurface anomalies, including those associated with dense and light non-aqueous phase liquids (DNAPLs/LNAPLs) and related dissolved phase contamination.
Our technology has also been used successfully for a number of other applications including mapping hydrogeology and locating other types of subsurface environmental impacts, including leaking pipelines, buried tanks, landfill/burial pit boundaries, subsurface voids/caves/tunnels, and the presence or absence of contaminant transport channels or preferential migration pathways.
As with any technology, Aestus GeoTrax Survey™ has some limitations. However, compared to most geophysical techniques, GeoTrax Survey™ is fairly bullet proof and is typically NOT affected negatively by the presence of:
- Above ground metallic objects such as vehicles, fences, power lines, scrap metal piles, etc.
- Below ground metallic objects that are discretely located and not extensive in nature.
- Traffic/vibrations, which can be a problem for seismic surveys.
- Varying geology: GeoTrax Survey™ has performed well at all geologies attempted thus far and does not have the attenuation issue experienced by ground penetrating radar or standard ERI when clay bodies are present, for instance.
The issues to be aware of when using Aestus GeoTrax Survey™ include:
We advise our clients that if Aestus GeoTrax Survey™ scans are performed overtop of an extensive grounding grid, such as may be found at electrical substations, this metallic grid can effectively blank out our survey images in areas that run across the grounding grid. Although localized grounding rods are not a problem for GeoTrax Survey™, traversing robust grounding grids should be avoided. We can evaluate this issue on a site-specific basis.
Parallel Metallic Pipelines (Electrically Connected to the Earth):
Historically, Aestus has been able to achieve project objectives by selectively working around known buried pipeline locations to achieve high quality survey images and avoid interference from buried metallic pipelines.
When performing GeoTrax Survey™ transects immediately adjacent and parallel to buried metallic pipelines, some degree of interference may be experienced to metallic interference. (See our FAQ for more details on this.) We typically use a 15-foot (4.6 m) buffer zone rule when surveying parallel to metallic pipelines.Under certain conditions, such as those that are geology dependent, we can successfully survey within the buffer zone. If metallic pipelines are coated with epoxy, bedded in gravel, extremely rusted, etc., they may not interfere with our data collection because the pipes are effectively insulated from the earth.
Should we perform a survey perpendicular to a buried metallic pipeline, the pipe may appear in the image as a conductive anomaly and can cause a narrow conductive shadow at the bottom of the survey image. (See our FAQ on this issue.) In this situation, the survey image on either side of the pipeline location is intact with good data quality. This situation happens at many of our projects sites due to the presence of unavoidable utilities and we are able to work around it.
Calibrating Electrical Imaging Data:
It is important for Aestus to work in concert with our clients through our Aestus GeoTrax CSM+™ process to calibrate our electrical images to existing site data via data integration work and/or follow-up confirmation drilling. This is because Aestus GeoTrax Survey™ subsurface imaging technology fundamentally collects only data related to electrical properties of the subsurface and is not a quantitative analytical tool. Consequently, it does not immediately identify or quantify the chemical, geological, and biological (bioactivity) composition of anomalies detected on a stand-alone basis.
Our Aestus GeoTrax VIZ™ process integrates data for calibration and interpretation purposes, which typically include, but are not limited to, boring log data (PID, geology, etc.), analytical sample data, and fluid level measurements. This process allows us to effectively convert our electrical signatures back to the subsurface features of interest to our clients, such as physical (geology), chemical (contamination presence/absence and relative concentration), and biological (indicating potential presence/absence of bioactivity) signatures.
Typical GeoTrax Survey™ Electrical Signature Hierarchy:
It is important to note that the level of complexity present during data interpretation increases as more of the below signatures are present and the first three items listed below can often partially overprint signatures of underlying geology.
- Biological activity (biomass and related groundwater chemistry shifts)
- Remedial action fluids and/or their effects (e.g., injectates, or chemistry shifts that occur as a result of injectates)
- Presence of contamination (e.g., NAPLs, aqueous phase impacts, etc.)
- Groundwater (quality/chemistry)
- Soil and rocks (geology/lithology)
This overprinting is normally acceptable because identifying contamination and issues relative to remedial attempts and presence/absence of bioactivity is helpful information and the drilling data helps constrain the geological contacts and variability. The Aestus GeoTrax VIZ™ data integration process helps parse out issues of interest relative to project objectives.
Once calibrated to the subject site, the Aestus GeoTrax Survey™ images form a very powerful tool to accurately predict locations and frequently relative concentrations (and/or NAPL saturation) of contamination in subsurface soils. In cases when our images show subsurface features or anomalies that are not part of the existing conceptual site model, additional confirmation boring data may be required to correctly redefine the CSM.
Additional confirmation borings, should they be required, are limited in number because the Aestus GeoTrax Survey™ images provide specific drilling targets and therefore facilitate a very focused confirmation drilling program.