SMD

Subsurface Monitoring Device

Monitoring tool of underground water

The SMD logs in and transmits data relative to the electrical conductivity of waters at all levels of depth of the aquifer by geotechnical means.
Conductivity measurements are autonomously carried out which enables a continuous monitoring of the properties of the aquifer.

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No immersed electronic equipment
Longevity of system
Low maintenance requirements
Autonomous and remote controlled
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Salt Water Intrusion Meeting (SWIM)

« Electrode cable measurement prove to be the most cost effective and accurate method for exploring the depth of the fresh/saline groundwater interface. »

How does it work?

The SMD consists of two parts :

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The Data acquisition

Computer

A low-power consumption embedded PC system running on Linux and a proprietary software processing measurements acquisition, operation processes, data transmission, remote controlling.

Electronic cards

measurement card for each electrode.

Power supply of measurements

a DC power source for current injection.

Electrical shaft

The electrical shaft consists of a shaft fitted with a string of electrodes made of stainless cupper-aluminium (for resistance to corrosion) to carry out resistivity measurements. A measurement requires 4 electrodes. One or several pressure and temperature sensors are fitted with the electrical cable inside the piezometre.

  • conductivity = up to 57 measurement points. Pressure and temperature : upon request
  • Depth : as far down as 200 m deep.
  • Vertical resolution : from 30 cm to several metres
  • Radial resolution : 40% to 50% of spacing between two electrodes.
  • Diametre of the electrodes : 30 mm
  • Spacing between two electrodes : from 15 cm to 2 m

Measurement principle

The data acquisition software operates the electronic cards enabling the injection of a DC between two electrodes (A and B) and the measurement of potential difference between two electrodes. (M and N).
By using Ohm’s law average resistivity of the physical environment is then measured around the piezometre between the 4 electrodes. Repeating this process all along the shaft enables a global measurement of resistivity around the piezometre.

Then by using Waxman-Smits’ equation (3) conductivity of the aquifer’s water can be deduced. This equation requires knowledge of the formation factor (F) and of surface conductivity (Cs) which are determined at the time of the installation of the SMD by use of gamma ray loggings for clay grounds, electrical resistivity and sonic P-wave velocity loggings for porosity.

Finally, an electrical conductivity profile of aquifer water around the piezometre, normalized in temperature, adjusted by sink effect is carried out at regular intervals (4).

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