SUSTAINABLE WATER

Groundwater is an integral part of life and survival. To find, protect and keep our groundwater deposits (aquifers) drinkable, we need to not just drill a hole in the ground, but to gain a deeper understanding of potential threats and opportunities. This is where our non-destructive solutions give you the upper hand.

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Groundwater Prospecting and Mapping

Finding an aquifer that is impractical or unusable will not suffice. Proper prospecting needs to determine accessibility, longevity, quality, and security of an aquifer. Drilling blindly is also ineffective, costly, destructive, and requires lots of energy and heavy equipment. Here, our non-destructive geophysical solutions are both safer and more sustainable.

Aquifer protection and pathways

An aquifer can be a good source of fresh water for many generations, but it needs to get replenished by a reliable supply. Gravity will carry pollutants down into the soil and these can be transferred via complex pathways into distant aquifers, causing contamination. This is why an aquifer can suddenly become polluted, even though no surrounding area seems to be responsible. By using our solutions to identify natural protective barriers or potential pathways the aquifer’s protection can be examined and secured, and even monitored over time to alert authorities to potentially problematic changes.

Salination

An aquifer that is too close to salt (saline) water, such as sea water, can be at risk of salination. Through over-use, a borehole connected to such an aquifer can get increasingly mineralized or saline water drawn into it from depth or by lateral pathways. This is why we need to understand the surrounding geology and how salination zones can spread over time.

Our solutions

All materials have an electrical resistance, which defines how difficult it is for an electrical current to pass through that material. Water has low electrical resistivity, lowering the resistivity of most materials in which it is present. The more saturated with water a material is, the more its resistivity will change. Dissolved salt has very low electrical resistivity, which means that the higher the salt content of water the lower the resistivity. This makes it possible for us to differentiate between fresh water and saline water. If water is contaminated with a pollutant, its electrical properties will change. Exactly how it will change depends on the pollutant, but typically it will lower the water’s resistivity.

Our solutions make it possible to identify aquifers and assess their properties and surrounding geology in two ways:

ABEM Terrameter LS 2 water product image

Resistivity

By pushing electrodes into the ground and injecting a current, the ABEM Terrameter LS 2 can map the subsurface using resistivity. Two electrodes are used to inject a current, and a minimum of two electrodes are used for voltage readings. Knowing the injected current and the measured voltage, the Terrameter LS 2 calculates the resistance of the ground. Depending on the positions of the electrodes, the data will be collected at different depths (Z coordinate) and beneath different surface positions (X and Y coordinates).

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ABEM WalkTEM water product image

Transient Electro Magnetics (TEM)

With our ABEM WalkTEM solution, a transmitter loop is laid on the ground, which will be used to generate a magnetic field. A receiver antenna then measures the rate of the magnetic field's diffusion, which is recalculated to resistivity. All data will be collected beneath the same surface position (X and Y coordinates) but at varying depths (Z coordinate).

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The 4-Step Guide to Good Data

Before it is possible to reach the final result in terms of distribution of geological layers, presence of groundwater, depth to the groundwater table or determining the presence of saltwater or not, the data has to pass through a sequence of processing steps.

Step 1 - Data acquisition

Everything starts with careful and well documented acquisition of the geophysical raw data. GPS receivers in the ABEM Terrameter LS 2 and the ABEM WalkTEM instruments will automatically store the positions of the instrument along with the raw data, geographically marking where every data set has been collected. Make a note on observations during fieldwork that may have an influence on data quality.

Step 2 - Processing / filtering

Field data will never be perfect, so in order to prepare and optimize the data for the inversion process, noisy and faulty data has to be removed. Some parts of the data may need to be removed, if it has been have been influenced by

  • natural electromagnetic noise, which hasn’t been removed through stacking
  • effects from coupling to electrical or metallic installations
  • geological noise in terms of near surface inhomogeneities
  • data uncertainty caused by inaccurate fieldwork

Step 3 - Inversion (generating a geophysical model)

To turn the collected data into a geophysical model, advanced inversion software is used to perform the calculations. The software programs Aarhus SPIA TEM / DC solves the 1D inversion problem for VES and TEM measurements, whereas Aarhus Workbench or Res2DInv solves the inversion problem for 2D resistivity data, and Res3DInv must be used to solve the 3D inversion problem for resistivity.

The inversion process will result in a geophysical model which, in terms of resistivities and depth, gives the best image of the subsurface. The model will always be a more or less good approximation to the real world – hence it is a model – and inconsistencies between the assumed model and the actual earth will exist. It is therefore always important to consider any errors that might have been introduced by choice of the simplified model and also the use of other data sources, either physical or geophysical, to help constrain and improve the model.

Step 4 - Geological interpretation

The model created by the software is based on parameters of resistivity and depth and must be converted into geological properties and groundwater related information. For this purpose, the relation between resistivity and geological layer properties is essential. The electrical resistivity is an indicative parameter that allows different geological formations to be distinguished from one another.

In groundwater mapping, clay (low resistivity) can be distinguished from sand (high resistivity), or saltwater (low resistivity) from fresh water (high resistivity), or wet sand (lower resistivity) from dry sand (high resistivity).

However, the resistivity to material translation is non-unique and the resistivity interval of the various geological components overlap. Often a certain level of ambiguity may have to be dealt with. Different geological layers in completely different geological scenarios can have the same resistivity, hence some prior knowledge of the geological scenario from geological maps and boreholes should always be a part of a geological interpretation of a geophysical survey.

 

Customer cases

Fresh water intrusion in saline aquifer

Examination of the refreshing speed of salinated water in the Netherlands, with the use of Resistivity.

Groundwater mapping

Effective Bolivian groundwater prospecting with ABEM Terrameter LS

Aquifer protection

Geophysical investigations to ensure the protection of aquifer during re-route of highway.

Find your solution

Which of Guideline Geo’s geophysical solutions that is optimized for your particular requirements, depends on several factors. The tutorial, below, with focus on TEM and ERT/VES*, highlights some of the most important considerations to keep in mind. Contact us at Guideline Geo and we will guide you to the right solution.

* TEM (Transient Electro Magnetics), ERT (Electrical Resistivity Tomography) in 2D profile/3D grid and  the one-dimensional VES (Vertical Electrical Sounding).

Do you need to investigate in a rural or urban area?

You need to consider disturbances related to urban areas. VES and ERT are suitable for both urban and rural areas while TEM is not to be undertaken near urban infrastructure, due to the electromagnetic noise and signals induced in utilities, metal infrastructure and objects.

  • URBAN AREAS: VES , ERT
  • RURAL AREAS: TEM, ERT, VES
  • SOLUTIONS:
    VES
    –  ABEM Terrameter SAS 1000, ABEM Terrameter LS 2 VES
    TEM –  ABEM WalkTEM
    ERT –  ABEM Terrameter LS 2

 

                                                                                                                             

 

What investigations depth?

TEM is suitable for all depths, while VES, ERT down to 500 m depth

  • TEM  – from shallow investigations down to more than 500 m depth
  • VES, ERT – may be able to measure down to 500 m depth  (using recommended configurations)
  • SOLUTIONS:
    VES
    –  ABEM  Terrameter SAS 1000,  ABEM Terrameter LS 2 VES
    TEM – ABEM WalkTEM
    ERT – ABEM Terrameter LS 2

 

                                                                               

 

 

              

 

 

 

Do you need to investigate zones of bare rock/highly resistivity overburden?

In case of hard/dry surface, the inductive method TEM is recommended, since it does not require insertion of electrodes into the ground.

Note: Customized plate electrodes or electrodes drilled into the rock is an alternative solution if to still use VES, ERT. Contact Guideline Geo for further information

  • Bare rock/highly resistivity overburden: TEM
  • SOLUTIONS:
    TEM – ABEM WalkTEM

 

 

Do you need to map (near-) vertical or confined structures?

The ERT method provides better identification of vertical features, e.g. narrow fracture zones or faults than VES and TEM.

  • SOLUTIONS:
    ERT – ABEM Terrameter LS 2

 

 

Do you need detailed results or a broader prospection?

ERT generates a more detailed picture of the subsurface since the method collects many more data points. Broadly spaced TEM and VES soundings can be a useful method of rapidly assessing the overall structure of large areas. It is also possible to generate 2D profiles from a series of VES or TEM soundings.

  • SOLUTIONS:

ERT – ABEM Terrameter LS 2
VES – ABEM Terrameter SAS 1000, ABEM Terrameter LS 2 VES
TEM – ABEM WalkTEM

 

                                                                                            

Do you need a solution for only groundwater or other applications as well?

  • Only groundwater: TEM, VES, ERT
  • Other applications in addition to groundwater: VES, ERT
  • SOLUTIONS:
    VES
    – ABEM  Terrameter SAS 1000, ABEM Terrameter LS 2 VES
    TEM – ABEM WalkTEM
    ERT – ABEM Terrameter LS 2

 

                                                                                                       

                                                       

 

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