Geology Reference
In-Depth Information
IPMR13a and IPMR13b measured at the same place but at right angles,
show very different water contents and depths, which could be explained by
the presence of 2D or 3D structures that are not revealed in the same manner
by both soundings. Due to a low signal to noise ratio, it may also be possible
that the IPMR13b result are erroneous.
The cross-section presented in Fig. 8b summarizes the water content and
decay time deduced from inversion of PMR soundings at KB Tanda. The
higher water content and longer decay time suggest that the largest amount
of water is stored in the northwestern part of the profile (IPMR9, IPMR11).
The maximum thickness of the water-saturated weathered zone was found to
be about 15 m. A relatively short decay time and low water content (IPMR3)
is most probably caused by a greater amount of clay or silt in the weathered
zone. The low water content and almost waterless areas, as observed on
IPMR14 and IPMR8 can be attributed to unweathered zones (fresh rocks).
WELL LOGGING
Geophysical well logging provides information on the geologic framework
and the groundwater system at disposal sites. Log data can be used to plan
the location of pits, trenches and monitoring wells. They can provide specific
information on completion problems in monitor or injection wells. Logging
provides more continuous data on the vertical and lateral distribution of
effluent from waste than can be obtained from samples at a lower cost. Logs
can also be used for monitoring changes in water quality. In order to plan
a cost effective remediation programme, a thorough understanding of the
hydrogeologic system is necessary and much of the needed information can
be obtained economically from well logs.
Self-Potential (SP) Logging:
The SP log is a measurement of the natural
potential differences or self-potentials between an electrode in the borehole
and a reference electrode at the surface. The development of SP in bore-
wells is contributed mainly due to the differences in salinity and other
parametres between the fluid filling the borehole i.e. mud or water and the
quality of formation water. The diffusion of ions of different salts from one
medium to another and flow potentials generate spontaneous polarization.
Normally, clay/shaly rocks produce positive anomalies while porous,
permeable layers such as sands, fractured sandstone, and compact sandstone
give rise to negative anomalies. The interpretation of SP logs mainly consists
of identification of different lithological horizons and marking their
boundaries. This is usually done in conjunction with the resistivity logs.
Point Resistance (PR) Logging:
One of the simplest and very useful
logging methods measuring resistivity variations is called as the point
resistance (PR) logging method. In this technique, the resistance between
two electrodes (one of them is kept on the surface and the other is moved
