How does MERIT work?
Surface locations are laid in an array in the same fashion as surface electrical resistivity and a survey is performed.
The length of the array and the spacing of the electrodes determines the depth and resolution of the images.
Implants electrodes are permanently installed in the subsurface at a designed depth and spacing at similar locations along the array.
Surface electrodes and implanted electrodes are connected to electrical resistivity survey instruments.
Measurements are made by creating a signal within the ground using two electrodes and receiving this signal at another two electrodes.
The two arrays (one on the surface and one implanted) create overlapping electrical fields in a tomographic configuration.
These overlapping electrical fields enable exponentially more data points to be collected. Where typical surface electrical resistivity survey may have hundreds data points MERIT has thousands of data points resulting in far higher resolution images.
Implants are left in the ground for long term monitoring.
What types of geologic conditions can MERIT be applied to?
MERIT is based on the geophysical technique called electrical resistivity. Most sites are resistivity friendly, and this geophysical technique has been successfully applied worldwide for last 100 years and in most geologic conditions including:
- Seepage in dams
- Groundwater depth
- Lithology favorable for groundwater
- Contamination of groundwater
- Depth to freshwater-saltwater interface
- Location of sinkholes and cavities
- Detection of fractures and dikes
- Thickness of overburden
- Geologic structure
- Archeological sites
Can MERIT detect seepage pathways?
Yes. Electrical resistivity is a well-accepted geophysical technique that has been utilized to detect seepage pathways in dams. Extensive geophysical surveys using ER have been conducted by universities, USACE, Bureau of Reclamation and water management districts to detect seepage pathways in dams and levees. MERIT is simply a dramatic improvement of this established technique.
How is MERIT implanted electrode placement determined?
The location of the implants is evaluated based on the desired target and depth needed. The spacing of the electrodes determines the resolution. Typically the resolution is ½ of the electrode spacing. So if we want to see a 5 foot target the spacing of the electrodes need to be 10 foot or less.
MERIT can typically reach depths with ½ the array length required of surface electrical resistivity arrays. Where possible in ‘top of rock’ surveys the implants are installed as near the rock surface as possible to give the highest resolution. For surveys where depth of investigation is considerable MERIT implants are placed between 50 to 100 feet deep.
What type of an area that can be investigated in a MERIT survey?
MERIT can be applied where surface electrical resistivity is typically used along profiles, reaching the same depth as surface electrical resistivity survey but in ½ the distance. This makes MERIT ideal for areas with limited space.
Does a MERIT survey require wells?
No. MERIT implants are placed into the unconsolidated material using direct push technology typically above the rock. The direct push tools are only 1.5 inches in diameter and are minimally invasive.
What are the advantages and limitations of the MERIT technology?
MERIT increases the resolution and accuracy by increasing the signal strength through a second array in the subsurface which places the electrodes closer to the target depth.
MERIT permanent implants provide the ability to monitor sites over years. Once installed, the cost or repeated surveys reduce significantly to only the geophysical data collection and analysis.
MERIT has been proven to obtain details of competent rock profiles to depth of over 150 feet.
MERIT can provide good stratigraphic profiling if the general stratigraphy consists of material of different electrical properties.
No specialized equipment is required to install MERIT implants. MERIT uses standard direct push tooling and supplies that are common.
MERIT Limitations
Like surface electrical resistivity, MERIT uses a contrast in stratigraphic layers of different electrical properties. For example, if stratigraphic layers are resistive/conductive/resistive contrast is high and results are good. An example of unfavorable stratigraphic layering would resistive/resistive/resistive where the contact between layers would not be well defined.
ER has some inherent limitations that affect the resolution and accuracy. Like all methods using measurements of a potential field, the value of a measurement obtained at any location represents a weighted average of the effects produced over a large volume of material, with the nearby portions contributing most heavily. This tends to produce smooth curves which may not lend themselves to high resolution for interpretations.
Another feature common to all potential field geophysical methods is that a particular distribution of potential at the ground surface does not generally have a unique interpretation. Although these limitations should be recognized, the ambiguity of the resistivity method is less than with the other geophysical methods. For these reasons, it is always advisable to use complementary geophysical methods in an integrated exploration program rather than relying on a single exploration method. (USEPA https://archive.epa.gov/esd/archive geophysics/web/html/resistivity_methods.html)
How does the MERIT method differ from other geophysical techniques?
MERIT is simply a unique application of a proven geophysical technique. It is designed to image the deep geology and detect anomalous features. The enhanced electrical resistivity signal produced by MERIT configuration increases the penetration and the resolution at depth. In this way MERIT far surpasses surface based geophysical methods.
MERIT not only has the capability to provide the stratigraphic layers, but also the structure of the subsurface in detail.
MERIT can penetrate the overburden and target the interface of the unconsolidated sediments and the rock, giving key insights into foundational base or karst development.
The implant system of MERIT is permanently installed into the subsurface for application of long term monitoring.
How long is the expected life time of the MERIT permanently installed electrodes?
Our designed implants and coaxial cable are built to military specifications and for use underground. Life expectancy is 50+ years.
How deep can the MERIT reach?
The depth of electrical resistivity surveys is dependent on the array length. Typically the depth is 1/5 the array length for surface electrical resistivity. However due to the implant technology increase the depth and decreases the array length MERIT can prove deeper surveys over shorter lengths For Example to reach 100 feet
Surface electrical resistivity would be requiring (500 ft. /5) = 100ft
MERIT with implants at 50 feet would require only (250 ft. /5) + 50ft. = 100ft
How long does it take to complete a MERIT Survey?
For small projects MERIT would require only a couple of days’ installation and geophysical data collection. Larger projects with more complex installation require one to two weeks. Data processing and report writing will vary from a days to weeks depending on the complexity of the project.
Can MERIT technology help identify seepage pathways?
Yes. MERIT implants can be modified to also receive self-potential signals. Self-potential is a passive geophysical method the help detects water flow, which also has a long history of applications at dams. Self-potential will not detect groundwater flow direction, but will indicate where potential seepage pathways exist. Self-potential studies are performed on the ground surface over a limited area like a dam. However, the signal strength on the ground surface is very faint. When equipped with self-potential MERIT implants are installed in the water table where the self-potential signals are much stronger. A full array of self-potential implants can provide a stable, long term monitoring platform for the detection of seepages and leaks.
How small of a target can the MERIT resolve?
The detection size of the target is a function of the electrode spread (typically ½ the electrode spread). So typical 10 foot electrode spacing would resolve a target 5 feet or more in size. Depth plays a role as well; targets near the end of the penetration depth are harder to see. A detailed study conducted MERIT was published in the journal of Applied Geophysics.
Can the MERIT technology be used to determine contaminant plumes or groundwater quality?
Yes. MERIT can identify conductive leachate plumes from contaminates such as landfills or saltwater intrusion. In addition, the identification of groundwater potential zones (clean water) using electrical resistivity technique is common. Identification of the fracture zone and its thickness is another application.
What is the vertical accuracy of the technology?
MERIT technology has strong correlations with verification drill data. Through the application of the tomographic array configuration and the increased number of data points collected MERIT has been able to correctly identify the stratigraphic units and depth. (See MERIT Case Study _Geotechnical)
Have MERIT results been independently verified?
Yes. Verification of the theoretical, mathematical, optimization, laboratory results and field trials have been peer reviewed in the Journal of Applied Geophysics.
MERIT results have been verified independently by FDOT, FDEP and geotechnical consultants. References are available.
What kind of deliverables are included?
Deliverables include a written report, (with scope, purpose, and results) along with figures, forward models, inversion data, and maps in an electronic format for use by the client.
Is MERIT affected by cultural features (ie. pipes, underground utilities)?
No. MERIT has been successfully applied near cultural features with little or no influence.
Is MERIT expensive?
No. MERIT permanent installation is comparable (or less expensive) than other high end geophysical technologies such as Seismic Reflection, Magnetometric Resistivity, or Cross Borehole. However, MERIT’s cost drop significantly with repeated use to typically less than 1/3 the initial installation cost. With a 50 year (or more) life cycle the return on investment is substantially higher than the cost of repeating surface methods that do not leave infrastructure for long term monitoring in place.
Is MERIT a valid geophysical technique and has it been vetted, peer reviewed, and published?
Yes. MERIT took over 3 years to develop and was assisted by an of international team of four prominent PhDs in geophysics. Some publications on MERIT include:
Improved imaging of covered karst with the multi-electrode resistivity implant technique. Authors: Kruse, S. E.; Harro, D.
Improved 2-D resistivity imaging of features in covered karst terrain with arrays of implanted electrodes Authors: Kiflu, H. G.; Kruse, S. E.; Harro, D.; Loke, M. H.; Wilkinson, P.B.
Optimized arrays for 2D resistivity surveys with combined surface and buried arrays Authors: M.H. Loke, H. Kiflu, P.B. Wilkinson, D. Harro and S. Kruse
Improving resistivity survey resolution at sites with limited spatial extent using buried electrode arrays, H. Kiflu, S. Kruse, M.H. Loke, P.B. Wilkinson, D. Harro
Has the MERIT technology ever not worked?
Yes. Some sites are not good for the application of electrical resistivity. However, forward models can be constructed based on available information such as previous geotechnical or geophysical reports and simulations insure that the client receives the desired results. If no information is available surface electrical resistivity can be performed to insure MERIT will achieve desired results.
Have regulatory agencies accepted MERIT?
Yes. MERIT has been used by Florida Department of Transportation (FDOT), Florida Department of Environmental Protection (FDEP), Orlando utilities Water ConservII and Florida Turnpike and Florida Gas.