Volume 8 – Mechanical and Electrical Services - Malaysia Geoportal

Chapter 3 GROUND WATER FACILITIES
Down hole logging is usually aimed at providing more information on the formations penetrated by
the hole than can be gained from the drilling. The formation characteristics will have been changed
to some extend by the drilling. The presence of the hole fluid in it will distort the results of the
measurements of formation characteristics.
Invasion of the formation by drilling fluids must be restricted. Wall cakes must be thin and stable.
The drilling fluid must be of drilling solid.
Resistivity is measured in OHM-meter. The resistivity of a material can be used to help identify the
material. When the log is recorded, the logging operator will be able to provide the driller with his
figure for depth of casing, borehole and major formation changes. General interpretations depend on
these points:
- Low resistivity is usually associated with saturated formations and saline water
- Medium resistivity may be indicatives of fresh water sand or porous rock
- High resistivity is exhibited by dense and impermeable rocks
‘Normal’ Resistivity Logs are recorded using one current electrode and, at least, one potential
electrode down the hole.
3.8.5 Gamma log
Gamma ray log records the natural radiation of the formations. It is easy to run and can be very
helpful to the driller, particularly the water well driller. The gamma log has the advantages of being
able to record useful information in dry and cases holes where electric logging method are useless.
They are especially useful in providing a log of an old hole. The gamma log usually distinguishes
between aquifer (sand) and no water product beds (clays) in unconsolidated formations.
3.8.6 Calliper logs
The calliper logs measures the diameter of the borehole. In some cases, it will detect casing
couplings and reveal the position of the screens.
3.8.7 Fluid flow measurement
Measurement of flow, up or down a borehole is usually done by a propeller type flow meter
3.9 DESIGN OF GROUND WATER WELL
Proper well design, in addition to determining the depth and diameter for best yield, includes casing
selection, selecting an appropriate intake section, procedures for well development, testing, and
disinfection. Well design guidelines:
- Design by professional groundwater engineer
- Distant requirement for the target user
- Choose materials that will provide good service with price consideration.
- Design and select screens and construction steps under same priorities.
3.10 INSTALLATION OF CASING
Casing is used in water well to provide a stable hole, to seal the walls of the hole to exclude
undesirable water. In pump well casing is also to accommodate pumping equipment. Casing in
pumped water well must also have a sufficient diameter to accommodate pumping equipment,
strength to withstand during installation and use, ability to resist corrosion and other deterioration.
March 2009 3-7