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Does Willowstick really work?
The short answer is, “Of course.” Willowstick has successfully completed over 250 projects, each of which affirms the efficacy of the methodology. The technology is based on the application of scientific principles that have been understood for decades and which have already been successfully employed in many other industries such as medicine, computing, and transportation.
Is the technology expensive to employ? What does a Willowstick survey cost?
The cost to perform a Willowstick investigation generally equates to the cost of constructing 2 to 4 exploratory / monitoring wells (depending on site access, type of terrain, cost of drilling for water, environmental and management issues, etc.). Based on documented case studies and repeated use of the technology, return on investment is normally positive and in many cases, very positive.
How does Willowstick work?
- Electric current is introduced directly into the water of interest at the elevation of interest.
- Earthen materials enhance the electrical conductivity of water.
- The electric current uses the groundwater as a path of least resistance to complete the circuit.
- These electrical pathways generate relative magnetic highs.
- The magnetic field is measured an recorded at the surface.
- This magnetic field reveals the location and depth of the groundwater connection paths.
For more information about how the technology works see the WHY-WILLOWSTICK page.
In what types of geologic settings can the Willowstick technology be applied?
The technology can be applied to most geologic settings where groundwater characterization is necessary. Such environments include unconsolidated sands and gravels (e.g. alluvial valleys and glacial deposits), sedimentary rock, fractured rock, porous basalts, and rock with dissolution channels.
What is the rationale for assuming that the signature electric current will follow preferential groundwater flow paths?
How is electrode placement determined?
Groundwater always follows paths of least resistance from areas of high potential to areas of low potential through areas of greatest transport porosity. Electrodes are specifically placed to introduce electric current in the natural direction of the groundwater flow (from up-gradient to down-gradient). This characteristic of groundwater flow and our ability to take readings and reposition electrodes based on real-time results is one of the key elements to a successful investigation.
The technology relies on the conductive nature of the groundwater to map preferential groundwater flow paths. In what range of subsurface conductivities, does the technology work best and can it be negatively impacted by saturated clays, sulfides, high TDS groundwater, salt water, etc?
Conductivity contrasts normally exist between zones of higher and lower saturation and/or transport porosity. The technology has proven successful in characterizing relatively clean groundwater aqueous systems (typical of domestic well water having conductivity values of roughly ±300-500 µS/cm). If groundwater is too ion free, with conductivities less than 200 µS/cm, it is difficult for the technology to delineate preferential flow paths.
Highly ionized groundwater such as seawater and brackish water can lend to high-contrast environments that can be very suitable for the technology. In locations where salt water interacts with fresh water our technology has been used successfully to identify the boundary between the two types of water.
Clay is highly conductive when saturated. Surveys have been successfully performed where clay was the dominant soil type. In each case, the signature electric current followed pathways or zones of greatest transport porosity where groundwater preferentially concentrates and flows through the clay and other materials. Hence, clay rich soils do not present problems for our technology.
Geologic semi-conductors, such as graphite-bearing shale, sulfide ores or iron rich formations, are rare in significant quantities and normally do not impact the use of the technology. As a matter of fact, the technology has been used successfully in a wide variety of mining environments.
Are there limits on the size of an area that can be investigated in a Willowstick survey?
Practically speaking, large areas (greater than 1 square mile), are best broken into smaller areas for more effective mapping and model creation. The only limiting factor is whether a proper electrical circuit can be established within the groundwater of interest. Large surveys generally investigate deep aquifers whereas small surveys investigate shallow aquifers.
Are electrodes required to be placed in wells? And if so, does a Willowstick survey require drilling new wells?
Electrode placement is essential to accurate groundwater characterization. However, Electrodes can be placed in wells and in reservoirs, ponds, seeps, springs, canals, or other sources where they can establish direct contact with the groundwater. In some cases electrodes are most effectively placed on the ground. Electrode placement depends entirely on site-specific conditions and the overall purpose of the investigation. Normally, wells are required for deep groundwater characterization and confined aquifers. However, for shallower applications (unconfined aquifers), wells are often ideal, but not required. In every case, it is preferred that at least one electrode be placed in contact with the groundwater of interest.
Every geophysical method has its limitations. What are the strengths and limitations of the Willowstick technology?
The Willowstick technology is superior to other geophysical methods in the following aspects:
- The technology was specifically designed for work in groundwater characterization
- The signature electric current targets groundwater of interest and identifies preferential flow paths
- Water bearing zones can be accurately characterized at more significant depths (hundreds of feet) than is available with other geophysical methods
- The methodology can be applied to inaccessible areas (steep terrain, developed areas, etc.) where traditional well drilling equipment cannot gain access or is not economically viable.
- Data interpretation is tuned to characterize preferential groundwater flow
- The technology is non-intrusive (leaving only footprints when the investigation is complete)
- Results of a survey can be used to located wells that must be drilled; avoiding the use of wells as a means of primary discovery
- Surveys can be completed quickly—sometimes in days, but never taking more than a matter of weeks
- The Willowstick equipment is light, mobile, non-invasive, and will not damage the environment
- The results of an investigation can improve traditional hydrogeologic models by adding key information that further defines the acuity of the model
Like all geophysical methods, the Willowstick methodology can have limitations in the following situations:
- Existing conductive features, such as grounding wires to power lines, can interfere with a survey. Generally, the effect of these features can be mitigated by our modeling and data filtering technologies
- Near-surface groundwater flow paths (if they become energized) can sometimes mask deeper flow paths.
- The technology cannot determine water quality, volume or direction of flow.
- The technology is unable to identify and map non-conductive materials (VOCs, non-conductive water, PCBs, NAPLs, etc.)
- The groundwater and host geology must have a contrast in electrical conductivity. If there is no such contrast, a survey simply cannot be conducted. This happens very rarely.
How does the Willowstick method differ from other electrical based geophysics?
There are several differentiating factors, but four stand out:
- First, the Willowstick technology is a unique application of Magnetometric Resistivity (MMR) designed specifically to identify paths of highest transport porosity in the subsurface.This methodology produces a primary signal in the groundwater of interest that can be measured from the earth’s surface.Most other electrical-based methods energize the earth and groundwater as a whole, and they often require resistive conditions (non-conductive overburden) to achieve good penetration and return of signal.
- Second, Willowstick measurements can be acquired from the surface of water, where technicians take measurements while in a boat.The technology can also be used to target a confined aquifer that lies beneath an aquifer or multiple aquifers.
- Third, the Willowstick method targets the groundwater of interest at specific depths.
- Finally, the technology measures the magnetic field instead of the electric field at the earth’s surface. The magnetic field penetrates conductive overburden whereas the electric field is shielded by it.As a result, there is no other geophysical technique better suited to map preferential groundwater flow paths than the Willowstick method.
How deep can the technology go?
The technology can characterize groundwater at significant depths as long as the receiver can resolve the magnetic field emanating from the targeted zone of interest. We have done several projects where the technology has been successfully used at depths between 1000 and 3000 feet. Our primary limiting factors are that the receiver must be able to accurately receive our signature signal and we must have an environment relatively free from other interfering forms of electric conductivity.
Most of our projects include depths between 30 to 500 feet, with the majority of projects in the shallower half of that range.And while our technology can be used in shallower environments, we find that investigations shallower than 30 feet are usually done by digging test pits or drilling relatively inexpensive observation wells.
How long does it take to complete an investigation?
Fieldwork generally takes one to two weeks for a typical project. A preliminary magnetic field contour map can be generated within a day or two following the data collection. Following the completion of the fieldwork, data interpretation and report writing normally takes two additional weeks. The whole project can be completed in roughly four to six weeks.For larger investigations, or in situations of greater complexity, the process may take longer.
Can the technology identify the direction of groundwater flow?
The technology does not identify direction of flow. In many cases, the direction of flow is obvious—but when it is not, other techniques must be employed to resolve the direction of groundwater flow.
Can the technology estimate quantity of groundwater flow?
The technology cannot determine quantity of groundwater flow.
How small of flow path can the technology resolve?
Detection of electric current flow is not limited by size so much as it is by the total conductance or electric current carrying capacity of a feature and the contrast with its host material (e.g. the receiver could detect a wire carrying 1 amp from 10,000 feet away). While the size or cross-sectional area of a conductor does increase its electric current carrying capacity, the question depends on several factors all at once, including the depth and the total amount of electrical current flowing though a feature compared to that flowing around it.
As an example, the technology has been successful in identifying seepage flow paths conveying as little as few gallons per minute at depths of over 100 feet.
Can the technology resolve questions about groundwater quality (e.g., can contaminant plumes be mapped)?
In general, the technology does not distinguish between minute changes in groundwater qualities (when nonconductive contaminates such as VOCs, PCB’s, NAPLs, etc.—measured in parts per million—are present in the groundwater). It delineates only paths of greatest transport porosity where groundwater preferentially flows from areas of high potential to areas of low potential. However, these pathways represent zones through which contaminates preferentially migrate.
The technology can, however, characterize reaction fronts—where groundwaters from different sources have significantly different electrical conductivities (e.g. salt and fresh water or where high and low TDS water mix).
Is the technology safe for humans and animals? How much electric current is used?
The Willowstick technology is safe for humans, animals and plant life. It is also non-intrusive and friendly to the environment. There are no negative effects from using the technology. The exposure to electromagnetic energy is comparable to being around typical household appliances. Like household electrical systems, the power source and equipment used in the technology should be respected and not tampered with. The voltage, current and magnetic field used in the technology are described below:
- The voltage used in the technology is 300 VAC or less (typical of household appliances).
- The amperage used is 0.5-2 Amps, which is comparable to the electrical current used in a 100-Watt light bulb.
- The magnetic field generated by the electric current flowing in the subsurface is weaker than a refrigerator magnet.
- The electrodes are connected together with an insulated solid copper wire laid on the earth’s surface.
- A small generator is used to energize the circuit wire and electrodes. The transmitter is equipped with a GFCI-like protection circuit (which shuts down two times faster than a typical bathroom GFCI) in the event of a surge.
In addition to the generator (which is the size of a typical suitcase) and the insulated circuit wire (connecting electrodes and power source), we use a “highly sensitive” receiver with small, passive induction coils to measure and record the signature magnetic field. The receiver is mounted on a survey pole and is hand carried across the survey area.
What is the positional accuracy of the technology?
Typically, the horizontal accuracy is between ¼ to ½ the measurement station spacing. Station spacing typically varies from 15 to 150 feet (or approximately 5 to 50 meters) depending on the investigation.
The vertical position accuracy depends largely on the degree of electric current focusing. In some instances the preferential flow of electric current produces very tight and clean anomalies that can be modeled with a fairly high degree of accuracy (depths to within 10% error). In other cases, the flow patterns are not as distinctive and the depths can only be roughly estimated. In such case, other data such as well logs, piezometric data, or other geophysical and hydrological data are important for a good estimation of depth.
Have survey results been independently verified?
Yes. Willowstick Technologies, LLC has been in business since 2004. Over 250 projects have been successfully completed throughout the world—for a variety of industries—and in many different types of geologic settings. A significant amount of experience and confidence has been gained with the technology. Many of the projects have been ground-proofed (refer to Willowstick’s web site for a list of clients and documented case studies). Over 70% of clients are return customers. The majority of these clients are highly sophisticated industry leaders. This, in itself, is proof of third party verification.
What do the deliverables look like? What kind of information is included?
Deliverables consist of a written report detailing the survey objectives and results, along with maps and data in electronic format, including contour maps & shape files, inversion data, compiled Matlab® models, or other specific client requests of any data used to create the maps, figures, tables, and models presented in the report. This enables the client to view, compare and analyze the data and results on their own (or using a third party) as well as incorporate the findings of the investigation into in-house models, reports, memos, presentations, etc.
How well does the technology work in highly developed areas where conductive culture exists (i.e. industrial sites, cities, landfills, etc.)?
There have been a number of successful investigations performed beneath highly developed areas (i.e., industrial parks, oil fields, landfills, community developments, etc.). Nevertheless, these areas can be challenging.
Artificial conductors (or culture) can provide an alternate pathway for the signature electric current away from the targeted groundwater. This can cause large anomalies that hide or interfere with the magnetic signal coming from the groundwater paths.
Isolated pieces of metal are not problematic because they do not facilitate electric current flow from one electrode to the other. Only long continuous conductors are a concern.
The best approach to investigate an area with long conductive culture is to identify the artificial conductive features before a survey and strategically design the survey to avoid them—to whatever extent possible. In most cases, the location of conductive culture is known.
Wells or bore holes are often required to place electrodes at strategic depths to minimize stray electric current from flowing onto conductive culture. In most cases, the risks can be qualified for a specific site.
Has the technology been peer reviewed?
The Willowstick technology has been featured in a peer reviewed article that was published in the SEG’s The Leading Edge magazine, February 2011 issue.
Has the technology ever failed to provide intelligent information?
Yes, the technology has failed in a few instances. In one case, the circuit could not be established due to the high purity of groundwater. In another case, man-made culture caused too much interference making it impossible to distinguish the signal and interpret groundwater flow paths near the conductive features.
Can the technology be used to map leaks in synthetic liners?
The technology can map leaks through liners or cut off walls if electrodes can be positioned strategically on either side of the liner. In some cases this is impractical. Nevertheless, if electrodes can be positioned on either side of the liner and an electric circuit established between the two electrodes that flows through holes in the liner in amounts that exceed any capacitive effects, then the electric current flow patterns will reveal the location of the leaks.
Has the technology received acceptance from the regulatory bodies?
The technology has been mandated for use by both State and Federal regulatory agencies for specific projects. This does not mean, however, that the technology is endorsed by such regulatory agencies. Governments are typically very careful not to endorse specific techniques or products.
Why should I use the Willowstick method instead of Electrical Resistivity Tomography?
The Willowstick method has been specifically designed to identify groundwater flow paths. Electrical resistivity tomography is designed to identify sub-surface structures from electrical resistivity measurements made from the surface. If you are looking for groundwater flow paths then the Willowstick method is the best choice for your application.