Environmental1

Groundwater and environmental: borehole logging


MONITORING

Geophysical logging of boreholes prior to the installation of piezometers permits a cost effective means of evaluating hydrogeological conditions, and aids in the optimum design of piezometers and groundwater monitoring boreholes. Fluid logging of monitoring boreholes aids in the selection of sampling depths and is a means of monitoring water quality and movement. Checks of the completed piezometer and borehole construction can also be made using geophysical logging techniques and downhole video cameras to ensure correct installation of screen, linings, gravel pack, and bentonite or grout seals.

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Monitoring of groundwater around a landfill site using fluid logs

  • Identification of flow zones
  • Groundwater quality monitoring
  • Identification of aquifers
  • Location of coal bands and worked ground
  • Location of fractures and fissures

NITRATE DETECTION IN GROUND WATER

The water quality log (below) shows a high nitrate water in flow/crossflow in an exploration borehole. The high level of dissolved oxygen would suggest that this water originated from near to the surface. The caliper log shows a large fissure corresponding with the nitrate and oxygen highs, which would indicate that this broken section of formation is the source of the nitrate. The results of the logging allowed the production borehole construction to be optimised so that this fractured formation, producing high nitrate surface water, could be cased out ensuring only water with a much lower nitrate content was produced.


Advantages of Nitrate Logging:

  • Quicker and cheaper than taking core or water samples for chemical analysis
  • Instant results on site, water column is profiled
  • Repeatable techniques for borehole monitoring
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Example water quaility log
key: Cal – Caliper mm, T – Temperature °C, EC25 – Electrical Conductivity µS/cm At 25°C, DT and DC Differnentials, DO – Dissolved Oxygen % saturation, ORP – Redox Potential mV, NO3 – Nitrate content ppm