How do I find groundwater?
You can find the groundwater level, estimate the volume of an aquifer, map water bearing fractures etc. or try to determine the water quality (i.e. fresh, brackish or saline) using one or several different geophysical techniques or by invasive geotechnical investigations such as drilling.
The choice of method will normally depend upon the local conditions at your investigation site, and whether the groundwater is found within loose soils, porous bedrock or bedrock fractures. In most cases, the best solution for you will come down to a choice of undertaking resistivity (VES and/or ERT) or TEM investigations, together with a program of physical testing such as drilling. Here are some considerations to make in order to settle upon the best investigation methodology from the Guideline Geo line-up of products.
Scale of investigation
If the investigation area is large, say more than a square 1km, then employing a “search strategy” using TEM or VES might be a suitable method to start with. These are both point-wise investigations, quickly giving a nice estimate of the depth to groundwater. These methods can be used in isolation but also as a way to pin-point interesting or complex areas where ERT investigation can be added for a more detailed picture.
Depth of investigation
The depth to the groundwater surface will impact heavily upon our choice of geophysical method as some will be more or less suitable:
- 0-10 meters – VES, ERT and, in some geological settings, GPR
- Less than 40 meters depth – VES, ERT, TEM, and GPR under some special conditions, see Overburden
- 40-500 meters depth – VES, ERT, and TEM
- More than 500 meters depth – TEM is suitable for deep investigations where VES / ERT might be limited by access, equipment power or general survey practicalities (e.g., need for very long cables).
Rural or urban location
Due to electromagnetic noise, as well as interference from signals induced in utilities, metal infrastructure and objects, TEM cannot be undertaken in urban environments or within 150-200 meters of buildings/infrastructure. Deep resistivity investigations may not be easy in urban areas (due to length of cables and need of grounding of electrodes), but it is at least possible to collect useable data.
Asphalt and bare rock, or other very hard or dry surfaces, are tough conditions for VES and ERT as the electrodes need to be grounded. The most effective way to deal with this is to drill a hole for each electrode, work that can be quite time consuming; however, use of specialist plate electrodes, for example, can improve efficiency. In such hard and arid areas, TEM might be a better solution, being an inductive method and thus not requiring the insertion of electrodes.
Groundwater in bedrock fractures
1D methods (such as VES and TEM) are not that well suited to identifying laterally confined features like fracture zones and intrusions compared to 2D methods (e.g., ERT) which offer more lateral detail. Choose ERT for the best way of identifying and dealing with the effects of vertical features, such as narrow fracture zones or faults.
GPR for groundwater
If the geology is predominantly coarse grained, giving a free ground water surface, without any capillary fringe, it may be identified by GPR survey. In other soils, the capillary fringe causes a gradual change in the dielectric constant, meaning that there will be a weak reflection or no reflection at all from the groundwater surface. GPR can of course be used for other groundwater applications instead, including mapping of the bedrock topography (to estimate storage volume), the extent of protective clay layers, the thickness of friction soils etc.
Seismic methods for groundwater
Depending on the aquifer type, in some situations it can be easy to detect the groundwater table using seismic methods as there is a marked velocity contrast when saturated material is encountered. But, depending upon porosity and water saturation, it can be hard to see a large or sharp enough contrast with the surrounding geology to be able to tell with a higher degree of certainty if groundwater is present or not.
If you have need of ground investigation comprising more than just groundwater prospection and mapping, resistivity is a more versatile method, compared to TEM. Electrical resistivity can be used for, among other tasks, geological mapping, stratigraphic studies, mineral prospection, contaminant mapping and monitoring, and archaeological applications.
And as a final tip; combine methods to gather the most comprehensive data of your groundwater source and improve interpretation!
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