How it works

The Willowstick method uses a low voltage, low amperage, alternating electrical current to energize the groundwater. Electrodes are placed strategically in wells, springs or surface water to inject electricity into the groundwater of interest. Because the groundwater is a conductor, the electrical current follows the groundwater between the electrodes. As the electrical current flows through the groundwater, the current generates a magnetic field characteristic of the injected electrical current. This unique magnetic field can be identified and surveyed from the ground surface using a tuned, sensitive magnetic field receiver.

The Willowstick instrument measures the specific magnetic field generated by the signature electrical current, filters out interference, and amplifies the signal. Repeated measurements are recorded over time to ensure consistent results. The recorded data is corrected to remove fluctuations due to natural phenomena and man-made interference. The horizontal and vertical magnetic field magnitudes and directions can be measured to further define the extent and paths of groundwater.

 

Survey data is normalized to show relative highs and lows. These are referred to as anomalies that represent areas of different physical conditions. After accounting for other conductive structures, the changes in conductivity represent an increase or decrease in the presence of groundwater. In the simplest terms, the Willowstick method identifies where groundwater is most present in the area of investigation. Willowstick has proven effective in delineating, mapping and modeling subsurface aqueous systems in many complex hydro-geologic settings for over 130 clients in a variety of industrial applications.

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Solution Offerings

Like an MRI before surgery, conduct a Willowstick survey before you drill, then focus your monitoring wells or exploratory drilling in problem areas. Reduce exploration and remediation costs and rest assured that you have an accurate characterization of your groundwater.

 

There are generally two deliverable levels for a Willowstick survey. We recommend you engage both phases, but if robust 3-D models are unnecessary, we will leave you with a clear, accurate footprint map or plan view map.

Magnetic Field Contour Map (MFC)

These maps provide an accurate view of the preferential flow of groundwater through all kinds of geology including fractured bedrock and karst. However, because these maps incorporate only measured magnetic field data from the ground surface, they do not provide information with regard to the depth of a particular flowpath. Some clients just need a target for a monitoring well or to gather data and do not require depth information, in which case a 2D MFC map would prove sufficient. Willowstick is happy to price your project to only this level of deliverable.

Advantages of MFC map:

• See preferential flow of electrical current along conductive features
• Identify locations of preferential groundwater flow and potential contaminant transport
• Target Monitoring wells on flowpath locations

Disadvantages of MFC map:

• No depth information
• Only a 2D solution

The figure above shows preferential seepage pathways coming from a large tailings dam. This information allowed the client to target the placement of monitoring wells.

3D Models

In most cases, understanding groundwater flowpaths from every angle is critical to enhancing site conceptual models so that better decisions can be made regarding the monitoring and remediation of a particular site. Willowstick generally employs two different modeling techniques when producing 3D models of groundwater flow.

Electric Current Flow Models (ECF)

Using the MFC maps as a guide, an electrical current is simulated in a modeling environment and a theoretical magnetic field contour map is created. Next, the modeler adjusts the depth and electrical current flow until he can match the theoretical magnetic field contours with the MCF map. Once a reasonable match is achieved, the modeler can assign depths to the individual pathways.

Advantages to the ECF model:

• 3D numerical solution
• Pinpoints flowpath location
• Simple interpretation showing permeable and impermeable areas
• Answers depth questions

Disadvantages to the ECF Model:

• It is not a unique solution
• Not as accurate as ECD models
• Sensitive to man-made conductive features on ground surface
• Base on simple wire model, which ignores homogenous electric current flow

The ECF model above depicts two seepage pathways (in blue) and how they move through the dam or under an existing grout wall. Depth information to any point along the flow path can be provided through this process.

Electric Current Distribution Model (ECD)

These models depict the distribution of electrical current through a given site. The data is subjected to an inversion algorithm (mathematical model) designed to predict the distribution of 3D electric current flow within the subsurface study area.

Advantages of ECD model:

• A 3D mathematical based solution for the entire survey area
• Visuals show slices at any horizon and depth
• More accurately resolves the depths of anomalous current flow than ECF model
• Much less sensitive to man-made surface interference

Disadvantages of ECD model:

• Shallow surveys require higher data density, thus increasing price
• Could be overkill for simple surveys
• Not a unique solution

The ECD model depicted above shows a distribution of electrical current through the foundation of an earthen dam. The horizontal slice shown above indicate a leak path through foundation rock.

Sensitive Resistivity Mapping

Sometimes we advise you use our proprietary RaMPS resistivity mapping in conjunction with the Willowstick method to help identify subsurface electrical properties of the subsurface rock and soils (horizontal and vertical changes). Understanding the geologic structure (its location, orientation and extent) along with groundwater flow paths is of significant value in understanding the overall groundwater regime.

RaMPS model above shows a fault and associated heat source contributing to a geothermal resource.

Proven results

View our case studies and see for yourself.