FAQ
Frequently Asked Questions
FAQ
Frequently Asked Questions
Health and Safety
Yes. For each measurement, our surveys use less than one amp of current for less than one second, which is considered safe for humans. Pets and livestock are kept clear of the area to protect the equipment.
Yes, particularly relative to other high-resolution site characterization (HRSC) methods. Electrode stakes are driven into the earth typically 12 to 16 inches deep, which is above most utility burial depths in theory. Aestus uses both public and private utility locating services to promote the protection of utilities and our field staff.
How it Works
GeoTrax Survey is a geophysical survey technique that provides high-resolution, two-dimensional (2D) images of the earth’s subsurface. It is analogous to a CAT Scan in the medical industry.
It is used to map geology, locate environmental contamination, target production water wells, identify potential geohazards, monitor changes over time (temporal imaging), and has other uses.
Electrical current (safe level) is injected into the ground, and the voltage is measured coming back out. Ohm's Law is used to calculate resistivity:
- Four electrodes yields one measurement data point (a “pixel”)
- Iterative measurements yield a matrix of data points, or “pixels”
- Proprietary software generates a subsurface image from the datapoints, which is similar to a digital camera taking an electrical picture of subsurface
The theoretical resolution is 1/2 the spacing of the electrodes.
Example: 1- meter spacing results in a 0.5 -meter data resolution. A 10 -meter spacing results in a 5 -meter resolution.
The depth of the image is 1/5 (20%) of the length of the transect line (electrode array) on the ground surface. So a 500-foot-long transect line yields an imaging depth of 100 feet below ground surface (BGS). Based on the imaging objective depth, we evaluate how to best obtain the data for the project objective.
We have imaged up to 1,000 feet BGS, but our stakes only go into the ground about 12-16 inches. The technique is indirect. The flow of current generates the image, not the electrode.
We use 3/8-inch diameter stainless steel electrode stakes, which are driven into the soil typically 12-16 inches deep. Cables are connected to the electrodes and to field computers that collect the data.
We pre drill a 1/2-inch diameter hole through the pavement such that our electrode stakes can be driven into the soil a short distance below the bottom of the pavement. We patch the holes in the pavement upon completing our work on site.
We use a standard 12-volt-deep cycle marine battery as a power source. A current of ~350 milliamps is “injected” into the ground. Because this is relatively low current, it is safe for pets and people to travel past our survey lines. People should avoid contact with electrode stakes and be careful not to trip over our geophysical cables.
Please do not drive over our cables as this will damage them. We have traffic ramps to place over the cables and allow vehicles to cross.
No. We can design our survey layouts around known utilities. You should avoid surveying immediately parallel to metallic pipelines, and if possible, turn cathodic protection off as a safeguard.
The physics of the technology dictates that the shape of the resulting survey images is trapezoidal. Therefore, if the surveys were performed with the ends touching and no overlap, there is effectively a hole in what would otherwise be a continuous image along the transect line.
Vegetation clearing in straight lines is required prior to GeoTrax Survey field work, if dense vegetation exists. While large trees can normally stay, mitigation of trip hazards due to low lying stumps, etc., is important.
Yes, provided the area is safe to walk on. Steep slopes, high to low elevation changes, piles of soil, concrete walls, etc. are all manageable obstacles.
Yes. Please notify us in advance of field work if particular site material or specification is mandated. Otherwise, we will clean stake holes so they are free of any loose debris. We will fill holes with caulk patch material (asphalt or concrete colored). Finally, we will smooth patches so they are level with the surface of the concrete or asphalt.
Special Site Considerations
Yes. Aestus can:
- scan a proposed slurry wall alignment to find zones of competent geology, which might cause problems during installation;
- locate slurry walls (post construction);
- image zones of slurry loss (during construction); and
- scan a known slurry wall alignment to check for integrity of the slurry wall.
It should be noted that slurry walls can be observed on a one-time basis to evaluate if flow is moving beneath or laterally around the wall. Additionally, temporal (GeoTrax Monitoring) data can be used to test the effectiveness of the wall at installation and over time. This is especially helpful if the wall is a permeable reactive barrier and the flow dynamics need to be assessed.
Yes. GeoTrax Survey lines can run across and parallel to railroad tracks. The survey lines can go over the tracks, and in many cases, electrode cables can be threaded through the track ballast beneath the track. This allows train traffic to continue during data collection.
Depending on the railroad’s governing body’s regulations, GeoTrax Survey work within ~25 feet of an active line or any electrode cables crossing over top of the rails will require no train traffic for a duration of approximately 3 hours per survey line. Sometimes, GeoTrax Survey work can be completed during non-operational hours for trains. We rely on our clients to attain the right entry permits and permissions necessary to work on railroad property.
Aestus field crew members currently have or have recently held the following railroad safety certifications:
- Roadway Worker Protection (www.railroadeducation.com)
- Railroad-specific safety courses
Depending on the railroad, additional training or safety classes may be required.
Typically, no special equipment is required. Aestus will follow safety regulations from the governing railroad, which usually includes wearing reflective safety vests, hard hats, safety-toed boots, and safety glasses.
No. Aestus relies on our consulting firm and/or railroad clients to provide one or more flagmen who will coordinate track time and communicate with any active trains in the vicinity of our work.
Yes. Electrodes along the cables are supported or floated at the water surface. Strong wave action or currents increase logistics.
Confirmation Drilling
Similar to the medical industry where scanning data is calibrated via biopsies, or the oil industry where geophysics is calibrated with borehole data, we need to calibrate our imaging data (via targeted drilling) to contaminant concentrations, geology, and bioactivity levels.
No. Our clients have local site drilling and laboratory experience and are typically closer to the site to perform confirmation drilling work more cost effectively.
All sites are different, and we can provide an estimate during the proposal process. The goal is to minimize drilling over time by focusing these efforts via targets from our electrical imaging.
Typically, we recommend installing monitoring wells to better assess groundwater chemistry and the presence or absence of non-aqueous phase liquids (NAPL), especially in fine-grained media. For example, it can take months for NAPL to show up in wells screened in clays. Most sites are regulatory driven by groundwater concentrations, so avoiding potential false negatives is important. An iterative discussion of confirmation drilling is recommended.
You should schedule confirmation drilling approximately 12 weeks after the last day of field work, which is just after we deliver your interim report with specific suggestions regarding drilling locations and sample intervals. This allows for data integration and discussions of confirmation drilling plans.
Aestus provides recommendations on sampling protocols that allow confirmation of GeoTrax Survey image results based on attaining regulatory endpoints. We provide a table of confirmation drilling locations and analytes for each project.